Axiom Home Page Forums General Discussion Technical Questions theater room acoustics

Well, I am building my theater now. Using the "golden ratios" for room dimensions, I am going to build my room to be 7'-3" high, 11'-7 1/4" wide, and 16'-10 3/4" long. The screen will be 96"X56" and I am planning to simply have 2 rows of 4 theater seats in the room. From what I have read, the dimensions I have chosen are best (as a ratio of 1:1.6:2.33) for minimizing room peaks and nodes. In conjunction with the room size, I also will be using my trusty BFD to further smooth the room response.

These measures should really tame the bass response (hopefully to nearly ruler flat!), but what about the rest of the spectrum. I have read that room reflections are what generally muddy the sound of even the best systems, so how should I treat my walls. Do I leave the front wall solid, to allow reflections towards the seating, and then deaden the ceiling, side and rear walls to eliminate reflected sound? Do I leave a portion of the side walls un-deadened to allow some reflected sound?

I had Kind of leaned towards the first idea. If you are outside, the sound only comes from the one direction. Any sound that should sound reflected, such as bounced off a building or the lively reflections in a gymnasium, should (I would think), be re-created by the surround speakers and therefore I would want all walls deadened. Then I wonder if I can get good width and depth imaging if the front wall is deadened. I am leaning towards a stiff front wall that will reflect sound, much like an ampitheater. Perhaps the front wall, and the first four feet of the side walls and ceiling should be reflective. This would provide for a reflective depth, width, and height, without being so far out as to allow first reflections to be directed at the listening positions.

I have read that there should be a balance between reflective and absorptive materials, but I have never read any good "tried and true" facts. Anyone have any good actual designs that are proven to sound great?

I think minimizing all reflections should be your goal. Think of the room where they master the sound. I've seen one of these rooms at a company my uncle used to work for (POP Studios in Santa Monica). They pretty much deaden the entire room so any sound you get comes directly from the speakers. I remember seeing a lot of anechoic acoustic foam on the walls.

With that said, there's plenty of things you can do to your walls to deaden them. Acoustic foam is the least pleasing to the eye. A popular choice for home theaters is thick velvety "movie theater style" drapes along all the walls. Another option is thin acoustic panels, which resemble drywall wrapped in fabric.

if you use foam, be sure it's fire retardent; remember the rhode island nightclub fire.
dan

Thanks for the replies. To deaden the room I will use 2X4 furring over the masonry, filled with acoustic sound deadening fiberglass insulation (more dense fiberglass batts) with fabric over the top. I guess I was wondering if the front wall should be reflective to give a depth to the sound. Why is it that speakers should be placed a few feet from walls? I kind of thought that the front wall and maybe the first few feet of the side walls and ceiling should be reflective for this reason. You are saying that Imaging would be better in a dead room?

Here are some sources, there seems to be a lot on the subject on the web. http://www.echobusters.com/page1.html
http://www.eighthnerve.com
http://www.apluselectronics.com/acoustical_treatment.htm http://www.acousticroomsystems.com/7834_eprint.pdf

Personally i find anechoic rooms to be rather deafening.
How ironic eh?

Conventional wisdom is that the ideal room for 2-channel (front speakers only) listening is one with the "dead/live" configuration. Thus, the front half of the room is relatively dead -- especially the front and side walls near the speakers, reducing early-reflected sound coming from front, which will adversely affect sonic quality. On the other hand, the rear half of the room is more reflective. So, this is actually opposite from what you've described.

I agree with Peter that, for a multi-channel surround setup, the entire room should be relatively dead. But I don't think it should be much more dead than typical North American living rooms with carpeted floor, fabric-upholstered furniture, drywalls with some shelves, and some fabric window treatments.

I've actually been looking into acoustics for my room before I get my system. By all accounts most say that room accoustics are the most over looked and one of the most important parts of a system. The most expensive high end systems will sound like crap with bad accoustics... Most seem to recommend balancing between reflection and absorbtion and warn against making a room accoustically "dead"... It appears dampening the primary and secondary reflections have the greatest impact and that you typically want to absorb in the front of the room and diffract in the back of the room. Even though I'm just a novice I'd bet that all those folks complaining about Axiom speakers being "bright" are really describing their room accoustics, since not everyone seems to find this / agree. Properly adjust the room accoustics and violas the Axioms are no longer bright but totally neutral, and revealing. I think it makes sense for speakers with the qualities of the Axioms to easily appear bright with the wrong accoustics....?

Here's some sites for your purusal:

http://www.diyacoustics.com/files/pdf/platinum.pdf

http://www.angelfire.com/sports/RCcars/acoustic_panels.htm


http://www.asc-hifi.com/acoustic_basics.htm

http://www.audiorevolution.com/equip/cheaptreatments/

http://ic.net/~jtgale/diy2.htm

http://pat.home.mchsi.com/SAP.html

http://t-3.cc/users/audioworx/page3DIYpanels.html

http://www.rivesaudio.com/Links/Linkframes.html

http://www.realtraps.com/index.htm

http://www.audioholics.com/techtips/setup/RoomAcoustics-RivesAudio.html

http://www.audioperfectionist.com check out the Free Journal section either 1 or 2 has an HT article and how to set up speakers and accoustically treat a room. This guy makes a number of interesting points I'll likely ask advice / opinions on from the local forum experts
at some point.

