### Doppler Questions

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March 2013

6. The siren of a stationary ambulance emits sound waves at a frequency of 850 Hz. An observer, travelling in a car at a constant speed in a straight line, begins measuring the frequency of the sound waves emitted by the siren when he is at a distance x from the ambulance. The observer continues measuring the frequency as he approaches, passes and moves away from the ambulance. The results obtained are shown in the graph below.

6.1 The observed frequency suddenly changes at t = 6 s. Give a reason for this sudden change in observed frequency. (1)
6.1 The approaching observer (higher f) passes the source at t = 6 s and moves away (lower f) from the source.

6.2 Calculate the:
6.2.1 Speed of the car (Take the speed of sound in air as 340 m.s-1 (5)

6.2.2 Distance x between the car and the ambulance when the observer BEGINS measuring the frequency (3)

March 2012

6 An ambulance approaches an accident scene at constant velocity. The siren of the ambulance emits sound waves at a frequency of 980 Hz. A detector at the scene measures the frequency of the emitted sound waves as 1 050 Hz.

6.1 Calculate the speed at which the ambulance approaches the accident scene. Use the speed of sound in air as 340 m.s-1. (4)

6.2 Explain why the measured frequency is higher than the frequency of the source. (2)
6.2 Waves in front of the moving source are compressed.
The observed wavelength decreases.
For the same speed of sound, a higher frequency will be observed.

6.3 The principle of the Doppler effect is applied in the Doppler flow meter. State ONE positive impact of the use of the Doppler flow meter on humans. (2)
Any ONE
• Determine whether arteries are clogged / narrowed so that precautions can be taken in advance to prevent heart attack or stroke.
• Determine heartbeat of foetus to assure that child is alive / does not have a heart defect.

Nov 2012

6. A bird flies directly towards a stationary birdwatcher at constant velocity. The bird constantly emits sound waves at a frequency of 1 650 Hz. The birdwatcher hears a change in pitch as the bird comes closer to him.

6.1 Write down the property of sound that is related to pitch. (1)
6.1 Frequency

6.2 Give a reason why the birdwatcher observes a change in pitch as the bird approaches him. (1)
6.2 There is relative motion between the bird and the bird watcher.

The air pressure versus distance graph below represents the waves detected by the birdwatcher as the bird comes closer to him. The speed of sound in air is 340 m.s-1.

6.3 From the graph, write down the wavelength of the detected waves. (1)
6.3   0,2 m

6.4 Calculate the:
6.4.1 Frequency of the waves detected by the birdwatcher (3)
6.4.1

6.4.2 Magnitude of the velocity at which the bird flies (5)

March 2011

6. The whistle of a train emits sound waves of frequency 2 000 Hz. A stationary listener measures the frequency of these emitted sound waves as 2 080 Hz. The speed of sound in air is 340 m.s-1.

6.1 Name the phenomenon responsible for the observed change in frequency. (1)
6.1 Doppler effect

6.2 Is the train moving AWAY FROM or TOWARDS the stationary listener? (1)
6.2 Towards

6.3 Calculate the speed of the train. (4)
6.3

6.4 Will the frequency observed by a passenger, sitting in the train, be GREATER THAN, EQUAL TO or SMALLER THAN 2 000 Hz? Explain the answer. (2)
6.4 Equal to (2 000 Hz)
The passenger moves at the same velocity as the train. / There is no difference in velocity of the passenger relative to the train.

Nov 2011

6. A train approaches a station at a constant speed of 20 m.s-1 with its whistle blowing at a frequency of 458 Hz. An observer, standing on the platform, hears a change in pitch as the train approaches him, passes him and moves away from him.

6.1 Name the phenomenon that explains the change in pitch heard by the observer. (1)
6.1 Doppler effect

6.2 Calculate the frequency of the sound that the observer hears while the train is approaching him. Use the speed of sound in air as 340 m.s-1. (4)
6.2.

6.3 How will the observed frequency change as the train passes and moves away from the observer? Write down only INCREASES, DECREASES or REMAINS THE SAME. (1)
6.3 Decreases

6.4 How will the frequency observed by the train driver compare to that of the sound waves emitted by the whistle? Write down only GREATER THAN, EQUAL TO or LESS THAN. Give a reason for the answer. (2)
6.4 Equal to
Velocity of train driver relative to the whistle is zero.

OR

Train driver has same velocity as whistle.

OR

There is no relative motion between source and observer.

March 2010

7. An ambulance with its siren on, moves away at constant velocity from a person standing next to the road. The person measures a frequency which is 90% of the frequency of the sound emitted by the siren of the ambulance.

7.1 Name the phenomenon observed. (1)
7.1 Doppler effect

7.2 If the speed of sound in air is 340 m.s-1, calculate the speed of the ambulance. (5)
7.2.

Nov 2010

6. The siren of a burglar alarm system has a frequency of 960 Hz. During a patrol, a security officer, travelling in his car, hears the siren of the alarm of a house and approaches the house at constant velocity.
A detector in his car registers the frequency of the sound as 1 000 Hz

6.1 Name the phenomenon that explains the change in the observed frequency. (1)
6.1 Doppler effect

6.2 Calculate the speed at which the patrol car approaches the house. Use the speed of sound in air as 340 m.s-1. (4)

6.3 If the patrol car had approached the house at a higher speed, how would the detected frequency have compared to the first observed frequency of 1 000 Hz? Write down only HIGHER THAN, LOWER THAN or EQUAL TO. (1)
6.3 Higher than

March 2009

7. Dolphins use ultrasound to scan their environment.
When a dolphin is 100 m from a rock, it emits ultrasound waves of frequency 250 kHz whilst swimming at 20 m.s-1 towards the rock. Assume that the speed of sound in water is 1 500 m.s-1.
7.1 Calculate the frequency of the sound waves detected by a detector on the rock. (4)
7.1.

