Axiom Home Page Forums General Discussion Technical Questions theater room acoustics

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