The resonant characteristics of an acoustic sound generator. For example, the distinguishing characteristics of the vowel sounds of a human voice, as determined by that person’s physical characteristics; what makes each voice sound unique. These characteristics are actually emphasized frequency bands, and are relatively fixed in frequency despite the pitch of the voice changing.
Like the human voice, a musical instrument also has a fixed set of formants, which give it a unique, recognizable tonal color or timbre. It is this set of formants that allow us to recognize an instrument regardless of the pitch it is playing; the tonal color remains relatively static.
This brings to light one of the potential problems with samplers: When you digitally transpose a sound up or down, you are also transposing the formants associated with that sound, giving us the infamous “Jolly Green Giant” or “chipmunk” effect. When you transpose a vocal sample, you are essentially changing the size of that person’s head (something they might not appreciate…)!
Some devices, such as Digitech’s Vocalist series of pitch transposers, attempt to control formants when changing pitch, resulting in a more natural sounding transposed note.