OK, this seems straightforward from searching AVS and studying theater wall treatment...

FRONT WALL: Acoustical treatment (1" Insul-Shield) floor-to-ceiling.
CEILING: No acoustical treatment - none, nada.
FLOOR: Thick, plush carpet is fine.

But here's where it gets confusing, and I need help...

SIDEWALLS
A) Acoustical treatment (1" Insul-Shield) from floor to ear-height (44"), with 16oz polyester batting above.
--or--
B) Acoustical treatment (1" Insul-Shield) from floor-to-ceiling on all 1st reflective surfaces.

These 2 theories seem to contradict each other. So which is it?


BTW, for those searching for Insul-Shield type product, here are the substitutes which seem to have identical acoustical absorption ratings:

Owens Corning Select Sound Black Acoustic Board
Owens Corning Fiberglas 703 Series duct insulation.
Johns Manville Insul-Shield
Johns Manville Linacoustic Permacoate rolls.
Certainteed Certpro Acoustaboard Black
Knauf Duct board EI-475
Knauf Duct liner EM

...personally, I found the Knauf EI-475 easiest to find (4' x 10' sheets @ $40) from a general heating and air conditioning company.

First, (B) is wrong. Virtually every surface is a first reflective surface (speaking of walls). "B" is actually treating those early reflections which meet the listener's ears within a time frame described by Helmut Haas (further researched by Toole and Olive). Depending upon the delay due to the longer path to the ear will result in the sound being perceived as a distortion, or echo. As well, sounds reflecting off a surface suffer timbre shift. Next, you don't care a twit about "early reflections" that don't intersect the listening position.

The 'early reflections' technique is more typically found in two channel playback environments where higher reverberation times in the room are required to make up for the absence of surround channels (or intelligent surround processing techniques).

JB,

I agree with Dennis that B is wrong. I'll go even further and say you should never cover any large surface area with material that absorbs the mids and highs. It makes the room too dead sounding, and does nothing to solve the inevitable low frequency problems. Much better is a mix of bass trapping and mid/high absorption, with no one area all live or all dead.

--Ethan

Now "A".

Requirements for multi-channel (more than 2) are different than that required for 2 channel.

In multi-channel, the entire wall behind the front speakers is treated. You want none of the back reflections to overlay the surround field or the bring the reverberent field forward (your reverberent field and surround field is created by the multi-channel processor or mix, not so much the room as is mandatory for 2-channel). Depending on speaker placement, this treatment is brought forward along the side walls. Wall treatments are floor to slightly above ear level (where exactly is also a function of front speaker heights). While one could argue the sound at their feet is of no concern, often that square footage of treatment is required to bring the room's RT60 down to the lower levels required for multi-channel playback.

If you have soffits, the bottom of the soffits is also treated...several reasons, right tricorners among them.

Sorry, two additional questions...

1) Is 1" Insul-Shield (or equivalent) adequate for the front wall, or should it be increased to 2" if possible?

2) If side wall treatment is only be up to early level, then what should be done to fill out the 1" fabric above ear level: polyester batting or 1" styrofoam insulation to maintain the full reflective surface? I've heard both argued, but don't know if there's much difference.

I would not use the 2" material...it will very likely be too absorptive.

The fill out is polyester batting. Don't use styrofoam.

The diferent reqiurements for 2 channel and multi channel is interesting. So obviously there is no way to make a room work as well for both uses as it would for each task specifically. However what should be the general guidelines for a room that would be used for both 2 and multi channel - so as to do the least damage to either format?

Compromise between the two playback scenarios is not a good option. Effectively you're saying you're (a) willing to spend a bunch of money and (b) happy to make the room sound poorly in either case.

If you have a good surround processor and a well set up multi-channel room, play your 2 channel recordings in multi-channel mode...a better result. I can assure you a good surround processor will do a whole bunch better job of creating the spaciousness than your room can accomplish.

Originally posted by Dennis Erskine
If you have soffits, the bottom of the soffits is also treated...several reasons, right tricorners among them.

Very informative thread as usual from Dennis. So are the undersides of the Soffits treated for absorption (Insulshield) or diffusion (batting)?

Tom

Since this thread is going, I also have a question -

You have mentioned the front wall and the side walls for acoustical treatment, but what about the rear wall? Is it different with a 5.1 setup vs a 6.1 or 7.1?

Pam

Originally posted by obie_fl
Very informative thread as usual from Dennis. So are the undersides of the Soffits treated for absorption (Insulshield) or diffusion (batting)?

Tom

Tom,

I don't want to speak for Dennis but the underside of your soffits should be treated for absorption. (Insul-Shield)

Treating the underside of cove lighting soffits will have to be explained to me... especially if they are at 8' height (with the majority of ceiling at 9'). If one is using dipole surround speakers, it would seem that treating the soffits could make the surrounds less diffusive and more identifiable to the ear.

I am also interested in the question Pam asked about the rear wall in a 7.1 setup. I will likely use duct wrap on the front wall and below ear level on the side walls. The consensus seems to be nothing on the rear walls or ceiling - correct?

Another question - A non-perf screen reflects quite a bit of sound, but does it pass enough to acoustically treat the wall behind it?

Dennis

I am curious about the room treatment used specifically on the front wall behind Omnipolar speakers(Mirage OM 10). Does too much apsorption defeat the Omnipolar characteristics of a speaker?

Does too much apsorption defeat the Omnipolar characteristics of a speaker? Yes. And, that's what you want.

Originally posted by JBS
These 2 theories seem to contradict each other. So which is it?

Hello JBS.

You may want to check my free Mediaroom spreadsheet. Send me email at proufo@cantv.net.

As Dennis points out, music listening has different requirements than HT, especially with stereo/mono sources.

At this point I believe the best arrangement for a multi-purpose room is to make it deadish for HT and perhaps m-ch mixes, and use the same setup for recreating or extracting ambiance for 2-ch music sources.

Best regards.

I would offer that treating the undersides of large soffits could overdamp a room. Ideally you'd leave it alone until tested.

Ted

But what about that rear wall guys???

Do you treat it or not? Do you treat it differently if it were 5.1, 6.1, or 7.1 surround?

(Can you tell I'm getting close to that part of my construction?)

Please help this pitiful female...
Pam

Mine is treated the same as the sides.

Of course, I was just following the plans, which was a great relief to me.

Randy

Thank you, Randy! I feel better now....

Crisis averted for now. Hysteria is gone.

Pam

Yep, That's classic Dennis construction right there!

There are two types of walls: Front walls and all others.

Again, this is for HT rooms, multi-channel playback not dedicated two channel.

Ted

For what it is worth, my opinion is that the rear wall is a bit different than the sides.

In smaller rooms, you can use the rear wall as a point to place diffusion. This helps with standing waves a little, helps RT60 a little, and does not overly damp the room making it sound too dead. You can mix a bit of absorbtion ( I do opposite the screen since you can't treat that on the front wall) but not so much that you kill the surround reflections you want/need.

Overall, I prefer strips of absorbtion, say 9-12" wide by 1- 1 1/2 " thick by 5' tall. Put these at first reflection points for the main fronts, and the rest every 3-5' along the side walls with the center of the panel (vertically) being about your ear level. You end up with about the same sq. ft. coverage as the whole bottom up to ear level but it is distributed throughout the room more effectively IMO. Yes, the front wall should be totally dead.

Keep in mind that mine is for music and for theater. Personally, I think HT can sound very good in a 'properly designed' 2 channel room with just a few mods that don't hurt 2 channel that much. I have not heard the same results from 2 channel in a 'properly designed' multichannel room. Usually sounds very dead and the soundstage is severely closed in. If I am going to err to one side or the other, I will err toward the 2 channel design.

As for having to play all my music through multichannel because I did the room for HT, I'll pass. I spent a ton of cash getting components that recreate music well without a lot of processing. I'm not going to pass them through multiple signal modifications.

Don't misunderstand me. Dennis is describing good practice, theory, and lots of experience with DEDICATED HT environments. If that is your goal exclusive of music listening, go for it. If you are like many of us who also use this space for a high quality music listening space, play with it a little and see what you like. After all, that is what it's all about - what YOU like. It's your room.

A couple of minor points: diffusors do not affect RT60 in the manner described nor will they have any impact on standing waves.

While vertical strips can create a moderate amount of diffusion, you cannot cover all the early reflection points to all seats effectively. In the 'ear level' scenario, you do have 'stuff' bouncing around above your head; but, if you watch your angle of incidence against where your speakers are placed (height), you've covered all the bases within the curve. This is also a much more effective method with rows of seating since you'll have more people close to the side/back walls than in a two channel, single seat of excellence environment.

In multi-channel...no bad seats not one good seat.

Not suggesting that you hit all the first reflection points, just the side wall points from the main L and R.

Diffusion (diffraction) does indeed assist with RT60 and elimination of standing waves. Look at any anechoic chamber. There is NO abosrbtion. ALL diffusion. No standing waves in there. What is the RT60 in an anechoic chamber (understand they are all different, rhetorical question.)

If the the absorbtion is placed appropriately (not directly across from other absorbtion) and diffusion allows the wave to be changed from a first axial to a tangential reflection onto various absorbtive features, you are in fact lenghtening the time till it hits another reflective or absorbtive surface, thereby extending the time and distribuiting the wave across more absorbtive surfaces (assuming good polar plots on the diffusors).

Like I said Dennis, don't get me wrong. For pure HT, your way is probably best. I want it all. I want 1 good seat AND no bad ones. Your 'non-bad' ones may be a tiny bit better but my 1 good one is killer when its just me sitting down listening to music.

An anechoic chamber is all absorption (ever done research in one?). Like this one for example:
http://www.bell-labs.com/org/1133/Research/Acoustics/AnechoicChamber.html

Sound decays (STP in air) at the rate of 1/r squared. The rate of decay is not affected by reflection (a form of diffusion). It is affected by distance and absorption.

The purpose of diffusion is to randomize reflections which will result in axial, tangential and oblique reflections.

RT60 is the time it takes for the sum of all reflections to decline 60dB once the sound source ceases. The calculation is RT60=0.049(V/Sa). [where V is volume, S is surface area, and a the absorption coefficient). In an anechoic chamber RT60 is statistically 0 since there is no reflection. Sound reflecting from a wall surface or a diffusor will decay at the same rate.

The location of absorptive materials (except in certain cases in small venues) has no impact on the RT60; however, the placement and type of diffusors will have impact on the proper creation of the reverberent field.

Originally posted by Dennis Erskine
An anechoic chamber is all absorption (ever done research in one?). Like this one for example:
http://www.bell-labs.com/org/1133/Research/Acoustics/AnechoicChamber.html

Sound decays (STP in air) at the rate of 1/r squared. The rate of decay is not affected by reflection (a form of diffusion). It is affected by distance and absorption.

The purpose of diffusion is to randomize reflections which will result in axial, tangential and oblique reflections.



Learn something every day. I always thought the wedges were all edge hardened in those chambers so they acted more like diffusors. Guess I will have to stand corrected on that one.

My comments were only made to present the poster with options, not to rebutt any other previous statements. They were also stated understanding that I was trying to provide a compromise solution not knowing what the poster's use was for the room.

Even though what I was describing may not be significant in pure RT60 terms, I still don't get how forcing a portion of a wave to travel through more air and through more absorbers before bouncing back at the user from the front can't help. Remember, I was only discussing diffusion on the rear wall, not the sides. If it takes the same amount of time to decay but each wave bounces back at my position fewer times and from more different directions within that space of time, how can that not help the situation?

Besides the above stated, it appears to my ears, to provide 2 other benefits. In surround mode, the rear portion of the surround field gets scattered more evenly thereby increasing the sense of envelopment. In 2 channel, forcing the wave to travel a longer distance before coming back off the front wall makes the space appear acoustically larger than it actually is. Not all rooms require this but you don't run the risk of sucking the life out of the sound if you put some up. Like you said, reflection does not effect the rate of absorbtion.

Also, if I the room is a bit live, the only time I would worry about it is when it is just me in there. At that point, I am probably listening to music so it works better that way. When we are watching movies, there are 4-12 wonderful abosobers that get added to the room called people spread throughout the middle of the space.

I have heard rooms done both ways. Both can sound very good when doing multichannel duty. Like I said before, I have just never been able to find that one sweet spot for stereo when you totally optimize for surround, even when sitting in the nearfield of the front speakers. Just my opinion but I'll sacrifice a tad on the ultimate in multichannel quality to get that sweet spot back.:D

Diffusion is helpful. Placement is important. The object is the creation or (or preventing the destruction of) the reverberent field.

I have a quick question for you guys regarding sound treatment....

I've read and understand that the area below ear level should covered with Insulsheild (or an equivalent) and above should be covered with batting. My question is, for the second and possibly third rows, should you raise the height of the insulsheid to accomodate what would be below ear level for those on risers? I.E. from front wall to first row Insulsheild is installed to the height of 4ft. Once you get to the row with a 12" riser, do you then increase the height of the insulsheild to 5ft or do you go with 5ft from front to back? What is the 'general' rule to follow?

TIA.

Some do, some don't. The idea is to cover a certain percentage of the wall with more absorbtion. If you go higher on shorter walls, you are actually increasing the percentage of the wall that is covered.

I don't know any general rule of thumb about this. Dennis could probably provide a more experienced answer for this type of implementation.

So to follow up on dpape's last question...

What goes above the lower panels? I was thinking of just wraping "board" with polyfill and then covering in GOM. I'm hoping to have all the panels just look the same size and appearace.

My concern is the sound absorption (or lack of) if I dont use the right materials for the foundation of the upper panels.

Anyone have a sugesstion?

Thanks

Polyester quilt batting is the recommended upper covering right on the drywall - the hard part is finding the 1" thick stuff. I have discovered it at Hancock Fabrics. Also check quilt stores.

pam

Thanks Pam,

It seems (from post & pictures) that people are just putting the batting directly on the wall and covering it with fabric? This seems pretty difficult to me?

I was thinking of using 2'x4' sheets of "board" (1/4" plywood?), covering them with the batting and wrapping them in GOM fabric. The trick (and I still don't know how I'll do this) is to then mount these on the wall. My thinking is that this will give me a nice "matched" look of panels (some sized differently for looks), and I should be able to get the upper wall panels and lower wall panels to match - much like the custom theater rooms I've seen.

This method also allow me to build the panels on the floor and glue/staple the fabric perfectly before I hang them on the walls. I think it should give a better result than trying to cover the batting directly on the walls?

Am I missing something here (other than how to attach the panels to the wall), or shouldn't this give a very finished look??

Dave

Originally posted by dmcvie
Thanks Pam,

It seems (from post & pictures) that people are just putting the batting directly on the wall and covering it with fabric? This seems pretty difficult to me?

I was thinking of using 2'x4' sheets of "board" (1/4" plywood?), covering them with the batting and wrapping them in GOM fabric. The trick (and I still don't know how I'll do this) is to then mount these on the wall. Dave

Dave,

This is a great idea. I seen a similar project on HGTV the other day. They were using 2X2 panels and alternating colors. They attached them with 2-3 inch wide Velcro. Makes it easy to change patterns?

This was not a theater project but would work just the same.

Just a thought.

Scott

Thanks Scott,

Good idea on the Velcro. I'm wondering if there any any "accoustical" concerns for this type of construction or method of hanging them? (Dennis:D )??

Sure it has to sound good, but it has to look good too.

Dave

Dave,

The reason for the polyfill on the upper wall is due to the fact most build out the wall with the firing strips and insulshield on the bottom. Covering the upper, drywalled half with fabric and no polyfill gives a sag in the middle of the stretched fabric.

The polyfill only serves as fill, it theoretically has little acoustical impact (some diffraction, eh?) but still lets the sound waves hit the dry wall and be reflective and alive.

One could due 'hard" reflective panels above and soft, absorbent below ear level (keeps dialog and such clean from first reflections).

There are some heavy duty picture mounting "spikes" which could be used for mounting purposes. We used them to secure our pictures so they wouldn't rattle.

Good Luck,
Adam

Thanks Adam, that helps my understanding. I took a look at your pictures, Great Job! I hope to accomplish a similar look.

Is the entire rear of you screen wall covered in Insulshield? Can you tell me where you got it? I was going to use sound board, but I think Insulshield does a better job at lower freqs.

Thanks again for you input.

Dave

Thanks.

Yes, we covered the entire area behind the screenwall (the screen is actually mounted on a false wall about 18" in front of the real wall. We also covered the side walls next to the speakers, top to bottom (the speakers are another couple inches in from of the screen wall).

Think of a theater stage and my speakers are in the "wings" in front of where the Theater curtains would be (my hope eventually is to mount some drapes).

I actually used the Owens Corning Sound Select due to some issues about getting the Insulshield locally. The specs are quite similar. Let me know if you need it, I have the PDF or web address around somewhere if you can't find it on their website.

Adam

Hello Dennis:

:confused:
Originally posted by Dennis Erskine

The location of absorptive materials (except in certain cases in small venues) has no impact on the RT60 :confused:

You REALLY shouldn't say that:

Look at picture (copyright) below:
Measured by me in official University Lab.
Method as per ISO 354 - standard reverberation room method (except for position) on the same day with exactly the same rigid rockwool slabs framed with very light metal frame to screen off edges.
You could almost ask: which one do you like best.
So the only varying factor is the position in the room and how it's used.

Also look here:
www.ib-neubauer.com/Literatur/17_th_ICA_2001_Rome_1.pdf
Predicting Reverberation Time: Comparison between Analytic Formulae and Computer simulation with CATT room acoustic program.

Sabine and Norris Eyring assume statistical distributed absorption, boundary conditions and reflections.
Millington allows assymetric absorption.

Fitzroy and the improved Higini Aray Puchades approach try to solve the shortcomings of the above approaches in function of assymetric distribution of absorption.

An official Reverberation Room is tuned with added diffusers to the MAIN material sample spot.
There is the edge effect and so much more.

Best regards
Eric

Originally posted by Eric Desart
Look at picture (copyright) below:


Sorry I've no idea how to enter the picture.
I resized it it's not too large (nor size nor bandwidth) but it doesn't appear.

If somebody can give a hint (no time to try for hours just for a picture).

Regards
Eric

Well, Eric, you are exactly correct. As it is true that making glittering generalities about any complex process can get one into hot water when shifting through the details later.

