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 convenience, the human frequency range is divided into three rough areas or bands. High frequencies (between about 5 kHz and 20 kHz), mid frequencies (between about 200 Hz and 5 kHz) and low frequencies (between about 20 Hz and 200 Hz).
Web Links 
