Compute the reverberation time T60 — how long sound lingers in a room after the source stops — with the Sabine and Eyring equations. Adjust the room volume, surface area, average absorption and acoustic treatment to see, in real time, the reverberation needed for a speech room or a concert hall.
Parameters
Room volume V
m³
Volume of the room's interior space
Surface area S
m²
Total area of the floor, walls and ceiling
Average absorption α
Area-weighted absorption of the surfaces (0 = fully reflective, 1 = fully absorptive)
Added absorption (treatment)
m²
Absorption added by panels, curtains, the audience and so on
Results
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Total absorption A (m²sabin)
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Effective avg. absorption ᾱ
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Reverb time T60 (Sabine) (s)
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Reverb time T60 (Eyring) (s)
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Mean free path (m)
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Use classification
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Room ray reflection & decay animation
Sound rays leave the source, bounce off the walls and grow fainter as they are absorbed. The lower graph is the sound-pressure level decaying over time; the point where it reaches −60 dB is the reverberation time T60.
The Sabine reverberation time T60. V is the room volume [m³], A the total absorption [m²sabin], S the surface area and ᾱ the average absorption. The ideal T60 depends on whether the room is for speech or music.
The Eyring equation. It treats wall absorption logarithmically and predicts the reverberation time more accurately than Sabine for highly absorptive rooms.
$$\ell=\frac{4V}{S}$$
The mean free path ℓ — the average distance a sound ray travels between two reflections, set purely by the room geometry.
What is Reverberation Time?
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I hear "reverberation time" a lot — what does it actually mean?
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Roughly speaking, it is "how long sound lingers in a room after the source stops". Clap your hands in a small carpeted room and the sound dies almost instantly. Do the same in a stone cathedral and it rings on for many seconds. That lingering tail of sound is reverberation, and the reverberation time describes how long it is. Precisely, it is the time it takes for the sound energy to decay by 60 decibels — down to one millionth — after the source stops. Everyone calls it T60.
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I see. So what determines the reverberation time?
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It is a contest between the room's volume and its total absorption. A larger room gives the sound more space to bounce around before it hits a wall, so it is absorbed more slowly and the reverberation grows longer. Soft, porous things — carpet, curtains, upholstered seats, even the clothing of the audience — soak acoustic energy out of the air at every reflection and shorten it. In the 1890s Wallace Clement Sabine found a beautifully simple relationship: the reverberation time is proportional to the volume divided by the total absorption. That is the Sabine equation we still use.
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So is shorter reverberation always better?
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Here is the interesting part — it depends entirely on the purpose, and the answer is the opposite for speech and music. Speech and lectures want a short reverberation, roughly 0.6 to 1.0 seconds, so that each syllable dies away before the next arrives and the words stay crisp. A classroom with too long a reverberation turns into a muddle of overlapping echoes. Music is the reverse: a symphony hall wants a long 1.8 to 2.2 seconds, because the lingering sound blends the notes, adds warmth and fullness, and supports the players.
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If speech and music need opposite values, what does a multi-purpose hall do?
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That is the world of compromise and clever tricks. A multi-purpose hall is often designed around a middle value of 1.1 to 1.8 seconds. On top of that, designers add movable absorbing panels or banners that can be deployed or retracted — short reverberation for a lecture, long for a concert — tuning the room event by event. What makes the Sabine equation so powerful is that it lets an architect predict and aim for exactly the right reverberation time before a single wall is built.
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The results also show an Eyring equation. How is that different from Sabine?
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The Sabine equation is a good fit for ordinary rooms where the absorption is not very high. But in a room where the walls are almost all absorbing material — a recording studio — Sabine over-estimates the reverberation. The Eyring equation treats wall absorption logarithmically, so it stays accurate in those highly absorptive rooms. When the absorption is small the two give nearly the same value. That is why this tool shows both side by side, so you can compare them depending on the room's character.
Frequently Asked Questions
The reverberation time T60 is the time it takes, after a sound source stops, for the sound energy in a room to decay by 60 decibels — to one millionth of its energy. Clap your hands in a small carpeted room and the sound dies almost instantly; do the same in a stone cathedral and it rings on for many seconds. That lingering tail of sound is reverberation, and T60 is the single most important descriptor of how a room sounds. This tool computes it with the Sabine equation T60 = 0.161V/A.
The Sabine equation T60 = 0.161V/A works well for rooms with low absorption (long reverberation), but it over-predicts the reverberation time when the average absorption is high. The Eyring equation T60 = 0.161V/(−S·ln(1−ᾱ)) treats wall absorption logarithmically and is more accurate for highly absorptive rooms such as recording studios. When absorption is small the two agree closely, and this tool shows both side by side.
The ideal value depends entirely on the room's purpose. Speech, lectures and classrooms want a short reverberation of roughly 0.6 to 1.0 seconds so that each syllable dies away before the next arrives and speech stays intelligible. A multi-purpose hall sits around 1.1 to 1.8 seconds. A symphony concert hall wants a longer 1.8 to 2.2 seconds, because the lingering sound blends notes and adds warmth. Speech needs it short; music needs it long.
