The two most commonly-used sound absorption materials are high-quality acoustic foam and specialized acoustic fiberglass (no, not the stuff you buy at the hardware store). For brevity, at times we’ll generically call acoustic foam just plain “foam,” although there are very dramatic differences in cell structure and density between acoustic foam and the thousands of other types we could manufacture. This is why you can’t just run down to the local Supermarket and buy mattress pads with which to acoustically treat your studio.