However, as I've noted in other postings, as room size decreases, traditional methods of predicting RT60 cannot be considered terribly accurate without use of ray tracing techniques. (In actuality, as architectural and other elements -- people for example -- become larger with respect to total room size. As you are aware, the same would hold true for any attempts to accurately model or predict modal resonances without both ray tracing and finite element analysis. As well, with variances in physical "as built and installed" properties, we find stochasic analysis in monte carlo simulation helpful. These models, however, have the down side of running for multiple hours. In the end, models are useful to prevent expensive errors and the effort is to be statistically accurate such that once we can measure the actual space we're faced with 'tuning' rather than rebuilding.

Originally posted by Eric Desart
Look at picture (copyright) below:


Tryed it once more. Hope it appears.
It's hard to believe this are results from the same material measured in environment designed to exclude as much as possible site phenomena.

Please respect the Copyright of this picture resulting (as part of) extensive study.
It mainly shows how difficult it is to transfer Sabine values to real life circumstances.
It also shows that one should be really careful when interpreting Sabines.

Sabines are only partly a material property.

Best Regards
Eric

Dennis,

I agree with you.

This subject goes to far in detail.
Real life alpha is indeed one of the most difficult things to define exactly.
A lot of it is empirical.
In geometric acoustics (ray or cone tracing) it's a bit easier (averiging affect), with BEM and FEM, however accurate the models, the problem indeed remains the input (boundary impedance).
A lot of it are statistical empirical approaches.

I'm just responding to general statements, which can cause confusion.

Best regards
Eric

Well, yes, we're always faced with the garbage in/out problem...the accuracy of models are highly dependent upon the accuracy of the data (and our assumptions). What we've found with out monte carlo in finite element analysis (data input by density function, point estimate or normal distribution) is we can begin to refine the estimates based upon a result history. I also have found in many cases the assumption is the data is right so when the measurements are done and do not follow prediction prediction, there's a tendency to be looking in the right place...sort of follows along with the issue that many are not properly interpreting the measured results. (Now, I am rambling. Sorry all.)

My research (grant approval not withstanding) now is in the area of various construction techniques (as applied to acoustic solutions) and am looking to generate some research into the issues of Timbre matching surround speakers.

I like the idea of using panels on the walls instead of a complete batting covering for the top half of the wall. In fact, if you put a ledge molding instead of chair rail around the walls, do you even need treatment on the upper half? If so, could you use the panels as dmcvie suggests? How far apart would you have to place them on the wall?

In reference to making panels for the upper half of the side walls, here's a link (http://www.avscience.com/acousticpannel.htm) that I've kept handy for quite some time. It's from the AVScience website and was written by Andrew Tierney. While it might not be the end-all beat-all way to do acoustic panels, it gives someone a good guideline to follow if they want to go this route.

As for which one is easier to do, I have no idea. I'm not even to that stage of construction (going to be putting up drywall in the next week or so). Someone else might be able to chime in and enlighten us on the matter.

Greg,

> it might not be the end-all beat-all way to do acoustic panels <

That's not a very impresssive design. You'd get much better results with a lot less work by simpling wrapping one or two inch thick 703 or 705 rigid fiberglass panels in fabric.

--Ethan

Like I said, it's not the end-all beat-all way...but it gives someone who has never seen an acoustic panel or has no knowledge on how to make one a guideline. I can neither confirm nor deny their effectiveness as I have not built any. I simply put the link out there, if for no other reason, to help someone figure out how to create their own acoustic panels.

Thanks for the link. Bottom line is if possible, I would prefer to NOT cover the entire upper wall with the batting, but if that is the easiest and cheapest way to do it, I will go that route. I just can't see it being the cheapest because of the wall fabric.

I am very interested in an answer to "gjlowe" question:

"if you put a ledge molding instead of chair rail around the walls, do you even need treatment on the upper half?"

My plan is to treat the whole lower half (ear level and below) of the room as suggested (except full treatment on front wall). However, I have read mix messges on the value of using cloth covered batting on the top. Some say it is for acoustical purposes and some say that it just pushes the upper wall out to meet the treatments of the bottom half. I personally would like to just paint the top half and build a ledge in the chair rail to save money if batting doesn't add any additional value. So which is it?

What is the rule about hanging pictures on the upper walls?

Some treat the lower half of columns and others do not. Is there a reason to go one way or the other?

I see many putting heavy curtains along the front wall which looks very nice. What impacts does this cause on the front wall treatment? No impact or cancel it out?

Finally, do people treat the wall area behind the screen?

Thanks for any feedback.

Greg,

> Like I said, it's not the end-all beat-all <

I wasn't criticising you! I just wanted to warn readers that half an inch of foam is not very useful as acoustic treatment, no matter how pretty it looks.

--Ethan

I really could you use some advice based on the following questions:

-----------------------------------------------------------------------------------------
I am very interested in an answer to "gjlowe" question:

"if you put a ledge molding instead of chair rail around the walls, do you even need treatment on the upper half?"

My plan is to treat the whole lower half (ear level and below) of the room as suggested (except full treatment on front wall). However, I have read mix messges on the value of using cloth covered batting on the top. Some say it is for acoustical purposes and some say that it just pushes the upper wall out to meet the treatments of the bottom half. I personally would like to just paint the top half and build a ledge in the chair rail to save money if batting doesn't add any additional value. So which is it?

What is the rule about hanging pictures on the upper walls?

Some treat the lower half of columns and others do not. Is there a reason to go one way or the other?

I see many putting heavy curtains along the front wall which looks very nice. What impacts does this cause on the front wall treatment? No impact or cancel it out?

Finally, do people treat the wall area behind the screen?

Thanks for any feedback.
-----------------------------------------------------------------------------------------

Any response from the experts would certainly be appreciated: Dennis or Eric?

now obviously we use the insulshield to get the RT60 down to a good level.

Is the general rule of using insulshiled up to ear level and below all the way around still followed when using a drop ceiling with acoustical tile?

I would think the acoustical tile would lower the RT60 somewhat and less insulshield would be needed.

Originally posted by gjlowe
I like the idea of using panels on the walls instead of a complete batting covering for the top half of the wall. In fact, if you put a ledge molding instead of chair rail around the walls, do you even need treatment on the upper half?

No, you don't. The batting, as I understand it, is not there for acoustical purposes, but simply to provide some support under the fabric that you would install so that the upper wall surface is flush with the lower wall surface. If you don't care about that, as I don't, then simply install any type of molding on the top of the ledge to hide the staples.

In my case, I simply stapled fabric directly to the drywall surface on the upper walls. If you do this, I highly recommend painting the drywall surface flat black first. Otherwise, the white drywall surface can sometimes reflect light back through the fabric.

Great thread! I have a complex situation. I'll provide some background, then a question:

We have (for now) a multi-use room in our basement. It is 14x30 and I have a theater in a section that is 14x16. The rest is a playroom for our daughters.

I have treated the front wall. The screen, front and center speakers are 30" from the wall. There is a 36" TV and 72" tall cylinder sub behind the screen and curtains on the sides.

Behind the seating area which is 12' from the screen are two MDF columns that are 4' apart. These columns are 16' from the front wall. There are MDF shelves running between the columns that actually form 2x2 cubes. The idea was to form a room divider and a diffusive surface with these (imagine a tic-tac-toe grid between the columns). The side walls are currently treated with acoustic panels at reflective points back to the rear columns (16').

I plan to rework the room and treat the sidewalls with GOM.

I know that's a lot of description and visualization.

My question:
Do I need to treat the entire 14x30 room or can I stop at the 16' mark? Remember, the rear columns/shelves form a room divider which is probably more visual than anything. But, the wall treatment could stop there and it would look fine. I'm just not sure about the sound.

Maybe I should just leave it as is until I can repossess the entire room from our daughters.

Jeff

OK. Into the fray regarding wall, ceiling, and floor treatments to tune the room. Construction of the walls and ceilings (assume a concrete floor) is a whole different issue.

The screen wall wall should be soft absorptive. Ditto for the first couple of feet from that wall along the sides. The side wires generally should hard absorptive, say absorptive below 1000 cycles, reflective above. However spot soft absorptive is absolutely required. The placement is determined by the first reflective point for each of the three front speakers for each seat in each row of chairs. The idea is that each seat wants to hear only direct sound from each of the front three speakers. NO refctive sound please. Go to a commercial theater. You will soft absoeption just above your head on each side. That is there to ensure that the row behind you just like the row in front of you hears onle direct sound. LCR speakers in a commercial theater and hopefully yours as well have wide horizontal dispersion and narrow vertical dispersion. I assume risers for the second and more rows in your HT. Dependent on room dimensions and seat locations, you will need some spot soft absorption for the first row, and behind it but higher some for the second row and so on. Assuming close rear wall location to the last row, the rear couple of feet of the side walls and the rear wall should be diffusion on the top half and hard absorption below that.

The ceiling is another issue and is dependent on ceiling hight (I assume about 8 ft) and whether your center speaker is above, behind, or below the screen. High mounted left and rights enter also. Basically, with an above screen center (i.e., with the center mounted very close to the ceiling), you will need some soft absorption right in front of the center (a 2x4 ft) panel. With high mounted left and rights but with some distance to the ceiling, you will need some soft spot absorption (say four 2x2 ft panels) probably 2 spots just in front of the center two chairs and above the chairs on either end assuming a four chair row of HT type recliners.

Side and rear surrounds are generally mounted near the ceiling and these locations cry out for ceiling diffusion in front of each speaker. The rest of the ceiling depends on the type of ceiling. You do not want a painted sheet rock surface. At a minimum use some acoustical bubble filler in the paint or a textured sand. Hard absorption is good but expensive. Acoustical tile say with an NRC of .75 but you should weight down the tile by gluing sheet rock to the back and then you should put some fiberglass wall or ceiling insulation over that. Treatment of the space between the joists is an issue of room isolation rather than room tuning though it will affect the tune.

Your floor should be carpeted but do not use a foam or rubber pad. Horse hair jute is the best but is generally no longer available because of youths smoking it for the hemp. 1/2 inch felt padding to me is the next best thing BUT put the bonding surface side down (normal carpet insulation is to put it up so that the tape the installers use to hold pad sections together will not tear the pad).

So how do you figure the first reflective points for the LCR speakers?

You can pay a qualified acoustician to calculate the room modes, reflection points etc, and recommended surface treatments and the locations for those treatments. We charge our clients $1700 for this service and do the interfacing with the acoustician for the client. There are a variety of of the shelf programs which can also be used.

A simple way to locate first reflective location points if the room is built is to replace each front speaker with an incandescent light bulb. Use the woofer for the center point assuming a 2 way speaker. Next sit in each seat and have an assistant move along the walls with a small mirror. You will see a reflective point for each of the 3 front speakers along each of the 2 side walls for each chair. These points are the first reflective points. Ditto for ceiling reflections.

The mirror method is "close enough for government work". Some thoughts:
1. be sure the mirror is flat against the wall/floor/ceiling;
2. consider all the seats (no bad seats);
3. check height as well as distance down the wall;
4. the tweeter is more critical than the woofer, none-the-less, your speaker or light bulb should be at the exact position of the speaker once installed (height, distance from wall etc.);
5. recognize you are *not* interested in just the first reflections...it's early reflections which would include reflections from the wall behind the speaker (unless you're treating that surface).
6. recognize that not all early reflections are bad...you're looking at the initial time delay gap along with the "Haas" effect, so "time" is more important than "first".

Dennis,

> recognize you are *not* interested in just the first reflections...it's early reflections which would include reflections from the wall behind the speaker (unless you're treating that surface). <

That's a good point, and brings up a question you can probably answer for me about the importance of absorption on the front wall.

There's no question that loudspeakers radiate low frequencies in all directions. So those will bounce off the front wall and collide with direct sound from the speakers. But since mid and high frequencies are radiated from the front of the speaker, why is mid/high frequency absorption so often used on the front wall behind the speakers?

--Ethan

Mark,

So you can't get the floor treatment because bozos are smoking it to get high?

That is hilarious!

There's no question that loudspeakers radiate low frequencies in all directions
Sound from the speaker radiates as a sphere. Thus all frequencies radiate in all directions. The speaker cabinet causes defraction but does not "cast a shadow". (One of the reasons the formula for determining acoustic energy decay (SPL) is 1/r^2.)

[If sound at all frequencies did not radiate spherically, you couldn't hear a speaker standing behind it. Although, I do claim I cannot hear what my son is saying when he's looking the other way. ;) ]

I agree with Dennis.

Normally HTs are small, ranging from say 300 to 700 sq ft. The smaller the room, the more need for treatment behind the seats (rear wall and side walls). As to reflection points between the speakers and the seat, many will fall very close together so it's not like you have to treat a lot of spots. For a two row theater with the second row on risers, say 20 ft wide and 25 ft in depth, the side-wall soft treatment need not exceed about 24 sq ft per side.

The wall behind the front speakers needs to be dead. You want imaging. So by deadening the front wall you increase the point source characteristics of the front array. The problem here is that the screen often occupies a significant portion of the front wall and that sucker is reflective.

And yes, horse hair jute, in my many years of 2 channel experience, the best carpet pad for making a room sound good, is no longer commercially available. Too many "utes" were smoking it with bad side effects.

So treatment is a room 13ft by 20ft would need to be more aggressive on the side and rear walls than in a room 20X25ft.

I would say as a generalization, the closer you sit to the rear wall, the more it needs to be treated (with diffusion). Obvioiusly, if the rear is an an infinite distance behind you, there is no need to treat it. Likewise for the side walls. On the other hand, the bigger your room, the further away you will likely sit from the screen, say 15 or 16ft away at the closest instead of say 12 1/2 ft. The further away the more near reflections rather than just a first order you will need to treat, so there really is no clear generalization.

As Dennis says below (the power of an edit :)), you really can't get a small room to sound (e.g., decay) like a large room. However, diffusion will help.

As Mark pointed out, this really not a good thing for generalizations. Having said that, as a room gets smaller the time delay between the initial sound and the reflected sound decreases and the relative dB SPL increases. This would suggest different treatments. Part of the difficulty in smaller rooms, is developing the capability to create the reverberent field. As the delay becomes shorter and the difference in dB SPL between direct and reflected sound decreases, you can easily have problems.

My room is very small compared to most here. My finished dimension will be approx. 9'3"W x 21'L with 18' in the length visible since I will have a false screen wall with an acoustically transparent screen.

I am going to cover the front wall as suggested with sound absorbing material, but given the width of my room, would I need to add more insulsheild to the side walls than the standard 4' (or ear level) height? Also, when it's mentioned that the insulsheild is placed below ear level, does this ear level refer to those on the riser(s) or just those in the first row?

Proudx,

> So treatment is a room 13ft by 20ft would need to be more aggressive on the side and rear walls than in a room 20X25ft. <

You need to distinguish the type of treatment. There's low frequency absorption, mid and high frequency absorption, and mid and high frequency diffusion. Smaller rooms need more absorption. Diffusion should never be closer than ten feet from your ears, so that rules out diffusion entirely in very small rooms. In your 13x20 room you should be sitting forward of the half-way point anyway, so diffusion on the rear wall could be useful.

--Ethan

Mancubus:

Haven't seen your room...but, I'd not suggest you use more Insul-Shield. I'd suggest you start six inches higher from the floor and bring the top higher.

Ear level...whether or not it's in the nose bleed seats or orchestra pit.

I disagree with Ethan. The closer you sit toward a rear wall, the more you need diffusion.

Sorry to lower the tone of the conversation, but I'm a little confused about how this stuff is used. Is it a layer that would be placed over sheetrock (and then covered, for example, with GOM), or is it used in place of sheetrock? Or is it something else entirely?

Mark,

> I disagree with Ethan. The closer you sit toward a rear wall, the more you need diffusion. <

On what do you base that? If anything, the closer you are to the rear wall the more you need bass traps because of the acoustic interference issues discussed here in great detail recently.

Dr. Peter D'Antonio of RPG wrote an excellent tutorial on diffusion, which you can download from his company's site. He's sort of the father of modern diffusion theory, and he is very clear that you should never place diffusion on any wall that's closer than ten feet from where you sit.

Here's a direct link to that PDF file:

www.rpginc.com/news/library/TechBullDiffCritList.pdf

--Ethan

jschlege
I know how you feel. I stay semi-confused about acoustics all the time. I have some 4x8 Homasote that I bought from Home Depot. I'm going to use it on my front wall only. It's too expensive at nearly $18/sheet. I'll try to find something else for the sides.

This is a question for the experts: I'm placing mine over the sheetrock but shoud I place firring strips over the sheetrock first and leave a space or is it OK to screw it directly to the sheetrock. My screen and speakers are all 30" from the wall. Also, I see lots of HT sites that don't have fabric on the walls. I've also seen some in AV Interiors that really look like paint. I assume that it is not absolutely necessary to cover this stuff with fabric?
Jeff

Originally posted by jschlege
Sorry to lower the tone of the conversation, but I'm a little confused about how this stuff is used. Is a layer that would be placed over sheetrock (and then covered, for example, with GOM), or is it used in place of sheetrock? Or is it something else entirely?

My room is 14x20 with a drop ceiling at just under 8 feet. I plan on putting Linacoustic on the entire front wall (except where the screen is), from top to bottom on the first 2 feet on the side walls, and from ear-level down on the other three walls. I plan on painting the sheetrock on the upper half of the rest of the walls with some sort of treatment at the first reflection points of the front speakers. I also plan to use a thick carpet pad. Does this sound like effective treatments?

Jeff,

> I'm placing mine [homosote] over the sheetrock but shoud I place firring strips over the sheetrock first and leave a space or is it OK to screw it directly to the sheetrock. <

Homosote and other thin absorbent materials are effective only at higher frequencies. But all rooms need absorption at all frequencies. You can improve the performance of homosote a little by spacing it away from the sheetrock. Much better is to space it six to twelve inches away, and pack the resulting air gap entirely with fiberglass. This way the fiberglass can do some serious absorbing, and the homosote gives an exposed surface that's easier to make look nice.

--Ethan

I guess I'm still a little confused. I was planning on using Insul-Shield. Is that installed on the studs like sheet rock, or over the sheet rock. I appreciate that you could probably set it off from the sheet rock and insulate in between, but that would give me insulation insul. shield over insulation over sheetrock over insulation, and I'm not sure I want to go there. If insul. shield can replace sheetrock, I would prefer just to do that (with the appropriate insulation behind.

