In the latter years of my time as a boat builder I had migrated to Aluminum work. Mostly large motor yachts. Sound and vibration were major concerns. The engineer found a company called EAR Corp. as a maker of sound barrier material.
Sound is energy and to eliminate it you have to break the trail so to speak.
We used principally two products from EAR, ISODAMP a very heavy rubber/vinyl/neoprene material approx. 1/2 inch thick. It was cut actually bandsoared into pieces for placement between stanchion bases and the stanchion and as part of machinery mounts such as airconditioner compressors. For buikheads particularly on the opposite side from the engine room we used a thin but godawful heavy Barium filled neoprene sheet glued onto the vertical bulkheads and layed atop the bottom tanks. The insulation in the engine room itself was a foam filled perforated metal material approx. 1 1/2 to 2 inch in thickness.
To the best of my recollection we did not have any complaints from owners about sound/noise.
Well Lo and Behold! I found a photo on one such application of the Barium sheet and Isodamp pads.
Sheet is on top of double bottom tanks and engine room bulkhead, pads the blue layer, are atop the supports for cabin/stateroom subfloor.
"Lord, grant that I may always desire more than I can accomplish"