Frequency is a measure of how many times a given event happens in a given period of time. It is directly related to wave mechanics, and is the reciprocal of period. The SI unit of frequency is the hertz (Hz), which corresponds to a single cycle per second, giving it the general unit of s−1. Frequency cannot be modified by a physical process, although phenomena such as the Doppler Effect can change how frequency is perceived by a person in motion towards or away from an audio source.