Sabine Formula: RT60 Calculation Fully Explained

The Sabine formula calculates reverberation time as T60 = 0.161V ÷ A in SI units. Room volume V is measured in cubic metres, while equivalent absorption area A is found by adding each surface area multiplied by its frequency-specific absorption coefficient.

The equation estimates sound decay and required absorption. It is a statistical prediction, not a substitute for measuring the completed space. Understanding what reverberation time represents prevents an estimate from being treated as an exact result.

What Is the Sabine Formula?

The Sabine formula relates room volume to total equivalent absorption. More volume lengthens reverberation time, while more absorption shortens it.

The metric and imperial equations

For SI units, use:

T60 = 0.161V ÷ A

• T60 = reverberation time in seconds
• V = room volume in cubic metres
• A = equivalent absorption area in square metres sabin

For imperial units, the common form is:

T60 = 0.049V ÷ A

Here, volume is in cubic feet and absorption is in square feet sabin. Do not use the metric constant with imperial measurements. Its rounded value can vary slightly with convention and air conditions.

What does T60 mean?

T60 is the time required for sound level in a room to decay by 60 dB after the source stops. A long T60 produces a persistent tail; a short T60 sounds drier. The suitable result depends on room volume and purpose, so an appropriate RT60 target for speech may differ from one for music.

How Do You Calculate Total Room Absorption?

Calculate equivalent absorption as A = Σ(Sᵢ × αᵢ): multiply each material’s area by its frequency-specific absorption coefficient, then add the contributions.

Symbol	Meaning	SI unit
Sᵢ	Area of one surface or material	m²
αᵢ	Absorption coefficient of that material	Dimensionless, normally 0 to 1
Sᵢ × αᵢ	Equivalent absorption contributed by that item	m² sabin
A	Sum of all equivalent absorption	m² sabin

A 10 m² surface with α = 0.20 contributes 2 m² sabin; it does not mean that only two physical square metres are active.

Why the calculation must be frequency-specific

Materials do not absorb every frequency equally. Calculate A and T60 separately for relevant octave or one-third-octave bands using frequency-specific sound-absorption coefficient data.

Using one Noise Reduction Coefficient, or NRC, for the whole spectrum conceals this variation. NRC is a single rating averaged from selected mid-frequency bands, not an absorption coefficient for every frequency.

Include more than the six room surfaces

Doors, windows, curtains, seats, people, furniture, and treatment may contribute absorption. Some are rated in sabins per item. Subtract doors and windows from the wall finish before adding them separately.

How Do You Use the Sabine Formula?

Use the Sabine formula in three stages: calculate room volume, find equivalent absorption for every surface at one frequency, and divide 0.161V by the absorption total. Repeat the process for each frequency band needed.

Worked example: a 5 × 4 × 3 m room

Consider an empty rectangular room that is 5 m long, 4 m wide, and 3 m high. The following coefficients are illustrative 500 Hz values, not specifications for every product with the same description.

Room volume:

V = 5 × 4 × 3 = 60 m³

The floor and ceiling are each 5 × 4 = 20 m². Total wall area is 2(5 × 3) + 2(4 × 3) = 54 m². If the door occupies 2 m², the remaining painted wall area is 52 m².

Component	Area	Example α at 500 Hz	Equivalent absorption
Hard floor	20 m²	0.05	1.00 m² sabin
Finished ceiling	20 m²	0.10	2.00 m² sabin
Painted walls excluding door	52 m²	0.05	2.60 m² sabin
Door	2 m²	0.10	0.20 m² sabin
Other furnishings	—	—	1.00 m² sabin
Total	—	—	6.80 m² sabin

Now calculate:

T60 = (0.161 × 60) ÷ 6.80

T60 = 9.66 ÷ 6.80 = 1.42 seconds

The predicted 500 Hz reverberation time is approximately 1.42 seconds. Real coefficients must come from suitable test data, and the result should be checked with an in-room reverb-time measurement method.

How Can You Calculate Absorption Needed for a Target RT60?

