An ambulance has a siren of frequency 880 Hz. You are sitting at a stop light as the ambulance passes you, traveling at a constant 22 m/s. If the speed of sound is 343 m/s, what is the change in the frequency that you hear as the ambulance passes you?

A :-60 Hz
B :-53 Hz
C :-113 Hz
D :-56 Hz
E :-116 Hz

B. 53 Hz

Explanation:

The change in frequency of the waves like light waves, sound waves etc. due to the change in the positions of the source and/or observer is known as doppler effect.

The equation of doppler shift is:

f₁ =

Here f₁ is apparent frequency heard by the observer, v is speed of sound, u is speed of the observer, s is speed of source and f is the actual frequency.

Put 343 m/s for v, 0 m/s for u, 22 m/s for s and 880 Hz for f in the above equation.

f₁ =

f₁ = 827 Hz

The apparent frequency is less than the actual frequency as the source is moving away from the observer. That's , why there is a plus sign in the denominator of the equation.

Change in frequency = f - f₁

                                   = (880 - 827) Hz

                                   = 53 Hz

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