Burglar Alarm Bell

In which case will the Doppler Affect cause a listener to hear a higher frequency?

a. a listener stands on a street corner and listens to a car engine moving away from her.
b. a listener is driving away from a stationary factory whistle as it blows.
c. a listener on a bicycle is moving toward a stationary burglar alarm bell as it is ringing,
d. a listener stands on a street corner listening to a policeman standing in the intersection blowing a whistle.

The answer is C because as the bicyclist is moving towards the the stationary burglar alarm they receive an increasing frequency which eventually decreases again as he moves away. In option D both are stationary so that wouldn’t work. A and B hear a decreasing frequency.

Inside intruder alarm bell box.mp4

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