Question

The sound from the whistle of a truck is 2500 Hz. The truck is moving at 30.0 m/s toward a building, which reflects the sound and is heard by the driver of the truck. As heard by the driver of the truck, how much does the frequency change from the original frequency of the whistle? The speed of sound is 340 m/s.

Answer 1

Using Doppler effect,

first when truck run towards stationary building, then frequency reached at wall will be

When Source is moving towards stationary observer, then

f1 = f0*V/(V - Vs)

V = Speed of sound = 340 m/sec

Vs = Speed of source(truck) = 30.0 m/sec

f0 = frequency of sound source = 2500 Hz

So,

f1 = 2500*340/(340 - 30) = 2741.9 Hz

Now this frequency f1 will be reflected by wall and heard by you, then

Now when Source is stationary and object is moving towards the source

Now you are observer and wall is source, So

f2 = f1*(V + Vo)/V

Vo = Speed of observer = 30 m/sec

So,

f2 = 2741.9*(340 + 30)/340 = 2983.8 Hz

frequency heard by driver after it's reflected off the wall = 2983.8 Hz

frequency change from the original frequency of the whistle = df = f2 - f0 = 2983.8 - 2500 = 483.8 Hz

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