Good Luck, El

great paragraph from the audioperfectionist.com

Would you accept medical advice from some anonymous @ on the internet? I guess some people do, but it?s risky business. And bad advice about audio is even more serious. Bad medical advice can only kill you. Bad audio advice can wreck the sound of your hi-fi and you?ll have to live with that!

What is everyones opinion on fabric-upholstered walls (all four walls) in a home theatre? Using the the fire resistant padding behind the fabric. Would this be a good idea or not?

I think it's definitely a better idea than nothing at all (i.e. just masonry or drywall), but you don't want to overdo it. If I were you, I'd leave the acoustic treatments until the end so you can experiment with what level of live/deadness you prefer in the room. Every space is different, so you may find a balance between live and dead is preferable to you.

Hi all,

Sushi and Eldwyn are correct, more or less. For a domestic home theater, you want a mix of absorptive and reberberant characterisitics. By the way, the "live end/dead end" thesis has been found to be dubious and not supported by acoustical tests.

Keep in mind that for music listening, in stereo or multichannel, the walls to each side of the main speakers should have some reflective qualities, because in all the NRC research on loudspeaker dispersion and its correlation in double-blind listening tests, the speakers that were rated to sound the most "spacious and open" and thus, more natural, had lateral off-axis responses that most closely approximated the on-axis frequency response. Those secondary reflections reach your ears a few milliseconds after the direct sound, but they will NOT reach your ears if the side walls are totally absorptive.

It's always something of a trade-off between movies and music, because you do want to limit ceiling reflections that limit dialog intelligibility (and aren't that important for music); on the other hand, you do want to preserve those lateral sidewall reflections from your main speakers because those will add to the spaciousness of music and movie scores.

The answer? A reasonable mix of domestic type furnishings that reflect and break up some sounds as well as some absorption (carpet or rugs, a few draperies, etc.) And yes, listeners that describe some Axiom speakers as "bright" are usually describing the unnatural acoustics of their room.

Regards,

So Allan would you have any recommendations in terms of strategically placing absorptive or diffractive panelling? I’ve read many recommend placing absorptive panels at both the primary and secondary reflections points, determined by using a mirror placed along the wall and behind and to the sides of the front speakers. In your opinion would this ruin the lateral off-axis reflections you’re referring too? Or is just a random placement of absorptive and reflective panelling just as good or better?

How about Bass traps… Are they a reasonable thing to include as well?

Thanks, El

Thanks, Alan, for pointing out that the "live-end/dead-end" theory has been unsupported by tests. I stand corrected -- and feel better, too!

Eldwynn, hope Alan will fill in again, but I just don't get this entire "bass trap" concept. You know, bass sounds have very long wavelengths -- ~5ft @ 200Hz, 10ft @ 100Hz, 20ft @ 50Hz, and so on... Basic physics textbooks will tell you that waves in general just do not "see" an object that is much smaller than the wavelength. In other words, there is NO physical basis whatsoever for a sound wave of 10ft wavelength to be affected in any way by those bass traps of mere 1ft or smaller in lateral dimensions.

I strongly suspect that "bass traps" are yet another bunch of audiophile snake oils.

Brian Cheney, the designer of the VMPS speakers, seems to be a big supporter of the "dead-end/live-end" thesis. This gives me yet another reason not to buy his speakers -- the VMPS RM40 ($6000/pair) did not impress me much, anyway...

Ya my exact thoughts.The whole idea sounded a bit odd to me as well,unfortunately, my first / second year physics is just to old to rationalize a good explanation. I did notice one DIY site for bass traps stressing the importance of having a sealed tube. From this I was infering that the sound waves where compressing the air within the tube causing a loss in amplitude??????????????? As well a Bass trap manufacture showed baffels in their models, meant to elimate standing waves. WIth a standing wave in the tube I guessed this would eliminate compression/expansion of the air in the tube, making it ineffective???????? But I'm just hand waving.........Any physicists out their? Hopefully Alan has some insights on Bass traps and proper placement of absorptive panelling.

Thanks, El

O.K. So then fabric upholstery on the walls is not a good idea?

Fire resistant pads are inundated with a chemical called polybrominated diphenyl ethers (PBDE) among others.
I would avoid putting that material in a place of constant human residence as much as possible. Considering alot of it is already used in chairs, tvs, monitors and other fabrics, i would recommend not making your walls a part of the toxic pool.

Dear Allan,

just received the Newsletter. Thanks so much for all the helpful advice about room acoustics. This is an example for me where a little information is a dangerous thing. Having read a bit on the internet about optimizing room acoustics and bass traps I was thinking I would have a few days of work ahead of me to optimize my room. I do have mostly bare dry-wall “walls” and perhaps a couple homemade absorbing panel may help optimize things but I’ll be starting with some pictures first. Your advice is very much appreciated!

Now I can relax and listen to the music…..

El