In audio, the number of repeating cycles of change in air pressure or oscillations in voltage, that occur in one unit of time usually a second. Complex sounds are made up of many pure tones of different frequencies. Measured in units originally called cycles per second (CPS), now called Hertz (Hz).

For comparison, the human hearing has a frequency range, roughly between 20 Hz and 20 kHz (20.000 Hz).

(see also sonogram)

Alternative form for frequency : kHz.