Final answer:
The attack phase is the part of an instrument's sound envelope that describes the time taken for the initial buildup of sound, contributing to the instrument's unique timbre.
Step-by-step explanation:
The part of the envelope of an instrument that refers to the time taken for the initial buildup of sound is known as the attack phase. This is a crucial aspect of an instrument's timbre, which is the character or quality of a musical sound distinct from its pitch and intensity. The attack phase, along with decay, sustain, and release, forms the complete sound envelope of a note. Instruments of different sizes produce varying sounds - for example, a small instrument like a tambourine will have a higher pitch due to higher frequency and shorter wavelength, while a bigger instrument like a drum will render a lower pitch with lower frequency and longer wavelength. The distinct timbre of each instrument also arises from the combination of fundamental and overtone frequencies, which can be affected by factors such as the instrument's material, design, and external conditions like temperature.