RT60 and reverberation time

Clap your hands in an empty flat and the sound hangs in the air. Do it in a furnished bedroom and it stops almost at once. The difference is reverberation time — how long it takes sound to die away once its source stops — and RT60 is the standard way of putting a number on it.

What the number means

RT60 is the time, in seconds, for sound to decay by 60 dB after the source stops — roughly from loud to inaudible. It was defined by Wallace Sabine over a century ago, and his equation still holds for ordinary rooms: reverberation time is proportional to the room’s volume and inversely proportional to the total absorption in it.

That relationship explains most of what you need to know. Bigger rooms ring longer. Adding absorption shortens the ring. And because absorption is frequency-dependent, so is RT60 — a room can be short at 2 kHz and long at 125 Hz at the same time, which is typical of a room treated mainly with thin porous panels.

Targets by room

There is no universal right answer; the target follows the use. Published guidance, measured at mid frequencies in an unoccupied room, clusters like this:

• Recording studio control rooms: about 0.2–0.4 s. The room must not add to the mix.
• Home theatres: about 0.2–0.3 s for typical room sizes, following the critical-listening-room formula in EBU Tech 3276 and ITU-R BS.1116 (about 0.25 s at 100 m³). Clear dialogue, tight bass, and the surround field doing the enveloping.
• Conference and meeting rooms: about 0.4–0.6 s, and lower for rooms used mainly on video calls. Speech intelligibility for people who may be on a microphone.
• Lecture halls and speech auditoriums: about 0.7–1.0 s. Enough support to carry an unamplified voice.
• Multi-purpose halls: about 0.8–1.2 s; where music is the main use, DIN 18041 targets rise with volume, to about 1.4 s at 1,000 m³ and 1.7 s at 5,000 m³, so adjustable elements are common.
• Concert halls: about 1.7–2.0 s occupied at mid frequencies, where the room is an instrument.

These are ranges from published guides, not a standard. Different sources shift them by a tenth or two; the shape of the pattern is what matters.

Why the whole curve matters

A single RT60 figure hides more than it shows. The measurement is made per octave band, from 125 Hz to 4 kHz at minimum, and the shape across those bands is what determines how a room feels. A flat curve sounds balanced. A curve that rises steeply at low frequencies sounds boomy even if the mid-band number is textbook. A curve that drops sharply at the top sounds dull.

When a target is quoted as one number, ask for the tolerance across bands. German standard DIN 18041, for example, allows the 125 Hz band to run up to about 1.45 times the mid-frequency target, and holds the bands from 250 Hz to 2 kHz within about ±20 % of it.

Small rooms: use with care

Sabine’s equation and the whole idea of a diffuse reverberant field assume a room large enough that sound has mixed thoroughly. Small rooms — most home theatres and control rooms — are mode-dominated at low frequencies, and a low-band RT60 there is often dominated by how long individual modes ring rather than by a diffuse reverberant tail, so read it with care. It is still useful as a comparison before and after treatment, but it should sit alongside a modal analysis rather than replace one.

Sources

1. 01ISO 3382-2:2008 — Measurement of room acoustic parameters, Part 2: Reverberation time in ordinary roomsInternational standard for measuring RT (T20 / T30).
2. 02Sabine, Collected Papers on Acoustics (Harvard, 1922) — public domainThe original reverberation equation; 0.161 is the modern metric constant.
3. 03Eyring, “Reverberation time in ‘dead’ rooms”, JASA 1(2A), 1930
4. 04UK Department for Education — BB93: Acoustic design of schools, performance standards (2015)Mid-frequency RT limits for lecture rooms, drama studios and halls (Table 6).
5. 05DIN 18041:2016 target formulas, as quoted in Germany’s federal BNB rating system (criterion 3.1.4)RT targets by room volume and use, and the tolerance band across frequency.
6. 06Beranek, “Aspects of Concert Hall Acoustics” — AES Heyser Memorial LectureThe best halls: occupied RT between 1.7 and 2.0 s.
7. 07EBU Tech 3276 — Listening conditions for the assessment of sound programme materialControl-room RT: Tm = 0.25 (V/100)^⅓ s, 0.2–0.4 s.
8. 08Kuttruff, Room Acoustics, 6th ed. (CRC Press, 2016)Standard textbook on room acoustics theory.

External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.