7.3 Doppler Effect for Sound Waves | CIE AS Physics Topic Questions 2025

1a

2 marks

State what happens to the frequency and pitch of a sound as the source moves away from an observer.

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1b

1 mark

State the name of the effect in which the frequency of a sound wave changes as the source of the sound moves relative to an observer.

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1c

4 marks

The change in frequency of a sound wave as the source of the sound moves relative to an observer can be described by the equation

$f_{o} = f_{s} \frac{v}{(v \pm v_{s})}$

State the meaning of the following terms and their associated units

(i)

f_{o}

[1]

(ii)

f_{s}

[1]

(iii)

v

[1]

(iv)

v_{s}

[1]

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1d

1 mark

State the equation as it would be used to calculate the frequency of an observed sound moving away from an observer.

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1a

3 marks

A train is travelling at 45 m s⁻¹ and blows a whistle at a frequency of 750 Hz. A person is waiting at a level crossing for the train to pass. Take the speed of sound in air as 340 m s⁻¹.

Calculate the frequency observed by the person:

(i)

In still air.

(1)

(ii)

When the wind is blowing at 10 m s⁻¹ towards the train and away from the person.

(2)

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1b

2 marks

After the train passes the level crossing, it slows down as it approaches a tunnel. Wind is funnelled down the tunnel towards the train reaching a velocity of 27 m s⁻¹. The observer at the level crossing hears the whistle at a frequency of 729 Hz.

Calculate the new speed of the train.

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1c

2 marks

After exiting the tunnel, the train stops at a station. The observer from part (a) is now walking to the platform to catch the train at a speed of 1.5 m s⁻¹. The human ear can distinguish between frequency differences of around 3.6 Hz.

The Doppler equation for a moving observer is:

$f_{o} = f_{s} (\frac{v \pm v_{o}}{v})$

Show that the change in frequency of the train's whistle, as heard by the walking observer, can be considered negligible.

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1d

2 marks

Once the train has left the station, it travels down a line with two tracks. Another train is approaching on the opposite track with a speed of 7.6 m s⁻¹. The speed of the departing train is 83% of the speed of the second train.

When both the observer and source are moving, the following equation can be used to determine the new frequency:

$f_{o} = f_{s} (\frac{v + v_{o}}{v - v_{s}})$

Calculate the frequency of this same whistle as observed by a passenger on the approaching train as a percentage of $f_{s}$ .

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1a

3 marks

A scientist is conducting an experiment on bees. Fig 1.1 shows the set up monitoring equipment near a hive, including a microphone which picks up the buzz from individual bees flying past.

9-5-hl-sq-medium-q2a

Fig 1.1

A single bee flies at a constant speed in a straight line past the microphone, and the frequency of the buzz is detected.

Explain the sound pattern detected by the microphone as the bee moves towards and away from the microphone.

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1b

3 marks

The speed of the bee is 8.9 m s⁻¹. The maximum frequency of sound recorded by the microphone is 271 Hz. The speed of sound in air is 340 m s⁻¹.

(i) Calculate the frequency of sound produced by the bee.

(ii) Determine the minimum frequency recorded by the microphone.

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1c

2 marks

When bees are about to swarm, they produce a higher-pitched buzz, known as 'piping'. The frequency of the piping sound is 550.0 Hz. The scientist moves the microphone closer to the hive during this process. Whilst they are moving, the change in observed frequency is 3.4 Hz. The scientist wants to calculate the speed with which they walk to the hive.

Explain the effect of any assumptions they would have to make in determining this speed.

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Doppler Effect for Sound Waves (CIE AS Physics)

Topic Questions