What is RT60?

RT60 is a measurement of how long it would take a sound to decay 60dB in a large room. The RT60 (reverberation time decay in 60 seconds) of a room is usually measured by a trained acoustician.

In an acoustical design of a room, it is essential to understand the shape, volume and purpose of the room. This helps us understand the room characteristics and the optimum RT60 for its intended use. For example, we would expect a large lecture theatre to have a high RT60 but would need a relatively low RT60 for sufficient speech intelligibility for lecture purpose. Post-construction, RT60 is used to characterise the acoustical quality of the room.

RT60 and Absorption

The optimum RT60 for the room’s intended use gives us insight on the amount of absorption necessary. The design process for the room interior begins here with materials selection, their strategic placement, and how much acoustical installation is needed. It is useful to have a list of acoustic panels and their NRC values at this stage.

Measurement of RT60

Projects with strict RT60 requirements typically need post-construction RT60 test (ISO 3382).

The RT60 test measures RT60 at different frequencies; a good room design should yield a smooth RT60 curve across the measured frequency range in addition to achieving the optimum RT60. This implies that the sound quality in the room is well balanced and is without any problematic sound frequency or characteristic echo.

Indicative Clap Test

Some projects do not have the need for strict adherence to the optimum RT60. Instead, rooms are sufficiently treated to the occupiers’ comfortable level of reverberation.

Post-construction RT60 test is not necessary; its measurements would also mean little for the occupiers. The occupiers can observe the result of the acoustical treatment by an indicative clap test. By clapping once in the room, the room’s reverberation and any acoustic flaws can be easily observed.

We also briefly mentioned about RT60 and other terms in our Acoustics Glossary Guide. If you are interested in other acoustic terms, you may download the pdf of the Acoustics Glossary Guide.