Final answer:
The release or decay phase represents the final decay of sound in an instrument's sound envelope. It's marked by a decrease in sound wave amplitude and is part of what gives an instrument its unique timbre.
Step-by-step explanation:
The part of the envelope of an instrument that represents the final decay of sound when the note has stopped being played is called the release or decay phase. This is one of the four stages in the sound envelope, known by the acronym ADSR, which stands for Attack, Decay, Sustain, and Release. During the release phase, the sound waves undergo a decrease in amplitude, which results in a gradual diminishment of the sound until it is no longer audible.
Musical instruments emit sound waves with varying qualities that contribute to their unique timbre. For instance, a violin and a piano can play the same note, yet they will sound distinct due to their different overtone structures and the way sound waves resonate in their respective physical forms. The characteristic resonance and decay patterns of sound are an integral part of an instrument's distinctive timbral qualities.