A fire engine is driving directly away from a solid wall at 35 m/s on a hot 30.0

Question

A fire engine is driving directly away from a solid wall at 35 m/s on a hot 30.0o C day while its siren emits a steady 465. Hz sound. The speed of sound on this day is 349 m/s.

a) Compute the frequency heard by a stationary listener standing near the wall.

b) Compute the wavelength of the waves as they approach the stationary listener.

c) A firefighter on the back of the fire engine hears the sound waves after they are reflected from the wall. What is the frequency of this reflected wave as heard by this firefighter?

Explanation / Answer

let us use the expression n' = [c - vo / c - vs ] * n

Here source will be at origin and observer is on positive X axis. vo - speed of observer and vs - speed of source

along positive X direction. c- velocity of sound

In this problem,

c = 349 m/s,

vo = 0 ,

vs = - 35m/s

n' = [c - vo / c - vs ] * n

Hence n' = 422.61Hz

So

lambda' = 349/422.61= 0.825 m

Relative speed of the listener with the source is 66 m/s
So

n'' = [349 / 415] * 465 = 391.04Hz

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