Note (music)

(PD) Image: John R. Brews
The amplitude of a musical note varies in time according to its sound envelope.

In music, a note is an abstract representation of the pitch and duration of a tone. The pitch designated by a note is objective only in the case of a simple tone (also called a pure tone) such as produced by a tuning fork, which consists of only a single frequency of vibration, in which case the pitch is uniquely related to that frequency at a given loudness. A musical instrument on the other hand, produces a tone, which is a superposition of various frequencies with various amplitudes and phases peculiar to the instrument, and also affected by the manner of play that determines the sound envelope of the note (referred to by Lamb below as "adventitious circumstances"). To quote Lamb:

One musical note may differ from another in respect of pitch, quality, and loudness. The pitch is usually estimated as that of the first simple-harmonic vibration in the series, viz. that of lowest frequency, but if the amplitude of this first component be relatively small, and especially if it fall near the lower limit of the audible scale, the estimated pitch may be that of the second component. By "quality" is meant that unmistakable character which distinguishes a note on one instrument from the note of same pitch as given by another...difference of quality, so far as it is not due to adventitious circumstances, can only be ascribed to differences of vibration-form, and so to differences in the relative amplitude and phases of the simple-harmonic constituents.