A filter cutoff envelope mimics the brightness-decay of acoustic instruments

Acoustic instruments are brightest at the moment of excitation — a struck piano string has more high-frequency energy at the instant of the hammer blow than seconds later. A filter cutoff envelope replicates this by driving the cutoff frequency of a low-pass filter: at note-on the cutoff opens (brighter), then closes (darker) as the envelope progresses. The cutoff envelope is typically expressed as a percentage of the user’s base cutoff setting (1 = 100%), not an absolute frequency, so cutoff tracks pitch and high notes are not silenced. A contour-amount control limits the sweep range (e.g. 80–100%) to avoid the repetitive sound of always starting from 0%.

Examples

On a sawtooth with a low-pass filter: set envelope attack=0, decay=300ms, sustain=20%, release=200ms and route it to cutoff with moderate amount. The note opens bright and closes dark — mimicking a picked bass or plucked synth string.

Assessment

Explain why a cutoff envelope is specified relative to the user’s cutoff (a percentage) rather than as an absolute frequency. What does the contour-amount control adjust?