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Frequency is measured by velocity of a traveling wave as given by:

v = D/t

where v is velocity, D is distance traveled, and t is time elapsed.  Suppose, for example, it takes an acoustical wave 2 seconds to travel the length of a single cycle D, resulting in the equation v = D/2.  Recall that the wavelength is the distance between successive crests and troughs (or between two consecutive troughs as shown above and labeled with the term "1 cycle").  The time taken to travel the distance of one "cycle" of a wave is referred to as the period T.  The period T of a wave is usually measured in seconds.  So, going back to the above equation, the time variable t has the value T, which is the period of the waveform:

v = D/T

Now, since T is defined as the time taken for a completion of one cycle, it follows that the number of complete cycles per second will be 1/T.  In other words, the velocity at which a wave travels the distance of a single periodic cycle is:

v = 1/T

and is called the frequency of the wave.