Final answer:
When holding a note on an instrument, the Sustain part of the envelope changes. It is the level of volume that should remain constant during the note's duration, but natural damping may cause it to decrease over time.
Step-by-step explanation:
When a note is being held out on a musical instrument, the part of the instrument's envelope that changes is 3) Sustain. The envelope of a musical note consists of four parts: Attack, Decay, Sustain, and Release. The Attack is the initial burst of sound when a note is first played; the Decay is the drop in volume immediately following the Attack; the Sustain is the level during which the note is held; and the Release is the diminution of sound as the note is let go. When a note is sustained, this part of the envelope remains constant, reflecting the steady state of volume as long as the note is held.
However, even during Sustain, some instruments exhibit a slight envelope change due to natural dampening effects, but in terms of the envelope parameters, Sustain indicates the period where no intended change in volume occurs. If we pluck a banjo string, as the note is held, the amplitude of the sound waves decreases due to energy dissipation—a process called damping. This results in the volume of the note gradually getting softer, even though the Sustain part of the envelope is designed to hold the sound steady.