The two common equations used to estimate a room’s reverberation time are the Sabine and Eyring formulas. Both use room volume and sound absorption, but they represent absorption differently. The right equation depends on the room and the assumptions behind the estimate.
For the basic metric Sabine estimate, use T = 0.161 × V ÷ A. Here, T is in seconds, V is room volume in cubic metres, and A is the total equivalent absorption area in square-metre sabins. For a step-by-step calculation, see How to Calculate Reverb Time.
What does RT60 mean?
RT60 is commonly used for the time associated with a 60 dB sound decay in a room after the sound source stops. In room-acoustics practice, formula-based RT60 is an estimate derived from room properties. A measured reverberation time comes from a measurement procedure and analysis of the room’s decay.
The distinction matters: a calculator can estimate a value from entered dimensions and absorption data, but it does not establish the room’s measured response. ISO 3382-2 specifies measurement methods for reverberation time in ordinary rooms.
The Sabine formula
The common metric equation is:
T = 0.161 × V ÷ A
Where:
| Symbol | Meaning | Unit in this form |
|---|---|---|
| T | Estimated reverberation time | seconds |
| V | Room volume | m³ |
| A | Total equivalent absorption area | m² sabin |
| 0.161 | Constant for this common metric form | Used with the units above |
Total equivalent absorption area is estimated by summing the contribution from each surface:
A = α₁S₁ + α₂S₂ + … + αₙSₙ
Here, αᵢ is the absorption coefficient for a surface and Sᵢ is its area. The absorption coefficient is dimensionless; the product αᵢSᵢ contributes an equivalent absorption area. In the metric form, the resulting A is expressed in square-metre sabins.
Sabine formula example
For a room with a volume of 120 m³ and a total absorption area of 30 m² sabin:
T = 0.161 × 120 ÷ 30 = 0.644 seconds
This is a model-based estimate. The input values and the room’s acoustic behavior determine how useful that result is.
Read the dedicated Sabine Formula guide for a fuller explanation of the variables and method.
The Eyring formula
The common metric form is:
T = 0.161 × V ÷ [−S × ln(1 − ᾱ)]
Where:
- T is estimated reverberation time in seconds.
- V is room volume in cubic metres.
- S is the room’s total surface area in square metres.
- ᾱ is the area-weighted average absorption coefficient.
- ln is the natural logarithm.
Ā is calculated by dividing the total equivalent absorption area by the total surface area:
ᾱ = A ÷ S
The Eyring equation uses the average absorption coefficient in a logarithmic expression. It is often used when a room has higher average absorption than the conditions for which the simpler Sabine estimate is most suitable. It still represents a simplified room model; it does not account for every geometric or spatial effect.
See the dedicated Eyring Formula guide for a more detailed explanation.
Sabine vs. Eyring: which formula should you use?
| Question | Sabine | Eyring |
|---|---|---|
| What does it use? | Volume and total equivalent absorption area | Volume, total surface area, and average absorption coefficient |
| What is its form? | Direct inverse relationship between absorption area and time | Logarithmic relationship with average absorption |
| When is it commonly considered? | A straightforward estimate for rooms with relatively lower average absorption | An alternative often considered for more absorptive rooms |
| What should you keep in mind? | Simplified assumptions can limit the estimate | It also relies on simplified room assumptions |
There is no single equation that guarantees an accurate result for every room. Consider the room geometry, how absorption is distributed, and whether the coefficients match the frequency and installation. If the result will inform a consequential design decision, seek appropriate measurement or professional acoustic advice.
Why absorption data matters
Both equations depend on the absorption information entered. A coefficient can vary with frequency, mounting, and the specific material or assembly. If you use a single average coefficient, the result is a simplified estimate for that input—not a full frequency-response analysis.
Use the Absorption Coefficients guide to understand the data, and the Absorption Calculator to total surface contributions. For room-specific context, see the Room Acoustics Guide and Ideal RT60 guide.
Why calculated and measured RT60 can differ
A formula predicts from simplified inputs. A measurement evaluates the sound decay in the actual room using a prescribed setup and analysis. Differences can arise when the room’s absorption is unevenly distributed, its geometry departs from the assumptions, or the input coefficients do not represent the installed surfaces at the frequency being considered.
Penn State’s room example shows why a calculated value and an observed decay can differ in a real space. ISO 3382-2 describes standardized measurement methods for ordinary rooms. A calculator result should therefore be treated as a planning estimate, not as a certified or measured room value.
Frequently asked questions
What is the RT60 formula?
A common metric estimate is the Sabine equation, T = 0.161 × V ÷ A. The Eyring equation is another model and uses room volume, surface area, and average absorption coefficient.
What is the difference between the Sabine and Eyring formulas?
Sabine relates reverberation time directly to volume and total equivalent absorption area. Eyring uses a logarithmic expression based on the room’s average absorption coefficient. Their assumptions and suitability depend on the room.
What is the unit of the RT60 result?
The result is time in seconds when the formula’s units are consistent. In the common metric forms shown here, volume is in cubic metres and surface or equivalent absorption areas are in square metres.
Does the RT60 formula calculate a measured value?
No. It estimates reverberation time from entered room and absorption data. Standardized measurement uses a separate procedure; ISO 3382-2 describes methods for ordinary rooms.
Can I use the same absorption coefficient at every frequency?
Only if your goal is a broad simplified estimate and you understand the limitation. For frequency-specific work, use coefficient data for the relevant frequency bands and calculate each band separately.
Noah Bennett is an audio engineering writer and acoustics specialist at Reverb Calculator. He focuses on reverb time calculation, room acoustics, studio sound design, and music production tools for producers, audio engineers, musicians, and home studio creators.