Function envelope_distance 

pub fn envelope_distance(a: &ClipMetrics, b: &ClipMetrics) -> f64

The envelope distance of two clips, in dB.

It is the mean absolute level difference over the frames where either envelope lies within ENVELOPE_RANGE_DB of the loudest frame of both. A frame below that range, or past the end of the shorter clip, counts at the bottom of the range, so a longer tail adds to the distance. The frames align at the two onsets.