The A above middle C normally has a frequency of 440 cycles per second, or Hertz. This means that it vibrates 440 times per second. An A, one octave higher has a frequency of 880 Hz., exactly twice as many vibrations per second. The mathematical expression of this relation is 880:440 or 2:1. The following table shows the mathematical relationship of several intervals, ordered from consonant to dissonant.
See Intervals in the Tutorial section.