Which Foam Is Best for Soundproofing?
Updated
The best foam for soundproofing a room's echo is open-cell acoustic foam, either polyurethane or melamine, sold with a lab-tested NRC. The thickest polyurethane panels hit NRC 1.10 (4 in. Auralex Studiofoam Wedge (opens in a new tab)). No foam blocks noise through a wall, though, and closed-cell foam like polystyrene is a poor sound absorber too (WorkSafe WA (opens in a new tab)).
Thickness matters more than which open-cell foam you pick. The same Auralex wedge goes from NRC 0.50 at 1 in. to 1.10 at 4 in. Buy 2 in. for voices and calls, and go thicker where bass matters, since low-frequency sound "can only be absorbed with thick foam layers" (BASF (opens in a new tab)).
Watch out: The Studiofoam Wedge lists only a UL 94 HF-1 fire rating, not an ASTM E84 class. If you need a rated surface, pick a Class A foam such as Auralex Studiofoam Pro (NRC 0.90 at 1.5 in., spec sheet (opens in a new tab)), or melamine, which BASF says chars in a flame without melting or dripping.
Wondering why foam can't stop a neighbor's TV? That's answered in do soundproof foam panels work. Shapes and mounting are compared in soundproof foam panels, what actually blocks sound is in the soundproof panels guide, and there are more answers in the soundproof panels FAQ.