Why doesn’t my wall stop the noise?
Did you know that sound leaks can destroy the soundproofing of a wall? If the wall leaks, it won’t work.
What is a sound leak? A sound leak is any gap or hole in a wall where air can pass through. If air can get through, noise will get through.
A gap that is equal to 1% of the wall area can lower the Sound Transmission Class of a partition from STC 50 to STC 40. A reduction of 10 STC points means the sound coming through the wall seems about twice as loud. A 1% gap is less than 1/8” wide across the bottom of a typical apartment wall.
So what should you do about it?
Any hole, gap, or joint must be sealed airtight. Gaps often occur where the wall meets the floor and the ceiling, around pipes, ducts, and conduits, and around joists or beams that cross the wall. Acoustical engineers usually recommend non-hardening sealants because other sealants will crack over time.
Make sure that you get the sound isolation that you expect and pay for. Specify that the wall be sealed well on both sides.
ASTM C919 Standard Practice for Use of Sealants in Acoustical Applications contains additional information.