Insulshield is placed over the sheetrock. Sheetrock is screwed to the studs.

Do some searches and you will find how all the acoustic treatments come together. The begining of this thread is a good start.

Thanks. The begining of the thread has very useful information on placement/type, but not installation. I just need very basic installation info. for the acoustic treatments and GOM.

All the comments have been very helpful. Although I have one question regarding corner treatment.
I am planning to put some corner bass traps in my HT. But at $500 each, It's not an option for me. Is there anything less expensive yet work as effectively? Thanks for your advice!

JS,

> The begining of the thread has very useful information on placement/type, but not installation. <

See my Acoustics FAQ, tenth in the list on my Articles page:

www.ethanwiner.com/articles.html

It explains these issues in detail, including how to mount rigid fiberglass.

--Ethan

Ethan Winer...Please remove from your signature your link to your site and also please stop directing people to you for business. Please be so kind to re-read the rules of the site for this is not allowed on AVS Forum.

Thank you kindly.

Originally posted by jmsun
All the comments have been very helpful. Although I have one question regarding corner treatment.
I am planning to put some corner bass traps in my HT. But at $500 each, It's not an option for me. Is there anything less expensive yet work as effectively? Thanks for your advice!

google.com


Cut and paste (many links)!! (http://www.geocities.com/jonrisch/a4.htm)

DIY will save significant $, and [done properly] give the same result.

Mark,

> I disagree with Ethan. The closer you sit toward a rear wall, the more you need diffusion. <

Some further clarification on this:

Last night I met with Wes Lachot, a top notch recording studio designer, and asked him about this. Wes said that ten feet should not be considered a hard cutoff, and he's seen diffusion be useful even when it's only eight feet from your ears. But he did agree with Dr. D'Antonio's basic premise that diffusion is always better when the round trip distance from head to diffusing boundary is at least 20 milliseconds, and 30 ms or more is even better.

--Ethan

Having a need and being able to use are two different things.

Dr. D's rule of 3 times the lower limt wave length, is, to the most causual of observers, dependent on the lower cut off limit of the diffusor panel. I often use a 1 1/4 inch deep diffusor panel directly mounted onto the rear wall, all covered by Guilford fabric. Dr. D's panels are considerably deeper and correspondingly have a much lower cutoff, thus requiring that the listener be a further distance away from the wall to negate the adverse near field effects of a diffusor.

When a listener is close to a rear wall (the optimum design would indeed be different if there were 8 to 10 ft between the rear wall and the last row listener), the wall needs to be treated-not with heavy absorption and cetainly not with a bass trap (although the room may indeed need bass traping, try thr rear corners with a couple of bass traps (and I do not mean something labeled by some company as a bass trap but something tuned to pick up the room's specific bass resonance). Diffusion is what is indeed needed. But with a close wall position we want shallow depth diffusers or diffusers with a rather high lower limit. Here a diffuser could be such things as a bookshelf full of books, or some half round ASC tube traps with the reflective side out with a few inches of spave between each one (which would offer some absorption below 300 cycles), a rough sand paint wall etc. The point I am making is that you do not want a smooth hard reflective wall but you certainly do not want heavy absorption. You do not want a Dr. D difffuser because you would be sitting to close to it. Just some light diffusion.

Originally posted by mark haflich:

When a listener is close to a rear wall, rhe wall needs to be treated-not with a lot of absorption and cetainly not with a bass trap (although the room may indeed need bass traping, try thr rear corners campers with a couple of bass traps (and I do not mean something labeled by some company as a bass trap but something tuned to pick up the rooms specific bass resonance). Diffusion is what is indeed needed. But with a close wall position we want shallow depth diffusers or diffusers with a rather high lower limit. Here a diffuser could be such things as a bookshelf full of books, or some half round ASC tube traps with the reflective side out with a few inches of spave between each one (which would offer some absorption below 300 cycles), a rough sand paint wall etc. The point I am making is that you do not want a smooth hard wall but you certainly do not wantabsorption to any extent. You do not want a Dr. D difffuser because you would be sitting to close to it. Just some mild diffusion.

Mark and Dennis,

I'm in the process of designing a home theater with dimensions 24'x15'x10'. I want to place two equipment racks on the rear wall along with a little kitchenette with sink, microwave, below-counter refrigerator with a few wall and floor-mounted cabinets.

With the exception of the rear wall, I plan on using the acoustic approach typical of Dennis’ designs with fiberglass on the front wall and front sides, and soffit. The rest of the side walls would be treated with fiberglass from ear level to the floor. However, in the rear of the room I’ve been considering some sort of diffusion since if left untreated the cabinets would be very reflective. I was toying with the idea of fastening half-round bamboo to the cabinets to serve as diffusers. An alternate absorption approach would be to hang heavy retractable drapes in front of all those reflective surfaces.

Short of forgetting about the kitchenette in the first place, do you have any opinions or recommendations regarding these approaches? The rear row of seating will be between 6 and 8 feet from the rear wall.

Thanks.

Larry

Larry:

The bambo cabinet doors would provide diffusion. You could also use something like http://www.rpginc.com/products/topakustik/index.htm (reflector style) for your cabinet faces. With a handy table saw you could like build something similar.

Originally posted by Dennis Erskine
Larry:

The bambo cabinet doors would provide diffusion. You could also use something like http://www.rpginc.com/products/topakustik/index.htm (reflector style) for your cabinet faces. With a handy table saw you could like build something similar.

Hi Dennis,

Thanks for the suggestion. If you were using this approach on one of your designs would you first partially complete the room's standard acoustic treatments, then measure the room's response, and finally select the appropriate Topakustik product to fine tune the reverberation time, and other acoustic parameters?

Although these RPG products are listed as diffusor systems, the literature mainly discusses the absorption properties of the perforated styles. Does the reflector style provide significant amounts of diffusion?

For my design the cabinet faces would be 4 to 6 feet from the rear seating positions. Do you share Mark's opinion that as the listener gets closer to the rear wall diffusion makes more sense? Would you mind discussing the pros and cons of rear wall diffusion versus absorption with regard to rear seating location?

Thanks very much.

Larry

Mark,

I still don't follow your logic. If the last row of seats is closer to the rear wall than you'd like, it seems to me the goal should be to make the rear wall seem acoustically farther back. This is exactly what happens when the wall is made absorbent. There's no difference acoustically between a wall that is infinitely absorbent and one that is infinitely far away. Now, you might say the goal is not to make it seem infinitely far away, but just somewhat farther back. Okay, but in that case...

> with a close wall position we want shallow depth diffusers or diffusers with a rather high lower limit. <

When you install a rear wall diffuser that operates only at the highest frequencies, that leaves the wall reflective at all mid and low frequencies. So you still have the original problem of being too close to the rear wall at the majority of frequencies. So listeners in the rear still suffer from comb filtering and echos at many important frequencies. And at low frequencies the 1/4 wave cancellation problems are even worse near a reflective wall.

> although the room may indeed need bass traping, try thr rear corners with a couple of bass traps (and I do not mean something labeled by some company as a bass trap but something tuned to pick up the room's specific bass resonance) <

Yes, most/all rooms need some amount of bass trapping, but I'm not sure what you're getting at. We already settled the issue of broadband versus targetted bass trapping two weeks ago, and broadband emerged the clear winner.

--Ethan

For my design the cabinet faces would be 4 to 6 feet from the rear seating positions.
But we're missing some critical data...how high above the floor are the cabinets...you have below counter cabs and above counter cab?

Do you share Mark's opinion that as the listener gets closer to the rear wall diffusion makes more sense?
I neither share it or reject it. There's plenty of other factors to be considered...characteristics of the room, distance of the wall from the source, height of the source vs ear height vs wall height, etc., etc.
Would you mind discussing the pros and cons of rear wall diffusion versus absorption with regard to rear seating location?

Again, there are no pros and cons, this is entirely a room dependent matter that would be studied during the design phase of a project.

I cannot say whether your room requires more or less absorption (for RT60 purposes) nor more or less diffusion. However, the product(s) I referenced can provide either absorption/diffusion or diffusion only. I believe the spec sheets can be downloaded from that page.

Once again, I agree with Dennis. Every room is different and that is why, if one is going to do it right, you need the services of an expert acoustician, I am not one! I do know where to go to get one or more, though my favorite is Norman Varney (I am biased, he is an old friend, too).

No, we haven't solved anything, Ethan, Every HT room to be optimized requires some strong absorption. Very broadband. This is required on the front wall, the front corners, and on the side walls (to kill the reflections between the LCR speakers and the listeners). That is every listener should hear these three speakers only directly and not also bounced off both side walls. That is very very clear.

HOWEVER, and note well, the side wall absorption need not go deep into the bass region. The 3db down point of the front speakers should be sufficient for this absorption. Elsewhere along the sides we do not want much if any absorption above 1000 HZ or so. Below 1000 hz we indeed want broadband absorption. You can go as low as you want here but there are practical reasons as to how low you can go. There are limits as to how much absorption you have room for in wall or on wall.

To deal with very low bass absorption by side wall treatment is generally not praticable. Dedicated broad band bass absorbers (say from 300 cycles down (upper mid bass and below). We need to depressurize the bass waves so they do not inhibit the ability of the front speakers to work optimumly. High bass back pressure will really screw the midrange up. Fact. Most rooms will have a bass resonance peak, A flat broadband absorper will tame this but the peak will still be there because of the other bass frequencies attenuation. So measure where the peak is is, or you can easily calculate what it will be from the room dimensions, and then treat it if is severe. Nothing really wrong with broad band bass absorption but it is not the end all.

By no means do we want rhe rear wall to be highly absorptive. That would be a very big mistake. The room will sound overly dead, decay times will go to hell and a handbasket. We want the wall to sound like it is relatively far away but seill there, and not like the listener is in an open field or an anachoic chamber. Diffusion (and many diffusors will absorb as well) is what is called for.

And not that I waqt to give everything away, but on the rear wall my mild diffusion goes on the upper half of the wall and below that I generally treat with broadband diffusion (below 1000 HZ). No reason this broad band can't be as low as you want. The lower rear wall is generally hidden acoustically by the seats so you can make it as dead as you want, Hide you deep bass absorption there if you want. Just do not make the wall above halfway up absorptive. OK you can have some absorption but mostly diffusion

Originally posted by jmsun
All the comments have been very helpful. Although I have one question regarding corner treatment.
I am planning to put some corner bass traps in my HT. But at $500 each, It's not an option for me. Is there anything less expensive yet work as effectively? Thanks for your advice!

Jmsun,

See this thread,

http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=278701

A couple of minor points: diffusors do not affect RT60 in the manner described nor will they have any impact on standing waves.

While vertical strips can create a moderate amount of diffusion, you cannot cover all the early reflection points to all seats effectively. In the 'ear level' scenario, you do have 'stuff' bouncing around above your head; but, if you watch your angle of incidence against where your speakers are placed (height), you've covered all the bases within the curve. This is also a much more effective method with rows of seating since you'll have more people close to the side/back walls than in a two channel, single seat of excellence environment.

In multi-channel...no bad seats not one good seat.

Dennis,

In the deciding the height for the insulation on side and rear wall where should the refelection point for the front speakers to the listening positions be with respect to the top of the insulation? Above, at, or below? Which elememt of a 3way speaker to use as the reference?

Thanks

Dale

Thought I would give this a bump as a great thread, thanks everyone!

Originally posted by Dennis Erskine
The mirror method is "close enough for government work". Some thoughts:
1. be sure the mirror is flat against the wall/floor/ceiling;
2. consider all the seats (no bad seats);
3. check height as well as distance down the wall;
4. the tweeter is more critical than the woofer, none-the-less, your speaker or light bulb should be at the exact position of the speaker once installed (height, distance from wall etc.);
5. recognize you are *not* interested in just the first reflections...it's early reflections which would include reflections from the wall behind the speaker (unless you're treating that surface).
6. recognize that not all early reflections are bad...you're looking at the initial time delay gap along with the "Haas" effect, so "time" is more important than "first".

Does this hold true when using Bi-Polar speakers for LR? Would the treatment of the front wall then change?

Originally posted by smithb

My plan is to treat the whole lower half (ear level and below) of the room as suggested (except full treatment on front wall). However, I have read mix messges on the value of using cloth covered batting on the top. Some say it is for acoustical purposes and some say that it just pushes the upper wall out to meet the treatments of the bottom half.

Regardless of the intended purpose, polyester batting is a very efficient sound absorber. We have certified lab test data, but it is proprietary. Sorry! :(

Regards,
Terry

Polyester Acoustical Blanket Absorption Coefficients

inches mm 125 250 500 1000 2000 4000 NRC
1/2" 12.7 0.05 0.1 0.18 0.28 0.39 0.48 0.25
1" 25.4 0.08 0.18 0.33 0.49 0.61 0.66 0.40

from: http://www.jm.com/insulation/performance_materials/products/ei51_insultone.pdf

For more polyester data, please see
http://www.bobgolds.com/AbsorptionCoefficients.htm

patrickwebb,
Thanks for bumping this. Great information!

I have a question though . . . where is everyone buying their insulshield?

This is a great thread...

I have a question.

How much of an acoustical difference will it make if I treat the upper walls and have painted sheet-rock on the lower three feet of the wall?
My thinking is, I have a 5 year old and sooner or later he or one of his toys will make its way through the GOM. Actually it just seems like it would be vulnerable to accidentally being kicked. My ceilings are 9+ feet tall and the room is going to be about 16X23. (Roughly)

Thanks

Craig :)

Craig,
The typical thinking is (in part) the "lower wall" treatments are to minimize the fronts first reflections (indirects) of the speakers back to your ears muddying up the sound due to timing differences. Where one draws the line how far up to go depends on multiple factors that others are more qualified to answer but one important one is how tall are your seats and the people who sit in them (ie ear level), vs tweeter/speaker level.

Good luck,
Adam

Originally posted by adamf
Craig,
The typical thinking is (in part) the "lower wall" treatments are to minimize the fronts first reflections (indirects) of the speakers back to your ears muddying up the sound due to timing differences. Where one draws the line how far up to go depends on multiple factors that others are more qualified to answer but one important one is how tall are your seats and the people who sit in them (ie ear level), vs tweeter/speaker level.

Good luck,
Adam

So this is a bad idea then? About 2/3 of the wall would be treated. Roughly three feet from the floor to the crown molding.
Thankns

Craig

Hi Craig,

Use the mirror method. If a mirror placed flat against the wall at your treatment area will reflect the front speakers when viewed from your sitting position(s), you are OK for early reflections from that wall.

- Terry

As Terry said.... :)

One compromise would be to do the drywall as you note but use just a couple treatment panels mounted on the walls (sort of like long pictures) in select areas of the first reflections (ie those you see in the mirror from your prime seat/sweet spot). There are examples of this in several HT mags showing this concept/idea.

Good luck.

Terry, adam... thanks! :)

Craig

My question was never answered, so I thought I would try again....

How does the acoustical treatment change (if at all) when you are using bi-polar speakers (Def's in my case)?

My guess is that the only change might be the treatment of the front wall. Would I then NOT want to treat the front wall OR would I want to treat the front wall with diffusion?

Also, Dennis or someone more familiar with it.....is the key to treating the side walls really only important for the ear levels of the listerners? Here's where I'm going with this.....can you INSTEAD of treating the ENTIRE bottom portions of the walls (as seen and described in this and many other threads) just treat a PORTION of the side walls (I am thinking of treating my walls similar to a racing strip on a car---i.e. the treatment would not go all the way to the floor. Would this provide the same benefit as treating the entire bottom below ear level?

I think this might be of benefit to others who don't or can't go back and retro-fit adjustable outlet boxes etc (although I installed them--I was merely thinking of others in this case). If this would provide the same results, it might be a way for others to achieve the same benefit without all the hassle of trying to modify or work around things that might not have been planned for.

It's a VERY crude illustration, but see attached for what I am talking about. Thanks for your help.

-Jason

Jason:

There are two matters to consider when looking at placement of the acoustical materials:

1. The total amount of absorption across the frequency spectrum required to meet target reverberation times in the room; and,
2. The placement of those materials to address such considerations as early reflections, speaker boundary interface response, etc.

You can certainly create a "racing stripe" type of layout. On the other hand, if the entire wall is fabric covered (without regard to the colors, placement of colors, etc), there is no reason to restrict the placement of acoustic materials to match the layout of the fabrics.

Bipolar speakers would not be handled significantly different than dipolar other than perhaps placement of the speakers.

Where in the Atlanta area can I purchase the Insul-Shield type insulation?

Dennis:

Way back on 05-04-03 in this thread in this post (http://www.avsforum.com/avs-vb/showthread.php?postid=2177054#post2177054) you wrote when given a choice between 1" and 2" I would not use the 2" material...it will very likely be too absorptive.
That has me very confused.

I thought that thicker made it more broadband (more lows), which is usually a good thing.
The only reason not to go with thicker I thought was economics -- it just costs twice as much to double the thickness.

Jeff - The JM Coated Insul-Shield Black (or Theater Shield) is available from Alpha Insulation in Marrietta.

Bob - 2" is not enough for broadband (low frequency) applications when placed on a wall surface. On the other hand, in most of our rooms, using the 2" material provides more absorption than we need (or want). It is a balancing act. Once we determine all the absorptive surfaces we want and model 2" we're too absorptive.

Bob,

> I thought that thicker made it more broadband (more lows), which is usually a good thing. <

That's how I see it too. Aside from the larger edge surface, which is not irrelevant, switching from rigid fiberglass one inch thick to rigid fiberglass four inches thick absorbs exactly the same amount above 1 KHz. All that changes is how low in frequency "complete absorption" extends to.

--Ethan

I agree with you, Ethan.

Furthermore, if you cover a wall with an absorber which abuts other surfaces, there is no extra edge absorption effect like that in a reverberation room measurement. The absorption coefficients cannot go above 1, as when the edges are exposed. So the extra thickness simply extends bass absorption.

Regards,
Terry

When you do the calculations for the other bass absorption in the room which likely cover <250Hz - adding a treble absorber that also goes down to 250Hz would cause excess absorption at 250Hz. An HT has much more potential for bass absorption in architectural features and furnishings that must be accounted for.

