The Book of Acoustics

Chapter 2.0 Essential measurments

2.2 What to measure


Reverberation time (RT60) is the lifetime of a sound in a space, or the amount of time it takes for the sound to decrease by 60 decibels (dB) once the source of the emission has stopped. Reverberation time is measured in time (seconds) and sound pressure (dB). Spaces with an RT60 less than 0.3 seconds are considered to be acoustically ‘dead’, while spaces with an RT60 more than 2 seconds are considered to be ‘echoic’.

A long reverberation time may make a room sound echoey, live, and full. A short reverberation time may make a room sound dull or dry. Generally, a large room will have a longer reverberation time than a small room. While a room equipped with many soundabsorbing materials, like carpets and textiles, will have a shorter reverberation time than a room with fewer sound-absorbing materials. Likewise, a room with many reflective materials will have a higher reverberation time than a room that does not. The ideal reverberation time depends on the intended use of the space.

A simplified calculation of reverberation time (T) can be conducted using the volume of the room (V) and absorption surface area (A), according to Sabine’s formula: T = 0.16 x (V/A)


The clap test Anyone can use the simple ‘clap test’ to quickly get a sense of the reverberation of a space. Start by doing a single clap outdoors away from buildings to give your ears some context (there should be no echo). Then return indoors and do a single clap in different parts of the space you are assessing. If there is a lot of reverberation the sound will echo back loudly, and continue resonating for some time. If there is little reverberation, the sound will die out quickly. Perhaps there are other acoustic qualities you notice? Practice, observe and learn.


The Book of Acoustics

This is a handbook for architects and interior designers who want to create acoustically beautiful spaces that make people feel happier, healthier and more productive.