A friend who wanted to sound proof the garage at her new house, so that her band could use it as a practice space, was asking for advice for how to do that cheaply via Facebook. The recommendations she got were to put up acoustic foam or egg crate material on the walls, and that’s totally wrong. The misconception people have is they don’t understand the difference between sound proofing versus treatment.
When you see a picture of a recording studio with acoustic foam on the walls, what that’s there to do is to cut down on uneven reflections and make the room sound better when you’re in it1. What foam or other acoustic treatment does not do is keep the sound in the room from escaping to the outside. In order to sound proof a space, you need to add a lot of mass – as in concrete or brick walls. Anyone who tells you something different doesn’t know what they’re talking about.
- Foam is pretty bad for treating rooms too, because it's too low density to stop bass reflections. Bass frequencies cause just as many problems as higher ones, so if you only treat high frequencies you end up with a room that just sounds kind of dead. Good studios use higher density material, like rigid fiberglass, and also diffusors to scatter the reflections and stop what's called nodes. ↩