Reverberation Time

Sabine's equation

Wallace Sabine found in the 1890s that the time a sound takes to die away by 60 dB depends on just two things: how much air the room holds and how much absorption its surfaces offer.

$V$ is the volume in m³, $S_i$ each surface area in m² and $\alpha_i$ its absorption coefficient, from 0 for a perfect mirror to 1 for an open window.

Eyring's correction

Sabine's formula overestimates in well-damped rooms; it predicts a finite decay even when every surface absorbs everything. Eyring's version handles that correctly:

The two agree when the mean absorption is small and drift apart as it grows. Use Eyring for treated rooms.

Targets and limits

For two-channel listening in a domestic room, 0.3 to 0.5 s across the midrange is a widely used target. Much shorter sounds lifeless; much longer smears detail.

Coefficients vary with frequency, and most materials absorb far less bass than treble. This tool uses one mid-band value per surface. In small rooms the low end is governed by individual modes, so a single decay time only makes sense above the Schroeder frequency.

References

1. W. C. Sabine, Collected Papers on Acoustics, Harvard University Press, 1922. HathiTrust· Internet Archive
2. C. F. Eyring, “Reverberation Time in ‘Dead’ Rooms,” J. Acoust. Soc. Am., vol. 1, no. 2A, pp. 217–241, 1930. ASA