Though if edge effects are measured with data above 1 - that means it impacted the data below 1 as well. And you do not know what that edge delta is!

what is the most cost effective way to measure a rooms RT60 Value?

proud: you can do it crudely, but cheaply with an impulse response program (i have a cheesy one if you want it), a cheap mic, and some loud noise like a clap, a wood clap, a starter pistol.

As the fiberglass increases in depth, it's absorption characteristics at lower frequencies improves (the greater improvement occurs from 1" to 2"); however, one is wanting to achieve a reasonably uniform RT60 across the frequency spectrum requiring a balance in the materials utilized in the room. The would include the absorption characteristics of people in upholstered seats, unoccupied leather covered upholstered seats, gypsum board, etc. These are also small rooms where typical RT60 prediction models are less than reliable. In our experience, we find we're over absorbed in the general area of the 250-500Hz range with the 2" product

Maybe it's just me but.........I have always sat in the front middle seats of my theater(see my gallery) and thought that it sounds great. However, we had company over the other night and I sat in the left end seat which is turned slightly sideways to the front wall because of the curvature of the seating arrangement. I swear that I picked up more detail in this seat than ever before. Is this because the angle of seat (and my ears) changed the reflection points? If so, and it's not just in my head, than I definitely need to get some treatments on my walls. I swear I heard things in the movie that I hadn't before!

Dennis, do you know of anyone in St.Louis that can come out and do a reading for me? I have DIYed my entire theater and would like to build my own panels, but need someone to lay it out for me.

Thanks so much for all your help!

Mike

Mike:

Gerry Lemay, founder of the Home Acoustics Alliance (www.homeacoustics.net) is based in the St. Louis area.

Dennis:

I was always curious of your preference for 1" on the side walls. Thanks for the explanation!

Andre

OK,
I had decided to use the OC703 but it is becoming a major pain to find it and once I did find it, the shipping was ridiculous. I assume Insul-Shield and Linacoustic are both similar products used for treating reflections. I'll now check DE's suggestion.

QUOTE]Originally posted by Dennis Erskine
Jeff - The JM Coated Insul-Shield Black (or Theater Shield) is available from Alpha Insulation in Marrietta.

Bob - 2" is not enough for broadband (low frequency) applications when placed on a wall surface. On the other hand, in most of our rooms, using the 2" material provides more absorption than we need (or want). It is a balancing act. Once we determine all the absorptive surfaces we want and model 2" we're too absorptive. [/QUOTE]

Jeff:

You can visit my showroom in Marietta and "hear" for yourself.

Dennis,
I plan to do just that! I can only hope I don't spend too much money while I'm there.

Originally posted by Dennis Erskine
Jeff:

You can visit my showroom in Marietta and "hear" for yourself.

It's been a couple of months and as members near this phase of their project this thread becomes VERY important....so BUMP!

And yes, horse hair jute, in my many years of 2 channel experience, the best carpet pad for making a room sound good, is no longer commercially available. Too many "utes" were smoking it with bad side effects.

I guess it's back to the mushrooms.

I will be putting velvet curtains on the front wall all around the screen. The generally accepted wisdom is to treat the front wall to absorb as much sound as possible ("dead end").

Is the addition of heavy velvet curtains along the front wall negate the need to put sound panels behind the curtain?

Also, since the velvet is not acoustically transparent what use would putting acoustically transparent ("expensive") fabric over the panels should I choose to treat the wall.

Hi Guys:

I read the entire thread and there was still one question that I think never got answered. And of course it applied to me, too :). My room:

Plan View (http://www.mlec.net/scott/HTBigRoom1.gif)
View toward room front (http://www.mlec.net/scott/HT3DNorth.jpg)
View toward room back (http://www.mlec.net/scott/HT3DSouth.jpg)

It's larger than several discussed in the thread and my seating is not against the back wall.

It appears from this thread that I should:
1) Treat the screen wall top to bottom (linacoustic or equiv).
2) Treat some portion of the side walls.

I will have a drop tile ceiling in the area inside the soffit.
I will have a carpeted and padded floor.

My side walls due to construction and foundation issued will have a curtain wall with a shelf-cap instead of chair rail. That is, it's 2-walls thick at the bottom for lack of a picture). I have no need to put fabric on the upper section of the wall if not needed, it woudl be drywall. Is it necesary to use the upper treatment?

I have a lot of flexibility on the side curtain wall's height. Minimum 12" off floor, maximum - ceiling. I planned on about 4' all the way back. Maybe thats too much and I shoudl only do first reflection area?

For my elevated rear sofa, do I need to elevate the sidewall first reflection points, too?

I'm not sure but since my rear wall is relatively far away, should I treat it or not?

As for my soffits, how woudl one treat them? RIgid fiberglass panels covered with fabric on the floor facing part, or up the sides of the soffit's too?

With all this absorpion will the room be too dead?

Thanks,
Scott

FD,

> The generally accepted wisdom is to treat the front wall to absorb as much sound as possible ("dead end"). <

Not necessarily. What matters most are 1) absorbing the first reflections which are at specific places on the side walls and ceiling, and 2) absorbing low frequencies to clean up the inevitable muddiness that exists in all untreated rooms.

> Is the addition of heavy velvet curtains along the front wall negate the need to put sound panels behind the curtain? <

No, because curtains absorb only higher frequencies. You still need bass trapping.

> since the velvet is not acoustically transparent <

It doesn't matter if a curtain absorbs a little extra on its own. Though this assumes you don't plan to put your speakers behind the curtain!

I don't know how deeply you want to get into the fun and exciting world of acoustic treatment. But if you want to learn more see the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

--Ethan

Curtains will do very little below very high frequencies. You still need to treat the front wall. The curtains do so little that it won't matter if they are in front of the treatments.

Actually, heavy curtains with a thick drape, covering 3.25 walls (floor to ceiling, rear wall, two side walls, some of front wall) can look very nice and have a huge effect on sound. Whether that is good or bad depends on the rest of the room. But you're right, even the best of them still absorb more highs than lows. But one can always put more absorbtion (wool and/or membrane) behind the curtains to extend into the low end and still look nice.

1105 Cotton curtains (0.5 kg/m2) draped to 3/4 area approx. 130 mm from wall (Ref. 17)
0.3000 0.3000 0.4500 0.6500 0.5600 0.5900 0.7100 0.7100

Thin curtains with no drape are worse than carpet
1107 Curtains of close-woven glass mat hung 50 mm from wall (Ref. SBI/13)
0.0300 0.0300 0.0300 0.1500 0.4000 0.5000 0.5000 0.5000

Scott,
The polybatting used on the upper half of rooms is usually just to provide a backing to keep the GOM or other fabric treatment even to the base. I don't believe Polyfill makes too much of a practical impact on diffusion/dispersion. Now whether you need treatments in this area (or around the soffits is a separate issue and depends on the RT60 and other features of your room. A couple well placed panels in the drywall areas once the room is built could be done to "tune" the room.

The first reflection points from your rear couch should be treated (theoretically) as that will improve the sound for those sitting there a well.

rear wall again varies. Most use dispersion/diffusion. I needed some increased deadening so I treated th bottom of my wall with soundselect. I do note some boomy base in the 2 back-corner seats and may some day try and find out what frequency and then apply a basstrap in those corners.

Dennis E. had addressed the soffit issue in the past, do a search in the builders area with soffit treatment and dennis's name and you might come up with it. Will try taking a look later if you can't find it.

Good luck,
Adam

Adam:

Thanks, no batting on the upper area is good for me.

I will look into varying the height of my first reflection point treatment to accomodate the 2nd sofa.

I'll plan to leave rear wall alone for now, or roll the dice and cover the same as side walls.

I couldn't find anything helpful about the soffit treatments. If you think you know where to look I'd love to.

With my walls at almost 50% coverage, presmuably with soffits treated, my ceiling with acoustic tile, my floor with carpet, and my front wall at 100% treatment, should I expect the room to be too dead? bpape indicated it woudl go a long ways toward deading, but being new to this I don't know how much is too much.

Thanks,
Scott

Originally posted by JBS
BTW, for those searching for Insul-Shield type product, here are the substitutes which seem to have identical acoustical absorption ratings:

Owens Corning Select Sound Black Acoustic Board
Owens Corning Fiberglas 703 Series duct insulation.
Johns Manville Insul-Shield
Johns Manville Linacoustic Permacoate rolls.
Certainteed Certpro Acoustaboard Black
Knauf Duct board EI-475
Knauf Duct liner EM

...personally, I found the Knauf EI-475 easiest to find (4' x 10' sheets @ $40) from a general heating and air conditioning company.

OK, I've done quite a bit of reading and feel like I know where in the room to apply the acoustic materials. However, I have a very basic question about the materials themselves. Are some of these more like rigid boards and others more like blankets? If so, it looks like there should be two different categories, with one for boards and the other for blankets, as some people may prefer one over the other. Is the following break-down accurate?

Board-style:
Owens Corning SelectSound Black Acoustic Board (http://www.owenscorning.com/around/sound/commercial_acoustics/black-acouboard.asp)
Johns Manville Insul-Shield (http://www.jm.com/insulation/building_insulation/products/hig1214dk_insul-shield.pdf)
Certainteed Certapro Acoustaboard Black (http://www.certainteed.com/CertainTeed/Pro/Property+Owner+Manager/Insulation/Prodindex/Commercial/CERTAPRO%E2%84%A2+AcoustaBoard%E2%84%A2+Black.htm)
Knauf Duct board EI-475 (http://www.knauffiberglass.com/index.cfm?fuseaction=prd.dspProdDetail&ID=13)


Blanket-style:
Owens Corning Fiberglas 703 Series duct insulation (http://www.owenscorning.com/comminsul/documents/Fiberglas700Series.pdf)
Johns Manville Linacoustic Permacote rolls (http://64.207.52.204/prod/jma/schools/ductliners.php)
Knauf Duct liner EM (http://www.knauffiberglass.com/index.cfm?fuseaction=prd.dspProdDetail&ID=15)
Owens Corning SelectSound Black Acoustic Blanket (http://www.owenscorning.com/around/sound/commercial_acoustics/black-acoublanket.asp)

Ethan,

I over generalized in my original post. I've been reading alot of the posts here about acoustics and about LEDE, first, second reflections, etc. I've seen quite a few theaters from posters on this site that put either insul-shield or OC 703 panels or similar material along the front wall (Dead end) and cover that with acoustically transparent fabric. I was planning on starting with the front wall and then move to the side and rear walls as time and money allow. I was intending for the panels along the front wall behind the heavy velvet curtain to absorb high frequencies only. I intend to build bass traps like Jon Risch's to cover the low end.

Regarding putting panels of OC 703 or similar material, What I REALLY want to find out is if I still need to put panels behind the heavy velvet curtains. It looks like I do not need to based on the numbers that Basement Bob posted regarding cotton curtains. I could and probably am wrong.

If I do put panels behind the curtains, would I still need to go through the expense of putting acoutically transparent fabric over the panels? I would think that since the curtains are NOT acoustically transparent that it would be a moot point. Again, I could be wrong.

I willingly bow the massive collective wisdom of the people who frequent this great site for help. Thanks again for the replies!

fdlozano:

What I REALLY want to find out is if I still need to put panels behind the heavy velvet curtains. It looks like I do not need to based on the numbers that Basement Bob posted regarding cotton curtains.
Those examples were the best and worst absobtion figures for curtains that I found. And even the best, the heavy cotton ones that are way out from the wall and huge drape, still absorb a lot more in highs than in lows.

OK, let's talk about what's going on here.

We've got modes, flutter, and SBIR (first reflection).

Curtains will do wonders for flutter.
Curtains will do nothing perceptable for modes.
Curtains will do a bit for side reflections (mid and high), but nothing for SBIR reflections off the front wall (which tend to be louder lower).

DE designs seem to have about 1" of linacoustic or InsulSheildBlack for the front wall. ( 0.09 0.29 0.67 0.89 1.03 0.99 ).
If you have lots of couches and thin walls then some of your modal issues may already be handled.

In the stereo control room world there are two designs for front speakers - baffles and thick absorbtion on the front wall on either side of the speakers. Either handle SBIR.

LEDE may not be appropriate for small rooms, unless you're doing a dual purpose room which is also for playing musical instruments.

OK, you want a bit of a dead room from all sides for 7.1
But you don't want to absorb everything because it has three bad effects:
1) it's expensive
2) it requires you to turn up your amplifier possibly to the point of distortion
3) it removes the reflected sound which is needed for spaciousness

So what you're really after is
a) absorb first reflections and SBIR for the best imaging
b) a flat RT60 (even absorbtion at all frequencies)
c) Any reflection path less than 20ms you might want to absorb or diffuse so that it hits the listener area down about 30db or more relative to the direct sound.

If I do put panels behind the curtains, would I still need to go through the expense of putting acoutically transparent fabric over the panels? I would think that since the curtains are NOT acoustically transparent that it would be a moot point.You got it. Curtains over absorbers are fine.

Yes. You should still put other absorbtion behind the curtains. When you cover the panels, just make sure it is a somewhat porous, non-shiny finish material. Muslin does wonderfully. The idea of the panels is to absorb. Don't cover them with a cloth that reflects and you'll do fine.

I agree with bpape and based on discussions with Dennis, you STILL will want to treat the front wall. You will also want to treat the first reflection points on the side walls, but it sounds like you plan on doing that anyways.

Ethan, I'm curious as to why you disagree with Dennis and others about treating the front wall. I've seen you state this in another thread---most recently above where you said "Not necessarily." in reference to the generally accepted practice of doing this in a Home Theater enviroment. It appears you disagree with this statement by Dennis from this thread (http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=255432:):

"In multi-channel, the entire wall behind the front speakers is treated. You want none of the back reflections to overlay the surround field or the bring the reverberent field forward (your reverberent field and surround field is created by the multi-channel processor or mix, not so much the room as is mandatory for 2-channel). Depending on speaker placement, this treatment is brought forward along the side walls. Wall treatments are floor to slightly above ear level (where exactly is also a function of front speaker heights). While one could argue the sound at their feet is of no concern, often that square footage of treatment is required to bring the room's RT60 down to the lower levels required for multi-channel playback."

Actually after re-reading that thread....I see YOU (Ethan) asked Dennis the front wall treatment question here:

"Dennis,

> recognize you are *not* interested in just the first reflections...it's early reflections which would include reflections from the wall behind the speaker (unless you're treating that surface). <

That's a good point, and brings up a question you can probably answer for me about the importance of absorption on the front wall.

There's no question that loudspeakers radiate low frequencies in all directions. So those will bounce off the front wall and collide with direct sound from the speakers. But since mid and high frequencies are radiated from the front of the speaker, why is mid/high frequency absorption so often used on the front wall behind the speakers?

--Ethan"

And Dennis responded with:

"Sound from the speaker radiates as a sphere. Thus all frequencies radiate in all directions. The speaker cabinet causes defraction but does not "cast a shadow". (One of the reasons the formula for determining acoustic energy decay (SPL) is 1/r^2.)

[If sound at all frequencies did not radiate spherically, you couldn't hear a speaker standing behind it. Although, I do claim I cannot hear what my son is saying when he's looking the other way. ]"

So again....my question Ethan, Why do you not agree with this?

-Jason

I believe what I said in the other thread was that with carpet, acoustic tile ceilings and 50% wall coverage, the room would likely be too dead in the upper frequencies and still too live at lower ones.

I believe what I said in the other thread was that with carpet, acoustic tile ceilings and 50% wall coverage, the room would likely be too dead in the upper frequencies and still too live at lower ones. Thank you for the clarification, that helps me understand what the problem might be anyway. If I have to stay with carpet and ceiling, what woudl be a course of action then? Wall treatment only at first reflection points? or just less overall treatment at some percentage of coverage? Then bass traps for lows? Would a drywall ceiling cure or help the overdeadened highs? Or what? :)

Thanks,
Scott

From the perspective of early reflections, it is generally not necessary to treat the front wall. A commonly cited figure for the sound level at which early reflections cause imaging shift problems is 10 dB below the level of the direct sound. This is the recommendation chosen by a few professional organizations.

Looking at the directivity patterns of typical baffled loudspeakers, the rear sound radiation does not reach -10 dB at medium to high frequencies. Non-directional low frequencies are too broad to contribute early reflection "spikes" to the impulse response. So the possibility of a -10 dB early reflection from the front wall of a home theater is pretty low.*

If you are acoustically treating a home theater room, the front wall is a readily available surface which, if treated, will help lower reverberation time. Also, the same research which showed that an approximately -10 dB reflection is necessary to shift the image away from a speaker also showed that lower levels can be perceived as changing the sound.

Any reflections from the front wall, while not usually causing specific acoustical harm, can do no good. It is not a hard and fast rule to treat the front wall of a home theater. It is generally just a good idea -- a no-brainer if your theater design allows it.

Regards,
Terry

* If you are using an "acoustically transparent" microperforated projection screen in front of speakers, this can bounce significant high frequency energy toward the front wall, which will reverberate between screen and front wall if not absorbed. In this case, front wall treatment is mandatory, IMHO.

SBIR is something different than "early reflections". None-the-less, have speakers (various from speaker to speaker) near a boundary can have unfortunate side effects unless that boundary (or the speaker) has been modified to adjust (or eq is used). It is a great place for absorption to achieve RT60 targets as Terry noted.

Whether or not the reflected sound from the front wall surfaces will be perceived as an echo or 'fused' to the direct sound will depend again, on speaker distance from the boundary. HAAS effects not withstanding, such out of phase early reflections can have a negative impact on intelligibility.

Flutter echo is seldom a problem in a home theater as is isolated to only a few seating locations should it occur. None-the-less, front wall absorption will reduce the opportunity for flutter echo to become an issue.

Originally posted by Terry Montlick
Looking at the directivity patterns of typical baffled loudspeakers, the rear sound radiation does not reach -10 dB at medium to high frequencies. Non-directional low frequencies are too broad to contribute early reflection "spikes" to the impulse response. So the possibility of a -10 dB early reflection from the front wall of a home theater is pretty low.*

Terry,

Thanks for your take on the front wall treatment. I guess in my particular case (having bi-polar mains) where the front wall has speakers directed at and bouncing off of it, it becomes even more important to treat the front wall. Regardless of speaker design, you basically said the same thing Dennis said and what I and bpape stated above.....treat the front wall! It still doesn't answer my question as to why Ethan disagrees with this. I can guess that he might think that treating that large of a surface will deaden a room too much and cause even muddier (if that's even a word) bass...but that's what I am trying to find out.

