Question

What frequency (in Hz) is received by a person watching an oncoming ambulance moving at 118 km/h and emitting a steady 850 Hz sound from its siren? The speed of sound on this day is 345 m/s.

What frequency (in Hz) does she receive after the ambulance has passed? Hz

Answer 1

here,

(a)

Speed of the wave, c = 345 m/s

Speed of the person (receiver), r = 0 m/s

Speed of the ambulance (source), s = -118 km/h = -118 × 1000 / 3600 m/s = -32.8 m/s

By Doppler effect, frequency received by the person, f = [(c + r)/(c + s)] f0

f = [(345 + 0)/(345 - 32.8)] * 850 Hz

f = 939 Hz

(b)

Speed of the wave, c = 345 m/s

Speed of the person (receiver), r = 0 m/s

Speed of the ambulance (source), s = 118 km/h = 118 × 1000 / 3600 m/s = 32.8 m/s

By Doppler effect, frequency received by the person, f = [(c + r)/(c + s)] f0

f = [(345 + 0)/(345 + 32.8)] * 850 Hz

f = 776.2 Hz