Frequency of a Sound Wave
Most sound generators produce recurrent waves which are generally similar to each other. These waves are propagated at a definite velocity. This velocity depends on the medium of propagation.

One cycle of a sound wave in air, consists of one compression of the air together with the subsequent rarefaction that occurs. The air molecules are forced together (compression or compaction) and then subsequently (in accordance with the 2nd law of thermodynamics) they immediately begin returning to their equilibrium state. The equilibrium state of the air molecules is the state in which they were before the compression under observation occurred. (Always taking into account that other disturbances of the atmosphere may have been occurring simultaneously with this compression.) In doing so they acquire momentum and thus become compressed again and so on.

Definition of Frequency
Frequency is the number of complete waves or oscillations or cycles of a periodic quantity occurring in unit time (usually 1 second).

Note the difference between Frequency and Pitch. Frequency is a measure of the rate of disturbance whilst pitch is what our heads do with this phenomenon. In defining frequency, note the fundamental reliance on the concept of time.

Wavelength of a Sound Wave
The wavelength of a sound wave is the distance the sound travels to complete one cycle. The symbol used to denote wavelength is the Greek letter lambda ().

Velocity of Propagation of a Sound Wave
The preceding examples have shown that a sound wave travels with a definite finite velocity. The actual velocity depends on the medium through which the wave is travelling. In fact the following can be observed to be true:

V light wave >> V radio wave< V sound wave >V water wave >> V earth's rotation

Since a wave advances a distance of one wavelength in a time interval of one period, it follows that the velocity of a wave is given by

but since T = 1/f, we can write

Now we know that the frequency of a sound wave is the number of cycles that pass an observation point per second. Thus the velocity of propagation of a sound wave is its wavelength times its frequency.

v = f or

where v = velocity of propagation, in centimetres per second
= wavelength, in centimetres
f = frequency, in Hz.

The frequency of a wave is independent of the waves' medium, however, the wavelength will depend on the wave velocity in the medium through which it is travelling.