Envelopes shape the sound.
Each musical instrument has a distinctive envelope .
This is described as ADSR=Attack, Decay, Sustain, Release.
Attack is the initial onset of the tone when the instrument begins to vibrate.
Decay is the decrease in amplitude immediately after the attack.
Sustain is the amplitude during the middle portion of the envelope just prior to its release.
Release is when the instrument stops producing a sound - the time it takes the instrument to stop vibrating or come to a state of rest.
ADSR describes the simplest four points of an envelope shape.
Trapezoidal envelopes are typical of instruments in which air is blown through a pipe such as wind or brass instruments.
Rectangular envelopes are typical of mechanical devices, like organ pipes opening and closing.
Triangular envelopes are typical of plucked or struck instruments like pianos, guitars and dulcimers.