Explain how the frequency of the sound waves is been calculated

If the sound is reflected from a moving object then the frequency will be shifted higher or lower depending if the object is moving towards or away from the observer. This is the Doppler Effect.

I have assumed that the observer emits the sound and is not moving. If the observer is moving towards the reflector or away the frequency will also be shifted higher or lower.

If for the example the observer is emitting sound and moving away from the reflector then the frequency of the sound reflected back will be lower. To calculate this there are two steps. First, the frequency, f received by the stationary reflector needs to be calculated with the doppler formula

f=cc+vsf0

where f0 is the original frequency, c is the speed of sound in air and vs is the speed of the source moving away from the reflector.

Now the frequency, f′, observed by the moving observer is given by

f′=c−vocf

where vo is the speed the observer is moving away from the reflector.

Now putting the two equations together and remembering that the observer is the source of the sound (so that vs=vo) we get

f′=c−vscf=cc+vsf0

f′=c−vsc+vsf0

Frequency is the number of oscillation per second. It’s SI unit is Hertz(Hz).

Frequency= 1/ Time period