Everyday Physics

Doppler Effect

Have you ever wondered why, when an ambulance or any car with a siren passes you, the sound of the siren seems to first get higher in tone as it approaches and lower once it moves away?

This is explained by the Doppler Effect:

The Doppler effect (or Doppler shift) is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.

Let us take the example of the Doppler Effect on sound. We must first understand that sound travels in waves. It needs particles(atoms) to pass through the air. Sound is nothing but the passing on of vibration by air particles between a source and your ears. When your ear receives these vibrations, your brain interprets them as what we know as sound.

Figure 1: How sound waves travel through air (source is in the center)

Now, when the source, say a car, is moving, these waves are not as symmetrical as in figure 1. They get distorted because of the speed of the car, as illustrated in figure 2.

Figure 2: Distorted sound waves of moving source(the source is the orange dot in the middle of the waves, the arrow shows the direction of motion)

Now the high-ness or low-ness of these waves is defined by the frequency. Frequency, simply put, is the number of vibrations that happen in one second. In figure 2, the wavy lines moving up and down show the frequency: the closer packed they are, the higher the frequency, the further they are, the lower the frequency.

Now, for a moving source, as seen from figure 2, the frequency at the source will not be the same as:

  1. the frequency according to someone who is standing in front of the source.
  2. the frequency according to someone who is standing behind the source.

This effect can better be seen in figure 3.Figure 3(a) shows a source which is not moving while 3(b) shows a source which is moving.

Figure 3(a): The frequency is the same all around.
Figure 3(b): The frequency is different, according to whether you hear the sound at the source, in front of it or behind it.

Consider the following analogy: Someone throws one ball every second at a man. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if the thrower is moving away from the man. So the frequency is increased at the receiving end when thrower is moving towards the receiver because the balls have less distance to travel to reach the receiver. The same works the other way around: if the thrower is moving away, balls need to travel farther, and frequency as measured by receiver is reduced.

Similarly, the original question I asked can be answered by the same phenomenon. As the ambulance comes towards you with the siren switched on, the distance that the sound waves need to travel is progressively decreasing, but the waves travel at the same speed, the time taken is also decreasing. This results in a listener who is front of the ambulance hearing a sound with higher frequency than someone in the ambulance will hear. Conversely, for someone who is behind the ambulance, the sound will have lower frequency. This is because the sound waves still have the same speed, but the distance is increasing, and so the time taken by the sound to travel is increasing. So the listener behind the ambulance hears a lower sound than that heard by someone sitting inside the ambulance.

Figure 4: Doppler effect of car with siren

 

 

  • Christian Andreas Doppler (29 November 1803 – 17 March 1853) was an Austrian mathematician and physicist. He is celebrated for his principle — known as the Doppler effect — that the observed frequency of a wave depends on the relative speed of the source and the observer.

 

Figures and definitions taken from:

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