It's helpful (at least for me) to seperate room acoustic treatments in two categories: high and mid treatments (i.e. 703 and the like) and bass treatments/traps. The front wall treatment I am curious about (with regards to Ethan) is the high and mid treatment of the front wall.

-Jason

Drywall ceiling will significantly reduce the amount of high frequency absorbtion. Look at the square footage of the ceiling in relation to the square footage of all the room's surfaces.

If you stay with tiled ceiling, you might do more scattered absorbtion and hit the reflection points. It is a good thing to have absorbtion throughout the room for more effective decay control. Do some calculations on what you think your room will be like. There is a spreadsheet floating around from Eric Desart that has a lot of good stuff in it regarding decay time calculations. Fill in the blanks and see where you fall at given frequencies. You'll see quickly where you still need help.

Threads Merged

Originally posted by Terry Montlick
From the perspective of early reflections, it is generally not necessary to treat the front wall. A commonly cited figure for the sound level at which early reflections cause imaging shift problems is 10 dB below the level of the direct sound. This is the recommendation chosen by a few professional organizations.

Exactly. High frequency, off-axis response is significantly attenuated. Tweeters, typically above 2.5k-3k Hz have dispersion patterns like flashlights. The point where midrange off-axis response is only 6dB down can be as low as 200 Hz.

Originally posted by jasplat88
Thanks for your take on the front wall treatment. I guess in my particular case (having bi-polar mains) where the front wall has speakers directed at and bouncing off of it, it becomes even more important to treat the front wall.

Don't you mean dipole mains? Dipole mains (open baffle speakers), by design, rely on the contribution of the backwave. That is why proper placement is critical.

Originally posted by nirvana_av
Don't you mean dipole mains? Dipole mains (open baffle speakers), by design, rely on the contribution of the backwave. That is why proper placement is critical.

Well in my case I actually meant bi-polar (I own Def Tech BP2000's). But in either a bi or di-polar situation my point is still vaild (I think the only main difference between bi and di-polars is the drivers opperate out of phase in a di-polar setup and in phase in a bi-polar setup---but I might be wrong). The point I was making is now you definitely have early reflection points off the front wall and it's still important to treat it (the front wall).

-Jason

But if the speaker was designed for bipolar operation - are you not destroying the speaker designers intended sound by covering up the front wall?

Why use the bipoles then - best go for monopole! In other words - if reflections are bad - then why choose a speaker that is designed for reflections?

Originally posted by jasplat88
Well in my case I actually meant bi-polar (I own Def Tech BP2000's). But in either a bi or di-polar situation my point is still vaild (I think the only main difference between bi and di-polars is the drivers opperate out of phase in a di-polar setup and in phase in a bi-polar setup---but I might be wrong). The point I was making is now you definitely have early reflection points off the front wall and it's still important to treat it (the front wall).


Ah, the Def Techs. Some of the top-end Wilsons do this with a tweeter on the back. I might be wrong, but I think the point of this alignment is to improve imaging and "space". I would agree with Kras, they might not be the best for an HT application.

From a acoustical treatment (in regards to the front wall) standpoint, I already asked this question and was provided the following answer by Dennis:

"Bipolar speakers would not be handled significantly different than dipolar other than perhaps placement of the speakers." (see page 6 of this thread)

I realize some people THINK these speakers are not ideal for HT.....I happen to THINK they are. That's why there are so many manufacturers of speakers....so people can pick and choose what they like :)

-Jason

The point is that that the rear facing tweeter of a bipolar (or dipolar) speaker is useless if the front wall is absorbing those treble frequencies. It essentially becomes a monopole.

If you like the bipolar (or dipolar) sound - then you should NOT treat early reflections. You are wasting money on an unused tweeter if you do treat.

With the Def Tech, you're not just wasting the tweeter, you're pretty much wasting a whole MTM.

Originally posted by krasmuzik
The point is that that the rear facing tweeter of a bipolar (or dipolar) speaker is useless if the front wall is absorbing those treble frequencies. It essentially becomes a monopole.

If you like the bipolar (or dipolar) sound - then you should NOT treat early reflections. You are wasting money on an unused tweeter if you do treat.

Kras...I am well aware of your version of "the point".... however, you are wrong. Treating the front wall with 703 or similar WILL NOT absorb 100% of the sound from the rear facing tweeters and drivers, and warrant them "useless." It will help tame them, which is what I want. I have yet to hear a speaker that sounds as good as the Def's I have. If achieving the sound I like is "wasting my money" oh well, I can live with that. BTW, it's NOT the sound of bi-polars or di-polars that I like....it's the sound of Def Techs.

It's the Holidays....you need to stop being so Anti-Def Tech and live a little!

-Jason

Originally posted by nirvana_av
With the Def Tech, you're not just wasting the tweeter, you're pretty much wasting a whole MTM.

Oh well....I guess I will have to rip them out (the rear facing MTM's) and sell them on Ebay to retrieve some of my wasted money:D

From BasementBob's web page.

Product thickness density mounting 125hz 250hz 500hz 1000hz 2000hz 4000hz NRC

703, plain 2" (51mm) on wall 3.0 pcf (48 kg/m3) 0.17 0.86 1.14 1.07 1.02 0.98 1.00

Ignoring the excess absorption of exposed edges which will not occur with a covered wall - OC703 absorbs pretty much 100% of the treble frequencies. Of course an absorption coefficient like this has insufficient accuracy to tell you how many -dB down the reflection will actually be.

I never said anything negative about Def. Tech speakers (my opinion of bipolar front speaker is irrelavent to this discussion). The point is if YOU like the expansive reverberant sound-stage that a rear facing bipolar gives you - treating the wall to absorb treble makes that sound-stage go away.

If you want to just tame that sound-stage a bit and like the bipolar sound - then use a less aggressive absorption scheme that leaves some % of that front wall facing reflective.

Kras,

I'm not using 2" 703 and Dennis does not recommend using 2" material as it absorbs too much (but I'm sure you knew this if you have already read this thread--it's in his third post on the first page). I am using a 1" equilivant (it's not 703), and it does not absorb the entire high end frequency and even less in the mids.

Not that's it's important, but I did set up the 1" ductboard and spent a few hours listening to different music and movies with and without the 1" material on the front wall before I permanently covered it, and I do prefer it treated.

-Jason

Well OK 1" maybe down 10% at 1KHz - but the higher frequencies are still pretty close to 100% absorption - at least with the Linacoustic numbers I see on BasementBob's page. The difference with the thinner stuff is at mids at 250-500Hz.

IS Black 1" (25mm) 1.5pcf (24kg/m3) 0.09 0.29 0.67 0.89 1.03 0.99 0.70
Linacoustic RC 1" (25mm) 0.08 0.31 0.64 0.84 0.97 1.03 0.70

So I still maintain that you like it because the bipolar has been converted to monopolar (or technically a directional front firing tweeter)- at least in treble frequences the back wall wave is being absorbed.

Originally posted by krasmuzik
So I still maintain that you like it because the bipolar has been converted to monopolar (or technically a directional front firing tweeter)- at least in treble frequences the back wall wave is being absorbed.

Kras,

You may be correct, but I maintain that I still like the way my Def's sound even over other monopoles....so oh well.

The point I was making was......in general, the front wall of a HT should be acoustically treated. Period!

Kras is just making some valid technical points here. If the speakers work for you, then that's great. Enjoy. Given the HT application and desire for front wall treatment, it simply wouldn't be advisable for the next guy who's shopping around for speakers.

One must understand the genesis of speaker designs with a significant amount of "back radiation" or "back wave" (bipolar, rear firing tweeters, etc.) Back in the two channel only days, the only way to create room filling spaciousness was to rely upon reflected sound throughout the room. On method of augmenting this was to make better (more?) use of the front wall...which also helped fill the coverage hole in the middle.

You can get excellent sound stages with treated front walls and even better sound stages with three channels. In the multi-channel world it is most often very counterproductive to use such designs...effectively using your room as a surround processor...since ambiance extraction techniques and multi-channel recording can do a much better job.

(...and, I invite anyone to come and hear this in our demo facility in Atlanta)

FD,

> What I REALLY want to find out is if I still need to put panels behind the heavy velvet curtains. <

Yes, at least for bass trapping. All rooms need substantial absorption at low frequencies. Curtains won't do that, but you can certainly put traps behind the curtains.

> I would think that since the curtains are NOT acoustically transparent that it would be a moot point. <

Bass waves go right through curtains, no matter what they're made of.

--Ethan

Jason,

> I'm curious as to why you disagree with Dennis and others about treating the front wall <

I still have not gotten around to measuring the free-field response of my Mackie HR624, and I plan to do that eventually. I'm not convinced that loudspeakers radiate a significant amount of mids and highs out the rear (or that mids and highs wrap around) enough to worry about treating the front wall. Listening rooms I've been in that had absorption over the entire front wall always sounded too dead to me.

I guess my main objection to treating the entire front wall is that it puts all of the absorption in one place, rather than spread evenly around the room. Also, many people wrongly believe that if they treat the entire front of their room that's all they need. The thin materials often used for this absorb only down to the midrange, and that's just not adequate.

--Ethan

Originally posted by Ethan Winer
I still have not gotten around to measuring the free-field response of my Mackie HR624, and I plan to do that eventually. I'm not convinced that loudspeakers radiate a significant amount of mids and highs out the rear (or that mids and highs wrap around) enough to worry about treating the front wall. Listening rooms I've been in that had absorption over the entire front wall always sounded too dead to me.
--Ethan

I see a conundrum here, Ethan. If there is minimal direct radiation of mids and highs to the front wall, why does treating it make the room too dead? Of course, it may be that anyone who covers the entire front wall has also put too much treatment elsewhere.

Kal

Kal,

Sorry, I was still editing/adding while you were replying.

> If there is minimal direct radiation of mids and highs to the front wall, why does treating it make the room too dead? Of course, it may be that anyone who covers the entire front wall has also put too much treatment elsewhere. <

The only reason to need mid/high frequency absorption anywhere is to handle early reflections. Everything else is for more general ambience and echo control, and to my way of thinking that absorption should be spread around evenly.

--Ethan

OK. So are you saying that concentrating it on the front wall deadens more than spreading it around? What I am really asking is why the front wall is any more or less effective a site for mid-, hi- absorbtion if its proximity to the speakers is not an issue?

Kal

Kal,

> So are you saying that concentrating it on the front wall deadens more than spreading it around? What I am really asking is why the front wall is any more or less effective a site for mid-, hi- absorbtion if its proximity to the speakers is not an issue? <

The front wall is no less effective than any other surface if the goal is to reduce ambience generally. But treating the entire front wall and leaving the entire rest of the room reflective gives an unbalanced sound. You get obvious echoes off the rear wall, floor to ceiling flutter echo if there's no carpet, and also flutter echo between the side walls where they're not treated.

All of this depends entirely on the room in question! There are tiny rooms and huge rooms, so that too is a factor. I'm speaking in general terms about smallish rooms like you find in most homes.

--Ethan

Kal:
In the overall scheme of things, deadens more or less is less a function of placement than it is a matter of the total amount of absorption (and characteristics) in the space. In small rooms, placement of absorption becomes significantly important. Now with respect to your specific question above, the answer becomes "well that depends". While you elminated the proximity issue to the front wall, we have other factors to consider...for example distance to side and rear walls as we manage not only room reverberation times but path differences and their related effects. Typically, however, as long as you're NOT using full range speakers, you'll find the L/C/R in the 3' range from the front wall...then considering the other issues, the front wall becomes a very effective place for absorption.

You get obvious echoes off the rear wall, floor to ceiling flutter echo if there's no carpet, and also flutter echo between the side walls where they're not treated
You may have an overly reverberant space; but, flutter/slap echoes will not be the issue (unless you have a speaker on the floor or ceiling, this cannot happen).

Dennis how far is your business from Buckhead? I will be in Atlanta late January early February....

Edit for not reading before posting:
What happens Dennis in the case that your L/R are full range?

The front wall is no less effective than any other surface if the goal is to reduce ambience generally. But treating the entire front wall and leaving the entire rest of the room reflective gives an unbalanced sound. You get obvious echoes off the rear wall, floor to ceiling flutter echo if there's no carpet, and also flutter echo between the side walls where they're not treated.

All of this depends entirely on the room in question! There are tiny rooms and huge rooms, so that too is a factor. I'm speaking in general terms about smallish rooms like you find in most homes.

--Ethan Ethan:

Then can you offer a suggestion on a good starting point for this room for treatment? I was planning on entire front and 42" of side all the way around. Floor=carpet, ceiling=tile, soffit=drywall.

http://www.mlec.net/scott/HTBigRoom1.gif
http://www.mlec.net/scott/HT3DNorth.jpg
Thanks,
Scott
The more I listen the less I know

I am finishing out our basement and am putting a theater/ media room down stairs. Due to family consideration I cannot dedicate the space to just a theater. I was thinking about using a curtain on a track as a means of "creating" a back wall when i want to watch a movie. The benefit of course would be that when the theater is not in use the curtain could be pulled back, opening the whole space. The question is what would the effects be of the curtain on the acoustics of the room? Thanks for your help!!

Ron

The curtain is a great idea for light control, but it will absorb a lot of HF and do nothing about the bass. Bass traps or thicker panels elsewhere behind the curtains can help with the bass absorption.

Dennis how far is your business from Buckhead? I will be in Atlanta late January early February....
About 11 miles north.

Edit for not reading before posting:
What happens Dennis in the case that your L/R are full range?
As you move a speaker closer to the wall the SBIR notch frequency increases. With non-full range + a subwoofer, you can move the sub toward a boundary to raise the notch frequency above 80 Hz. You move your mains out from the wall (about 3') to move the notch below 80 hz. With full range, your speakers need to be on the order of 15 feet from any boundary. Further, the locations required for your main speakers to achieve the best imaging will not be the best location for LF to achieve smooth bass response in the room.

but it will absorb a lot of HF and do nothing about the bass. Bass traps or thicker panels elsewhere behind the curtains can help with the bass absorption.
But you also have a much larger room from a bass point of view so you must deal with modal response, smooth bass reponse, and amplifier power based upon the larger space.

I've noticed a lot of abbreviations used in the Home Theater section of AVS, is there a sticky or a Faq somewhere that has the definitions?

HH

Since Dennis brought up the SBIR (Speaker-Boundary Interference Response), some of you may be wondering how to measure it. You can do this with a program like ETF.

The idea is based on the fact that SBIR is a short-time effect. It happens within the first tens of milliseconds of the sound, where 1 millisecond = about 1 foot of sound travel. So if you take the frequency response using a short impulse-response time window (20 milliseconds or so), you will see what is going on during the time period when the direct sound from the speaker interferes with its reflection from a nearby wall.

On the other hand, if you look at a long-term frequency response (time window of a hundred or more milliseconds) it will span several wall reflections back and forth through the room. You now see the effects of room modes added to the SBIR, since the modes need time to build up over several wall reflections. Get it?

Now, there is one gotcha. With a small time window, you automatically get less detail in the frequency domain. It is a fundamental trade-off in the signal processing world. So if you set the time window to 20 milliseconds for example, you won't see any frequencies below 50 Hz. ETF will show you a smooth response above this, but there is still limited low frequency information, and you will not see a lot of detail. Nevertheless, you still should be able to spot any dip corresponding to the 1/4 wave cancellation which Dennis described.

Apologies to those for whom this discussion was too technical, but it seemed like a good opportunity to explain more about the "mysterious" SBIR.

- Terry

Originally posted by Dennis Erskine
As you move a speaker closer to the wall the SBIR notch frequency increases. With non-full range + a subwoofer, you can move the sub toward a boundary to raise the notch frequency above 80 Hz. You move your mains out from the wall (about 3') to move the notch below 80 hz. With full range, your speakers need to be on the order of 15 feet from any boundary. Further, the locations required for your main speakers to achieve the best imaging will not be the best location for LF to achieve smooth bass response in the room.

Originally posted by Terry Montlick

Since Dennis brought up the SBIR (Speaker-Boundary Interference Response), some of you may be wondering how to measure it. You can do this with a program like ETF.

The idea is based on the fact that SBIR is a short-time effect. It happens within the first tens of milliseconds of the sound, where 1 millisecond = about 1 foot of sound travel. So if you take the frequency response using a short impulse-response time window (20 milliseconds or so), you will see what is going on during the time period when the direct sound from the speaker interferes with its reflection from a nearby wall.

On the other hand, if you look at a long-term frequency response (time window of a hundred or more milliseconds) it will span several wall reflections back and forth through the room. You now see the effects of room modes added to the SBIR, since the modes need time to build up over several wall reflections. Get it?

Now, there is one gotcha. With a small time window, you automatically get less detail in the frequency domain. It is a fundamental trade-off in the signal processing world. So if you set the time window to 20 milliseconds for example, you won't see any frequencies below 50 Hz. ETF will show you a smooth response above this, but there is still limited low frequency information, and you will not see a lot of detail. Nevertheless, you still should be able to spot any dip corresponding to the 1/4 wave cancellation which Dennis described.

Apologies to those for whom this discussion was too technical, but it seemed like a good opportunity to explain more about the "mysterious" SBIR.

- Terry

Hi Dennis and Terry,

Thanks for the great information on a subject that, as Terry notes, is rather mysterious to the layperson.

I have been attempting to use EFT to aid in selecting my crossover points. I’ve been using a long time window and comparing the logarithmic frequency responses of various pairs of crossover points for my mains and LFE subwoofer. Based on this long time window it appears to me that selecting 60 hz for my mains and sub yielded the smoothest frequency response. However, based on your information it would appear that I am missing the short time effects of Speaker Boundary Interference. Should I be attempting this with a 20 ms time window? I realize I won’t see the lowest bass response, but I assume the object would be to see how smooth I can get the responses at the crossover points?

Due to practical trade-offs of accommodating a moderately sized non-perforated screen, my floor standing main speakers are located less than the ideal distance from the side walls. The manufacturer’s instructions recommend maintaining more than three feet from the side walls to the sides of the speaker cabinets. The sides of my speakers are 1’4” from the side walls or 2’ from the walls to the center of the bass driver. (Surprisingly they state that the back of the speaker cabinet are permitted to be as close as 1-1/2” from the front wall. The back of my speakers are about 5” from the front wall.)

