The Doppler Shift, when associated with sound, is the change in frequency of a source as it moves: the frequency will appear to increase as the source comes towards a listener and will appear to decrease as the source moves away from a listener. (This formula is also used to calculate the motion of stars.)
f = fs (v + vL)/(v – vs)for sound
f = frequency heard by listener
fs = frequency of the source
v = velocity of sound
vs = velocity of the source
(positive if moving towards listener, negative if moving away from listener)
vL = velocity of listener
(positive if moving toward the source, negative if moving away from the source)
Doppler Shift Formula Questions:
1) An ambulance has a velocity of 50 m/s and its siren produces a steady frequency of 250 Hz. What is the frequency of sound heard by an observer who is in front of the ambulance, assuming the velocity of sound equals 343 m/s ?
Answer: The velocity of sound, v = 343 m/s, and the velocity of the ambulance, the source, vs = 50 m/s. The frequency of the source, fs = 250 Hz. The velocity of the listener is vL = 0. Use the formula.
f = fs (v + vL)/(v – vs)for sound
f = (250 Hz)(343 m/s + 0 m/s) / (343 m/s – 50 m/s)
f = (250 Hz)(343 m/s)/293 m/s
f = (85750 Hz m/s)/ 293 m/s
f = 292.66 Hz
Note, the frequency heard by the listener is higher than that actually being emitted by the ambulance.
2) An ambulance comes to a stop, but its siren is still on with a steady frequency of 250 Hz. What is the frequency of sound heard by an observer who driving away from the ambulance at 50 m/s, assuming the velocity of sound equals 343 m/s ?
Answer: Because the listener is moving away from the ambulance, his velocity, vL = – 25 m/s. The velocity of the ambulance, vs = 0.
f = fs (v + vL)/(v – vs)for sound
f = (250 Hz) (343 m/s – 50 m/s)/(343 m/s – 0)
f = (250 Hz)(293 m/s) / (343 m/s)
f = 73250 Hz m/s / 343 m/s
f = 213.56 Hz
Note: the sound heard by the driver is at a lower frequency than that actually being emitted by the ambulance.