Doppler Effect for Sound Waves (CIE AS Physics)

A police car siren emits a sound wave with a frequency f_s of 440 Hz. The car is travelling away from an observer at a speed of 30 m s⁻¹.

 The speed of sound is 340 m s⁻¹.

 Which of the following is the frequency the observer hears?

Figure 1.1 shows object P, moving at velocity v_s. It emits a sound wave of frequency f_s. The speed of sound is v.

Fig. 1.1

At which point on the diagram will the observed frequency be ?

A source moving at constant velocity v_s, emits a sound wave of frequency 450 Hz. A stationary observer records a frequency of 300 Hz.

What is the value of v_s, and is it moving towards or away from the observer?

Speed of sound, v = 340 m s^-1

	vs / m s-1	Towards / Away
A.	850	Away
B.	170	Towards
C.	170	Away
D.	850	Towards

A car of terrified observers try to get the car to start to drive away from a hungry T. rex, running toward them at a speed of 10 m s⁻¹. They hear the roar of the T. rex at a frequency of 170 Hz. The speed of sound in air is 340 m s⁻¹.

What is the frequency of the sound as emitted by the T. rex?

A car travels in a straight line towards a stationary observer at a speed of 0.2v, where v is the speed of sound. The car's horn emits a pure sound of frequency f_s.

What is the frequency of the sound of the horn as heard by the observer, f₀?

The sun emits light of a particular wavelength λ_s. At any instant, there is a range of wavelengths from less than λ_s to greater than λ_s­ that can be observed on Earth. This is due to the Doppler effect.

 Which of the following statements would explain this Doppler effect?

Two runners, A and B, run past each other in opposite directions, both at speed u. A police car has its siren on. It is traveling at a speed of 10u, in the same direction as Runner A. Runner A and Runner B observe the sound wave from the siren at frequencies of f_A and f_B respectively. The speed of sound is v.

What is the ratio of ?

Fig. 1.1 is a graph of the observed wavelength against time of a sound wave emitted from a single source. The emitted sound wave has a frequency . The sound travels at a constant velocity directly towards an observer and a frequency of is observed. It then travels directly away from the observer and a frequency of is observed.

Fig. 1.1

What is the value of the ratio?

Fig 1.1 is a diagram of a source that has a mass of 500 g and is connected to a rope of length 20 cm. The rope is attached to point Q and has a tension of 800 N. The source travels around point Q anticlockwise in a circular path with a velocity of v_s. It emits a sound wave of frequency f_s. When the source is at point P, an observer stands directly in line to the direction of motion at point O.

Fig 1.1

What is the value of v_s and  when the source is at point P?

Speed of sound, v = 340 m s^-1

	vs / m s-1
A.	17.9	1.06
B.	17.9	0.950
C.	320	17.0
D.	320	0.515

A source moves directly in line with an observer at a constant velocity v_s and emits a sound wave with a wavelength of λ_s. The observed wavelength of the source is λ₀ and the ratio  is equal to .

What is the value of v_s?

Speed of sound, v = 340 m s^-1