Reverb Time Calculator

RT60, equivalent absorption and the two models

RT60 is the time required for the sound-pressure level in a room to fall by 60 dB after the source stops. For boundary surface i, multiply its area Sᵢ by its absorption coefficient αᵢ. The sum is the equivalent absorption area:

A = Σ(Sᵢ × αᵢ)

With volume V, total boundary area S = ΣSᵢ and mean coefficient ᾱ = A / S, the metric Sabine estimate is:

T₆₀,Sabine = 0.161V / A

The Eyring estimate replaces the linear absorption term with a logarithmic one:

T₆₀,Eyring = 0.161V / [−S ln(1 − ᾱ)]

For an optional target time T, the inverse Sabine estimate is Arequired = 0.161V / T. The inverse Eyring estimate is Arequired = S × [1 − exp(−0.161V / (ST))]. Subtracting current A shows the modelled absorption surplus or shortfall; it does not specify a panel product or placement.

Use cubic metres and square metres with 0.161, or cubic feet and square feet with the customary approximation 0.049. The calculator converts units before applying one consistent formula. MathWorks’ room-acoustics model describes the Sabine relationship, equivalent absorption and its frequency-dependent inputs. Eyring’s original Journal of the Acoustical Society of America paper explains why the logarithmic model was introduced for more highly absorbing rooms.

Worked example

Consider a 5 m × 4 m × 3 m room. Its volume is 60 m³ and its six boundaries total 94 m². At one chosen frequency band, suppose the surfaces are represented as follows:

Sabine gives 0.161 × 60 / 16.70 ≈ 0.58 s. Eyring gives 0.161 × 60 / [−94 ln(1 − 0.178)] ≈ 0.53 s. The difference is expected: the models converge at low mean absorption and separate as absorption rises.

Read the estimate responsibly

Absorption coefficients are not universal material constants. They change with frequency, mounting, air gap, thickness and test method, so do not mix values from different bands and call the result broadband RT60. This calculator treats the floor and ceiling separately but assigns one shared coefficient to all four walls. Both equations assume a reasonably diffuse sound field and reduce a complex room to averaged boundaries. They do not model source or listener position, scattering, air absorption, flutter echo, isolated room modes, doors, coupled spaces or unevenly concentrated treatment.

Use this result to compare early design scenarios, not to certify a completed room or promise speech intelligibility. Measure the actual decay when decisions matter. ISO 3382-2 specifies measurement procedures, positions, evaluation and reporting for reverberation time in ordinary rooms. For public venues, schools, workplaces, fire-rated assemblies or costly construction, follow applicable local requirements and consult a qualified acoustician.