### Doppler 2 Background

Active

(Click the question to get the answer.)

1. What is meant by the Doppler Effect?
This is the effect where a listener hears a frequency different from the actual source frequency. This happens because the source and listener are moving relative to each other.

The car is the source and emits a siren frequency of 400Hz. But because the car is also moving towards the stationary man (listener), the man hears a different frequency. This effect is called the Doppler Effect.
Audio 1

2. How does the frequency that the listener hears change :
answer the following questions

2.1 if the car approaches him?
A higher frequency would be heard by the listener. As the distance between the source and listener decreases, there is a kind of "concentration" of waves, causing an increased frequency.
Audio 2.1

2.2 if the car moves away from him?
The listener will hear a decrease in frequency. This is because now there is a "dilution" of waves.
Audio 2.2

3. Write down the Doppler Equation.
4. What does each variable in the Doppler Equation mean?
Listener
fL = frequency the listener hears
vL = speed at which listener moves

Source
fs = frequency the source emits
vs = speed at which source moves

v = speed at the sound moves

Audio 4

5. How does the speed of the source affect the speed of the sound?
The speed of the source (eg a car) has no affect on the speed of the sound that it makes.
Assume a car was at rest and the siren emits a sound that moves at 330m.s-1.
The sound would still move at 330m.s-1 even if the car had been moving. It is only the frequency of the sound that gets affected.
Audio 5

6. A police car moves towards a listener at a speed of 20m.s-1.
The police sounds the siren at a frequency of 400Hz. The speed of the sound is 330m.s-1.
Calculate the frequency at which the listener will hear the siren.

7. A police car moves away from a listener at a speed of 30m.s-1.
The police sounds the siren at a frequency of 420Hz. The speed of the sound is 340m.s-1.
Calculate the frequency at which the listener will hear the siren.

8. A man runs at 5m.s-1 towards a police car which is approaching him at 10 m.s-1. The police siren emits a frequency of 450Hz.
Calculate the frequency at which the man will hear the siren if the speed of the sound is 340m.s-1.