About Reverberation Time Calculator (RT60)
The reverberation time calculator returns the RT60 of a room, the time in seconds for the sound pressure level to decay by 60 dB after the source stops. It evaluates both the classical Sabine equation RT60 = 0.161 V / A and the Eyring-Norris equation, which is preferred when the mean absorption is high and the Sabine form over-predicts the decay time.
Enter the room volume (directly or from length, width, and height) and a list of surfaces, each with its area in square metres and its Sabine absorption coefficient alpha. The tool sums the absorption S_i * alpha_i into the total absorption A in metric sabins, computes the mean absorption coefficient, and compares the result against an optional target RT for speech, music, or critical-listening spaces.
How It Works
- Provide the room volume directly, or enter length, width, and height to compute V = L*W*H and the enclosing surface area S = 2(LW + LH + WH).
- Add one row per surface or material with its area (m^2) and Sabine absorption coefficient alpha (0 to 1).
- The calculator sums A = sum(S_i * alpha_i) plus any optional air absorption, then evaluates Sabine RT60 = 0.161 V / A and Eyring RT60 = 0.161 V / (-S ln(1 - alpha_avg)).
- Enter an optional target RT to get an assessment of whether the room is on target, too reverberant, or too dead for its intended use.
Worked Example
A 10 x 8 x 3 m classroom has volume V = 240 m^3 and enclosing surface area S = 2(80 + 30 + 24) = 268 m^2. Plaster walls of 130 m^2 at alpha 0.05 give 6.5 sabins, a concrete floor of 80 m^2 at alpha 0.02 gives 1.6 sabins, and an acoustic ceiling of 80 m^2 at alpha 0.70 gives 56 sabins, so A = 64.1 sabins and the mean alpha = 64.1 / 268 = 0.239. The Sabine reverberation time is RT60 = 0.161 * 240 / 64.1 = 0.603 s. The Eyring value is 0.161 * 240 / (-268 * ln(1 - 0.239)) = 0.527 s, lower as expected at this absorption level.
Formulas
- Sabine reverberation time
RT60 = 0.161 * V / A- Total absorption
A = sum( S_i * alpha_i ) + A_air- Mean absorption coefficient
alpha_avg = A_surface / S- Eyring-Norris reverberation time
RT60 = 0.161 * V / ( -S * ln(1 - alpha_avg) + A_air )
Standards & References
- Sabine, W. C. (1900) reverberation equation
- Eyring, C. F. (1930) reverberation in dead rooms
- ISO 3382 acoustics - measurement of room acoustic parameters
Frequently Asked Questions
What is the difference between the Sabine and Eyring formulas?
Both predict RT60 from volume and absorption. Sabine (RT60 = 0.161 V / A) is simple and accurate when the mean absorption is low. Eyring uses -S ln(1 - alpha_avg) and gives shorter, more realistic times in rooms with high average absorption, where Sabine over-predicts.
What is a sabin and how is total absorption calculated?
A metric sabin is one square metre of perfectly absorbing surface. Total absorption A is the sum over every surface of its area times its absorption coefficient, A = sum(S_i * alpha_i), with units of m^2-sabins. Air absorption can be added at high frequencies in large rooms.
What reverberation time should a room have?
It depends on use and volume. Speech and classrooms typically target 0.4 to 0.8 s, general offices and studios around 0.3 to 0.5 s, while concert halls for music aim for roughly 1.5 to 2.2 s. Enter your target RT and the tool assesses whether the room is on target, too reverberant, or too dead.
Why must the mean absorption coefficient be below 1?
The Eyring formula contains ln(1 - alpha_avg), which diverges to negative infinity as the mean absorption approaches 1. A mean alpha of exactly 1 would imply a fully anechoic room with zero reverberation, so the calculator requires alpha_avg < 1 and flags inputs that reach the limit.