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This is probably a dumb idea: if you go to the homecenter and look at sinks, they have this black sheet thing stuck to the bottom (if it were roofing, I'd call it ice and water shield). The only reason I could think of why its there was for sound deadening. I've seen the same stuff on dishwashers.
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Another reason is to cover tiny cracks in the stainless steel. Any engineer
wanting to make a sink is going to realize that the harder the steel he uses,
the more prone it's going to be to cracking when stamped into as radical a shape as a kitchen sink, but the more resistant it will be to being scratched up by even harder materials like the harder stainless steels that kitchen knives are made of, and perhaps the porcelain of dishes.
So, another good reason to coat the underside of the sink is to allow the manufacturer to use hard enough steel so that the cracks that form only penetrate to 10 percent of the steel's thickness (say), and NOT have every sink they produce being returned to Home Depot or Lowes because "it's cracked in the bottom corner, SEE!" and the customer wanting to go pick out one that isn't cracked (cuz they'll go through the whole lot of them by the end of the day). The
coating would hide those cracks, and the sink with tiny cracks will last just as long and work just
as well and what the customer doesn't know won't hurt him.
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From memory, sound 'stops' and gets reflected at a boundary between
dissimilar materials. Therefore for best sound 'stopping' consider
alternating layers of material, rather than relying on one, single
THICK layer of something.
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I've never heard that. LIGHT both reflects and refracts at the interface between materials with different refractive indices. Seismic waves reflect off different layers in the Earth's crust, but whether that's due to differences in density, hardness or whatever I don't know.
When it comes to sound, MASS is king. The more mass something has, the less it moves when it gets hit by anything (including a sound wave) and the more slowly it accelerates as a result of getting hit. Since noise is nothing more than pressure waves in the air caused by things moving, the more massive the steel (or the combination of steel and the coating on the steel) of that sink, the less it will vibrate when you pour water into it, and the less noise it will make when you drop something in it, and the lower the frequency of that noise (making the sink sound less "tinny"). And, that lesser amount of movement and speed of movement both translate into less "noise" in the air as a result.
I'm thinking that the same technology that makes acoustical coatings reduce noise in cars would work equally well on a sheet metal sink.