Pitch

For example, middle C in equal temperament = 261.6 Hz

Sounds may be generally characterized by pitch, loudness, and quality. The perceived pitch of a sound is just the ear's response to frequency, i.e., for most practical purposes the pitch is just the frequency. The pitch perception of the human ear is understood to operate basically by the place theory, with some sharpening mechanism necessary to explain the remarkably high resolution of human pitch perception.

The place theory and its refinements provide plausible models for the perception of the relative pitch of two tones, but do not explain the phenomenon of perfect pitch.

The just noticeable difference in pitch is conveniently expressed in cents, and the standard figure for the human ear is 5 cents.

Details About Pitch

Although for most practical purposes, the pitch of a sound can be said to be simply a measure of its frequency, there are circumstances in which a constant frequency sound can be perceived to be changing in pitch.

One of most consistently observed "psychoacoustic" effects is that a sustained high frequency sound (>2kHz) which is increased steadily in intensity will be perceived to be rising in pitch, whereas a low frequency sound (<2kHz) will be perceived to be dropping in pitch. (More detail)

The perception of the pitch of short pulses differs from that of sustained sounds of the same measured frequency. If a short pulse of a pure tone is decaying in amplitude, it will be perceived to be higher in pitch than an identical pulse which has steady amplitude. Interfering tones or noise can cause an apparent pitch shift.

Further discussion of these and other perceptual aspects of pitch may be found in Chapter 7 of Rossing, The Science of Sound, 2nd. Ed.

Effect of Loudness Changes on Perceived Pitch

A high pitch (>2kHz) will be perceived to be getting higher if its loudness is increased, whereas a low pitch (<2kHz) will be perceived to be going lower with increased loudness. Sometimes called "Stevens's rule" after an early investigator, this psychoacoustic effect has been extensively investigated.

With an increase of sound intensity from 60 to 90 decibels, Terhardt found that the pitch of a 6kHz pure tone was perceived to rise over 30 cents. A 200 Hz tone was found to drop about 20 cents in perceived pitch over the same intensity change.

Studies with the sounds of musical instruments show less perceived pitch change with increasing intensity. Rossing reports a perceived pitch change of around 17 cents for a change from 65 dB to 95 dB. This perceived change can be upward or downward, depending upon which harmonics are predominant. For example, if the majority of the intensity comes from harmonics which are above 2 kHz, the perceived pitch shift will be upward.

Perfect Pitch

"Perfect pitch" or "absolute pitch" refers to the ability of some persons to recognize the pitch of a musical note without any discernable pitch standard, as if the person can recognize a pitch like the eye discerns the color of an object. Most persons apparently have only a sense of relative pitch and can recognize a musical interval, but not an isolated pitch.

Rossing suggests that less than 0.01% of the population appear to be able to recognize absolute pitches, whereas over 98% of the population can do the corresponding visual task of recognizing colors with no color standard present.

See Rossing, 2nd Ed, p122, sec 7.7