I have 1” of fiberglass on the front and side walls. Given that I can’t change the horizontal orientation of the mains, is there anything I can do to mitigate SBIR effects?

Would merely setting the crossovers higher tend to overcome these effects?

In addition, would filling the space between the speakers and the side walls with rolls of my left-over 1” fiberglass be helpful? If so, since my overall reverberation time measures fairly low, would it be wise to cover the rolls with paper or some other reflective covering to avoid further reductions in the higher frequency reverberation times?

In order to center my main’s tweeters vertically on the screen, I have placed them on concrete blocks. However, that puts the centers of the bass drivers at the same distance to the stage floor as they are from the side walls. Do these common dimensions aggravate SBIR effects?

Thanks in advance.

Larry

Originally posted by Dennis Erskine
As you move a speaker closer to the wall the SBIR notch frequency increases. With non-full range + a subwoofer, you can move the sub toward a boundary to raise the notch frequency above 80 Hz. You move your mains out from the wall (about 3') to move the notch below 80 hz. With full range, your speakers need to be on the order of 15 feet from any boundary. Further, the locations required for your main speakers to achieve the best imaging will not be the best location for LF to achieve smooth bass response in the room.


In my room I have full-range towers and am forced to put them approx 14" from the sidewalls. I also measure using ETF a null at ~220Hz. This to my thinking is SBIR (1130/(220*4) = ~15"). Where would wall treatment go to correct this - directly to the sides of the speakers, or more towards where the first reflection points are?

I did try heavily trapping the side walls beside the speakers with 2" fiberglass at one point, and it made the speakers sound dead. However, I may have used too much of it. I just checked my speaker specs - the woofer crossover is at 250Hz. I may try just treating the wall down low where the woofer is.

From a general point of view, do you recommend non-full range + sub even for 2-channel audio?

Thanks,

Andy K.

Originally posted by kromkamp
In my room I have full-range towers and am forced to put them approx 14" from the sidewalls. I also measure using ETF a null at ~220Hz. This to my thinking is SBIR (1130/(220*4) = ~15").

Hi Andy,

What did you set your time window for your measurement?

Thanks.

Larry

Larry -
The idea with the crossover frequency and SBIR is to arrange sub and main speaker so that neither generates an audible 1/4 wavelength frequency. Just a simple calculation based on speaker distance to the wall is necessary.

Take the distance of the speaker or subwoofer to the wall in feet, and divide 282.5 by this distance. (282.5 = 1130 feet/sec, the speed of sound at room temperature, * 1/4 wavelength). The result is the frequency in Hz that would generate 1/4 wave cancellation.

This frequency calculated for your subwoofer, should be above the crossover point. This frequency calculated for the mains should be below the crossover point. Then, both sources completely avoid the problem.

Andy -
Wall treatment should be centered around the first reflection points, but it also needs to cover a broad area since the wavelength is relatively large. Without doing the math, I would guess that dimensions of 1/2 wavelength, or around 30", would be pretty safe.

One idea would be using a narrow tuned bass absorber for this specific frequency. That way, it will not affect the overall sound that you like. A 220 Hz tuned bass absorber (either Helmholtz or limp acoustic mass panel) shouldn't require a great deal of physical depth.

- Terry

Originally posted by Terry Montlick
Larry -
The idea with the crossover frequency and SBIR is to arrange sub and main speaker so that neither generates an audible 1/4 wavelength frequency. Just a simple calculation based on speaker distance to the wall is necessary.

Take the distance of the speaker or subwoofer to the wall in feet, and divide 282.5 by this distance. (282.5 = 1130 feet/sec, the speed of sound at room temperature, * 1/4 wavelength). The result is the frequency in Hz that would generate 1/4 wave cancellation.

This frequency calculated for your subwoofer, should be above the crossover point. This frequency calculated for the mains should be below the crossover point. Then, both sources completely avoid the problem.



Hi Terry,

Thanks for the response.

When measuring the distance from the speaker or subwoofer to the wall, are we measuring from the center of the bass drivers, or the side of the cabinet nearest the wall?

Thanks.

Larry

Originally posted by LarryChanin

When measuring the distance from the speaker or subwoofer to the wall, are we measuring from the center of the bass drivers, or the side of the cabinet nearest the wall?


Center of the bass drivers, mostly. There will be diffraction at the edges of the cabinet which will create secondary sound sources, but these will be lower in volume.

However, in the case of a side wall, you need to consider the time DIFFERENCE between direct and reflected sound, since the sound is not reflecting at right angles as it would be (more or less) from the front wall. Use the tape measure technique that Dennis suggested to determine this difference.

CORRECTION: When using the measured difference method, use 1/2 the wavelength, not 1/4 the wavelength. This means dividing 565 (2*282.5) by this distance difference.

- Terry

Is there is general forumla or theory on what % of sq/ft needs to be covered with (1") acoutic treatment to achieve correct RT60? Or is this just WAY too dependent on too many moving variables? Basically I am looking to see if it is generally accepted that treating the front wall and ear level down to floor on the side walls in most HT's is appropiate does that equate to ~30-35%? More? Less? Any general rules that would be helpful in getting close (or in the ballpark) without measuring?

-Jason

You can use some of the better spreadsheets and calculate it based on all of the materials in the room. It's far from exact but will get you in the ballpark.

How much you cover and in what depends so much on furnishings, concrete or wood subfloor, wall construction, # of people, etc. that it's awfully hard to give even a ballpark number. Also, the 'correct' RT60 is dependent on what you want to do with the room. The correct number is different for music listening than it is for HT.

Originally posted by Terry Montlick
Center of the bass drivers, mostly. There will be diffraction at the edges of the cabinet which will create secondary sound sources, but these will be lower in volume.

However, in the case of a side wall, you need to consider the time DIFFERENCE between direct and reflected sound, since the sound is not reflecting at right angles as it would be (more or less) from the front wall. Use the tape measure technique that Dennis suggested to determine this difference.

CORRECTION: When using the measured difference method, use 1/2 the wavelength, not 1/4 the wavelength. This means dividing 565 (2*282.5) by this distance difference.

- Terry

Hi Terry,

Thanks.

Just to be sure I know what you mean by Dennis' tape measure technique I've attached a crude diagram showing the speaker, listener and walls. The object is to measure the direct path sound (red line) and the reflected path of sound (black lines) and then use the difference in your half wave calculation. Correct?

Thanks.

Larry

Exactly, Larry, where the angles hitting the wall are equal.

Scott,

> I was planning on entire front and 42" of side all the way around. <

That's not the way I'd do it. All rooms need substantial bass trappng, and what you describe does nothing at all for the bass. At the risk of sounding like a broken record, see the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

It explains how bass trapping works best in the corners, and there's a sidebar that explains the importance of placing absorption at the first reflection points.

--Ethan

Jason,

> what % of sq/ft needs to be covered with (1") acoutic treatment to achieve correct RT60? <

Unfortunately, that's too simplistic an approach. One-inch thick rigid fiberglass does little for the midrange and below, so already you're starting off on the wrong foot. A complete treatment solution has to consider all frequencies, not just those above about 1 KHz. Also, in most rooms there really is no reverb. What dominates more than anythng else are the individual reflections.

(Sorry, I know this doesn't really answer your question!)

--Ethan

Originally posted by Terry Montlick

The idea with the crossover frequency and SBIR is to arrange sub and main speaker so that neither generates an audible 1/4 wavelength frequency. Just a simple calculation based on speaker distance to the wall is necessary.

Take the distance of the speaker or subwoofer to the wall in feet, and divide 282.5 by this distance. (282.5 = 1130 feet/sec, the speed of sound at room temperature, * 1/4 wavelength). The result is the frequency in Hz that would generate 1/4 wave cancellation.


What do you do with a dipole main that has its null aimed aproximately at the side wall?

Thanks Terry. To sum up, the first reflection points need to be treated for two reasons:

1)For high frequencies, you get a time domain reflection that can be seen in the impulse response

2)For low frequencies, you get boundary interference at a certain frequency causing cancellations to occur.

Ethan has great plans for a high-bass membrane trap that is centered precisely where I need it (150-300Hz).

Originally posted by Drew Eckhardt
What do you do with a dipole main that has its null aimed aproximately at the side wall?

You don't have to worry much about dipole mains and side wall SBIR. Unlike a
monopole, a dipole doesn't become a uniform spherical radiator at low frequencies.

Originally posted by kromkamp
Thanks Terry. To sum up, the first reflection points need to be treated for two reasons:

1)For high frequencies, you get a time domain reflection that can be seen in the impulse response

2)For low frequencies, you get boundary interference at a certain frequency causing cancellations to occur.


Yup. And they are both dependent on distance from the wall, or more correctly, path difference to your ears, as in Larry's diagram. This path difference determines:

1. For high frequencies, the time at which an early reflection can occur. If this time is greater than 15 milliseconds or so, it does not get merged by the brain into a single spatially-shifted sound image, and you don't have to worry about it screwing up your front sound stage.

2. For low frequencies, the frequency at which SBIR phase cancellation can occur.

- Terry

Originally posted by Ethan Winer
Scott,

> I was planning on entire front and 42" of side all the way around. <

That's not the way I'd do it. All rooms need substantial bass trappng, and what you describe does nothing at all for the bass. At the risk of sounding like a broken record, see the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

It explains how bass trapping works best in the corners, and there's a sidebar that explains the importance of placing absorption at the first reflection points.

--Ethan Thanks very much for the reply. OK< all the way around is out. What about front wall?

I read the FAQ yesterday in it's entirety. I admit I started getting glazed over when I got to the second sidebar and will start over there. I got the gist of the bass traps and got a pretty good understanding of the methods used to control stuff. I got the hows, but still am not sure which I need to plan for as I start construction. Unfortunately I'm still not sure where to go. After seeing a post where someone found out the better ceiling tiles are $$$$ and with my square footage, drywall is getting a rethink. I could place a whole lot of conduit and pay a drywaller to come poke and patch a lot of holes if the poster was correct on cost of good tiles (about $2000 for my room tiles only).

So, what I got out of your FAQ so far is bass traps will be necessary. I also saw your studio pic with the membrane traps. Those look most interesting to me beasue I have 2 little ones and am not to keen on filling the room with semi-exposed fiberglass. At least yours was split between the trp types.

But as I try to get this beast going, I'm still unsure what to plan for.

Above my ceiling is a vary large space, whether it be a drop-tile or drywall. It is made of 21" tall trusses 16" OC. Filling that cavity with fiberglass isn't impossible, but that would be a huge volume of fiberglass. You can't really fill inside the truss members without heroic efforts which I'm not going to be able to do. I could get the spaces between the trusses at least partially filled (13x16" batts maybe). I could potentially pay a insul company to stuff the spaces with 13" thick batts on their sides except where I have mechanicals (ducts).

Would that be one huge bass trap? From your FAQ and the mostly over my head discussions with the engineers and experts here, I gather that:

1) If I drywall the ceiling, the insulation above would be rendered almost moot? If I drywall, I may be better served to double it up, isolate it and get some sound control for upstairs (not a requirement, but if a fringe benefit, ok).
OR
2) If I use a drop-tile ceiling (presume cheap tiles) and insulate above, it will make the ceiling dead for a broad spectrum? giving it the illusion of no ceiling which is apparantly good. But I have a dead floor I'm worried about:

All these questions but one of my main concerns is that I do want carpet. It's a personal thing and we will use the space for more than home theater. The carpet will make the otherwise cold, hard floor more little-human and female-human friendly. If I MUST to keep the floor live from hosing the acoustics I could use one of the prefab wood floor types but I'd strongly rather not.

I'm happy to build the room and add the treatments later. The membrane sealed type traps look attractive to me as a solution if I can figure out which ones to make and where to put them.

The FAQ directs that the corners are the best spots for the traps. My room Link here (http://www.mlec.net/scott/HTBigRoom1.gif) has 2 corners occupied by small 45deg walls with doors in them. One corner is tight for space. One corner (front left) is wide open for a trap.

How would I best deal with these corners?

Mainly for now I need help in deciding a fundamental basic plan I can tweak later. To cover walls or not, drop tile or drywall ceiling, and all considering I want carpet.

Thanks very much for your and all else's help,
Scott

--------------------------------------------------------------------------------
Originally posted by Ethan Winer
Jason,

> what % of sq/ft needs to be covered with (1") acoutic treatment to achieve correct RT60? <

Unfortunately, that's too simplistic an approach. One-inch thick rigid fiberglass does little for the midrange and below, so already you're starting off on the wrong foot. A complete treatment solution has to consider all frequencies, not just those above about 1 KHz. Also, in most rooms there really is no reverb. What dominates more than anythng else are the individual reflections.

(Sorry, I know this doesn't really answer your question!)

--Ethan
--------------------------------------------------------------------------------



Thanks Ethan and bpape,

I kind of knew the answer, but was holding out hope.

Ethan,

Does having more than one sub (three (3) in my case) make adjusting bass response exponentially more difficult? As you may know I own Def Tech BP2000's with 15" powered subs in them that serve as my main (L/R). I have set them to "large" on my current reciever (i've tried both large and small, but MUCH perfer the large setting). I could also if it would be better set them to small and feed them the LFE/sub out which would mean they would sync up with the dedicated sub, but currently I do not have them run this way. I also have another (Def Tech) powered 15" sub located in the rear of the room which is feed from the sub out on my receiver (soon to be replaced by a PRE/PRO).

When I take test measurements and attempt to identify where and how much bass traping I need, should I be isolating my mains from my dedicated sub and testing these independently? While that might help me identify which sub is causing a peak at specific frequencies (i.e. mains or dedicated sub), ultimately when I use my system they will all be on, so that would suggest I should adjust my bass response curve in total? Shouldn't it? Just trying to understand. Any suggestions on setup or testing? I included a pic of my Def speaker inputs. Thanks!

-Jason

I am kind of in the same boat as jasplat88, in regards to proper setup to determine accurate measurements because I have NHT T6's and a W1 sub to recreate bass...

The position of a sub in the room will affect the level of excitement of the room's modes. For example, if you have a peak at the seating position, placing the sub in a null for that frequency will eliminate the peaks for that mode. Having said that, placing two or three subs in the room will create a "virtual" sub at some altogether different location in the space. In other words, the three subs together will behave as though there was a single sub in some other location.

Yes, setting up a room with multiple subs is more difficult than with a single sub...although significant benefit can be gained with this method. HOWEVER, you have a difficult problem. You cannot move two of your subwoofers. To do so would place your sound stage at risk. The best you can do is move the single sub around to optimize (favorably) your bass response at the listening position(s). That may leave the sub in the middle of the floor somewhere. Next, using parametric equalization, you tame the remaining peaks at the listening position. You are now left with nulls. These nulls can be addressed with specific band frequency absorption.

Using full range speakers in a small room (residential sized, for example) will simply mean that if you don't like the sound, move 6" and it'll change.

Dennis....thank you for the response. That helps---kind of :) You are right about my two front subs and the fact that they are pretty much set. I do have the option with my mains (L/R) since they are side firing to have the subs fire inside the room or towards the side walls. I have already tested both of these positions and found better bass for the rear seats by having them fire at the side walls. The front seating location bass did not improve much if any by this change--although I think it got a little more boomy in the higher bass freq's when the subs were firing towards the center of the room.

I have not moved my dedicated sub down there yet, but will do that this weekend. I will also graph my bass response once all subs are down there. After our earlier discussion about the front wall treatment with my particular speakers I have decided to continue testing different acoustic treatments for the highs and mids (1" 703 equilivant), but am more concerned about the bass at this point.

I did not mention, but I am limited on where I can place the sub IF I choose to place it in the rear of the HT. Otherwise, I can put it up front as well, but want to try the rear location first (and possibily settle for a acceptable vs. preferred) bass response curve---aesthetics reasons. I will try to chart out some response curves tomorrow and post them.

-Jason

Terry,

Just out of curiousity, does SBIR repeat across multiples of the fundamental? (ie. If I get cancellation at 220, will I also get it at 440,660,880 etc)

Andy K.

I am trying to find either 703, insulshield or comperable product somewhere in CT. I have spend the last hour calling around and NO one seems to cary anything. Help!! I am redoing the front of my theater and really want to do it right. Any suggestions.

Great thread!

Early on someone asked about perf screeens ...

I'm treating screen wall with 4" 703 mounted 4" off the wall.

Would a perforated screen (mounted approx 4" in front of the 703) improve overall sound quality? Seems like it should, as it eliminates a largely reflective surface over a large fraction of the front wall.

On the other hand, my inuition vis-a-vis acoustics is often confounded by physics.

ps -- if answer is "it depends": room is 18' by 15' by 12'; first reflections off the side walls are being handled by 2" RPG binary diffusor panels or 7" skylines (will decide by ear); rear wall ceiling has soffit with bass trapping.

Originally posted by kromkamp
Terry,

Just out of curiousity, does SBIR repeat across multiples of the fundamental? (ie. If I get cancellation at 220, will I also get it at 440,660,880 etc)

Andy K.

Yes, it does. However the higher frequency nulls are generally not a problem. They get lost in the increasingly large number of room modes which form the reverberant sound field.

- Terry

Thanks, thats what I thought. If I take a linear plot of my room with ETF, I do see these multiples of the fundamental. However, on the logarithmic graph the multiples are not an issue (you can see a dip at 440 but its small)

I have already tested both of these positions and found better bass for the rear seats by having them fire at the side walls.
This actually has little to do with the low frequency performance of the room...or impacts on modal response. We aim the drivers away from the audience and place a 1" acoustic (fiberglass) panel on the wall the driver points to. The problem is the mechanical noises of the sub, and the slap of the driver against the air creates an artifact of the sub's motion. That artifact can be (a) distracting and (b) fool you into thinking you can localize sounds below 80Hz.

Originally posted by Dennis Erskine

Yes, setting up a room with multiple subs is more difficult than with a single sub...although significant benefit can be gained with this method. HOWEVER, you have a difficult problem. You cannot move two of your subwoofers. To do so would place your sound stage at risk. The best you can do is move the single sub around to optimize (favorably) your bass response at the listening position(s).

Hi Dennis,

I was hoping you could help me with a theory question.

