Envelope
class Envelope(attackPerSecond: Float = 14.0f, releasePerSecond: Float = 2.4f, initial: Float = 0.0f)
Rises fast and falls slowly, which is what the bars in every music player do.
Both rates are in units per second rather than per frame, so it looks the same at 60 Hz and at 120 Hz. That matters more than it sounds: rates per frame would make it twice as twitchy on a fast display.