Behaviour of Sound
Reflection, absorption, reverberation — and Sabine's formula.
Sound in a room reflects, absorbs, transmits, diffracts and diffuses, and the balance decides how it sounds. The key number is the absorption coefficient (α, 0 to 1; an open window is 1.0); summed over the surfaces it gives the total absorption. And the master quantity is the reverberation time, RT60, given by Sabine's beautiful formula. Size a room with the calculator below.
Learning objectives
By the end of this lesson, you will be able to — mapped to the course outcomes for Acoustics in Architecture:
Describe the five behaviours of sound in a room.
Define the absorption coefficient, the sabin and total absorption.
Apply Sabine's formula to find the reverberation time.
Select an optimum reverberation time for a given use.
Behaviour & absorption
Sound reflects, absorbs, transmits, diffracts and diffuses; the absorption coefficient α (open window = 1.0) summed gives the total absorption A = Σ S·α.[1, 2]
What sound does
Meeting a surface, sound REFLECTS (angle in = angle out off hard surfaces), ABSORBS (energy turns to heat in the material), TRANSMITS (passes through to the next room), DIFFRACTS (bends around edges and obstacles — strongest at low frequency) and DIFFUSES (scatters off irregular or convex surfaces into an even field). Designing a room's sound is balancing these — enough reflection for liveliness, enough absorption for control, enough diffusion for evenness.[1, 2]
Reverberation & Sabine
The reverberation time RT60 is given by Sabine's formula RT60 = 0.161·V/A; choose the optimum for the use. Try the calculator.[2, 3]
Reverberation · Sabine RT₆₀ = 0.161·V / Σ(S·α)
Surfaces (area × material)
Reverberation time
1.30 s
A = 248 sabins
Too dead — remove absorption
RT60 — the master number
REVERBERATION is the lingering of sound by repeated reflections after the source stops; the REVERBERATION TIME (RT60) is how long it takes to decay 60 dB (to a millionth of its energy). RT60 is the single most important acoustic measure of a room — too long and speech blurs; too short and music sounds dead. It rises with the room's volume and falls with its absorption.[2, 3]
At a glance
| Aspect | One | The other |
|---|---|---|
| Reflect vs absorb | Reflection: bounces off hard surfaces | Absorption: energy turns to heat (α) |
| Scatter vs pass | Diffusion: scatters into an even field | Transmission: passes to the next room |
| The key number | α: 0 (reflector) to 1 (open window) | A = Σ S·α: the room's total absorption |
| Sabine | RT60 = 0.161·V / A | Rises with volume, falls with absorption |
| Optimum RT | Speech/classroom: ~0.4–1.0 s (short) | Concert hall: ~1.8–2.2 s (long) |
Key terms
The fraction of sound a surface absorbs, 0 to 1 — an open window is 1.0.
The unit of sound absorption — one square metre of open window (1 m² of total absorption).
A = Σ(S·α) — each surface's area times its coefficient, summed over the room.
The lingering of sound by repeated reflections after the source stops.
The time for sound to decay 60 dB — the master measure of a room's acoustics.
RT60 = 0.161·V/A — reverberation time from volume and absorption.
The right reverberation time for a use — short for speech, long for music.
The scattering of sound by irregular or convex surfaces into an even field.
Studio task
Use the calculator above to take a 2000 m³ hall to the concert-hall band (1.8–2.2 s), then to the speech band (0.7–1.0 s) — note how much absorption each needs. Which surfaces would you change, and with what material?
Self-assessment
1. An open window has an absorption coefficient of —
2. Sabine's formula for reverberation time is —
3. Compared with a classroom, a symphonic concert hall wants a reverberation time that is —
Recap
References & further reading
- [1]Leslie L. Doelle, Environmental Acoustics (optimum-RT tables). McGraw-Hill, 1972.
- [2]M. David Egan, Architectural Acoustics. McGraw-Hill / J. Ross Publishing.
- [3]Wallace Clement Sabine, Collected Papers on Acoustics. Harvard University Press, 1922. https://archive.org/details/collectedpaperso00sabiuoft
Further reading
- M. David Egan, Architectural Acoustics. J. Ross Publishing.
- Wallace Sabine, Collected Papers on Acoustics. Dover.
- Leslie Doelle, Environmental Acoustics. McGraw-Hill.
Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.