What is RT60?
RT60 (Reverberation Time) measures how long sound takes to decay by 60 dB after the source stops — essentially how long a room "rings." It determines whether a space sounds live and echoey or dead and controlled.
Target values vary by use: Recording studios need short RT60 (0.25-0.3s) for clarity. Classrooms need moderate (0.4-0.6s) for speech intelligibility. Concert halls need longer (1.5-2s) for musical richness. This calculator helps you determine how much acoustic treatment you need.
Formula Reference
Eyring: RT₆₀ = kV / (-S × ln(1-ᾱ))
V = room volume, A = total absorption in sabins (Σ area × α), S = sum of the surface areas listed above, ᾱ = A / S
Sabine's equation is W.C. Sabine's, from work begun at Harvard in 1895 and collected in Collected Papers on Acoustics (1922). The Eyring form is Carl F. Eyring, “Reverberation Time in Dead Rooms”, JASA 1(2), 1930. Both constants are stated in this form by the Acoustical Society of America in Classroom Acoustics for Architects (Appendix E). This is a design estimate, not a measurement — see ISO 3382 below.
🎯 What RT60 measures — and what ISO 3382 actually governs
Reverberation time is how long a sound takes to decay by 60 decibels after the source stops. Wallace Clement Sabine established both the concept and the first predictive equation from work he began at Harvard in 1895, after the new Fogg Art Museum lecture hall turned out to be unusable for speech; his papers were gathered into Collected Papers on Acoustics in 1922, and the unit of absorption — the sabin — is named for him.
Two standards get quoted on reverberation pages and they do different jobs. ISO 3382-1 (performance spaces) and ISO 3382-2 (ordinary rooms) are measurement standards: they specify how the room is excited, where microphones go, and how T20 and T30 come off a decay curve. Neither predicts reverberation from a drawing. Prediction is what this calculator does. A reverberation time that has to be defended — a specification, a commissioning report, a compliance check — is measured, not calculated.
🔬 The formula this calculator uses
The two constants are not interchangeable. The Acoustical Society of America states them in exactly this form in Appendix E of Classroom Acoustics for Architects: k = 0.161 s/m with volume in cubic metres and absorption in square metres; k = 0.049 s/ft with volume in cubic feet and absorption in square feet. Absorption is counted in sabins — an area times a dimensionless coefficient — so an imperial sabin is a square foot of perfect absorption and a metric sabin is a square metre. The unit toggle changes the constant as well as the labels, because feeding square metres to the imperial constant inflates the answer roughly elevenfold.
Sabine or Eyring?
Sabine's equation treats absorption as if smeared evenly over every surface and never returns zero: a room whose every surface absorbs perfectly still shows a finite time. Carl F. Eyring's 1930 paper Reverberation Time in Dead Rooms replaced the linear absorption term with −ln(1 − ᾱ), which diverges as ᾱ approaches 1 and drives the predicted time to zero. Below about ᾱ = 0.2 the two agree closely; as a room gets deader Sabine reads long, and the calculator suggests switching once the average coefficient passes 0.25.
One detail before you trust the Eyring figure: S and ᾱ come from the surfaces you enter, not from the room's geometry. If your listed areas do not add up to the room's real surface area, ᾱ is the mean over what you listed. List every surface, including the ones you are not treating.
📊 Choosing a target
Of the eight room-type targets in the panel, exactly one is a published requirement. ANSI/ASA S12.60/Part 1-2010 (R2020), Table 1, limits reverberation time in an unoccupied, furnished core learning space to 0.6 s at 283 m³ (10,000 ft³) or less and 0.7 s from 283 to 566 m³ (10,000–20,000 ft³). The limit applies in each of the 500, 1000 and 2000 Hz octave bands — the booklet's Table 2, note (a), states it for “reverberation times in octave bands with midband frequencies of 500, 1000, and 2000 Hz”, and its Appendix E works an example in which “the maximum reverberation time is 0.6 s at each of the three specified frequencies”. It is not an average, so a room that averages 0.6 s and runs long at 500 Hz does not comply. Above 20,000 ft³, and in ancillary spaces, the standard sets no reverberation limit. Table 2 carries a second requirement that is easy to miss: a core learning space of 283 m³ or less “shall be readily adaptable to allow reduction in reverberation time to 0.3 sec”. That is why the classroom preset is 0.6 s.
That is what the Analysis Frequency tabs are for. The calculator holds a separate absorption coefficient for every material at 125 through 4000 Hz and solves one band at a time, so checking a classroom against the standard means reading the 500, 1000 and 2000 Hz tabs and taking the worst of the three — not the average of them. The RT60-vs-frequency chart below the results shows all six at once. In practice 500 Hz is the one to watch, and the standard's own note says so: “it will generally be found that conformance to the reverberation-time requirement of table 1 at 500 Hz will also ensure conformance at the two higher frequencies.”
