What RT60 tells you about a room

If you have read anything about room acoustics, you have met RT60. It is the number quoted in product brochures, in acoustic reports and in arguments on forums — usually with very little said about what it actually describes.

RT60 is reverberation time: how long sound takes to decay by 60 decibels after the source stops. In plain terms, how long the room keeps ringing once the music, the voice or the clap has finished.

It is a decay time, not a quality score

A long RT60 means sound hangs around. In a cathedral that is the point; in a boardroom it is why nobody can follow the meeting. A short RT60 means sound stops quickly, which helps speech but can leave a music room feeling lifeless.

So there is no universally good value. The right range depends on the size of the room and what happens in it. A room built for speech wants a shorter decay than a room built for orchestral music, and a small room wants a shorter decay than a large one.

One number hides the problem

RT60 is usually published as a single figure, but rooms do not decay evenly across the frequency range. Most untreated rooms decay quickly at high frequencies — soft furnishings and people absorb treble easily — and slowly at low frequencies, where absorption is much harder.

That is why a room can measure a respectable average and still sound wrong: the bass is ringing long after the top end has stopped. Measured by octave band, the same room tells a much more useful story than it does as one number.

• A single RT60 figure averages away the frequency balance
• Bass decay is usually the longest and the hardest to fix
• Two rooms with the same average RT60 can sound completely different

What RT60 does not tell you

Reverberation time says nothing about where sound is arriving from, or when. Early reflections — the sound bouncing off a nearby wall or table within the first few milliseconds — affect clarity and stereo imaging, and they barely move the RT60 figure.

It also says nothing about background noise, about how the response varies from seat to seat, or about how much sound is escaping to the room next door. Those are separate measurements, and in many rooms they matter more than the decay time.

How to use it properly

Treat RT60 as one instrument on the dashboard. Measure it by frequency band rather than as an average, at the positions people actually use, and compare it against a target chosen for the room's size and purpose.

Then measure it again after the work. A before-and-after pair of curves shows what the treatment changed, which is something no amount of description can do.