Acoustical Treatments Master Thread - Page 8

Bob,

Just a little nitpicking:

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

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:

Another good thread, other than the one Larry provided, IMHO, is:
http://www.avsforum.com/avs-vb/showt...60#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

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....d%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)

Is my question not detailed enough or just stupid?

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.


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.

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 <<br />
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.

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

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...

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.

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.

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