Frequency

The rate of repetition of the cycles of a periodic quantity, such as a sound wave. Thus, frequency is the inverse of the Period. More generally, frequency can be thought of as the rate of change of phase.

See also: Click, Law of Uncertainty.

Frequency is denoted by the symbol f, and is measured in hertz (Hz) - formerly called cycles per second (cps or c/s) - kilohertz (kHz), or megahertz (mHz).

See diagrams under radio spectrum, simple harmonic motion, spectrum.

The only sound which consists of a single frequency is the pure sine tone such as produced by a sine wave oscillator or approximated by a tuning fork. All other sounds are complex, consisting of a number of frequencies of greater or lesser intensity. The frequency content of a sound is its spectrum.

See: Complex Tone, Critical Bandwidth, Eigenton, Formant, Fundamental, Harmonic, Inharmonic, Interval, Octave, Partial, Resonance, Sideband, Sympathetic Vibration, Timbre.

The ear can hear all frequencies from approximately 20 to 20,000 Hz, this often being called the audible range, or range of hearing (see audio frequency). The frequency response of the healthy ear is documented by the Equal Loudness Contours. Persons with hearing loss due to age (presbycusis) or other causes have a reduced sensitivity to usually the high frequencies. See diagram under audiogram.

Compare: Infrasonic, Ultrasonic, Vibration.

The subjective sense of frequency is called pitch. That is, frequency is an acoustic variable, whereas pitch is a psychoacoustic one. For the relation between musical pitches and frequency, see Appendix C.