Doppler Notes 3

Questions
1. The Doppler Effect is the apparent change in:
A. amplitude
B. frequency
C. speed
D. direction
B. frequency

2. Frequency is the:
A. maximum amplitude of a sound wave
B. number of vibrations per second
C. duration of each and every successive wave
D. distance between two consecutive points in phase
B. number of vibrations per second

3.1. What is meant by the Doppler Effect?
3.2. Does the Doppler Effect occur for light waves?
3.3. Does the Doppler Effect occur for longitudinal waves?
3.1. The Doppler Effect is the apparent variation in frequency of any emitted wave, such as a wave of light or sound, as the source of the wave approaches or moves away, relative to an observer.
3.2. Yes. the Doppler Effect occures for all types of waves.
3.3. Yes, the Doppler Effect occurs for both transverse waves and longitudinal waves.

4. Write down the Doppler Equation for sound waves, and state what each symbol means.

fL is the apparant frequency, what you actually hear (Listener)
v is the speed of the sound
vL is the speed of the listener
vs is the speed of the source (the object making the sound)
fs is the actual frequency of the sound

5. A police-car is moving towards you with its siren on.
5.1. Would you expect to hear a higher or lower frequency of sound? Explain
5.2. Would the wavelength of the sound that you hear be greater or lesser than at the source?
5.1. Higher frequency
5.2. Lesser - since the speed of the sound does not change,
from v = f λ , if f has increased, λ must decrease.

6. A police car is stationary some distance away from a stationary listener. The police sounds the siren at a frequency of 400Hz. The speed of the sound is 335ms-1 .

6.1. What is meant by the term frequency of 400Hz?

6.2. At what frequency will the listener hear the siren?

6.3. Now the police car moves towards the listener at a speed of 22ms-1.

6.3.1. What would be the speed of the sound when the police car moves at 22ms-1 towards the listener?

6.3.2. Calculate the frequency at which the listener will hear the siren.

6.3.3. Why is the frequency different from the actual frequency of the emitted siren?

6.3.4. At what frequency will the listener hear the siren if the police car was moving away from him at a speed of 22ms-1?

6.1. There are 400 vibrations per second
6.2. 400 Hz. This is because the source and listener are both stationary.
6.3.1. 335ms-1 The speed of the sound will not change if the car moves. The sound speed depends of the air temperature and nature in which it is moving, and not the car itself.
6.3.2.
Draw a diagram. (move mouse over diagram / tap on diagram if on phone ... tap away to hide)


7. A car is moving at an unknown speed towards a stationary listener in a 60km.h-1 zone. The car emits a siren at a frequency of 430Hz, and the speed of the sound is 330ms-1. The listener records that he hears frequency of the sound at 451Hz.

7.1. Calculate the speed of the car in ms-1.

7.2. Is the car breaking the speed limit?

7.1.



7.2. speed = 15,36 m.s-1
In kilometers per hour :
15,36 x 3,6 = 55,296 km.h-1
Since the speed limit is 60 km.h-1 he is not breaking the speed limit because he is slower.

8. A police-car moves towards a listener at a speed of 30ms-1 and emits a siren with a wavelength of 1,2m.
The speed of the sound is 330ms-1. The listener moves directly towards the police-car at a speed of 4ms-1.

8.1. What is meant by the term wavelength?

8.2. Are sound waves longitudinal or transverse?

8.3. Calculate the frequency of the siren at the source.

8.4. Calculate the period of the siren sound.

8.5. At what frequency will the listener hear the siren?

8.6. What should be the velocity of the listener in order for him to hear the frequency of the siren at the real frequency? Explain.

8.1. distance between two consecutive points in phase
8.2. longitudinal
8.3. v = f λ
      330 = f (1,2)
      f = 275 Hz
8.4. T = 1/f
      T = 1/275
      T = 0.0036s
8.5.



8.6. He must run at the same velocity as the police, i.e. he must run at 30ms-1 in the same direction as the police car.