Doppler Effect

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Doppler Effect
Helena Lee

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Background
Doppler Effect is named
after Austrian Physicist
Christian Doppler (1803-
1853)

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What is it?
 Imagine standing on the side of the road and watching cars go by
 You’ll notice that as the car is approaching, the frequency of the
noise generated is higher and as it passes by, it shifts to a lower
frequency
 This is the phenomenon known as the Doppler Effect
 If there is relative motion between the source of the sound and the
receiver of the sound, the frequency at the receiver is different from
the frequency that is transmitted
 If the two are moving toward each other, the received frequency is
higher, and if they are moving apart the received frequency is lower

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Key equation
f(0) is the frequency at the source and f’ is the
frequency at the receiver.

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 The above equation allows for consideration of all 3 scenarios:
 Stationary source, moving receiver
 Stationary receiver, moving source
 Moving source and receiver
 There is a ± in the numerator and a in the denominator
because if the receiver is moving toward the source, we observe
an increased frequency and use the top sign in the numerator.
 If the source is moving toward the receiver we also observe an
increase in the frequency and use the top sign in the
denominator
 In both cases, if the motion is toward, we use the top sign
 Basically the signs are there because the equation
accommodates all 3 scenarios

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Check Your Understanding
Japan has high speed bullet trains also known
as Shinkansen trains can travel up to
320km/hr. If the frequency perceived by
someone standing at the train station is 872
Hz as the train approaches the station, what is
the frequency of the whistle on the train?

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What do we know?
 Speed of train: 320km/hr
 The person (receiver) is stationary thus the speed is zero
 The speed of the sound (medium: air) is approximately 343m/s
 The frequency of the receiver is 872 Hz
 Solve for frequency of source

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Solution
1. Convert km/hr  m/s
2. 320km/hr 88.9m/s
3. Plug everything into formula to get

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Answer
645.99
Thus the frequency of the
train whistle is 646.0 Hz