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-   -   Acoustical Treatments Master Thread (https://www.avsforum.com/forum/19-dedicated-theater-design-construction/255432-acoustical-treatments-master-thread.html)

JBS 05-04-2003 12:13 AM

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...

A) Acoustical treatment (1" Insul-Shield) from floor to ear-height (44"), with 16oz polyester batting above.
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.

Dennis Erskine 05-04-2003 05:16 AM

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).

Ethan Winer 05-04-2003 06:54 AM


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.


Dennis Erskine 05-04-2003 07:10 AM

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.

JBS 05-04-2003 08:06 AM

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.

Dennis Erskine 05-04-2003 09:13 AM

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

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

gregr1 05-04-2003 10:24 AM

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?

Dennis Erskine 05-04-2003 11:17 AM

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.

obie_fl 05-04-2003 08:39 PM


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)?


PamW 05-05-2003 04:25 AM

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?


chs4 05-05-2003 05:07 AM


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)?



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

JBS 05-05-2003 07:50 AM

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.

DennisBP 05-29-2003 05:24 AM

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?


RRBOOGIE 05-29-2003 01:56 PM

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?

Dennis Erskine 05-30-2003 04:57 AM


Does too much apsorption defeat the Omnipolar characteristics of a speaker?

Yes. And, that's what you want.

proufo 05-30-2003 05:27 AM


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 [email protected].

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.

Ted White 05-30-2003 07:10 AM

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


PamW 05-30-2003 01:01 PM

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...

CptnRandy 05-30-2003 01:14 PM

Mine is treated the same as the sides.

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


PamW 05-30-2003 01:17 PM

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

Crisis averted for now. Hysteria is gone.


Ted White 05-30-2003 01:48 PM

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.


bpape 05-31-2003 06:05 AM

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.

Dennis Erskine 05-31-2003 01:31 PM

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.

bpape 05-31-2003 02:34 PM

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.

Dennis Erskine 05-31-2003 06:39 PM

An anechoic chamber is all absorption (ever done research in one?). Like this one for example:

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.

bpape 05-31-2003 08:05 PM


Originally posted by Dennis Erskine
An anechoic chamber is all absorption (ever done research in one?). Like this one for example:

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.

Dennis Erskine 06-01-2003 04:47 AM

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

Mancubus 06-01-2003 08:52 AM

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?


bpape 06-01-2003 09:15 AM

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.

dmcvie 06-01-2003 05:52 PM

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?


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