Reverberation Time

Enclosed Spaces

When a sound is generated in an enclosed space, it echoes from solid surfaces such as walls, floors, ceilings, hard furniture and so on. When the noise source is switched off or taken away, the echo gradually gets absorbed by its surroundings until the noise stops altogether.

The reverberation time – sometimes referred to as RT60 – is the time it takes for the sound to completely fade away. The exact time this takes will vary depending on the contents of the room. Soft furnishings like upholstery, carpets and curtains will absorb the sound and cause it to fade away quicker than hard surfaces. This is why an empty room always sounds more echoey than a fully furnished one.

Long reverberation times can have the effect of rendering speech unintelligible, whereas short times can lead to a dead sound that is poor for listening to music. In venues like concert halls and lecture theatres, therefore, control of reverberation is vital.

Measurement and Control

The ideal level of reverberation is dependent on the use the space is going to be put too. Measurement is the first step in determining what noise control measures will be needed. There are standards for measuring RT60 times: these are ISO 3382-1 used for performance spaces, and ISO 3382-2 used for normal rooms.

Measurement is usually carried out with a noise source and an acoustic analyser. The source is often an omni-directional speaker that sends equal levels of sound out in all directions. However, it can be something much simpler like a starting pistol, a clapper board or a bursting balloon. The analyser detects when the sound has ceased and measures the level of decay.

Measurement is carried out by first establishing the ambient noise level – the level of background noise. You can then begin testing. Ideally, you need to carry out two or three tests to get an accurate measurement. There should also be no more than two people in the room while the test is under way, and they should be at least a metre away from the analyser – people absorb sound too.

Armed with a measurement of reverberation time, you can take steps to counteract any problems. The ISO standards offer optimal RT60 times for different types of usage, including classrooms, concert halls and so on. Absorbent materials like curtains or dedicated sound-absorbing panels can be used to shorten RT60 times.