Q: “I read somewhere that the STC of drywall and plywood is the same as Auralex’s SheetBlok – I thought SheetBlok was used to create isolation, why would I use it if it equals drywall?”
A: First of all, drywall makes a fairly decent sound barrier when applied correctly. Unfortunately STC ratings, which are meant to provide a standard for sensible comparison of materials, are too generalized to apply to the transmission of full range music signals. While the STC of drywall, plywood and SheetBlok are roughly the same (around STC-26 or STC-27), these three materials do not yield the same acoustic isolation results in all frequencies. A review of how STC ratings are determined will reveal the flaw in putting too much emphasis on them for music purposes (see WFTD STC).
To get a more realistic idea how a material will really perform in a music application, it’s better to compare the individual transmission loss values in different octave or 1/3-octave bands to get a better idea of the performance of one barrier or partition versus another. For instance, two materials defined as STC-24 could have transmission loss values of 19 and 29 respectively at 125 Hz, which is dramatically different – especially where music is concerned.
All this leads us to the simple answer, which is that SheetBlok delivers better STC across the entire frequency range than both plywood and drywall and is therefore a more effective material for isolating sound. Further, because it is made of a limp-mass vinyl type material, it’s also pretty effective at providing mechanical isolation and decoupling when used between other, more rigid materials. It’s actually ideal when used in conjunction with materials like drywall or plywood.