HT acoustic treatments?

[charcoal grey]

I am currently designing a dedicated home theater room. Construction will start within the next few weeks. I just needed some input regarding the acoustical treatments before I start.

First, the room info. Room width varies, 140” at the widest point, and 128” at the narrowest. Length is 174” with a ceiling height of 96”. The room is an existing room located on the second level of my home.


Second the audio. I will be going with a 7.1 setup, using towers for all the speakers. Subwoofer is still in design phase. I am looking to use an IB setup. But since I have not been able to actually perform any room analysis as of yet I don’t know as to where the sub will be located within the room. My preference would be at the front of the room and possibly centered. Looking to do either a single manifold with 4 drivers, or dual manifolds with 2 drivers in each manifold.

For video I am planning to use a projector and an AT screen of an undetermined size. I already have the screen material. I have enough to build a screen as large as 100” diag. With my size of room I don’t think I will go with anything larger than in the low 90’s range though.

For treatments I am considering the following:

Front wall: 2” OC 703 along entire wall. Possibly covering that with 1” linacoustic.
Side walls: 2” OC 703 spaced out from the wall 2” at the first reflection points.
Rear wall: 4” OC located behind the rear towers and at any possible reflection points from side speakers.
Ceiling: I am uncertain of this. I would like to treat first reflection points. I am also considering building soffits. I could fill the area within the soffit with OC 703 for additional bass trapping.
Floor is a wood laminate. I will use a large area rug throughout the room. If needed, I will try to talk my wife into letting me put in carpet.

Any input would be appreciated. Thanks.

[bpape]

Bass control - good.

2" on all the walls - too much surface area.

Rear wall and side walls - depends on a lot of variables.

If you're doing 7.1 for HT, you'd be better off with at least the surrounds as monitors and up at 6' or so off the floor. Towers are a waste as you should be cutting them off at 80Hz or so to feed to the sub - not to mention being able to get them up where they need to be.

Bryan

[salvasol]

Bryan, I'll appreciate your opinion on this:

He will be using 2" OC 703 in the entire front wall ... Do you think this will be enough to stop the sound traveling to the adjacent bedroom???

I have a similar situation (2" x 4" wall without insulation and just one panel of drywall in each side); I can hear the movie in the bedroom, and I'm also planning to add 1" or 2" of OC 703, but I'm not sure is this will be enough ... I found at Lowes/Home Depot something called Soundstop panels (www.knightcelotex.com); they recommend using this panel attached to studs and then cover it with drywall (kind of a double layer on each side), but if we already have the drywall in place; Do you think is fine to use it over it and then add OC 703 to stop the sound traveling to the bedroom???
I got a page with the information, they give a STC 60 when you use Soundstop + Fiberglass Batts + Resilient channels + 5/8" drywall ... and to get STC 50 just use soundstop panel + 5/8" drywall ... What do you think???

I don't want to highjack this tread, but I think your opinion with help both of us

[bpape]

Sry - adding to the surface will do nothing to stop transmission to the other room. A single wall with no insulation is basically a drum. It's going to transmit.

If you're serious, cut holes at the top of each stud cavity, go to home depot and rent a blower, and fill the cavities with insulation. Then add Green Glue and another layer of drywall. That's about a 50% solution.

Bryan

[charcoal grey]

Thanks for the info Bryan. Just for clarification, I don't plan to cover the sidewalls entirely with 2", just at the first reflection points. I downloaded a free software that helped me locate those. It seemed for the side walls it was for about 12" up from the floor to a total height of about 48" and extends from the screen wall to just before the seating area. The rest of the side walls will be left untreated.

As for the towers speakers I have most of them already. Some were are backorder and I am waiting shipping for the rest of them. I do plan to build risers for the rears and sides though to raise them to a more appropriate height.

For my front wall I was not worried about sound proofing it just making sure it has no reflections problems. I do plan to add insulation within all the walls for the room to help with noise in the adjoining rooms. I realize this will really only help some at low volume levels if any.

