Sound Absorption Coefficient Calculator

Sound Absorption Coefficient

The sound absorption coefficient $\alpha$ measures how much incident acoustic energy a surface absorbs instead of reflecting. Values range from 0 (perfect reflection) to 1 (perfect absorption).

From Sound Intensity

$$\alpha = 1 - \frac{I_r}{I_i}$$

$I_i$ is incident intensity and $I_r$ is reflected intensity, both in watts per square meter. If 100 W/m² arrives and 20 W/m² reflects, $\alpha = 0.8$.

From Sound Pressure Level

$$\alpha = 1 - 10^{(SPL_r - SPL_i)/10}$$

When intensity is not known directly, sound pressure levels in decibels can be converted. A 6 dB drop in reflected level corresponds to one quarter of the incident intensity.

Frequently Asked Questions

What does alpha equal to 0.8 mean?

80% of incident acoustic energy is absorbed and 20% is reflected. The surface is fairly absorptive but not anechoic.

Can alpha exceed 1?

Ideal coefficients stay between 0 and 1. Measured values can appear slightly above 1 due to edge diffraction or measurement uncertainty.

Why use SPL instead of intensity?

Sound level meters report SPL in dB, which is easier to measure in rooms and labs. The logarithmic formula links SPL difference to intensity ratio.

Is alpha the same at all frequencies?

No. Most materials are frequency dependent. Published tables list absorption coefficients per octave band.

How is this used in room acoustics?

Designers sum absorption from walls, ceiling, floor, and furnishings to estimate reverberation time and speech clarity in a space.

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Sound Absorption Coefficient Calculator