The frequency of a sound is how high or low it sounds. The non-technical and musical term for frequency is 'pitch'. A high pitch sound is a high frequency sound and a low pitch sound is a low frequency sound. Frequency is the number of sound wave cycles per second and is measured in Hertz (Hz). 1Hz = 1 cycle per second. Human beings can hear a frequency range of sounds from 20Hz to 20,000Hz (20kHz) although we lose the ability to hear higher frequencies as we get older.