The other seven targets — control room, studio, podcast, home theatre, office, conference, worship — are defaults chosen for this tool. They sit inside the range commonly used for those spaces, but we could not attribute any of them to a standard or published criterion, so we do not claim one. Where a client or specification names a figure, use that figure.
🛠️ Where absorption coefficients come from
A coefficient α is the fraction of incident energy a surface does not send back into the room. It is measured in a reverberation chamber under ASTM C423 — a paywalled standard we have not read, named here to identify what governs the test — and manufacturers publish the results. Owens Corning's 700 Series data sheet reports 0.11, 0.28, 0.68, 0.90, 0.93 and 0.96 at 125 through 4000 Hz for Type 703 board 25 mm thick laid against a solid backing — an NRC of 0.70.
Two things follow from how that number is produced. Mounting changes it, so a data sheet that does not state the mounting is not telling you enough. And coefficients above 1.00 are routine — the same sheet lists 1.14 at 500 Hz for 51 mm 703 — because the test panel absorbs through its exposed edges as well as its face.
The material list here holds typical published coefficients for classes of material, good enough to size a job. They are not test data for any particular product, and for ordinary building surfaces — plaster, glass, timber, carpet — we could not reach the original laboratory reports behind the figures that circulate in the literature. Where you have the C423 report for the exact product and mounting you are buying, enter those numbers instead.
⚠️ What this estimate cannot do
- It assumes a diffuse field. Both equations assume energy spread evenly and absorption distributed around the room. All the absorption on one surface, an extreme shoebox or a corridor will not behave as predicted.
- It says nothing useful below the modal region, where a room rings at discrete resonances rather than decaying statistically. The room modes calculator covers that.
- There is no air-absorption term in either formula, so large volumes at 2 kHz and above will read long.
- It is not a compliance result. A classroom that has to satisfy ANSI/ASA S12.60, or a room commissioned against a contract, needs a measurement made to ISO 3382 with a calibrated system. Use this to decide what to buy, then measure.
❓ Common questions
How do you calculate RT60?
Multiply each surface area by its absorption coefficient at the frequency you care about, add the products to get the total absorption A in sabins, then divide k × V by A, where V is the room volume. The constant k is 0.049 working in feet and 0.161 working in metres, and mixing the two is the commonest error in the calculation. The result is a design estimate for a diffuse sound field; a reverberation time that has to be verified must be measured to ISO 3382 instead.
What is the Sabine RT60 formula?
RT₆₀ = k × V / A, where V is the room volume, A is the total absorption — each surface area multiplied by its absorption coefficient, summed — and k is 0.049 for feet or 0.161 for metres. Wallace Clement Sabine derived it from measurements begun at Harvard in 1895, published in Collected Papers on Acoustics in 1922. It assumes absorption is spread evenly and reads progressively long as a room gets deader, so above an average coefficient of about 0.25 the Eyring form is the better choice.
What is a good reverberation time?
It depends on what the room is for, and only one common case is fixed by a standard: ANSI/ASA S12.60 Part 1 caps reverberation time at 0.6 seconds in an unoccupied furnished classroom of 10,000 cubic feet or less, and 0.7 seconds from 10,000 to 20,000 cubic feet. The limit applies in each of the 500, 1000 and 2000 Hz octave bands separately, not to their average, so a room that averages 0.6 seconds can still fail. Figures quoted for studios, home theatres, offices and worship spaces are design conventions rather than requirements, so use the number your client or specification actually names.
How can we reduce RT60?
Add absorption. In Sabine's equation reverberation time is inversely proportional to total absorption, so doubling the sabins in a room halves the predicted time. In practice that means porous absorbers — mineral wool or glass fibre panels, heavy curtains, carpet over an underlay, upholstered seating — placed where they intercept reflections. Thickness decides which frequencies you affect, which is why a room treated with thin foam alone ends up dull without ever getting less boomy.
What is a good sound absorption coefficient?
There is no single good value, because a coefficient describes one material at one frequency on one mounting rather than scoring a product. Painted masonry sits near 0.02, while Owens Corning publish 0.11 at 125 Hz rising to 0.90 at 1000 Hz for 25 mm Type 703 board against a solid backing, and values slightly above 1.00 appear in laboratory reports because the test panel absorbs through its edges. What matters is the total absorption you accumulate across the room, in sabins, at the frequencies your problem lives at.
The reverberation constants and the total-absorption method on this page come from Appendix E of the Acoustical Society of America's Classroom Acoustics for Architects, which also reproduces the classroom criteria of ANSI/ASA S12.60/Part 1-2010 (R2020). The equations are attributed to W.C. Sabine's Collected Papers on Acoustics (1922) and to C.F. Eyring, JASA 1(2), 1930. Published coefficients and the ASTM C423 mounting are illustrated from the Owens Corning 700 Series data sheet. ASTM C423 and ISO 3382 are named to identify what governs each measurement; both are paywalled and we have not read them.