Time after which the amplitude is only 37% of the original.
Sampling rate.
Note: Using fast_exp yield a decay-factor within -180 dB (at 384000hz) of the one obtained with expm1. The alleged "innacuracies" of plain exp just did not show up so we don't prefer expm1 anymore. It doesn't change the length of an iterated envelope like ExpSmoother.
Multiplier for this RC time constant and sampling rate.
Gets a factor for making exponential decay curves, which are the same thing as a 6dB/oct lowpass filter.