I also read some other posts by you Bryan where it was suggested to set up the room with the seating and speakers and perform sound tests to determine the best speaker placement and seating placement. I am going to try to get that done by the end of next week. I just need to pick up a mic and external sound card by then. However, since I don't have all my speakers yet, is it ok to do this in just stereo mode? Is the procedure ever done in a multichannel mode?

[bpape]

Mark.

The isolation stuff was an answer to the poster above my post - Salvasol (David).

Treatments on the side wall at the reflection points is good though there won't be any at 1' off the floor (unless you have hard floors). We usually start them about 2' and go up to about 6' for visible panels. I don't know where your seats are to know how much 'screen to just before seating' is.

You can run your tests with just the L and R and Sub as long as the rest of the room is pretty much done (carpet, seating, etc.) For the surrounds, their' location is not so much driven by smoothing response as it is proper placement for accurate surround recreation.

Bryan

[charcoal grey]

Bryan, even though sound proofing the room is not really what I am after, does adding a second layer of drywall with green glue make a significant impact? I also read that it is supposed to help with low end nulls the room might have, according to green glue's website. I just figure if it will help with sound quality within the room also it might be something for me to consider.

[bpape]

Double drywall and green glue will help isolation for sure.

As for changing the inside, yes it will. However, it's a double edged sword. There is more bass kept in the room and the resonant frequency of the structure is lower. So, you have more bass control to accomplish, but, the walls are absorbing at a lower frequency where it's harder to treat (although the efficiency per sq ft of double drywall is only about half of single drywall).

Nothing's free...

Bryan

[alluder]

What about making the major surfaces non-parallel? Many recording studios are more than 4-sided, with angled ceilings such that no two surfaces are parallel and none meet in a right angle. Many performance spaces are designed with diverging side walls, a sloping ceiling, and multi-plane back walls. All these funny angles are to prevent standing waves in the recording studio or performance space. One of the main goals of this fancy acoustic treatment is to alleviate the bad effects of standing waves. Wouldn't it be better to start with a non-square no-standing-wave room to begin with?

[bpape]

Yes, I know. I design studios too. Trust me, you don't have the room to splay the walls enough to do more than mess up calculations. To make a dent, you'd lose about 28" of width at the front of the room. Not worth it. It would cramp the screen, mess up speaker placement, etc. Remember, studios are NEARFIELD listening environments. This is not.

Also, even though that gets rid of direct slap, it still does nothing to reduce decay times. That just requires absorbtion. You'll still have standing waves, just a matter of where they are and at what frequency. There are also other modes which involve 4 and 6 surfaces.

Sorry

[alluder]

bpape wrote:

Yes, I know. I design studios too. Trust me, you don't have the room to splay the walls enough to do more than mess up calculations. To make a dent, you'd lose about 28" of width at the front of the room. Not worth it. It would cramp the screen, mess up speaker placement, etc. Remember, studios are NEARFIELD listening environments. This is not.

Also, even though that gets rid of direct slap, it still does nothing to reduce decay times. That just requires absorbtion. You'll still have standing waves, just a matter of where they are and at what frequency. There are also other modes which involve 4 and 6 surfaces.

Sorry

Thank you for the quick response! OK, I see your point about studios being nearfield. but performance spaces are not, and I see many newer halls with a "keystone" shape and non-parallel back wall. And even older theatres often have a keystone shaped stage with a rectangular seating area. Also, you mentioned messing up the calculations. I assume pros use some kind of room response prediction software. Can these packages handle non-rectangular rooms? Finally, I agree, too much room reverberation is a mess, but a little can be helpful. The other day I was testing some headphones, and the Steely Dan song "Black Cow" was available, so I listened. On headphones the opening sounds completely dead! tons of reverb is added later in the song, but the opening needs some room response to sound decent. So it makes me think that a little bit of room response is not a bad thing. It might even allow people to get away with smaller subwoofers and less overall audio wattage.

[bpape]

Again, you're talking apples and oranges. Performance venues are calculated and treated as 'large' spaces where the rules and goals are completely different than in smaller residential spaces. You can predict things with special software in non-rectangular spaces but it's very very expensive.