Rearrange the metric equation as A = 0.161V ÷ T60 to estimate the total equivalent absorption required. Subtract existing equivalent absorption from that result to estimate the additional absorption needed.

For the 60 m³ example room, suppose the target at 500 Hz is 0.60 seconds:

A required = (0.161 × 60) ÷ 0.60 = 16.10 m² sabin

Existing absorption is 6.80 m² sabin, so:

Additional A = 16.10 − 6.80 = 9.30 m² sabin

Equivalent absorption is not automatically the same as panel area. If a tested absorber has α = 0.80 at 500 Hz under the intended mounting condition, the simplified coverage estimate is:

Panel area = 9.30 ÷ 0.80 = 11.63 m²

That estimate applies only to the studied band and mounting. Repeat it across the frequency range, then choose a practical acoustic-treatment plan instead of sizing panels from one result.

When Is the Sabine Formula Accurate?

The Sabine formula is most useful when the sound field is reasonably diffuse, average absorption is not high, and treatment is distributed around the room. Accuracy declines when those assumptions do not describe the space.

Small rooms may be dominated at low frequencies by room modes rather than a diffuse field. Highly absorptive rooms, unusual geometries, and concentrated treatment can also depart from the prediction. A room-acoustics assessment should consider modal decay, early reflections, and placement as well as RT60.

Measured reverberation time is often extrapolated from a shorter decay. T20 uses a 20 dB evaluation range and T30 uses 30 dB; both can estimate a 60 dB decay when noise prevents a full-range measurement.

Sabine Formula vs. Eyring Formula: Which Should You Use?

Sabine is generally better suited to rooms with relatively low average absorption. Eyring accounts for the increasing effect of absorption as the average coefficient rises, although both equations still rely on statistical, approximately diffuse-field assumptions.

Criterion	Sabine	Eyring
Common SI form	T60 = 0.161V ÷ A	T60 = 0.161V ÷ [−S ln(1 − ᾱ)]
Absorption condition	Lower average absorption	Moderate to higher average absorption
Inputs	Volume and equivalent absorption area	Volume, total surface area, and average absorption
Shared limitation	Assumes a reasonably diffuse field	Assumes a reasonably diffuse field

At low absorption, the results are similar; as absorption rises, they diverge. This comparison of Eyring’s equation also explains why changing formulas cannot correct a non-diffuse room.

What Are the Most Common Sabine Formula Mistakes?

The most common errors are mixing unit systems, using one absorption value across all frequencies, and treating predicted T60 as guaranteed. Other avoidable mistakes include:

• Counting the full wall area and then adding doors or windows again.
• Confusing physical absorber coverage with equivalent absorption area.
• Omitting occupants, seating, curtains, furniture, or existing treatment.
• Using product data from a different mounting condition.
• Applying the equation to low-frequency modal behaviour as though the field were diffuse.
  Keep the surface schedule and coefficient source with the calculation. This helps compare the prediction with RT60 fundamentals and measurement limits.

Frequently Asked Questions

What does the constant 0.161 mean in the Sabine formula?

The SI constant combines the equation’s decay relationship with the speed of sound and metric units. It is commonly rounded to 0.161, though slightly different values may appear depending on temperature and convention.

What is a sabin in room acoustics?

A metric sabin is one square metre of equivalent absorption area; an imperial sabin is one square foot. A 5 m² material with an absorption coefficient of 0.60 contributes 3 m² sabin at that frequency.

Should the Sabine formula be calculated at each frequency?

Yes. Absorption changes with frequency, so calculate reverberation time separately for relevant octave or one-third-octave bands. One broadband or NRC-based result can hide excessive low-frequency decay or over-absorption at high frequencies.

Can the Sabine formula calculate how many acoustic panels are needed?

It can estimate the additional equivalent absorption required for a target T60. Converting that value into panel quantity requires frequency-specific test data, panel size, mounting details, and a treatment layout; one coefficient cannot guarantee balanced results across all bands.

Why does measured RT60 differ from the Sabine prediction?

The room may not have a diffuse field or uniformly distributed absorption. Coefficient uncertainty, room modes, furnishings, measurement positions, noise, and construction details can shift the measured decay.