Sound Absorption Coefficient Calculator
The sound absorption coefficient α describes what fraction of incident sound energy a surface absorbs instead of reflecting. Calculate α from measured incident and reflected SPL values, or multiply α by surface area to get the total absorption in metric sabins for room acoustics design.
Calculation mode
dB
dB
Fraction of incident sound energy absorbed (0 = none, 1 = all)
- 1
SPL difference
80 − 70 = 10 dB - 2
Pressure reflection coeff r
10^(−10 ÷ 20) = 0.3162 - 3
Energy reflectance r²
0.3162² = 0.1 - 4
Absorption coefficient α
1 − 0.1 = 0.9000
How does this calculator work?
The absorption coefficient α = 1 − r², where r = 10^((SPL_r − SPL_i)/20) is derived from the incident and reflected SPL. α ranges from 0 (all reflected) to 1 (all absorbed). Multiply by surface area in m² to get total absorption in metric sabins, used in Sabine's RT60 = 0.161 V / A formula.
Formula
How this is calculated
When a sound wave strikes a surface, the incident energy splits into absorbed, reflected and (for non-solid surfaces) transmitted components: α + ρ + τ = 1, where α is the absorption coefficient, ρ is the reflectance and τ is the transmittance. For solid opaque surfaces τ ≈ 0, so α + ρ ≈ 1. The absorption coefficient α ranges from 0 (perfect mirror) to 1 (perfect absorber like an open window or anechoic wedge).
From SPL measurements, the pressure reflection coefficient r is 10^((SPL_r − SPL_i)/20), and the energy reflectance ρ = r². Therefore α = 1 − r². A 10 dB SPL drop on reflection corresponds to r = 10^(−0.5) ≈ 0.316 and α ≈ 0.9 — very high absorption. SPL values are typically measured in an impedance tube or reverberation room per ISO 10534 or ISO 354.
In room acoustics, the total absorption A = α × S in metric sabins (m²) is used in Sabine's reverberation time formula RT60 = 0.161 × V / A, where V is the room volume. Real rooms combine many surfaces with different α values; sum each α_i × S_i across all surfaces and room contents to get total A. Absorption coefficients are frequency-dependent (typically tabulated at 125, 250, 500, 1000, 2000 and 4000 Hz).
Frequently asked questions
The NRC is the arithmetic average of a material's α values at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. It is a single-number summary for comparing materials; a product with NRC 0.85 absorbs roughly 85% of mid-frequency sound energy.
A metric sabin (m²) is the unit of sound absorption equal to one square metre of perfect absorber (α = 1). Total room absorption A = Σ(α_i × S_i) in metric sabins feeds directly into Sabine's RT60 formula. The imperial sabin is in square feet.
Laboratory measurements (ISO 354 reverberation room method) occasionally report α > 1 for highly absorptive materials due to diffraction at sample edges — sound from outside the sample area also strikes the absorber. It is a measurement artefact; real absorption is physically limited to α ≤ 1.
Also known as
TG we-Calculate Editorial Team. (2026). Sound Absorption Coefficient Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/sound-absorption-coefficient-calculator
TG we-Calculate Editorial Team. "Sound Absorption Coefficient Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/sound-absorption-coefficient-calculator.
TG we-Calculate Editorial Team, "Sound Absorption Coefficient Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/sound-absorption-coefficient-calculator
@misc{wecalculate_sound_absorption_coefficient_calculator, title = {Sound Absorption Coefficient Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/sound-absorption-coefficient-calculator}}, year = {2026}, note = {TG we-Calculate} }
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