Reverberation time (RT60) is how long sound lingers in a room: specifically, the time it takes for a sound to decay by 60 decibels after the source stops. It is a property of the room, set by its size and how much absorption it contains, and is estimated by Sabine's equation, RT = 0.161 × V / A. Hard, empty rooms have a long reverberation time and sound loud and echoey; adding absorptive surfaces shortens it and makes speech clearer.
What is reverberation time?
Reverberation time — often written RT or RT60 — is the time it takes for a sound to fade away in a room once the source has stopped. Formally, it is the number of seconds for the sound level to fall by 60 decibels, roughly the drop from a raised voice down to inaudibility. Clap your hands in an empty tiled bathroom and then in a furnished living room, and you are hearing the difference in reverberation time directly.
The '60' is why it is also written RT60: a 60 dB decay is a large, consistent drop that gives a repeatable measurement across very different spaces. It is a property of the room itself, not of the sound — the same room reverberates the same way whether you clap, talk or play music in it.
Sabine's equation: what sets the time
The reverberation time of a room is estimated by Sabine's equation: RT = 0.161 × V / A. Here V is the room's volume in cubic metres, and A is the total sound absorption in the room, measured in square-metre sabins — each surface's area multiplied by how absorptive it is. The constant 0.161 simply carries the units; the physics is all in the ratio of volume to absorption.
The equation says two intuitive things. A bigger room reverberates for longer, because sound travels further between surfaces before it is soaked up; and more absorption shortens the time, because each reflection loses more energy. That is why a large, empty hall rings while a small, soft room does not. A reverberation calculator applies this equation so you can estimate a room's current time and how much absorption to add to reach a target.
Why hard rooms sound loud and smear speech
Hard surfaces — glass, painted plaster, concrete and hard floors — reflect almost all the sound that hits them, so a room full of them has very little absorption (a small A) and therefore a long reverberation time. Sound builds up instead of dying away, which is why such rooms feel loud: energy from every past syllable is still bouncing around while new sound keeps arriving.
That lingering energy also smears speech. Consonants carry most of a word's meaning, yet they are quiet and brief; when a long reverberant tail from the previous vowel is still sounding, it masks them, so words run together and intelligibility falls. Adding absorptive surfaces — panels, ceilings, soft furnishings — raises A, shortens the tail and lets consonants come through. How much each panel contributes depends on its measured αw or NRC at the mounting used.
What reverberation time feels comfortable?
There is no single 'good' reverberation time — the comfortable value depends on what the room is for. Speech spaces such as classrooms, offices, meeting rooms and call centres want a short time so words stay crisp; for ordinary new-build classrooms in England, BB93 sets the target at around 0.6 seconds. Music spaces are the opposite: a concert hall or church wants a longer time so notes bloom and blend, and a room tuned for clear speech would sound dry and lifeless for orchestral music.
So acoustic design aims at a target, not at maximum absorption. Over-treat a room and it can feel dead and muffled; under-treat it and speech suffers. Multi-use spaces settle on a compromise, and regulated or performance-critical rooms are modelled by a qualified acoustician against the intended use, rather than judged by a headline panel figure alone.
Reverberation is not the same as echo
In everyday speech 'echo' and 'reverberation' get used interchangeably, but acoustically they are different. Reverberation is the blur of countless reflections overlapping and merging into a smooth decay — a general property of the whole room. An echo is a single, distinct reflection that arrives late enough after the direct sound to be heard as a separate repeat, usually bouncing back off one far-off hard surface.
The distinction matters because both describe sound behaving inside a room, and neither is soundproofing. Absorptive panels reduce reverberation and tame echoes within a space, but they do not block sound travelling between rooms — that is a job for mass and construction, governed by separate standards. If your problem is noise coming through a wall rather than ringing within the room, reverberation time is the wrong measure.
Frequently asked questions
How is reverberation time measured?
On site, a calibrated sound source is switched on and then stopped while a microphone records how quickly the level falls once the source goes silent. Because reaching a full 60 dB drop above background noise can be difficult, the decay is often measured over a smaller, cleaner range and scaled up to the equivalent 60 dB time.
What is a good reverberation time for an office?
Offices are speech spaces, so they generally want a fairly short reverberation time to keep conversations and calls clear without the room feeling loud. There is no universal figure — the right target depends on the room's size, use and layout, which is what a reverberation calculator and, for critical spaces, an acoustician help you set.
Does reverberation time depend on frequency?
Yes. Most rooms reverberate for longer at low frequencies than at high ones, and absorptive materials also behave differently by frequency. That is why acoustic design looks at per-band data rather than a single average, and why a panel's absorption is reported as a curve across frequencies, not just one number.
Can acoustic panels reduce reverberation time?
Yes. Panels add absorption, which raises the value of A in Sabine's equation and so shortens the reverberation time, making speech clearer and the room feel quieter. They do not soundproof — they reduce reverberation within a room, not sound passing between rooms — and how much they help depends on their tested data and the room's volume.