The rate at which a repetitive signal repeats, measured in cycles per second or Hertz (or its convenient multiples). Audio frequencies are generally in the range of 20 to 20,000 Hertz because those are the frequencies to which the human ear sensibly responds. Equipment measurements often extend this range to 0 to 100,000 Hertz. Video frequencies typically deal with a range of 0 to 7 MHz because that is the bandwidth available for PAL and NTSC signals. The human eye responds to light in the frequency range of 385,000 GHz to 789,000 GHz. The frequency of a signal can be calculated by dividing the speed with which the signal propagates through a medium by its wavelength.