What stops noise is Mass. Foams and Fiberglass and other "soft" insulations "absorb" noise, but are not noise "barriers."
Lead has significant mass, and is an excellent noise barrier. Mass Loaded Vinyl (MLV,) which Comodave used in the Soundown product, when used in the same weight, presents the same "mass" as lead. Noise doesn't care what the mass is, 2lb sq ft of Lead or 2lb sq ft of MLV reduce noise the same. Lead is more expensive, and as noted, is toxic, so there is no good reason to use lead instead of MLV on these type boats.
When a mass layer is incorporated into a composite with a decoupler layer of foam against the structure (hatch/bulkhead) and an absorption layer exposed to the noise source, the mass layer's performance as a noise barrier is significantly increased.
Lead is still used is some specific applications like Coast Guard inspected vessels and large class yachts, but these are the exceptions, not the rule.
Originally Posted by fgarriso
SoundDown Inc makes a lead foam for sound abatement. It comes in 1"- 4" and they also have sound absorbent underlayment for the cabin soles.
Originally Posted by Comodave
They donít use lead but rather a mineral loaded vinyl in the middle since lead is a hazardous material.