Acoustical Treatments Master Thread - Page 2

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

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 )??

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:





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:
http://www.ib-neubauer.com/Literatur...001_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

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.

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 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 <<br />
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 <<br />
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.

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). <<br />
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!