Open cell foam is what we recommend to soundproof for over the air noise. It works for soundproofing in the same way that it works for thermal insulation. It expands and seals all the crevices and openings that let sound through. It has a high STC (sound transmis-sion class) rating and it absorbs the sound being transmitted, very well.NOTE: Insulation will only help for sound that travels over the air, not impact noise such as walking or heavy Bass playing. That sound travels through vibration of the structure and has to be controlled by isolating the different areas that the vibration should not continue.