From the Sabine equation T60 = 0.161V/A, you shorten reverberation by reducing the volume V or increasing the total absorption A. Since volume is hard to change in practice, adding absorption is the usual approach. Soft, porous materials — carpet, curtains, acoustic ceiling tiles, upholstered seats and the audience itself — all add absorption. The "added absorption" slider in this tool lets you enter the absorption [m²] gained from such acoustic treatment directly.
Real-World Applications
Classrooms, lecture rooms and meeting rooms: Where speech intelligibility is the top priority, the target reverberation time is around 0.6 to 0.8 seconds. Acoustic ceiling tiles, wall panels, carpet and upholstered chairs are combined to provide enough total absorption. School classrooms in particular suffer when reverberation is long — pupils at the back cannot make out the teacher's voice — so many countries set an upper limit on classroom reverberation time.
Concert halls and theatres: A hall for symphonic music wants a long reverberation of about 1.8 to 2.2 seconds to give the music richness and warmth. The basic strategy is a large volume with many reflective surfaces and little absorption. An opera house, which also needs the words to be intelligible, is kept slightly shorter at about 1.4 to 1.6 seconds — the target shifts with the type of performance. The Sabine equation is used early in design to balance volume against absorption.
Recording studios and anechoic chambers: For recording and measurement, you sometimes want to remove the room's sound entirely. Large quantities of absorbing material on the walls and ceiling raise the absorption so high that the Sabine equation becomes inaccurate, and the Eyring equation is used instead. An anechoic chamber pushes the reverberation time close to zero so the source itself can be measured cleanly.
Multi-purpose halls, gymnasiums and exhibition spaces: A multi-purpose hall used for both lectures and concerts adjusts its reverberation time with movable absorbing banners or rotating reflective panels. Large spaces with mostly hard surfaces, like gymnasiums and exhibition halls, easily become over-reverberant; retrofitting absorption to make announcements intelligible is a classic renovation. The "added absorption" input in this tool lets you estimate that effect in advance.
Common Misconceptions and Pitfalls
The biggest misconception is that reverberation time is a single value independent of frequency. The absorption coefficient of a material varies strongly with frequency. Porous absorbers (glass wool, carpet) work well at high frequencies but absorb little in the bass. Low frequencies, conversely, are hard to absorb unless you use a resonance-based structure (panel or cavity absorbers). For this reason, professionals calculate the reverberation time octave band by band, from 125 Hz to 4 kHz. This tool treats the average absorption as one representative value, so use it as a first estimate to grasp the overall picture.
Next, confusing absorption with sound insulation. The absorption coefficient α is the fraction of sound energy "lost rather than reflected" when sound hits a surface, and it sets the reverberation time. Sound insulation, by contrast, is how little sound passes through to the next room — a completely separate property. Adding curtains or absorbing panels does shorten reverberation, but it barely reduces leakage to the neighbouring room. That is why "it is noisy, so let's put up absorbers" does not fix an insulation problem. Reverberation control and sound insulation must be treated separately.
Finally, the misconception that "shorter reverberation is always better". If you think only about speech intelligibility, shorter seems better, but a "dead" room with reverberation near zero makes voices sound thin and weak and is tiring to spend time in. Even a meeting room is more natural with about 0.4 to 0.5 seconds of reverberation. For music spaces it matters even more — too short a reverberation makes the performance cold and flat. Reverberation is not a villain; it is something to "tune" to the right length for the purpose. That is the core of architectural acoustic design.
How to Use
Enter room volume in m³ (typical office: 50–500 m³, concert hall: 10,000–50,000 m³)
Specify surface areas and absorption coefficients for each material (drywall α=0.05, acoustic panel α=0.8, carpet α=0.6)
Click Calculate to obtain T60 (Sabine and Eyring formulas), total absorption in m²sabins, mean free path, and room classification (dead, live, optimal)
Worked Example
Meeting room 12 m × 8 m × 3 m (volume 288 m³): walls 120 m² bare concrete (α=0.04), ceiling 96 m² with suspended acoustic tile (α=0.75), floor 96 m² vinyl (α=0.03). Total absorption A = 120(0.04) + 96(0.75) + 96(0.03) = 78.72 m²sabins. Sabine T60 = 0.161 × 288 / 78.72 = 0.59 s. Eyring T60 (with ᾱ=0.27) = 0.161 × 288 / (−96 × ln(1−0.27)) = 0.52 s. Mean free path ≈ 4V/A = 14.6 m. Classification: optimal speech.
Practical Notes
Sabine formula underestimates T60 in highly absorptive rooms (ᾱ>0.5); use Eyring for accuracy above 40% average absorption
Add soft furnishings (chairs, people) as additional absorption—occupied vs. unoccupied T60 can differ by 30–50%
Speech intelligibility requires T60 0.4–0.8 s; music venues typically target 1.2–2.0 s depending on genre