Calculate the reverberation time (RT60) of a room with the Sabine formula, or solve for the absorption in sabins needed to reach a target RT60.
Sabine Reverberation Time Formula
RT60 = 0.161 * V / A (metric)
RT60 = 0.049 * V / A (imperial)
A = a1*S1 + a2*S2 + ... + an*Sn
- RT60 is the reverberation time in seconds, the time it takes the sound level in the room to decay by 60 dB after the source stops
- V is the room volume, in cubic meters (metric) or cubic feet (imperial)
- A is the total absorption of the room in sabins, in square-meter sabins (metric) or square-foot sabins (imperial)
- a1, a2 … an are the absorption coefficients of each surface material (0 to 1)
- S1, S2 … Sn are the areas of those surfaces, in square meters or square feet
The calculator applies this formula in three directions. In the reverberation time mode, you enter the room size and its absorption, either as a total in sabins or as an average coefficient times the surface area, and it returns RT60 = 0.161*V/A (or 0.049*V/A in feet). In the absorption mode, it rearranges the formula to A = 0.161*V/RT60 and tells you how many sabins the room needs to hit a target reverberation time; if you also enter your current RT60 or current absorption, it reports the shortfall and converts it into an approximate number of standard 2 ft by 4 ft acoustic panels. In the volume mode, it solves V = RT60*A/0.161 from a measured time and a known absorption. The advanced results option adds the Eyring time, RT60 = -0.161*V/(S*ln(1 – a)), which corrects Sabine’s overestimate in heavily treated rooms, and the critical distance, the point where direct and reverberant sound are equally loud.
Recommended RT60 Targets and Absorption Values
Reverberation time targets depend on what the room is for. Speech needs a short decay so syllables stay distinct, while orchestral music benefits from a longer one. The mid-frequency targets below reflect common design practice.
| Room type | Typical RT60 target (s) |
|---|---|
| Vocal booth / recording studio | 0.2 – 0.4 |
| Home theater / control room | 0.3 – 0.5 |
| Classroom | 0.4 – 0.6 |
| Conference room / open office | 0.5 – 0.8 |
| Lecture hall | 0.8 – 1.1 |
| Multipurpose auditorium | 1.2 – 1.8 |
| Concert hall (orchestral) | 1.8 – 2.2 |
| Church / cathedral | 2.0 – 5.0+ |
The next table answers the practical question the formula alone does not: how many sabins a real object adds to the room. Values are approximate mid-frequency (about 500 Hz) figures. To use it, take the absorption shortfall from the calculator’s target mode and divide by the sabins per item.
| Item added to the room | Absorption added (ft² sabins) | (m² sabins) |
|---|---|---|
| Acoustic panel, 2 ft x 4 ft x 2 in | 7.6 | 0.71 |
| Acoustic panel, 2 ft x 4 ft x 4 in | 8.0 | 0.74 |
| Heavy velour curtain, per 10 ft² draped | 5.5 | 0.51 |
| Carpet on pad, per 10 ft² of floor | 5.7 | 0.53 |
| Upholstered chair, unoccupied | 3.0 | 0.28 |
| Adult person, seated | 4.5 | 0.42 |
Sabine Reverberation Time Example Problems
Example 1. A classroom measures 10 m long, 7 m wide and 3 m high, and its mixed surfaces average an absorption coefficient of 0.20. The volume is V = 10*7*3 = 210 m³ and the total surface area is S = 2*(70 + 30 + 21) = 242 m², so the total absorption is A = 0.20*242 = 48.4 m² sabins. The reverberation time is RT60 = 0.161*210/48.4, which is about 0.70 seconds. That sits just above the 0.4 to 0.6 second classroom target, so the room needs a modest amount of extra absorption.
Example 2. A home theater has a volume of 2,400 ft³ and currently measures an RT60 of 0.9 seconds, and you want 0.4 seconds. The required absorption is A = 0.049*2400/0.4 = 294 ft² sabins. The current absorption is 0.049*2400/0.9, or about 130.7 ft² sabins. You therefore need to add about 163 ft² sabins, which works out to roughly 22 two-inch 2 ft by 4 ft panels at a coefficient of about 0.95 each.
FAQ
When should you use Eyring instead of Sabine? The Sabine formula assumes sound energy decays smoothly in a diffuse field, which holds when the mean absorption coefficient is below about 0.2 to 0.3. In heavily treated rooms Sabine overestimates the reverberation time, because it predicts a nonzero RT60 even for a room that absorbs everything. The Eyring formula, RT60 = -0.161*V/(S*ln(1 – a)), accounts for the discrete loss of energy at each reflection and correctly approaches zero as absorption approaches total. For a typical furnished room the two agree closely; for studios and theaters, trust Eyring.
What exactly is a sabin? A sabin is the absorption provided by one square unit of a perfect absorber, equivalent to an open window of that size, through which all sound leaves and none returns. One square-foot sabin corresponds to 1 ft² at a coefficient of 1.0, and one square-meter sabin (also called a metric sabin) to 1 m². Since 1 m² equals 10.76 ft², multiply metric sabins by 10.76 to convert to imperial. Real surfaces contribute their area times their coefficient, so 10 m² of material at 0.3 contributes 3 metric sabins.
Why does the calculated RT60 differ from a measured one? Absorption coefficients vary strongly with frequency, so a single mid-band calculation is only a summary; carpet that absorbs well at 2 kHz does little at 125 Hz, which is why measured low-frequency decay is often much longer than the Sabine estimate. Furniture, people, air absorption in large halls, and uneven distribution of absorption all shift the result as well. For serious work, run the calculation separately per octave band with band-specific coefficients and treat the output as a design estimate, not a measurement.
