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Pitch is the property of a sound that allows the construction of melodies; pitches are compared as "higher" and "lower", and are quantified as frequencies (cycles per second, or hertz), corresponding very nearly to the repetition rate of sound waves.
The perceived pitch of a sine wave is directly related to its frequency. More complex sounds also have pitch, notably speech and musical notes. Many such sounds are approximately periodic, and the perceived pitch is directly related to the period. They also have a spectrum that is (approximately) a stack of harmonics, and the perceived pitch is related to the harmonic spacing. The lowest harmonic in the stack is called the fundamental frequency, and its frequency is also strongly correlated with the pitch, though a strong pitch may be perceived even when the fundamental is missing.
Other complex sounds may have several pitches. A complex tone composed of two sine waves of 1000 and 1200 Hz will have three pitches. Two spectral pitches at 1000 and 1200 Hz, derived form the physical frequencies of the pure tones; and one "virtual pitch" at 200 Hz, derived from the repetition rate of the waveform. Sounds that do not have a period may also have a pitch, often dominated by the near-periodicity of the envelope or fine structure of the waveform.
Historically, the study of pitch perception, and especially of pitch in the case of the missing fundamental, has been a central problem in in psychoacoustics, and has been very instrumental in forming and testing theories of sound representation, processing, and perception in the auditory system.
Some theories of pitch perception hold that pitch has inherent ambiguities, and therefore is best decomposed into a pitch chroma, a periodic value around the octave, like the note names in western music, and a pitch height, which may be ambiguous, indicating which octave the pitch may be in.
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