Soundproofing = Block the Sound + Absorb what was Blocked

Most people think that soundproofing means all you have to do is block the sound. Those people didn’t go to engineering school. They probably got their degree cruising through Google Tech. Any sound containment project has two parts. First you have to block the sound you want to contain and then you have to absorb that sound. There are variations in this theme but it is the basic rule. If you don’t absorb the sound you contained, it stays contained and continues to hammer on the walls over and over and over again, until it rubs itself out against the walls.

Essentially sound is the audible part of the flow of acoustic energy. If something inside the room is continuously emitting sound, then the sound inside the room gets louder and louder until a balance of power is reached; where the sound power into the room equals power being drained out of the room. The sound power absorbed equals the sound level times the surface area of the containment room times its absorption coefficient.

Here’s the problem, a soundproofing project introduces a fixed number of attenuation between two rooms, for example 35 dB. If the sound in a bare noisy room is 80 dB, the noise in the next room is 45 dB. If the first room has good sound absorption, the sound does not build up so loudly so much and maybe only reaches 70 dB and so the noise in the neighboring room is down to 35 dB. This is why the first thing anyone says to do about soundproofing is to add building insulation between the wall studs. Building insulation is almost worthless as a real sound absorber but still it is has enough absorbing power to improve the soundproofing rating of a normal stud wall by 5 dB.

We always make sure that we absorb the sound we are blocking. That’s why heavy walled soundproofing projects are so problematic, they sound like racquetball courts.

Copyright 2010 Acoustic Sciences Corp.