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(normal) walls reflect the lower frequencies, wheras those frequencies easily pass (acoustically) thinner material like a desk * higher frequencies are reflected by a desk
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Actually, low frequencies will diffract around an object like a desk, not pass through. You can't reflect a wavelength larger than the dimension of the object, which is why using a 4x8 gobo (baffle) in the studio won't isolate the bass amp from the sax player's mic on the adjacent side. Transmission Loss and difffraction are distinct, though each accounts for the bass getting through in different circumstances.
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the mass law starts to break down at certain frequencies, and the wall will exhibit a striking amount of acoustic transmission over a very narrow frequency range. This is called "coincidence".
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The coincidence dip will vary with the mass and rigidity of the wall, which is of course what was mentioned with the thinner sheet performing better at a certain frequency than the thicker. As the wall gets thicker and more rigid, the frequency of the coincidence dip lowers. For speech-like frequencies, a boundry of a certain type may exhibit a marked dip, and making it thinner may increase the TL for that frequency, even while making the TL across the rest of the range a little worse. If you are only interested in that frequency, then thinner will work. If you need it to perform across a wide spectrum, thinner may not be the best solution for the system as a whole.
Working in concert with the coincidence dip is the mass-air resonance. This is why three panes of glass is worse than two panes for a control room window. Separating the span into two air-spaces instead of one gives worse performance for the same amount of material, and indeed even if you use more material (mass) in the version with the extra divider. Same with walls. Put your mass on the outside, and don't divide the air space.
There are ways to damp resonances, and also ways to influence them lower or higher, and considering the whole system and the overall goals, you can make decisions that minimize weaknesses and feature strengths. More mass is better than less mass for TL in general, but mass law reaches a point of diminishing returns in required materials, space, weight, and cost of construction. This is why studio designs combine mass and split construction for isolation.
I know this has turned into a long post, but one more important point to make on the subject.
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Surely you aren't saying you double the sheetrock as a matter of course in order to accurately predict room modes! That would be far more costly than just measuring the room response. Can you elaborate on your thought process here?
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As you make a more massive and rigid shell, more low frequency energy is contained in the room. Gypsum board (aka drywall or sheetrock) on studs is a flexural absorber. A typical wall not only lets some low frequency energy through, but also absorbs some. A wall in and of itself is a bass absorber to some extent. As you add mass and rigidity, less is transmitted, and less is absorbed. Therefore, while the predictions inside the room are easier, you also need more bass trapping, or at least better planned bass trapping to create an even response within the room. In the end, I think you'll have a better room overall, but you do need to pay attention to trapping all the more in a room with a "heavy" inner shell.
Whew...