EDUCATION

RT60 Explained: How Long Should Sound Decay in a Room?

What reverberation time tells you, what it misses, and how to use it without chasing one perfect number.

RT60 is one of the most useful ways to describe how a room holds on to sound. It is also remarkably good at producing a tidy number that looks more definitive than it really is. Used properly, it can show where a room is too lively, too dry or uneven across the frequency range. Used alone, it can send you off in pursuit of a decimal place while the bass continues doing whatever it likes.

The aim is not to make every room silent. It is to create a decay that suits the room's purpose and remains reasonably balanced from bass through to treble.

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What does RT60 mean?

RT60 is the time it takes for sound in a room to decay by 60 decibels after the source stops. A room with an RT60 of 0.3 seconds loses its reflected energy quickly. A room at 1.2 seconds continues to carry sound for much longer, so speech, music and new sounds overlap more heavily.

In practice, a full 60dB decay is not always available above the background noise. Measurement software often analyses a shorter part of the decay and extrapolates it. T20 uses a 20dB section, T30 uses 30dB, and early decay time looks at the first 10dB. You do not need to memorise the alphabet soup, but it explains why two readouts may differ slightly without either being broken.

Reverberation time also changes with frequency. A room can decay quickly above 1kHz while low-frequency energy rings for much longer. One broadband average can hide that imbalance completely.

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What is a good reverberation time?

There is no universal target because room volume and purpose matter. A vocal booth, mixing room, living room and restaurant should not all sound alike. Smaller critical-listening spaces generally need a shorter, more controlled decay, while a music room may retain a little more life.

These are broad starting points, not pass marks. A larger room can support a longer decay more comfortably than a tiny one. More important than landing on exactly 0.35 seconds is avoiding sharp changes between neighbouring frequency bands and very long low-frequency tails.

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Why small rooms make RT60 complicated

Traditional reverberation time assumes sound has become reasonably diffuse, with reflections arriving from many directions. Small studios and listening rooms do not behave that neatly, especially at low frequencies. Individual room modes dominate, so one bass note may decay rapidly while the next hangs around.

This means the low end is better understood by looking at frequency-specific decay plots, such as a waterfall or spectrogram, alongside the RT readout. The measurement position matters too. Move the microphone and a modal peak or null can change dramatically.

RT60 is still useful in a small room, particularly through the mid and high frequencies. It simply needs to be treated as one part of the evidence rather than the acoustic equivalent of a blood test.

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How to measure decay sensibly

A clap test is a good first check for flutter echo and obvious liveliness, but it cannot show decay by frequency. For a more useful measurement, use an omnidirectional measurement microphone, room-analysis software and a swept test signal through the loudspeakers.

Measure around the listening or working area rather than trusting one position. Keep the room in its normal state, including the furniture that will remain there, and make sure the background noise is low enough for a clean decay. Repeat the sweep so an interrupted measurement or passing lorry does not become part of the acoustic design.

Phone apps can indicate whether a room is broadly lively or controlled, but the microphone, automatic processing and limited dynamic range make them less reliable for detailed decisions. They are useful for curiosity, not precision surgery.

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Treat the imbalance, not just the number

Broadband panels reduce mid and high-frequency decay and control early reflections. Deeper wall traps and corner bass traps absorb further down, helping shorten resonant low-frequency tails. Ceiling clouds add useful coverage without occupying more wall space.

The common mistake is to keep adding thin absorption because the high-frequency RT number responds quickly. The room becomes dull at the top while the bass remains slow and uneven. Treatment depth, placement and total coverage need to work together.

Measure before treatment, make a sensible change, then measure again under the same conditions. The most encouraging result is not necessarily the lowest line on the graph. It is a smoother decay that supports the room's job and matches what you hear.