Given an input T, returns a number X such that, after T multiplications of a signal by X, that signal would be attenuated by 60 decibels. Roughly, -60db = 0db * pow(X, T). This could be used as a per-sample multiplier (X) to ensure a decay time (of T samples), for example. The name t60 is borrowed form the RT60 time used to measure reverberation time, which specifies the time taken for a signal to decay by 60db, as an approximation of fading to inaudibility.