If I understand Jason's situation correctly two of his subwoofers, which are integral to his mains, are on the right and left main channels. His third sub is a dedicated LFE subwoofer.

In the white papers I've read about using multiple subwoofer to smooth bass response I was under the impression all the subs were operating on the same signal.

My guestion is even if Jason could reposition his mains without adversely affecting their sound stage, wouldn't the fact that his three subs are on three different channels mean there would be little or no effect on smoothing bass?

In other words for purposes of discussion suppose his mains were comprised of a right and left satellite and a right and left subwoofer (with suitable crossovers so the subwoofers integrated well with the satellites). In this example the three subs could be repositioned independently without adversely effecting the sound stage of the satellites. However, would it be reasonable to expect to see improvement in the bass response if the subs were operating independently on three different channels?

If we ran test sweeps on the above configuration in an attempt to test this no doubt we could come up with an arrangement that would appear to smooth the bass response. That might be due to the fact that during testing the same signals are being delivered to all speakers. However, in normal operation when the subs were on different channels would the smoothing effects of opposing subs disappear?

Thanks.

Larry

Here's a quick follow-up on my room and sub situation. I moved down my dedicated sub which I have running through the BFD. The mains (with subs) are still run only feeding full channel to each main (no LFE feed). I placed my rear sub where I would *like* it to be and graphed the response curve. I then used the BFD to adjust for the TOTAL room peaks (i.e. I did NOTHING to change the bass response feed or produced by the front main speakers).

Prior to the BFD, the room was pretty boomy. After the BFD adjustments the bass response is significantly better. A lot of the boominess is gone and I am able to crank deep bass songs and notes much louder without the ringing.

HOWEVER, is it possible, I (and potentially others) *like* <---perhaps it would be more appropiate to say "are use to"---- some boominess? If you look at my response curve, it could still use some work (sorry I only had a couple of hours, and there were at least two peaks being caused by the front speakers becuase no change to the rear sub would impact the response curve. Even with the response curve the way it is now, i turned up the sub after getting it to where it is now, and still would prefer a tiny bit more "boom."

Dennis/Terry/Ethan, I know we have talked about adjusting the response curve, but have you noticed in your experience that some people actually prefer a response curve that is not flat? I might try to run LFE to the front mains and remove the jumper to the lows (which will allow me to run it through the BFD and adjust my curve a bit more). I'm interested to hear your thoughts on the boominess issue and any other observations you can read into this.

-Jason

Edit: How will the "phase" knob on my dedicated sub impact changes to this curve (it is currently set at 180)? The x-over on this same sub is currently set at ~@55Hz.

Originally posted by jasplat88

Dennis/Terry/Ethan, I know we have talked about adjusting the response curve, but have you noticed in your experience that some people actually prefer a response curve that is not flat?

Yes. For example, the characteristic Bose sound has boomy bass, and some people love it.

It is ultimately a matter of personal preference. However to me, the original meaning of "high fidelity" is something to consider. Audio equipment has improved over the years so as to be capable of much more accurate response -- that is, fidelity to the original sound recording.

But we are currently experience a boom in bass (sorry! ;) ) due to unprecedented ease in reproducing very low frequencies. In the older days of hi-fi, you struggled to get much of any very low bass any way you could. You put speakers right in the corners to get the most room mode boost, regardless of uneven frequency response.

So IMHO, the emphasis on huge bass is in some sense a current fashion. Also, something very important to consider is that psychologically, we vertebrates perform accommodation to sensory stimuli. Without both high and low levels of any stimulus, we tend to just tune it out because it becomes the norm.

A good movie sound mix will make judicious use of loud bass, so that when it is important dramatically, you really hear it, and the movie becomes that much more exciting. The bass should really be there when it is needed, hence the need for power and accuracy in subwoofer response. But to have it cranked up all the time is not necessarily desirable.

Regards,
Terry

The fact that each sub is playing something different is no really relevant to smooth bass reponse. Each sub is still producing frequencies which excite room modes. Now, having said that, if each of three subs is playing something different, you cannot use mutual placement as a means to smooth response between them. Full range speakers are a real problem in small rooms...everyone in the room will hear something different whether discrete bass channels are used or not. The non-directionality of bass makes it rather moot do to this in any case.
---
Your mains and your subs typically have a path difference between the drive and the listening position. At the crossover point (usually 80Hz) the sub and the main speaker are playing exactly the same sound. At the crossover you want the sub to be in phase with the main with respect to the listening position. If you don't, you'll have a peak or null at the crossover point. The phase knob allows you to adjust the phase of the sub to match the mains at the listening position. In your case, they must be in phase at 55Hz.
----
It is true that most do not prefer flat frequency response. We prefer a roll off in the high frequencies and, today, there's a lust for boom. You might do a search on "X Curve" for some background on this area of psychoacoustics.

http://www.screensound.gov.au/glossary.nsf/Pages/x+curve?OpenDocument

It does come down to preference. Also, most peoples' reference material (music or otherwise) is probably boomy. Currently, fashionable as Terry states:) Many people listen to music in cars or in areas which aren't close to being accurate. And many people have not heard a reference system. So, when one listens to something in a room with a relatively flat frequency response they will wonder. Where's the bass?

I agree with Terry that you will hear the bass if it was meant to be there. Another consideration is how boosting the bass affects the rest of the frequency spectrum. If you are having a conversation in a quiet room and each person is talking at the same volume level say 75db it is pretty easy to understand them. Now, would you be able to understand them if you were sitting in a Dodge Viper zipping down the road or just at idle for that matter without raiding the volume of your voices. That low frequency rumble is interfering with the intelligibility of your dialogue. The same thing happens when you boost a frequency range. You have raised the overall SPL of the room and this can make dialogue intelligibility and detail tougher to discern.

Now whether you'd rather be in the Viper or able to hear the conversation is your choice. OK, maybe a bad analogy! But, you get the point.

Bob


Bob

Terry/Dennis/Bob...thanks for the responses. I think in my personal experience (as a DJ for serveral years) I was suckered into the "pump up the bass" mentality where you help entertain your audience by helping them *feel* the music. Since that time (that was about 12 years ago) I have become accustomed to bass (although I have never owned a car where you could "feel" the bass a car or two away....but I have always had sub in my cars and in my home system. I think I (and at least many of my friends) prefer some boom <---- and to better define that for me at least....it is the ~35-60Hz boom I really enjoy (especially when it is clear).

Dennis,

Are you suggesting here:

Full range speakers are a real problem in small rooms...everyone in the room will hear something different whether discrete bass channels are used or not. The non-directionality of bass makes it rather moot do to this in any case.

that in my case I would be better off running my mains as "small" and then feeding them the LFE as opposed to running them as large full range? I think you are saying that it would be best to not have full range speakers, but at least for me that's just one more constraint I have to work with.

The x-over set at 55Hz was more by mistake, as before I had taken any measurements it was SO boomy I just turned it down to minimize the output from the sub and I forgot to change it back before measurements. I like running my sub x-over in the 80-90Hz range. I will make that change and re-measure.

I also downloaded EFT5 last night, but since I am not familar with that program yet, I will need to play with it. Can someone please provide some guidelines on what RT60 should be in:

1) A home theater enviroment?
2) A music enviroment?

Bob.....bad analogy...I'd take the Viper :)

-Jason

I am installing rigid fiberglass on the screen wall of my family room/HT (plus 2' back on the sidewalls from floor to ceiling). For each of the 2 front wall/wall corners I have 2'X5'X4" rigid fiberglass to span the corners and act as base traps. How high off the floor should the bottom edge of the 'traps' be?

Continuing from the trap up to the ceiling, is spanning the corner with the 1" rigid fiberglass for mid/high absorption ok or should it be placed flat on the walls? I save a little material by spanning and I think I am cutting it close as it is. If I span with the 1", should I leave a gap between it and the trap?

Originally posted by Dennis Erskine
The fact that each sub is playing something different is no really relevant to smooth bass reponse. Each sub is still producing frequencies which excite room modes. Now, having said that, if each of three subs is playing something different, you cannot use mutual placement as a means to smooth response between them. Full range speakers are a real problem in small rooms...everyone in the room will hear something different whether discrete bass channels are used or not. The non-directionality of bass makes it rather moot do to this in any case.


Hi Dennis,

Thanks very much for the response.

Aside from perhaps wasting money, is the simple solution to someone who has full range speakers to merely set them to small and set the crossovers sufficiently high?

Thanks.

Larry

Originally posted by jasplat88

Can someone please provide some guidelines on what RT60 should be in:

1) A home theater enviroment?
2) A music enviroment?



Hi Jeff,

Try this thread for a start.

RT60 what is a good value! (http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=332289&highlight=reverberation)

If you are refering to two-Channel music then there should be different Reverberation Times. However, I believe that multi-channel music can happily coexist in a room whose acoustics were designed for multi-channel movies.

Larry

Scott,

> if the poster was correct on cost of good tiles (about $2000 for my room tiles only). <

You shouldn't have to pay even 1/4 that much for good tiles! But one alternative is to use thin tiles and lay 6 to 12 inch thick fiberglass batts above them.

> Above my ceiling is a vary large space <

You don't need to pack the entire cavity. R38 batts are one foot thick, and that's plenty.

> Would that be one huge bass trap? <

Yes, but bass trapping needs to be distributed, not concentrated only in the ceiling or only on the back wall.

> If I drywall the ceiling, the insulation above would be rendered almost moot? <

Yes.

> If I drywall, I may be better served to double it up, isolate it and get some sound control for upstairs (not a requirement <

If you don't need isolation you'll do better to skip the sheet rock. But as Dennis Erskine always points out, preventing sounds from getting into a theater room is often just as important as keeping sound from getting out.

> I do want carpet <

That's fine. I have carpet in my home theater too. A reflective floor with absorbent ceiling is more important in a recording studio than a home theater.

> The FAQ directs that the corners are the best spots for the traps. <

Yes, but understand that wood panel traps and fiberglass-based traps work on different principles. Both need to be near the corners, but the exact mounting differs for the two trap types.

--Ethan

Jason,

> When I take test measurements and attempt to identify where and how much bass traping I need <

You already got good advice on sub placement, but I'll add that bass traps do more than flatten the low frequency response. Just as important is the way they reduce ringing, and in a way that EQ cannot.

--Ethan

You are probably better off setting your mains to small and connecting your subs to the LFE output from your BFD. Then I would try to find the best location for your subs and seating if that is a possibility. Before we work with three subs, let's start with one.

1. Put your sub in the corner and move your mic between the corners of the room and take a reading of Pink noise on the LFE channel only. Theoretically, that same corner should be fine. But, I like to double check especially if it is not rectangular. This should show all your room modes.

2. Now move your seating to get the smoothest response possible.

3. Place sub in your seating location and move the mic around the room and find the areas that give the smoothest response. This will be somewhere into the room. Not along the walls.

4. Next place your subs so that if you draw a line from each sub. That line would intersect one of your ideal sub loacations. This is probably a good starting point for your sub placement. Start moving each one individually now and find the best spot. This is the virtual sub technique. Note: if your fronts and seatings is fixed. I'd probably just use this technique first and just take measuments each time you move your third sub and find the best spot this way.

5. EQ out whatever peaks you have left.

Also, when doing this you might want to play with your crossover point a bit. Since the placement affects this. Changing your crossover might yield a smoother response as well. 80 works for the majority of small rooms but sometimes you can fix various problems by altering the crossover point.

About RT-60. Dennis gave great recommendations in the other thread referenced. But, remember RT-60 measures overall room reverberation. You can have a good RT-60 and still poor sound becuase you didn't treat the first reflections. So just having enough absortion to achieve a good RT-60 will not necessarily bring good sound. The absorptions has to be placed in the correct areas.

Larry,

Search Dennis' name. He had a relatively recent thread about why setting your speakers to small and using subs to correct for SBIR effect is useful. The answer to your question is probably yes. Again depends on placement and how the best placement for low frequency information isn't the best placement for the mids and highs.

Bob

Bob,

Just a little nitpicking:

> That low frequency rumble is interfering with the intelligibility of your dialogue. <

In a car there may be low frequency rumble, but there's a lot of midrange content in "road noise" too, and that's the frequency range that harms dialog the most. The principle is called masking, and one sound masks another the most when they both have similar frequency content.

--Ethan

Originally posted by Ethan Winer
ou already got good advice on sub placement, but I'll add that bass traps do more than flatten the low frequency response. Just as important is the way they reduce ringing, and in a way that EQ cannot.


I thought we'd been over this already, Ethan. :)

Remember when we were discussing the cancellation of room modes by EQ, and though you didn't intuitively understand it (quite reasonable, since it's not intuitively obvious), your engineer friend explained to you that there was such a thing as an "inverse filter" for a linear system? That such a filter could fix resonance in both the frequency and time domains simultaneously, and that the order of signal processing in a linear system doesn't matter?

Or maybe you are referring to something else by the term "ringing."

Regards,
Terry

LarryChanin/jasplat88:
Can someone please provide some guidelines on what RT60 should be in: 1) A home theater enviroment?
Another good thread, other than the one Larry provided, IMHO, is:
http://www.avsforum.com/avs-vb/showthread.php?s=&postid=4011194&highlight=RT60#post4011194
where RT60 based on room volume is used. (A control room is not like a music environment room.)

I read the first couple pages and was hoping someone could carify something that Dennis stated on the first page.

His Statement:


"Now A.

Requirements for multi-channel (more than 2) are different than that required for 2 channel.

In multi-channel, the entire wall behind the front speakers is treated. You want none of the back reflections to overlay the surround field or the bring the reverberent field forward (your reverberent field and surround field is created by the multi-channel processor or mix, not so much the room as is mandatory for 2-channel). Depending on speaker placement, this treatment is brought forward along the side walls. Wall treatments are floor to slightly above ear level (where exactly is also a function of front speaker heights). While one could argue the sound at their feet is of no concern, often that square footage of treatment is required to bring the room's RT60 down to the lower levels required for multi-channel playback.

If you have soffits, the bottom of the soffits is also treated...several reasons, right tricorners among them.


The statment that says "Wall Treatments are floor to slightly above ear level...". I plan not to use Linacoustic, but treat my room with Bass Traps and Absorption panels at the First Order Reflection Points.

Question 1. Should my Sound Absorption panel only be from the floor to above the ear, or should it extend the height of the wall?

Question 2. After reading the first few posts, I see that the front wall should be treated. I have checked my area for Linacoustic or Insulshield and cannot find it. So, if I was NOT to put drywall on my front wall, but instead, stuff R19 between the studs, and cover it with GOM....would that be an effective way to treat the front of the room?

Bob/Dennis/Terry/Ethan,

Here's a follow-up on our earlier discussion. I re-measured my room---this time with ONLY the dedicated sub, and I moved the sub from the rear of the room to the front. I ran one measurement set with the sub set about 1/3 the width of the room (attached blue line), and another with it very close to the center of the width of the room (pink like). I think both of these results are worse than what I had running all three when the dedicated was in the rear of the room. It's possible that the main L/R subs (full range) were helping mask that nasty null at 56Hz I guess. Is it strange the null did not move when moving the sub (just reduced the null---but added another null at 125? Should I keep moving it around the front stage and measuring? Should I avoid the front corners?

Pink noise generated no more than 2dB difference between the front left center and right corners. The rear corners also read within 2dB....so basically all the corners showed very similar dB levels (sub placed in the center of the front wall). Is that suppose to tell me anything? I have not tried putting the sub in my listening position. If I do that how do I tell if the bass is smoother (without measuring)? If measuring is envolved, that's ok, but just a PITA to do it in potentially dozens of spots around the room.

-Jason

Edit: It's helpful if I attach the chart huh?

Hi Ethan,

I agree it might not be the best analogy and I wasn't factoring the road noise/ wind noise for the analogy. Not to mention if I was driving the Viper I wouldn't care as much about conversation:) Masking is a bigger problem! A good analogy for that would be to trying to understand somebody in a crowded restaurant with everybody having conversations.

I was just pointing out that if you had the bass frequencies playing at 120db and dialogue at 70db the dialogue would be more difficult to understand than if the bass was at 70db. And that overly exaggerating the bass region or any region can affect the overall quality of the sound.

Bob

Originally posted by jasplat88
It's possible that the main L/R subs (full range) were helping mask that nasty null at 56Hz I guess. Is it strange the null did not move when moving the sub (just reduced the null---but added another null at 125? Should I keep moving it around the front stage and measuring? Should I avoid the front corners?


Good chart, Jason. There should be some sub position which doesn't fall in that 56 Hz null for your listening area. Of course, that position may have its own problems. What are your room dimensions and where are you putting the measurement mic?

As for the dip at around 125 Hz, why worry about it? It will be well above your crossover frequency.

Regards,
Terry

Terry,

My room is 11'2" x 23'6" x 7'2" (the 7'2" is an average of ceiling heights that range from 7'6" to 6'6"<---- 7'6" the first 13 feet from the screen wall; 6'6" under my measurement location and then it goes gack up the last 4 feet of the room to about 7'). The seating location I am measuring from is approximatly 4'6" off the back wall and about 3' off the side wall measured at ear level.

So you're saying I should keep moving the sub around up front (on the stage) and keep measuring huh? I was afraid you would say that :)

-Jason

A treatment question for all of you. Is there anything other than rigid fiberglass that will work for the front wall? How about those rigid insulation panels at the lumber yards? Rigid fiberglass is proving to be VERY difficult to track down.

Mark,

Have you tried your local HVAC dealers and ask for ductboard or ductliner...it has VERY similar properties as 703 and Linacoustic and is MUCH easier to find. I was able to get 2 boxs (10 2'x4' sheets per box) for $65. Works great!

-Jason

NO I haven't Jason, what does it look like? Is it rigid our a role? I will ask, but I did call two plumbing/heating places asking for ductboard or rigid fiberglass and they had no idea what i was talking about.

Another questions for everyone. Uf I plan on having curtains in the front to the sides of the screen, bo they negate any effects of using GOM and rigid fiberglass in the first place on the front wall?

Mark,

It is rigid and they use it to line ducts. Here's a pic of what mine looks like on the wall before covering.

