Why "splay" walls 4 custom listening room design?

Wooterb, first I should comment on Russ H. HT home theater architect. Just because he designed IMAX doesn't mean he is qualified for small room acoustics--they are very different. The fact is Russ H. is very qualified for small room acoustics, but my point is, don't take someone that has done studios and theaters as an expert in home theater or small listening room applications. Small rooms (less than 8000 cubic feet) for home listening and HT are very different animals--but the tools of the trade are the same. In fact we are redoing some rooms that were done by studio engineers--they were perfect for studios, the reverb times and everything are right on--but they sound terrible for home listening.

Now on to your real question: splayed walls. Two parallel walls will produce room modes. Some of these modes can have very high Q factors (they are narrow band and highly peaked). When you splay the walls you don't eliminate the mode, but you distribute it. The Q factor goes down as does the peak. The more the splay the lower the Q factor, until at some point there is basically very little left of a room mode. Splaying walls if very effective in this matter.

Why don't people splay walls? It's costly. It doesn't work that great for the resale value of your home. It is "unpredictable". The last statement is not at all true, but has been quoted from Alton Everest and others. He also doesn't believe splaying is very effective: calling it nominal--but I disagree here as well. (Everest's books by the way are still excellent--I just don't agree with everything in them) Splaying the walls is very predictable, but the math is much more complicated and not easily described. If you do intend to splay the walls, it's a good idea to work with an acoustical engineer and make sure it's right. I don't recommend most people to go the splayed wall route alone.

A comment on the golden triangle: Most of the previous work in our opinion is not quite right. It evenly weights axial, transaxial, and oblique modes. Oblique modes are almost negligible and axial should have 3 times the weighting factor of the transaxial. We are working on new ratios and new methods that better distribute actual real world results. We are not done with this work yet, and still take each room as a case by case basis.

As to Bill E. room: It will be fantastic. Bill is fortunate enough to be able to spend what is necessary to build this room (in an existing house no less- far more expensive than new construction). There are no parallel walls save one in the rear which is such a small surface area it is negligible. It is designed for 2 channel only, and the difference in design for 2 channel vs HT is pretty great. We are very anxious to get this room complete--and listen.

Please do visit our website at www.rivesaudio.com. It has been mentioned and the tutorial in the acoustical issues section should be helpful.

Wooterb: Please read my response more carefully. I said he is VERY qualified. "The fact is Russ H. is very qualified for small room acoustics" My point was that some people--not Russ--who do large room acoustics and studios are not qualified for small rooms. Russ most definitely is--and I would never imply anything differently. As to where you can buy CARA 2.1 software, we sell it on our website, or you can call us directly 800-959-6553. It is a very good package.

Clueless and Jadem6: You all have brought up important points that absolutely deserve attention. In general, proper construction can be more important than all sorts of esoteric materials. Abundant screws is a must, but the real key is to create layers of varying density material. For this we generally use 2 layers of gypsum sandwiched by a third low density layer. This gets rid of the drum effect because for the drum to resonate it needs one somewhat uniform medium. Layering up gypsum does not get rid of the problem, but it does reduce it by building up mass and by staggering the layers. Another area, that you did not bring up was glass--it leaks bass badly and reflects mids and highs. On top of that--if left as is--it resonates very badly, and is usually a source of serious problems in listening rooms. There are solutions for this too, but I won't go into the details.
The other aspect about computer programs, not having the data to properly measure these things--that is absolutely correct. There is a book published by our illustrious government on noise control. It was published in 1980 and is out of print (and very hard to find). It has tables upon tables of absorption coefficients for most building materials and even sandwiched structures like those mentioned above. Doors, walls, floors--it's there. We use this in all of our calculations. Even with this abumdance of data--the computer can't quite do everything--here's the problem:
You want to retain your bass energy--so you build a rigid wall. Unfortunately, this rigid wall is going to create bass modes with a very high Q factor. So, you could go the opposite way, and have single layer gypsum. You won't have the high Q bass mode, but you will have resonance from the "drum" effect and you will lose and have sloppy bass. So what is the answer? There is no one answer. It depends on room dimensions, furnishings, speakers used, taste of listening, volume of typical listening. If there was one answer--why would people need acoustical consulting services? We could all just follow a simple method and be done with it, but it's not that simple.
The computer and calculations are great tools, they are just that--tools and need be used appropriately and whoever is using them needs to understand their limitations. In fact, we are redesigning a listening room that was designed by someone who had designed a number of studios (that does not make them a bad designer--but in this example it just didn't work). The room, purely from a calculations and frequency measurement point of view is excellent. The reverberation times and frequency response are just what they should be--but the room sounds terrible. I won't go into the details of the problems, but suffice to say--you can design a room that based solely on computer programs and caculations that will sound terrible. At the same time, however, the computer and calculations are necessary to determine if a particular design is going to work--you know if the frequency response and other measurements are off based on these--you have more work to do. But when you do get the right calculations--it doesn't mean you have the best possible room.