Reverberation time (RT60)
How long it takes sound in a room to decay by 60 dB after the source stops – the single most quoted number in acoustics.
It goes back to Wallace Clement Sabine, who around 1898 was asked to fix a dreadful lecture hall at Harvard and worked out that reverberation time grows with a room's volume and shrinks with its absorption. His formula, RT60 ≈ 0.161·V/A, is still the first estimate anyone reaches for. A concert hall sits around two seconds, a living room around half of one.
A full 60 dB is rarely measurable in practice, since background noise gets in the way first. So the decay is measured over 20 or 30 dB and extrapolated, as ISO 3382 prescribes. In reverb plug-ins the parameter is usually just labelled decay or time, and it is RT60 – whether or not the simulated room could actually be built.