Why "splay" walls 4 custom listening room design?

Hi all:

Immediately below is a quote from my post above.

>>>Jadem 6 said > "If you do however pursue this I would recommend you look into what wall materials your using. One layer of drywall over wood studs can give a soggy base response compared to a more rigid construction."

Me talking.> Make sure your dry wallers are generous in the use of screws. I had one person (who I consider fairly well informed) argue that the choice of stud/drywall and the screw schedule in the typical american room (fairly simple and cheap to implement from the start) was more important than most of the other exotic stuff combined. He also noted that this issue was almost never found in computer programs for room acoustics.

Well I got no response to my post (so here I try again) and since posting it I read Martin Collom's article in the October issue of AudioXpress. The byline of the article is as follow:

"Balance of treble and bass in your speaker system may have more to do with what materials were used in building your house than other factors-which is good news for audio amateurs" Martin Colloms 11/02 AudioXpress p4.

What are the "other factors"? Well how about the Theile-Small parameters which are usually touted as gospel?

Colloms says" "the entire low-frequency scenerio deserves consideration." as they "assume 2Pi space" and "really need rethinking."

He implies that the entire structure of T/S are faulty and that the underlying assumptions are based on the kind of house Theile lived in at the time he did his work....

""Theile confirmed to me that his seminal work on low-frequency alignments and design was based on 2Pi partly because thet studio building where he researched and listened was a relatively "leaky" timber structure."" Collom p 6. (Jadem6 is making sense eh?)

In any event, maybe my friend from my first post was right. The kind of materials chosen for your walls and the screw schedule is more important than, as he puts it, "the exotic stuff"... i.e. TS measurements and computer models.

What you think?

I remain

Clueless, I too remain clueless as to who (if anyone) actually knows the facts. Here are some random thoughts based on my personal observations and quite a bit of research and continuing education.
Buildings react to the low base signals in what I would call an organic way. Think of how wood responds in instruments and how tweakers have used woods of differing densities to "tune" there rooms. The material itself actually interacts with the sound wave and can take on it's characteristics for that moment. In the case of gypsum board (gyp. bd.) the material is only ½" thick in normal construction spanning 14½" of air over the height of the wall. This is in effect a long drum with a loose skin. The more screws used to fasten the gyp. to the studs may tighten the drum skin some, potentially changing the frequency but the issue remains of a thin skin over a wide space. I would assume more studs, 8" on center would alter the bass response quite a bit, although I've never tried it. The "experts" I've worked with suggest 4 layers of ½" gypsum board (each glued and screwed) as a minimum construction in a church, play house or theater. This is a minimum, they prefer 8" concrete block or poured concrete. The concrete has very little reaction by nature with the bass frequency thus allowing it to pass through. Base waves do not bounce and reverberate off materials like higher frequencies do. If you have ever been late for a concert in a stadium or hall built from concrete, you will know your late because the base is heard from the street. The problem for us with concrete is it reflects the high frequencies, but not a lot worse than gyp. bd. The paper on the surface of gyp. bd will tone some frequencies in a slap echo scenario. For most of us, concrete is not practical unless your listening space is in a basement. Then I would try to make sure both side walls were made of concrete. In an above ground situation with wood studs, the only practical solution is thicker gyp. bd. In the four layer scenario the net result is 2" of gypsum, in that it's glued to each preceding layer it acts as a solid 2" gypsum shell. Gypsum is a softer and a more absorbent material than concrete, but the two are very similar as a basic structure. I'm thinking the gyp may have some properties beneficial or possible a hindrance to the low midrange and upper bass where it may absorb some energy. Would this be released at a later time causing smear? Does a thicker stud wall with insulation help defuse this energy along with base signals? It's also possible to retrofit an existing room by adding three layers of gyp, the problem here is all the trim must be removes and the door and window frames would need to be extended.
Anyway, the idea as I understand it is to make the walls rigid but not created a drum. Once the space is created that has the least interaction to the base signal, we can consider wall materials. Much has been written on the wall coverings. I have used fabric pleated curtains, ceiling to floor on home theater designs. This is much like a movie theater, I think this is an attractive solution that responds to the greatest amount of frequencies due to the pleats. The added benefit of curtains rather than something applied to the wall is it's ability to double it's potential by absorbing not only first order but reflected waves.

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.