the effectiveness of fiberboard products, there are several, is limited at low frequencies Recently I posted a qoute from a marketing brochure where they said as much (They must have good lawyers).
If your goal is to contain subwoofer rumble, then Damped Isolated Mass is the way to go, that is why we always talk about double layers of 5/8 drywall (mass) with Green Glue between layers (Dampening) hung from a clips and channel system (Isolation)
Found the discussion of a similar product "Hushboard"
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