An abbreviation for Reverb Time -60dB. It is an expression used to more specifically state what a given reverb (see WFTD archive Reverb) time is. The reverberation decay time in a large empty concert hall may be as much as 15 to 20 seconds. This means that in practice it takes that long for the reverberations of a sound to decay into the ambient noise of the hall (which includes thousands of ongoing reverberations). Raise the ambient noise level in the hall by 20 dB (perhaps by turning on a heating system) and the reverb time will sound shorter because as it decays it gets lost in the sound of the heating system. The purpose of the RT60 specification is to provide an objective measure of reverb time. The spec says that reverb time is defined as the time it takes the reverb to go down in volume by 60 dB, or to 1 millionth of the original volume.