Multiplier for this RC time constant and sampling rate. Use it like this: smoothed += (v - smoothed) * result;
Note: Using fast_exp yield a decay-factor within -180 dB (at 384000hz) of the one obtained with expm1. The alleged inaccuracies of plain exp just did not show up so we don't prefer expm1 anymore. This doesn't change the length of an iterated envelope like ExpSmoother. Actually, small variations of a decay factor often results in a misleadingly large RMS difference, that doesn't actually change the sound quality.
Gets a factor for making exponential decay curves, which are also the same thing as a 6dB/oct lowpass filter.