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Discussion Starter · #1 ·
I have been doing some reading on acoustic treatments and was hoping some of the people here could help me out a little bit.


Acording to what I have read most designers use a "live-end, dead-end" treatment when setting up two channel systems. They plan for absorbing materials behind and to the sides (but just in front of) the speakers and for diffusing treatments along the back wall and to the sides of the listening position. The idea is to absorb any frist reflections from the speakers up-front and add ambient sound to the rear.


Obviously this is a little different when planning for a surround sound system as the rear channel adds the ambience and you wouldn't want to aid the first reflections of the rear speakers. Yet everything I have read has warned that an overly treated (or dead/dry) room may be as bad and possibly worse than an untreated room, so one wouldn't want to over due the absorption by putting up absorbtion treatments around the whole theater.


My idea is to use a "dead-end, live-middle, dead-end" strategy as a compromise between the two ideas. This would entail the standard two-channel absorption treatment both in the from and rear of the theater and difracting treatments to the sides of the listening positions.


My concerns about this are 1) I do not have a firm grip on acoustic theory in general and I may be way off in my assumptions 2) I also plan to use my theater as a listening room for two channel music and do not want to "over do it" and ruin the room's two channel characteristics 3) I do not want to limit myself to a 5.1 channel surround system as I may feel the need to step up to 7.1 (or beyond) before all is said and done ;) .


One other question...None of the articles I have read mention anything about treating the ceiling, at least aside from corner traps. I imagine this is just as important as treating the side walls and would like to believe that I can simply extend the treatments from one wall, across the ceiling to the other wall, but...The distance from the speaker to the ceiling will probably be greater then the distance from the speaker to the side wall. How is this propagated in room modes? Is there a calculator that takes this into acount?


Thanks for all the help your other posts have given me and for the responces I'm sure I'll receive.


Kirk


BTW, I do not yet have hard dimensions for the room as I am still in the design phase, however it will have to be smaller then the current space in my basement of 17'x21'x8'. Otherwise the dimensions used will be primarily determined by what will allow the best sound propagation.
 

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I love these acoustic room treatment questions.


kirknelson,


Your problem is not unique, a lot of folks wonder how to balance the home theater sound with the 2 channel sound. The only problem with your proposed solutions is you need to be careful with the amount of acoustic absorption you use on the back wall.


My recommendation is this.


1. Treat the main speaker wall with 75-100% coverage 2"-3".

2. Treat the side walls at the main reflection points 2".

3. Treat the rear wall with 10-40% staggered 1"-2".

4. Space the rear/surround channels further away from the walls OR mildly treat the walls near the surrounds with 1" acoustic foam.

5. Consider using diffusers or diffusers/absorption combo on the back wall.

6. Use bass traps on the main speaker wall (corner wedges work well)


The ceiling sound treatment is optional. I personally would at least treat the first reflection points, but they depend on a lot of variables.
 

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There is no way to have a room which is both optimal for two channel and multi-channel play back (by multi-channel playback, I also mean two channel recordings being played back by means of an ambiance extraction system ala Lexicon Logic7, Meridian TriField, etc). There is a significant different in the RT60 requirements, dispersion control, etc.


In any event, a relatively absorptive front wall (including about the first 3' of the front side walls) is helpful. Beyond that, you'd like to have absorption at ear level and below, reflection/diffusion above.
 

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Hi Dennis,


When you say "a relatively absorptive front wall," would heavy royal velvet curtains suffice, or would you still put open-cell foam and more behind the curtains?


Also, does the screen (in my case, a fixed wall Stewart) have any absorptive properties or would you treat behind the screen as well?


Thanks in advance for your insight.
 

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Gregg:


I cannot answer that question ... I don't know the charateristics of your room nor your intentions as to its use. As to the Stewart screen, one of the oft cited advantages of the Stewart MicroPerf is that it does not contribute to slap echoes in the room. Treating behind a non-microperf screen is of little benefit except for those frequencies that are not reflected by the screen itself.
 

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Discussion Starter · #6 ·
Gregg:


Screens are designed to reflect light and do a great job of reflecting sound as well (as Dennis mentioned). If you are planning to use the room for music listening as well as movies then I would suggest going with a retractable screen or installing some thick curtains which can be drawn in front of the screen when listening to music.


Kirk
 

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Discussion Starter · #7 ·
LordHz & Dennis:


Why only install the absorption material to ear level?

Is this type of material only usable to counter first reflections?


Does this answer my question regarding ceiling placed treatments? (i.e. the sound has a further distance to travel before reaching your ears so it should decay enough to be unnoticeable)


If so than this makes me curious about how to counter room modes as these treatments only effect first reflections. I doubt a diffuser would help when dealing with room resonance frequencies.


One side note from Russ Herschelmann's latest article in "Stereophile's Guide to Home Theater:"


"...if we mount a sheet of acoustic foam 3 3/8" thick on the wall the material will absorb frequescies at 1kHz and everything above 1 kHz but not below....we're using 8" thick layers of foam against the walls in your theater, since it's effective down to 423 Hz....The guys who did my theater originally used 2"...that means that the only resonances that were absorbed were above 1 kHz. The really nasty stuff- the problems in the midrange and bass- was completely unaffected...all they did was suck out the high notes and make your room sound dead and boomy...."


Comments on the above quote are welcome. I also encourage others in this forum to seek out Mr. Herschelmann's "Home Theater Architect" column in past issues of SGHT as it has proved quite helpful to me.