You don't want to kill the room completely and your statement of a little room being good is correct. Everything in moderation. Some needs to be dead, some can be left live. The trick is figuring out how much and where.

Counting on 'room boom' (lack of bass control) to offset a small sub isn't a good way to go. You'll get tons of midbass, no low bass, and muddy, difficult to understand dialog. Get a real sub, treat the room properly, and hang on to your hat.

Bryan

[mas]

alluder wrote:

too much room reverberation is a mess, but a little can be helpful. .

Just a note.

In a small acoustical space you do not have a true reverberant field (except perhaps at frequencies higher than what we are concerned). The references to RT60's and the such reverberant field measurements are pertinent only to large acoustical spaces (as defined by Schroeder.)

One method to achieve an increasingly diffused semi-reverberant sound field, aside from the use of diffusors ( about which you also need to be cognizant of their distance from your listening position in order to minimize the specular reflections) is to use Russ Berger's elegant trick of utilizing excess ceiling space or a closet or adjacent hallway or utility room, etc. by way of a phase grating. Sound passes out and through the phase grating depending upon its incident angle re-enters the room at a later time with lesser gain - lending an increased semi-reverberant tail and a sense of greater spaciousness. These phase grating also afford good off axis incident diffusion as well - and their use, coupled with a backing layer of absorption, offers some very interesting characteristics as well - especially in LF absorption - but i would suggest experimentation and measurements to verify the desired performance.. An example of these are the pArt Science Space Coupler. But these can be made without too much difficulty for additional cost savings

[bpape]

Sorry - but RT60 is still very useful in small room acoustics. You're correct that it's not a true reverberant field. However, it's still a useful way to calculate relative target decay curves. RT30 can also be used if preferred. The point is that no matter what method you use, broadband decay time must still be considered and balanced.

Bryan

[mas]

bpape wrote:

Sorry - but RT60 is still very useful in small room acoustics. You're correct that it's not a true reverberant field. However, it's still a useful way to calculate relative target decay curves. RT30 can also be used if preferred. The point is that no matter what method you use, broadband decay time must still be considered and balanced.

Bryan

Respectfully...

The ETC would be a much more useful and illustrative tool to determine the amplitude, source, and subsequent isolation and remediation of specular reflections as well as to determine the effective degree of remediative diffusive (or absorptive) treatment than a measurement that does not accurately address either.

Simply generating a number that fails to isolate any of the real factors (or behaviors) remains an abstraction that contributes little to isolating and addressing any of the real concerns upon which one should be focusing.

Such a statistical value for an RT60 or an RT30 fail to identify and isolate any specular reflections that are the source of the degradation in intelligibility that are predominate in a small acoustical space. Likewise, such calculations fail to assist in the practical identification, treatment and/or verification of effective remediation for room modes as well - rendering their practical usefulness of questionable value.

Rather it is the practical identification and effective treatment of the room modes and specular reflections that enable a greater 'well-behaved' semi-reverberant decay 'tail'. And to the degree that one can effectively address this, the finite acoustic energy available in the acoustical space can be most advantageously utilized.

A statistical abstraction that fails to provide for the practical identification of such phenomena is of limited value except as a point of discussion over a beer. ;-)

[bpape]

Agreed that decay times are only a piece of the puzzle. Just throwing up absorbtion to bring them into line without considering specific reflections would be a bad idea. Don't believe I ever said that though.

Just as bad IMO though, is only looking at specific reflections and ignoring balancing the decay spectra fully based on the size of the room AND it's usage.

Bryan

[mas]

bpape wrote:

Just as bad IMO though, is only looking at specific reflections and ignoring balancing the decay spectra fully based on the size of the room AND it's usage.

I must admit to being very curious as to where this was ever suggested, as I certainly did not!

There is a well established and accepted 'best practice' process for evaluating such data - a few cursory examples of which are suggested in the following diagrams.

I must admit to being a bit surprised that more are not familiar with this process that has actively evolved over the past 30+ years and about which information is readily available!