Acoustic foam panels, bass traps, and diffusers all work through absorption. Thick, porous materials are most effective at mid and high frequencies. For bass frequencies, specialized bass traps made of rigid fiberglass or rockwool are required because standard foam is too thin.
Absorption is measured by the Noise Reduction Coefficient (NRC) — a rating from 0 (perfectly reflective) to 1 (perfectly absorptive). Auralex Studiofoam has an NRC of 0.80 at 1 kHz, meaning it absorbs 80% of sound energy at that frequency.