-Jason

See here:

http://www.ethanwiner.com/acoustics.html#rigid%20fiberglass

Any competent commercial insulation/heating company or supplier should know what this is. Small home-type installers may not.

marjen:

You can use anything found on this page:
http://www.bobgolds.com/AbsorptionCoefficients.htm
(fiberglass, rockwool, cotton, polyester, even open cell foam)

Originally posted by Newk2
I am installing rigid fiberglass on the screen wall of my family room/HT (plus 2' back on the sidewalls from floor to ceiling). For each of the 2 front wall/wall corners I have 2'X5'X4" rigid fiberglass to span the corners and act as base traps. How high off the floor should the bottom edge of the 'traps' be?

Continuing from the trap up to the ceiling, is spanning the corner with the 1" rigid fiberglass for mid/high absorption ok or should it be placed flat on the walls? I save a little material by spanning and I think I am cutting it close as it is. If I span with the 1", should I leave a gap between it and the trap?

Is my question not detailed enough or just stupid?:confused:

How high off the floor should the bottom edge of the 'traps' be?

Shouldn't really matter at all. Obviously resting the panels on the ground with gravity is easier than suspending them in the air, but there's no performance difference or nuthin.

If I span with the 1", should I leave a gap between it and the trap?

Maybe I'm confused, but it sounds like you are using thicker panel across the corner for your trap right? And thin fiberboard elsewhere on the walls? All spanning the corner does is help get more of the bass, and the thicker the better (for ANY panel, including on the walls). Continuing a corner-spanned trap up to the ceiling makes for a neat corner of course, ideally you'd want the whole thing with thicker paneling there, but if you're running low there's no "problem" with using a thinner amount on the upper part of your corner span.

Chris,

Thank you. I was starting to feel like a pariah. I posted long ago in the Home Theater Builder Forum with no response.

When I purchased the Certainteed 4'X10'X2" ductboard I kind of freaked at the price. I decided, after seeing that Ethans traps were only 4' long, that 5' ones would work and quarterd the sheet to make (2) 4" thick.

Realizing now what a relatively small amount of money that was, I would go ahead and buy 2 sheets to do both corners the full 9' 6". But alas, I live in Bremerton and the business I deal with is in Renton. It is only open during regular business hours. It ends up being an almost all day venture and a hard way to justify a day's use of leave.

I will run my curtain diagonally across the corner to conform to the spanning of the trap so thought that if there was no problem I would span the corner the same with the 1" material from the top of the trap to the ceiling.

I remember reading somewhere that someone recommended leaving a space at the bottom of the trap. I did not know if the space was necessary, but if so, how much?

Again, thanks. After your post, I'm feeling a little more secure and a little less sensitive... :) ... possibly a little silly as well.

Originally posted by jasplat88

My room is 11'2" x 23'6" x 7'2" (the 7'2" is an average of ceiling heights that range from 7'6" to 6'6"<---- 7'6" the first 13 feet from the screen wall; 6'6" under my measurement location and then it goes gack up the last 4 feet of the room to about 7'). The seating location I am measuring from is approximatly 4'6" off the back wall and about 3' off the side wall measured at ear level.


Hmm, there is no obvious modal source for that deep 56 Hz null, given the info you provided. The mic position is kind of close to the rear wall, and I suspect SBIR. It's hard to diagnose exactly, because an n'th octave frequency response is typically a composite of multiple FFTs with their own time windows. It's pretty "processed", and one doesn't know the details of the short-term vs. long-term analysis characteristics -- how much SBIR vs. modal response is represented in the display. If the distance to the back wall is more like 5', a 1/4 wave null at 56 Hz can occur.

How can SBIR be caused by a particular listening position, rather than a particular speaker position? Well, the same fundamental acoustical principle (called "reciprocity") which allows you to place your subwoofer in your seat in order to find a good position for it doing a "crawl" also implies that whatever room boundary effects occur at the speaker position also happen at the listening position! So direct vs. reflected path differences produce exactly the same effect at the listening position.

To determine if this null is modal vs. SBIR, just move the mic position one foot closer to the wall. If SBIR, the null should jump to a higher frequency -- about 71 Hz.

BTW, when we do a standard acoustical analysis for a client, we directly analyze the impulse response, using a variety of both commercial and custom-written software tools. The impulse response is the "rawest", most unprocessed acoustical measure for a room. There is a wealth of information in it, if you know what to look for!

- Terry

Please explain the term right tricorner. Is this were a soffit meets the corner formed by the walls or is it the staggered corner formed by both the soffit facial and walls?
Along these same thoughts is it best to make a trayed ceiling into an octagon rather than a rectangle?

Thanks you so much guys for the list of alternatives and additional info on the duct board, etc. Looks like I have some phone calls to make.

Tri-corner describes where 2 walls and the floor or 2 walls and the ceiling meet (or any other 3 surfaces for that matter). If you can point to a position and have it be the end of 3 different dimensions, it's a tri-corner.

That's why that place is particularly effective for treatments. It is at the end of 3 different dimensions and therefore has the opportunity for maximal absorbtion of all frequencies AND modes from all 3 dimensions.

Terry,

> your engineer friend explained to you that there was such a thing as an "inverse filter" for a linear system? That such a filter could fix resonance in both the frequency and time domains <

Yes, believe me I remember that well. But you may not remember an equally important point that came out of that discussion. My expert EE friend Bill sometimes lurks here, so maybe he'll chime in too. Here's the gist of what Bill told me, watered down to a level I can understand. :D

Yes, it's possible in theory for an electronic device to reduce ringing after the fact. The key words being "in theory." The two main problems are:

1) In order to reduce ringing the filter must provide the precise inverse of the room. This is all but impossible with a standard EQ, though it could be done with more sophisticated DSP processing.

2) Much more important is the small physical size of the corrected area. If you aim to reduce ringing a little, you can do that for a reasonably large area. Let's say one or two cubic feet. But in order to make a meaningful improvement, the physical size of the corrected "zone" becomes very small. So unless someone is willing to clamp their head in a vise while listening to music, treating the room with bass traps is a more practical choice.

--Ethan

Ethan,

Bill is right on both points, however for low frequency room modes, the theory works quite well in practice.

For point 1, inversion of the low frequency room modal response can be done with extremely simple EQ. This works because low frequency room modes are (for all practical purposes) minimum phase. All you need is another minimum phase filter to cancel them. Any parametric equalizer provides such filters - they are simple 2nd order sections, with symmetrical pairs of poles and zeros. So for low frequency mode cancellation, the fanciest DSP offers no advantage over a simple analog filter.

As for point 2, the space that the filter is effective over is proportional to the wavelength of the room mode. So here again low frequencies make it practical. EQ for can cover not just a very precise position (such that you have to keep your head in a Mayfield clamp), but a reasonable listening area.

I too am skeptical of practical inverse filtering for high frequencies, at least for the near future. Current research has not show a whole lot of progress. The preciseness of head positioning is indeed a problem in this case. Particularly troublesome are the nasty artifacts which are audible BEFORE the main portion of a sound is heard.

Regards,
Terry

Originally posted by Terry Montlick
Hmm, there is no obvious modal source for that deep 56 Hz null, given the info you provided. The mic position is kind of close to the rear wall, and I suspect SBIR. It's hard to diagnose exactly, because an n'th octave frequency response is typically a composite of multiple FFTs with their own time windows. It's pretty "processed", and one doesn't know the details of the short-term vs. long-term analysis characteristics -- how much SBIR vs. modal response is represented in the display. If the distance to the back wall is more like 5', a 1/4 wave null at 56 Hz can occur.
- Terry

Terry, thanks for the post. I thought 4'6" off a back wall in a 23'6" deep room was actually pretty far away from the back wall....no? I measured this morning and it is entirely possible that the mic location has been closer to 4'10" (maybe even 5') off the back wall. So you are saying if I move the mic back....say to 4' or even 3'6" (i.e closer to the back wall) and the null moves up in frequency, then the null is casued by SBIR? I will try taking some measurements at and around my previous null location and report back.

-Jason

Bob has been helping me behind the scenes, and here is part of one of his pms:

[qoute]Anyway, you have room modes at:

Length
24, 48, 72 hz

Width
51, 102 hz

Height
79 hz Basically don't have your ear height at about 1/2 this dimension, this would create a null around 80hz. Not a good spot for a nulll with the crossover at 80hz.

Now as I said real world measurments are not as perfect as calculated. Now using your seating measurement you have basically placed yourself in a null at 48 and 51 Hz. Which between the 1/6 octave measurment and real world fudge factor I bet is the 56hz on your graph. You have a double whammy there causing that nice dip.

Move your seat forward (only the mic for now) 1.5 feet and move the sub about 62" from the side wall and then move it 44" into the room. Now take your measurements." [/quote]

Well Bob, I did as you suggested and moved my main (only using dedicated sub for these tests) sub 66" from the side walls (BTW this places it almost dead center in the middle of my stage, and then moved it ~44" out from the front wall (I will attach a pic in a minute so you can see physically where it is). Since I wasn't sure if the 44" was to the front of the sub, or driver or middle of sub enclosure, I placed it ~44" to where the magnet on the driver probably is. The front of the sub is 48" to the wall and the middle of the enclosure is 40" if that helps any.

Well the response curve (green) is much better and the 56Hz null is much improved and there is a slight dip now at 50Hz. Overall though this is better. Now the problem is....the sub can't stay there (see next pic...it's exactly where the center channel will go), and the listening position is fixed at slightly 1' back from where I moved the mic (the mic in this measurement was 6' from the back wall, and 3' from the side wall). But I think we are just trying to determine the IDEAL sub location and understand my room modes. Based on Bob's curve (green one) I should be able to flatten out at about 80dB from 25Hz-111Hz using the BFD no?

I have been measuring from what will be my music listening postion (rear row), however, my main movie watching location will be on the front row and I have not taken any measurements there....should I?

-Jason

Here's a follow-up pic to show where the sub is located in the latest measurements I took (green line from previous post).

Newk: you must be in the navy then? I see how that would be a little difficult to make it all the way to renton. You might be getting your stuff at the same place I got my fiberboard, i forget the place's name, like JR or something i forget.

In any case, I don't know of any reason for gaps for your trap. The only thing I could think of was maybe airflow to behind the trap so it doesn't get stale, but it's not like people are breathing behind the trap in your corner, and the thing is porous, so...

Originally posted by jasplat88
Here's a follow-up pic to show where the sub is located in the latest measurements I took (green line from previous post).


I'm wondering if that "extra" insulation that's leaning against the walls in this pic has been there all along for all the measurements you taken so far? I think that could effect your results. Best to either put it where it belongs, or take it out of the room, I think.

Originally posted by Terry Montlick

For point 1, inversion of the low frequency room modal response can be done with extremely simple EQ. This works because low frequency room modes are (for all practical purposes) minimum phase. All you need is another minimum phase filter to cancel them. Any parametric equalizer provides such filters - they are simple 2nd order sections, with symmetrical pairs of poles and zeros. So for low frequency mode cancellation, the fanciest DSP offers no advantage over a simple analog filter.



Terry,

I appreciate all your posts, but would it be possible for you to explain the above paragraph in layman's terms? I'm trying to digest all of this information, and do not understand what you wrote.

Thank you.

Originally posted by HT-DJ
I'm wondering if that "extra" insulation that's leaning against the walls in this pic has been there all along for all the measurements you taken so far? I think that could effect your results. Best to either put it where it belongs, or take it out of the room, I think.

It has not moved during any of these tests, but it should have little to NO impact on the frequencies I am measuring (bass). It is there as I am thinking I will need to add more acoustic panels (and that is where they would go or at least where I would like them to go) to drop my RT60 when I get to that phase. I did remove all the panels I had along the front wall prior to any measurements (i.e. my front wall is reflective and not treated with anything at the moment). Good observation:)

-Jason

A right tricorner is a reflector as well (that's how bicycle reflectors are made)...it has the characteristic of reflecting something (light, HF sound) back at the same angle it originated from.

Chris,

Not in the Navy but work for the Navy as a civil servant. The place I purchased my ductboard from is actually in Kent and the business name is Paragon Pacific. They sell to individuals but are really set up to sell in large quantities to contractors etc. It is kind of a pain to get one or two sheets (mostly for the warehouse guys). They are friendly and helpfull though. One sheet of 2"X4'X10' rigid liner board with tough guard on the face was about $93 with tax. If I thought there would be much improvement by going all the way to the ceiling, I might go back and buy another sheet on Jan 6 because I will be taking that day off with leave I must take or lose.

Originally posted by HT-DJ
Terry,

I appreciate all your posts, but would it be possible for you to explain the above paragraph in layman's terms? I'm trying to digest all of this information, and do not understand what you wrote.


That's a tall order, but I'll try.

A minimum phase filter is one which alters the phase delay of a signal through it down to its theoretical minimum. There are implications for stability of the filter (its response is guaranteed to be stable and not blow up over time) as well as causality (no sound through the filter can create an earlier effect in time, which is, believe it or not, not true for every possible filter).

Every linear filter (one for which 2X the input produces 2X the output) can be decomposed into two different filters: a minimum phase filter and what's termed an all-pass filter. The latter introduces a delay across all frequencies. One example of a process which cannot be modeled without an all-pass filter is slap-echo, or any other discrete echo.

But the cool thing about minimum phase filters is that any particular one always has an inverse filter, which is also minimum phase. This means that you can cascade a minimum phase filter with its inverse (in either order), and completely undo the filtering process.

So what has all this got to do with room modes? Simply that the resonance of a single room mode, the back and forth wall reflection and amplification at a particular frequency, is minimum phase. So that means it can be completely canceled by its inverse filter. You can readily make such an inverse filter using an off-the-shelf parametric equalizer. You just dial in the frequency, bandwidth, and gain which reverses the effect of the room mode, and you have zapped it. And it doesn't matter that the inverse filter of the EQ came BEFORE the room resonance. The minimum phase principle doesn't care about what comes before what.

Hope this helps,
Terry

Guys, I have a bit of a situation in my new room. Tonight, I finished hanging all the rigid fiberglass. I'm using CertainTeed UltraGold Duct Board. My room is 26'L x 14'W 9'H (in center). There are two soffits on each side of the room. They extend 2ft into the room and down 1ft. I have covered the entire front wall from floor to ceiling. I have also extended the floor to ceiling treatment out three feet on each side wall. The rest of the room is covered from floor up to 4ft. I clapped my hands and was horrified to hear that awful concrete echo. My room has three exterior/underground walls that are concrete (front, left and right). The right concrete wall is separated by an airspace (alley) that is 26" wide and then a 2x6 wall with drywall that forms the HT interior wall. The left wall has a much smaller airspace of about 2" and is framed with 2x4. The front wall is also similar to the left. The rear wall is an interior wall and has a room on the other side. It is framed with 2x6 studs. ALL walls have R19 batts and one layer of drywall (damn!). The ceiling also has insulation. Could this echo be caused by the soffit?

My fiberboard properties can be seen here:
http://www.bobgolds.com/AbsorptionCoefficients.htm

Originally posted by BasementBob
jasplat88:
There's no axial mode for that room near 56hz, but there is a tangental mode at 55.8hz (1,1,0).

Yes, but a center front subwoofer position cannot excite this mode. Yet there was a significant null in the corresponding frequency response.

jasplat88: My room is 11'2" x 23'6" x 7'2" ... Hmm, there is no obvious modal source for that deep 56 Hz null
There's no axial mode for that room near 56hz, but there is a tangental mode at 55.8hz (1,1,0).

First few modes (axial, tangental and oblique): 24, 47.3, 50.4, 55.8, 69.6, 71.3, 78.6, 82.2, 87.8, 92, 93.4, 95.9, 96.4, 100.9, 103.7, 105.0, 106.5, 108.3, 111.7

OK, I went back and read a little about right tri-corners. I think I definitely need to treat the bottom of my soffits. On the other hand, it's quite different if I clap or whistle while seated in my chair...there is virtually no echo. I guess that's why you treat up to ear level while seated. Maybe, I'll only treat the soffit area in front of the seating area.

Originally posted by Jeff Hovis
Guys, I have a bit of a situation in my new room. Tonight, I finished hanging all the rigid fiberglass. I'm using CertainTeed UltraGold Duct Board. My room is 26'L x 14'W 9'H (in center). There are two soffits on each side of the room. They extend 2ft into the room and down 1ft. I have covered the entire front wall from floor to ceiling. I have also extended the floor to ceiling treatment out three feet on each side wall. The rest of the room is covered from floor up to 4ft. I clapped my hands and was horrified to hear that awful concrete echo. My room has three exterior/underground walls that are concrete (front, left and right). The right concrete wall is separated by an airspace (alley) that is 26" wide and then a 2x6 wall with drywall that forms the HT interior wall. The left wall has a much smaller airspace of about 2" and is framed with 2x4. The front wall is also similar to the left. The rear wall is an interior wall and has a room on the other side. It is framed with 2x6 studs. ALL walls have R19 batts and one layer of drywall (damn!). The ceiling also has insulation. Could this echo be caused by the soffit?

My fiberboard properties can be seen here:
http://www.bobgolds.com/AbsorptionCoefficients.htm

jasplat88:
If you get bored and find yourself with nothing to do, what happens with the subwoofer/mic at these locations:
- subwoofer at (6', 2'6.6", 1') and the microphone at (13'3.1"", 7', 3'8.9")
- subwoofer at (5'11.9, 1'11.9", 1') and the microphone at (12'4.7"", 7', 3'8.9")
( RPG Room Optimizer, 20hz to 80hz)

Originally posted by BasementBob
jasplat88:
If you get bored and find yourself with nothing to do, what happens with the subwoofer/mic at these locations:
- subwoofer at (6', 2'6.6", 1') and the microphone at (13'3.1"", 7', 3'8.9")
- subwoofer at (5'11.9, 1'11.9", 1') and the microphone at (12'4.7"", 7', 3'8.9")
( RPG Room Optimizer, 20hz to 80hz)

Hey Bob,

I will do this. Probably tomorrow (Monday) night and report back. I assume these are locations off the BACK wall correct? Read....for example in your first measurement (6' from the back wall, 2'6.6" from either side wall, 1' off the floor)? Just want to make sure before I get started. Thanks!

-Jason

jasplat88:
The program assumes a symetrical/rectangular room. So they are measurments off either the back or the front wall, as you prefer. They are all (23'6" dimension, 11'2" dimension, 7'2" dimension). I would have tried them from (front, left, floor), because then the microphone positions seem about right for a chair. I wouldn't swap start points between sub and microphone (i.e. sub measured from back and microphone measured from front -- would be bad).