Absorption Coefficient (α)

Every surface in a room absorbs some fraction of the sound energy that strikes it and reflects the rest. The absorption coefficient α quantifies this ratio: α = absorbed energy / incident energy. A surface with α = 0.05 (polished concrete) reflects 95% of incident sound; a surface with α = 0.95 (thick fiberglass absorber) reflects only 5%. Absorption coefficients are measured in a reverberation chamber per ISO 354. A sample of the material is placed in a room with known reverberation time, and the reduction in RT60 is used to compute the Sabine absorption coefficient. This random-incidence measurement can occasionally exceed 1.0 due to edge diffraction effects, which is physically meaningful (the material absorbs more energy than would fall on its geometric area alone). Absorption is frequency-dependent. Porous absorbers (fiberglass, mineral wool, foam) are most effective at mid and high frequencies, with absorption increasing with material thickness. A 50mm absorber might have α = 0.2 at 250 Hz but α = 0.9 at 2 kHz. Membrane absorbers (plywood panels, drywall on studs) are most effective at low frequencies due to resonant vibration. Helmholtz resonators (perforated panels with air cavity) can be tuned to absorb at a specific frequency band. The Sabine equation uses total absorption A = Σ(αᵢ × Sᵢ) to predict reverberation time: T = 0.161V/A. For accuracy, the Eyring equation is preferred when average absorption exceeds 0.2: T = 0.161V / (-S × ln(1-ᾱ)). Common NRC (Noise Reduction Coefficient) values, which average α at 250, 500, 1000, and 2000 Hz: carpet NRC 0.30, acoustic ceiling tile NRC 0.70, 50mm fiberglass NRC 0.75, open window NRC 1.00, concrete NRC 0.02. SonaVyx includes a 55-material absorption database and uses these coefficients in the treatment calculator.