The frequency of a repeated event is its number of instances per unit of time. It differs from angular frequency, and it sometimes implies temporal frequency for clarification. One event occurs every second, or one hertz (Hz) when measuring frequency. The time elapsed between events is measured by the period, which is the inverse of the frequency. For instance, if the reading of a heartbeat is 120 times per minute (2 hertz), the period, T, the space between beats, is half a second, i.e., 60 seconds divided by 120 beats. The frequency of vibratory and oscillatory phenomena, including audio signals (sound), mechanical vibrations, and light and radio waves, is an important criterion in science and engineering.