the time it takes for a sound made in a room to diminish by 60 decibels.
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British Dictionary definitions for reverberation time
a measure of the acoustic properties of a room, equal to the time taken for a sound to fall in intensity by 60 decibels. It is usually measured in seconds
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012