Clap your hands once in an empty tiled bathroom and the sound seems to hang in the air, smearing into a wash of echo. Do the same in a carpeted living room full of soft furniture and the clap is crisp and gone almost instantly. Same clap, same air, very different rooms. What separates them is a single measurable quantity: the reverberation time.
Reverberation time is the property that makes a cathedral feel vast and a recording studio feel dead. It governs whether speech in a classroom is intelligible, whether a concert hall flatters an orchestra, and whether an open-plan office is exhausting to work in. This article explains what reverberation time measures, how the Sabine equation predicts it, and how to use that prediction to shape a room before it is ever built.
Why this calculation matters
Reverberation time is the first number an acoustic designer reaches for, because so much else follows from it. Speech intelligibility, music clarity, perceived loudness, and noise build-up in a busy space all depend on how quickly sound energy decays. A room tuned for one purpose is often wrong for another: a lecture hall wants a short reverberation so consonants stay distinct, while a hall for symphonic music wants a longer one so notes blend and the sound feels full.
The calculation also turns acoustics into a design lever you can pull early. Because reverberation time depends on room volume and on the absorbing power of its surfaces, you can predict it from a drawing — before any material is bought or installed. That lets you decide how much carpet, how many acoustic panels, or how much soft seating a space needs, instead of discovering a problem after construction when fixes are expensive and disruptive.
The core formula
The reverberation time is defined as the time it takes for the sound energy in a room to drop by 60 decibels after the source stops — a factor of one million in energy. That specific definition is written RT60, or simply T60.
Wallace Clement Sabine, working at Harvard in the 1890s, found that this decay time depends on just two things: how big the room is and how much sound its surfaces absorb. His result is the Sabine equation:
T60 = 0.161 * V / A
Here V is the room volume in cubic metres, and A is the total absorption in square metres of absorption, a unit also called the metric sabin. The constant 0.161 carries units of seconds per metre and bundles in the speed of sound at normal room conditions.
The absorption A is the sum, over every surface in the room, of that surface's area multiplied by its absorption coefficient:
A = S1*alpha1 + S2*alpha2 + S3*alpha3 + ...
The absorption coefficient alpha runs from 0 to 1. A value of 0 means a perfectly reflective surface that returns all the sound; a value of 1 means a perfectly absorbing surface that returns none. Hard plaster might sit near 0.02, while a thick acoustic panel can exceed 0.9. Crucially, alpha varies with frequency — most porous absorbers soak up high frequencies far better than low ones — so a thorough analysis treats several frequency bands separately rather than reporting a single number.
The physical reading is straightforward. A larger room gives sound more distance to travel between encounters with a surface, so each "bounce" is rarer and the energy lingers — reverberation rises with volume. More absorption means each encounter removes more energy, so the sound dies faster — reverberation falls as A rises.
A worked example
Consider a small auditorium with a volume of V = 200 m^3. After tallying every surface — floor, ceiling, walls, seating — and multiplying each area by its absorption coefficient, the total absorption comes to A = 40 m^2 sabins.
Step 1 — write down the Sabine equation.
T60 = 0.161 * V / A
Step 2 — substitute the room's values.
T60 = 0.161 * 200 / 40
Step 3 — evaluate.
T60 = 32.2 / 40
T60 = 0.81 s
A reverberation time of about 0.81 seconds sits comfortably in the range usually targeted for speech-focused rooms such as classrooms and lecture halls, where values somewhere around 0.6 to 1.0 seconds keep speech clear without making the room feel lifeless.
Now suppose the room were too live — say 1.5 seconds — and you wanted to bring it down. The equation tells you exactly what to do. Reverberation is inversely proportional to A, so to halve T60 you must double the total absorption. Adding soft furnishings, carpet, upholstered seating, or acoustic panels raises A and shortens the decay. The volume is usually fixed by architecture; the absorption is the knob you can actually turn.
Common mistakes
Treating absorption as frequency-independent. A single alpha value hides the truth that most absorbers work mainly on high frequencies. A room can be well controlled at 2 kHz and still boomy at 125 Hz. Run the calculation band by band.
Confusing absorption area with surface area. The A in the Sabine equation is not the room's surface area — it is each area multiplied by its coefficient. A large glass wall has a big surface but a tiny absorption contribution.
Pushing Sabine into very dead rooms. The Sabine equation assumes a fairly diffuse, moderately reverberant field. In a heavily treated space where average absorption is high, it overestimates the decay time. The Eyring equation is the better choice when average alpha is large.
Forgetting the people. An audience is a substantial absorber. A hall measured empty and a hall measured full can have noticeably different reverberation times, which is why occupied and unoccupied conditions are specified separately.
Ignoring air absorption at high frequencies. In large rooms, the air itself absorbs energy at high frequencies. For modest rooms this is negligible, but in a big hall it shortens the high-frequency reverberation and should be included.
Try the interactive NovaSolver calculator
Working one band by hand is instructive once; iterating a real design across every octave is not something you want to do with a calculator. The Room Acoustics — RT60 Reverberation Time Calculator on NovaSolver lets you set room dimensions and pick a material for the floor, walls, and ceiling, then computes RT60 across six octave bands from 125 to 4000 Hz using both the Sabine and Eyring formulas. It also reports the room volume, the Schroeder cutoff frequency, and the count of room modes below 500 Hz, so you can see at a glance whether the space meets a concert hall, classroom, or studio target.
Related calculators
- Sound Pressure Level & Noise Calculator — to combine source levels and predict how loud a space will actually be.
- Sound and the Decibel Scale — a clear primer on the logarithmic decibel that underlies the 60 dB in RT60.
- Doppler Effect Simulator — for how relative motion between a source and listener shifts the frequency you hear.
You can browse the full set in the acoustics tools hub.
Closing note
Reverberation time is the clearest example of acoustics being a design discipline rather than a guessing game. The Sabine equation is simple enough to carry in your head, yet it ties three things together — volume, absorption, and decay time — tightly enough to plan a room around. Remember the inverse relationship: doubling the absorption halves the reverberation. Decide what the space is for, pick a target, and use the equation to size the treatment. Do that on paper, band by band, and the finished room will sound the way you intended on the day it opens.
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