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Sound is basically a series of vibrations. Limit the vibrations, then, and it follows that sound is reduced. That is the premise behind soundproof drywall, a relatively new addition to the contractor's and do-it-yourselfer's arsenal.
Of course, no living space can be completely soundproofed, any more than it can be made totally airtight. Whether guarding against cold air or annoying noise, however, the object is the same -- find out where the unwanted force is leaking in and seal it off. With sound, the bottom line is the sound-transmission class (STC) rating, which quantifies how much or how little sound is passing through the wall.
Anything above a 40 STC is considered at least adequately soundproofed, and soundproof drywall generally checks in at between 50 and 55. That marks a point at which a person standing on one side of a wall would not be able to hear a human voice on the other. Soundproof drywall accomplishes that by a process called constrained layer damping, first used in a construction context by the Quiet Solutions Company under the name Quiet Rock®.
The thicker the walls, the less penetration from sound. A thin layer of drywall tends to act like a drum head, actually amplifying sound vibrations. Soundproof drywall is around 5/8 inches thick (1.47 centimeters),and it is made even sturdier through a process called constrained-layer damping. Constrained layer damping has been around for decades, but it was previously used in applications other than home building, such as to reduce the vibrations in highway bridges or counteract engine noise in airplanes.
One way to achieve constrained layer damping is to alternate layers of the base material -- usually gypsum -- with layers of glue or plastic polymers. Other companies use ceramic materials or even metal. Whatever the material, the idea is to make the drywall stiff and less likely to quiver when sound batters it.
Soundproof drywall is more expensive than the thinner, more traditional variety -- just under $40 US Dollars per sheet. It also presents a challenge to do-it-yourselfers because things like studs and electric outlets have to be extended in order to be flush with the new surface. It might be cheaper just to ask the neighbors or the kids to keep it down.