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What Is Reverberation Time?
Reverberation time (RT60) is the time it takes for sound in a room to decay by 60 decibels after the source stops. It is the most important single parameter in room acoustics and determines how a space sounds for speech, music, or other purposes. A room with too long an RT60 sounds echoey and muddy, while too short an RT60 sounds dead and lifeless.
Wallace Clement Sabine developed the first mathematical relationship between reverberation time, room volume, and sound absorption in 1898. His work at Harvard University laid the foundation for architectural acoustics as a science. The Sabine equation remains the most widely used formula for estimating reverberation time in rooms with relatively uniform absorption.
The Sabine Equation
Where V is room volume in cubic meters, A is total absorption in metric sabins (m²), S is total surface area, and α is the average absorption coefficient. The constant 0.161 applies when using metric units.
Ideal RT60 Values
| Room Type | Ideal RT60 (s) | Notes |
|---|---|---|
| Recording studio | 0.2 - 0.4 | Very dry, controlled |
| Classroom | 0.4 - 0.6 | Speech clarity essential |
| Conference room | 0.5 - 0.8 | Speech intelligibility |
| Concert hall (chamber) | 1.4 - 1.8 | Warm acoustics |
| Concert hall (symphonic) | 1.8 - 2.2 | Rich, full sound |
| Cathedral | 3.0 - 8.0+ | Highly reverberant |
Absorption Coefficients
| Material | α at 500 Hz |
|---|---|
| Concrete / bare brick | 0.02 - 0.04 |
| Plaster on brick | 0.03 |
| Glass window | 0.06 |
| Wood floor | 0.10 |
| Carpet on concrete | 0.30 |
| Acoustic ceiling tiles | 0.55 - 0.80 |
| Heavy curtains | 0.50 - 0.70 |
Frequently Asked Questions
What is the difference between Sabine and Eyring equations?
The Sabine equation works well for rooms with low average absorption (less than about 0.3). For rooms with high absorption, the Eyring equation gives more accurate results: RT60 = 0.161V / (-S ln(1-α)). The Sabine equation overestimates RT60 in highly absorptive rooms.
How do I reduce reverberation time?
Add absorptive materials: acoustic panels, carpeting, curtains, upholstered furniture, or acoustic ceiling tiles. Each of these increases the total absorption (A), which decreases RT60 proportionally.
Does room shape affect reverberation?
The Sabine equation does not account for room shape, only volume and absorption. In practice, room geometry affects sound distribution and can create flutter echoes or focusing effects that the basic formula does not predict. Specialized acoustic modeling software handles these complexities.