Reverberation time versus frequency

"The reverberation time of the room is 1.2 seconds" is a convenient simplification. Real rooms have a different reverberation time at each frequency, and a good description gives a curve.

Reverberation time per octave band

RT60 is measured in octave or third-octave bands, because absorption depends on frequency:

• Porous absorbers (curtains, carpets, upholstery, mineral wool) absorb high frequencies far better than low frequencies.
• Panels and thin constructions absorb at low frequencies through resonance.
• Air absorbs high frequencies, mainly above 2 kHz, which shortens high-frequency reverberation in large halls, see the air absorption tool.

The result is that, in typical rooms, the reverberation time rises towards the bass and falls at the top. Concert halls are often designed so that bass reverberation is up to 20 to 30 % longer than at mid frequencies, which gives warmth. In a studio, the aim is usually a flat curve.

Reading the numbers

A single value, such as the "mid-frequency" reverberation time, is typically the average of the 500 Hz and 1 kHz bands. The RT60 tool shows the curve for the surfaces you select.

Reverberation time and reverberation level are different

Two ideas are often mixed:

• Reverberation time is how long the sound takes to die away: the slope of the decay.
• Reverberation level is how loud the reverberant sound is in comparison with the direct sound at your position.

Three decay curves can have different starting levels but the same slope, and so the same reverberation time. Conversely, the level of the reverberant field depends on the sound power of the source and the total absorption, see Direct and reverberant field. Two rooms with the same RT60 can sound differently reverberant, because the level differs.

Decay rate

The reverberation time is just the time for 60 dB of decay, so the decay rate in decibels per second is 60 divided by RT60. A room with RT60 = 1.5 s has a decay rate of 40 dB/s. See the decay rate tool. Measurements often evaluate only 20 or 30 dB of the decay (T20, T30) and extrapolate.

Early decay

The first part of the decay (the early decay time, EDT, measured over the first 10 dB) correlates better with perceived reverberance than RT60 does, and it can differ from RT60 when the field is not diffuse.

Practical advice

1. Always look at the frequency dependence, not just one number.
2. Check the bass: a room with plenty of high-frequency absorption and none at the low end sounds muddy.
3. Adjust the balance of treatment across the bands, not only its total amount.

Draft: written by an AI assistant, awaiting technical review.