Kirk Nelson
 

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Thanks Dennis, I know you can't make an exact recommendation without knowing the things you cited. I was really asking more along the lines of, when you used the term "relatively absorptive", are you just trying to tame the highest frequencies or were you talking about using absorptive materials sandwiched together to catch a wider spectrum of frequencies.


The intended use for my room is 75% movies, 25% multi-channel music. My goal is not to have the perfect room. Its just not practical in my case (WAF and all, ya know). If I were to use an analogy, I'd say I would like the audio performance in my room relative to an ideal room to be similar to that of a Porsche 911 relative to an Indy race car. Far better than the average but not so single of purpose that it becomes impractical for anything outside of its single intent.

Kirk,


Thanks for the 2 cents. I've got a fixed wall screen so it sounds like I've got a more complex curtain design to contemplate.
 

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I'm getting very close now to having to decide which way to go with the room treatments.


I have read perhaps too much on this topic and am confused - there are different schools of thought which at times seem actually to be conflicting.


My situation is:


Room: 15' W x 24' L x 8-9'H with an STC rating of 60


Seating: up to 3 rows of 4 people - 90% of time - just me and/or with wife..the other chairs can be brought into the room as needed.


Speakers: All full range - 7.1 setup - one 15" Sub


Listening: 100% in Logic 7 - no 2 ch only - about 60% music 40% movies


I had the President of a local accoustic design company drop by - they have done work on a number of studios and movie cinemas and a few home theatres. His recommendation was accoustic insulation covered with GOM equivalent fabric - 1" thick on the sides and 2" thick on the rear wall - the absorption panels to start AT ear level and be ABOVE to the ceiling - NO treatments below ear level..


I have to say I like the concept as I'll be in the sweet spot, but then it seems to be the exact opposite of what Dennis recommends.


so not sure really which way to go now..
 

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Quote:
"...if we mount a sheet of acoustic foam 3 3/8" thick on the wall the material will absorb frequescies at 1kHz and everything above 1 kHz but not below....we're using 8" thick layers of foam against the walls in your theater, since it's effective down to 423 Hz...
I was waiting for this one to be quoted. In fact, it's darn near verbatim from a paper by Floyd Toole (with more elaboration). This statement is only partially true. In *real* science, we use math, etc., to predict an outcome. Then we measure actual results to see if our theorical results match the real world. Then we must reconcile the results.


Let's look at 1" Insul-Shield for example. Using ASTM methods, type "A" mounting (directly on the wall), it has a measured coefficient of absorption of 0.24 at 250Hz. If the (I'll call it the "thickness statement") were correct, then it's coefficient of absorption would have to be zero. But its not. 1/2" gypsum board (drywall) has a coefficient of absorption of 0.29 at 125Hz! Again...the "thickness statement" would predict 0 at 125Hz. So either the theory is wrong or what we have built doesn't match theory. I'm not going into 30 pages of math, to explain what's happening here; but, to keep it simple, what we see and perceive as the boundary (wall) is something other than what is the acoustic boundary AND remember (I've harped on this dozens of times) all of this presumes a 100% reflective surface.


I have no idea where the acoustic consultant in Canada is coming from with respect to his suggestion of absorption above ear level and reflection below. This is absolutely the first time I have heard of such, and, on the surface, does not make sense to me. It also rather flies in the face of the work at Skywalker Range, TMH Labs (Tomlinson Holman), Toole, Olive and others in the fields of psychoacoustics. We want all the reflection/diffusion above ear level to avoid any chance of localization. It may be also worth pointing out that the needs of studios, commercial theaters, concert halls, and auditoria are significantly different than the needs of the residential sized room. The "rules" of architectural acoustics are radically different in small rooms.


First Relections ... if you're using speakers without any form of dispersion control, you may have a first reflection problem in a multi-channel system ... ie, we're spending alot of energy (and money?) to spead sound out to places it doesn't belong so we can just convert it to heat. Understand, however, translating two-channel needs to a multi-channel environment is a classic error.
 

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I viewed their suggestion as not necessarily wrong but as a different concept. I think you hit the nail on the head and that they are applying the same approach to a home theatre as they have done for commercial cinemas.


I am definitely sticking with the original plan of absorption below ear level plus at the points of first reflection.
 

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Quote:
Originally posted by John the Depot Dude


Listening: 100% in Logic 7 - no 2 ch only - about 60% music 40% movies


so not sure really which way to go now..
As Dennis has mentioned here and elsewhere, there are optimum room characteristics for different uses.


My guess is that optimizing the room for Lexicon reverb (I lean towards synthesized ambiance over Logic-7) and let multichannel music and movies fall where they may would be a safe bet for you.


This would be a fairly dead room that might require the ambiance channel to be on at all times for listening to 2-channel sources. Which is what you want anyway.


I recently learned that there are recent ambiance-synths from Yamaha that are sold only in Japan. Yamaha introduced a studio reverb box which is supposedly the best ever. That technology is not yet in any Yamaha consumer product.


Check
www.ambiophonics.org
 

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Thanks for the spreadsheet..


My approach has been to do this in stages now that the sound transmission is taken care of..


I'll be doing the absorption only around the room then once I'm used to the sound, investigate whether other treatments are necessary.


I've seen your posts on Ambiophonics and it does look interesting. I am though really impressed with Logic 7 - so much so that I wouldn't be interested in trying something else..
 
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