+

Doppler Effect

Helena Lee

+Background

Doppler Effect is named

after Austrian Physicist

Christian Doppler (1803-

1853)

+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

+Key equation

f(0) is the frequency at the source and f’ is the

frequency at the receiver.

+ 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

+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?

+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

+Solution

1. Convert km/hr m/s

2. 320km/hr 88.9m/s

3. Plug everything into formula to get

+Answer

645.99

Thus the frequency of the

train whistle is 646.0 Hz