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Old 8th June 2020
Here for the gear


Thanks Andre, Jens and Kyle for your kind responses. I appreciate you looking out for me and others who may be reading, by pointing out the limitations of STC. To answer Andre's question, the space is a recording studio, so the point is well taken. However, I still believe STC is a useful metric for planning the wall treatment in my space. I'm sorry I didn't mention that I had already taken measurements and found that by far, most of the sub-125 Hz energy getting into the space (which, in that range, is of more concern to me than what's leaving the space) is through the floor and ceiling. I had already been planning for how to address that and other flanking noise problems, while taking care to integrate appropriately with the wall treatment. So, I appreciate you taking care to underline those important considerations, and I'm sorry I wasn't careful to include more information in my OP.

Thanks for mentioning the Canadian NRC paper. I have seen similar tests, in terms of comparing STC to more broad-band transmission-loss measurements, but never so many test samples in one place. A tremendous resource, so thanks again for that. Unfortunately, there isn't a test sample in that study with more than two layers of drywall per side, so I'm still left to speculation with regard to the benefits (or detriments) of adding additional layers. Even if it would help, I understand that adding more than two layers of drywall/Green Glue is invoking the law of diminishing returns to a degree that may be too sharp and painful to justify. On the other hand, I will sometimes continue to buy microphones/preamps/compressors/etc. that cost exponentially more than others - and still consider those expenditures worthwhile, even though they may not perform exponentially "better." So I am open to at least considering the scenario proposed in my OP. . . and I am equally open to abandoning it. (Money is always a consideration, Andre!)

As for my question about whether using fiberglass thicker than R13 would yield any add'l STC benefits. . . The only pattern I can see in the NRC study seems to be in harmony with the general advice stated pretty widely elsewhere (including Jens's links, if I understand them correctly?): As long as the insulation is appropriate for the application/build and is not compressed beyond what is appropriate for the structural parameters of that application/build - e.g., cramming R38 fiberglass into a 2x4 stud wall; or "dense packing" cellulose insulation when the intention is to reduce sound transmission through a wall, rather than thermally insulating it - generally speaking, more insulation will yield better sound attenuation, than less insulation of the same type.

In any case, with regard to the specific scenario in my OP and its similarities to my room and planned build, my conclusions are these:

* If there's room in the wall cavity to add fiberglass thicker than R13, it will increase STC. By how much? Apparently, nobody knows, but it seems that it would not harm STC performance due to some counter-intuitive principle of physics. Would it harm TL performance at frequencies outside of the STC range? I can only speculate, but I highly, highly doubt it.

* Adding a third or even fourth layer of drywall/Green Glue (directly to the 1st/2nd layers on the hat channel side of the build) will indeed increase STC - with the same caveats as above.

Any corrections to or disagreements with my conclusions?

Thanks again.