Reverberation Time (Sabine / Eyring)
Octave-band RT60 of a rectangular room from its surface finishes and occupancy.
Room dimensions
Finishes
| Hz | A m² | ᾱ | Sabine | Eyring |
|---|---|---|---|---|
| 125 | 3.1 | 0.01 | 10.84 s | 10.77 s |
| 250 | 5.5 | 0.02 | 6.10 s | 6.03 s |
| 500 | 5.5 | 0.02 | 5.59 s | 5.54 s |
| 1k | 7.3 | 0.03 | 4.17 s | 4.12 s |
| 2k | 9.0 | 0.04 | 3.07 s | 3.03 s |
| 4k | 10.0 | 0.04 | 2.25 s | 2.22 s |
Reverberation time
RT60 (T_{60}) is the time for the sound energy in a room to decay by 60 dB after the source stops. It is the single most important descriptor of how a room “sounds”: short times give clarity for speech, long times give warmth and envelopment for music.
Sabine's formula
Wallace Sabine (1898) found empirically that RT is proportional to volume and inversely proportional to total absorption:
with V in m³, A in m² (“metric sabins”) and 4mV the air absorption term, significant above 2 kHz in large rooms.
Eyring's formula
Sabine over-predicts RT in absorbent rooms (as ᾱ → 1 it still gives a finite time, while the true answer is zero). Eyring's correction is more accurate when the mean absorption exceeds about 0.2. Both assume a diffuse field: absorption spread evenly and no extreme room proportions.
Using the result
- T_\mathrm{mid} is the average of the 500 Hz and 1 kHz bands, the usual single-number figure.
- Classrooms: ~0.4–0.6 s (e.g. BB93, ANSI S12.60). Concert halls: ~1.8–2.2 s. Churches: 2–4 s or more.
- Rooms with hard surfaces and only a carpet often show a long low-frequency RT, porous absorbers are weak below 250 Hz.
Absorption coefficients are typical literature values; product data from manufacturers (ISO 354 tests) should be used for design.