7.2 When the dolphin is 50 m from the rock, another ultrasound wave of 250 kHz is emitted.
How will the frequency of the detected sound waves compare with the answer calculated in QUESTION 7.1? Write down only HIGHER, LOWER or REMAINS THE SAME. Explain your answer. (2)
7.2 Remains the same
The detected frequency is independent of the distance between the source and observer.

Nov 2009

7. A fire truck, with its siren on, is moving at 20 m.s-1 towards a burning building. A person standing next to the road with a detector, measures the frequency of the sound emitted by the siren to be 450 Hz. The measured frequency is HIGHER than the frequency of the sound emitted by the siren.

7.1 Is the fire truck moving toward or away from the person? (1)
7.1 Towards the person

7.2 Explain why the registered frequency is higher. (2)
7.2 When the source moves towards a stationary observer waves in front of the source is compressed:
• resulting in a shorter wavelength
• resulting in a higher frequency
(speed of sound constant)

7.3 Calculate the frequency of the siren if the speed of sound in air is 340 m.s-1. (4)

Nov 2009 Unused

7. The siren of a police car produces a sound of frequency 420 Hz. A man sitting next to the road notices that the pitch of the sound changes as the car moves towards and then away from him.

7.1 Write down the name of the above phenomenon. (1)
7.1 Doppler effect

7.2 Assume that the speed of sound in air is 340 m.s-1.
Calculate the frequency of the sound of the siren observed by the man, when the car is moving towards him at a speed of 16 m.s-1. (4)

7.3 The police car moves away from the man at constant velocity, then slows down and finally comes to rest.
7.3.1 How will the observed frequency compare with the original frequency of the siren when the police car moves away from the man at constant velocity? Write only GREATER THAN, SMALLER THAN or EQUAL TO. (2)
7.3.1 Smaller than

7.3.2 How will the observed frequency change as the car slows down whilst moving away? Write only INCREASES, DECREASES or REMAINS THE SAME. (2)
7.3.2 Increases

Nov 2008

8. An ambulance travelling down a road at constant speed emits sound waves from its siren. A lady stands on the side of the road with a detector which registers sound waves at a frequency of 445 Hz as the ambulance approaches her. After passing her, and moving away at the same constant speed, sound waves of frequency 380 Hz are registered.

Assume that the speed of sound in air is 343 m.s-1.
8.1 Name the phenomenon that describes the change in the frequency observed by the lady. (1)
8.1 Doppler effect

8.2 Calculate:
8.2.1 The speed at which the ambulance is moving (7)

8.2.2 The frequency at which the siren emits the sound waves (3)

Nov 2008 Prep P1

8. An ambulance moving at 40 m.s-1 approaches a traffic light where a blind man and his dog wait to cross the road. The siren of the ambulance emits sound waves at a frequency of 350 Hz. The pitch of the sound that the man hears gets higher as the ambulance moves towards him and decreases as the ambulance passes him and moves away.

8.1 Use a sketch of wave fronts to show why the pitch of the sound that the blind man hears is:
8.1.1 Higher as the ambulance approaches him (2)
8.1.1

8.1.2 Lower as the ambulance moves away from him (2)
8.1.2

8.2 If the speed of sound in air is accepted as 340 m.s-1, determine the apparent frequency of the sound waves that the man hears while the ambulance approaches him. (5)
8.2

8.3 Explain how this effect can benefit a blind person. (2)
8.3 When crossing a street , a blind person can determine whether a car is moving towards or away from him

Exemplar 2008 P1

8 During an experiment to determine the speed of sound, learners are given a siren that sounds a single note of frequency 426 Hz. They attach it to a remote controlled car and move it at constant speed past a stationary tape recorder which is mounted in the middle of a runway. Ignore the effects of friction. The tape recorder records the sound of the siren.

The learners make the following observation:
The pitch of the sound from the siren as it moved towards the tape recorder was higher than the pitch as the siren moved away from the recorder.

8.1 Name the effect which explains this observation. (2)
8.1 Doppler Effect /

In one of the trials the speed of the remote controlled car was noted as 31 km.h-1. Two notes from the siren were recorded: one with a frequency of 437 Hz and the other note with a frequency lower than 426 Hz.

8.2 Convert 31 km.h-1 to m.s-1. (2)
8.2

conversion factor is 3,6

8.3 Determine the speed of sound in air. (5)
8.3

8.4 Give a reason why the observed frequencies are respectively higher and lower than the frequency of the source (426 Hz). (2)
8.4 Higher frequency: source is moving towards observer.
Lower frequency: source is moving away from observer.

8 The sketch below shows a stationary ambulance. The siren of the ambulance emits sound waves of frequency 700 Hz. The driver of a car approaching the ambulance and passing it at constant speed, observes the frequency of the emitted sound waves to change by 80 Hz.

8.1 Name and state the wave phenomenon illustrated above. (3)
8.1 Doppler effect
A change in observed frequency (pitch) due to relative motion between observer and sound source.

OR

A change in observed frequency (pitch) because the sound source and observer have different velocities with respect to the medium.

8.2 Take the speed of sound in air as 340 m.s-1 and calculate the speed at which the car passes the ambulance. (5)

OR