RT60 is the time for the sound pressure level in a room to fall by 60 dB after the source stops; below about 0.5 s a room reads as dry, and the number you compute with Sabine will be optimistic in a furnished room.
RT60: what reverberation time measures and how it is read
checked 2026-09-16responsible Room Tone Register (a named register, not a person)
What it is and is not
RT60 is a single number standing for a slope. It is defined over a 60 dB fall because the slope is close to straight in the middle of the decay, but a real room's curve bends near the top and the bottom, so the measurement is usually taken over the −5 dB to −35 dB portion and extrapolated. That extrapolation is why two people measuring the same room with different analysis windows get numbers half a second apart.
RT60 is not 'how long you hear the sound'. It is how long the level takes to fall by 60 dB, which in a normal room means the tail is inaudible long before the full 60 dB has elapsed.
The formula and its limits
Sabine's relation assumes the sound field is diffuse — energy arriving equally from all directions — and that absorption is spread evenly and is small. When the average absorption coefficient climbs above roughly 0.2, Sabine over-predicts the decay time and the Eyring form is preferred:
RT60 = 0.161 · V / (−S · ln(1 − ᾱ)). Same units. Use it when ᾱ exceeds about 0.2, which is common in a heavily treated room.
Frequency by frequency
A single RT60 hides the thing you care about. Porous absorbers work at high frequencies and do very little at low frequencies, so a room with a single quoted figure of 0.4 s can easily be 0.35 s at 2 kHz and 0.9 s at 125 Hz. That is the room that sounds 'boomy and dead' at the same time. Always ask for the octave-band table.
| Band | Typical untreated room | After 2 in porous panels on 20% of walls |
|---|---|---|
| 125 Hz | 0.9 s | 0.85 s |
| 500 Hz | 0.7 s | 0.45 s |
| 2 kHz | 0.6 s | 0.30 s |
The table is the argument for bass traps in one line: the panel area that fixes the midrange does almost nothing to the octave where the room sounds worst.
Near kin: EDT, T20, T30 and the reverberation radius
EDT is measured over the first 10 dB of decay and correlates better with perceived clarity than RT60 does; a room can have a good RT60 and a poor EDT. T20 and T30 are the same slope estimated over 20 dB and 30 dB windows, and they differ from each other in the same room. The reverberation radius is the distance at which direct and reverberant energy are equal, and it is what actually decides how close you must work.
| Term | What it is | How it differs from RT60 |
|---|---|---|
| EDT | Decay over the first 10 dB | Better correlates with perceived clarity |
| T20 / T30 | Slope estimated over 20 / 30 dB | Same room, different numbers |
| Reverberation radius | Distance where direct equals reverberant | A distance, not a time; decides working position |
| Early decay | First reflections within about 50 ms | Arrives before the tail and reads as part of the source |
The common misread is treating a single broadband RT60 as a description of the room. A room quoted at 0.4 s can be 0.35 s at 2 kHz and 0.9 s at 125 Hz, and the octave that sounds worst is the one the single number hides. Ask for the table or treat the figure as a summary, not a specification.
Source log
- 01Small Reverb Room (RT60 demonstration), Penn State Acousticssupports the definition and the decay-curve reading of reverberation time
- 02Energy Decay Curve, CCRMA (Stanford)supports the use of the energy decay curve rather than a single level reading
- 03ISO 3382-1: Measurement of room acoustic parameters, performance spacessupports reporting RT60 per octave band with a stated measurement method
Each line points at one specific document. No line is a home page.
Leave this page Close this page if you want to know what to do about the decay rather than what it is — start with the first-reflection job.