Reverberation Time Calculator (Sabine & Eyring)

Understanding the reverberation time calculator — concept and principle

Reverberation time (RT60) is the time it takes for sound in a room to decay by 60 dB after the source stops. It largely decides whether speech is clear in a classroom, whether music sounds full in a hall, and how much absorption a room needs. This calculator gives RT60 by both the Sabine and the Eyring formulas.

Every square metre of absorption acts like a square metre of open window: more of it, shorter echo.

How it is calculated

Sabine: T = 0.161 × V ÷ (S × ᾱ)
Eyring: T = 0.161 × V ÷ (−S × ln(1 − ᾱ))
V is room volume (m³), S the total surface area of walls, floor and ceiling (m²), and ᾱ the average absorption coefficient (0 = fully reflective, 1 = fully absorbing).
Absorption area A = S × ᾱ, in m² Sabine. For mixed surfaces, A = Σ(Sᵢ × αᵢ), and ᾱ = A ÷ S.

The principle

In a diffuse room, sound energy fills the volume evenly and is absorbed each time it hits a surface. More volume stores more energy, so decay is slower; more absorption removes energy faster. Wallace Sabine found experimentally around 1900 that T is proportional to V ÷ A. The constant 0.161 s/m comes from 24 ln 10 ÷ c with c ≈ 343 m/s. Eyring (1930) treated the sound as a series of reflections, each removing a fraction ᾱ, which gives the −ln(1 − ᾱ) term. For small ᾱ the two agree; in absorbent rooms Eyring is more realistic, and at ᾱ = 1 it correctly gives zero while Sabine does not.

Worked example

Inputs: V = 200 m³, S = 220 m², ᾱ = 0.2.
A = 220 × 0.2 = 44 m² Sabine.
Sabine: T = 0.161 × 200 ÷ 44 = 32.2 ÷ 44 = 0.732 s.
Eyring: −220 × ln(0.8) = 220 × 0.2231 = 49.09; T = 32.2 ÷ 49.09 = 0.656 s.

Each point is the calculator re-run with only “Total surface (m²)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Total surface (m²)	RT60 Sabine (s)
55	2.927272727
110	1.463636364
165	0.9757575758
220	0.7318181818
275	0.5854545455
330	0.4878787879
440	0.3659090909

How to read the result

For speech, shorter is clearer: ANSI S12.60 sets a maximum of 0.6 s for unoccupied classrooms up to about 283 m³. Offices and meeting rooms are commonly kept below about 0.6–0.8 s, while concert halls for orchestral music typically aim near 1.8–2.2 s. The example room at about 0.7 s would be fine for many uses but slightly too live for a classroom. To shorten RT60, add absorption; each extra m² Sabine reduces it.

Common mistakes

Using the absorption coefficient at one frequency for the whole spectrum; RT60 is usually calculated per octave band, especially 125 Hz to 4 kHz.
Forgetting the absorption of people and furniture, which can change RT greatly.

Keep in mind: These formulas assume a diffuse sound field and ignore air absorption, which matters in large halls at high frequencies. Acoustic design of schools, halls and studios should follow local standards and involve a qualified acoustician.

Frequently asked questions

Which result should I use, Sabine or Eyring?

For average absorption below about 0.2 they are close; for more absorbent rooms, Eyring usually matches measurements better.

What is a typical absorption coefficient?

Painted concrete or plaster is a few hundredths, carpet roughly 0.1–0.3 and acoustic panels often 0.7 or more at mid frequencies.

Sources: W. C. Sabine, Collected Papers on Acoustics, Harvard University Press, 1922 · C. F. Eyring, "Reverberation time in 'dead' rooms," Journal of the Acoustical Society of America 1, 217–241, 1930 · ANSI/ASA S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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