What reverberation time is
When a sound stops, it keeps bouncing between the walls, floor and ceiling and fades away gradually. The reverberation time, RT60, is the time it takes for the sound level to drop by 60 decibels. A long reverberation time makes a room sound echoey, blurs speech and raises the noise level; a very short one makes music sound dry.
Sabine’s formula
RT60 = 0.161 × V ÷ A (metric) or RT60 = 0.049 × V ÷ A (imperial)
where V is the room volume and A the equivalent absorption area: the sum of each surface area multiplied by its absorption coefficient. In imperial units, A is measured in sabins (square feet of perfect absorption).
The calculator uses an average absorption coefficient for all surfaces. Eyring’s formula, 0.161 × V ÷ (−S × ln(1 − α)), is more accurate when the room is absorbent (average α above about 0.2).
Example
A room 30 × 20 × 10 ft with hard surfaces (average α = 0.10):
- volume: 6,000 ft³;
- surface area: 2,200 ft², so A = 220 sabins;
- RT60 ≈ 0.049 × 6,000 ÷ 220 ≈ 1.3 s (1.27 s with Eyring).
To reach 0.6 s, the room needs about 490 sabins in total, or 270 more: about 340 ft² of acoustic panels with α ≈ 0.9.
Recommended reverberation times
| Space | Target RT60 |
|---|---|
| Classroom (ANSI S12.60, under 10,000 ft³) | 0.6 s or less |
| Office, meeting room | 0.4 to 0.8 s |
| Home theater, studio control room | 0.2 to 0.5 s |
| Restaurant | 0.8 to 1.2 s |
| Concert hall | 1.6 to 2.2 s |
Typical absorption coefficients (at 500 Hz)
| Material | α |
|---|---|
| Concrete, plaster, glass | 0.02 to 0.05 |
| Wood floor | 0.10 |
| Carpet on pad | 0.30 to 0.60 |
| Heavy curtains | 0.50 |
| Acoustic ceiling tiles | 0.60 to 0.90 |
| 2 in fiberglass panel | 0.80 to 1.00 |
Tips
- Absorption varies with frequency: thin panels absorb high frequencies but little bass. Measure or calculate at several octave bands for critical rooms.
- Spread absorption over several surfaces rather than treating a single wall.
- People and furniture add absorption: an empty room sounds more reverberant than when occupied.