6.2 The Doppler effect with sound (ESCMN)

You, the human being hearing the sounds, are referred to as the observer or listener and also the thing emitting the sound is referred to as the source. As discussed in the introduction, there room two cases which lead to the Doppler effect:

When the source moves family member to a stationary observer.

You are watching: You are standing on a train station platform as a train goes by close to you

When the observer moves relative to a stationary source.

In point out 1 and also 2 above there is loved one motion between the resource and the observer. Both the source and the observer can be moving at the same time but we won"t deal with that case in this chapter.

Doppler effect

The Doppler effect is the adjust in the observed frequency that a wave once the resource or the detector moves family member to the transmitting medium.

The Doppler effect occurs once a source of waves and/or observer relocate relative to every other, bring about the observer measuring a different frequency of the waves than the frequency that the source is emitting. The medium that the waves room travelling through, the transmitting medium, is likewise stationary in the instances we will certainly study.

The inquiry that probably concerns mind is: "How go the Doppler impact come about?". We deserve to understand what is continue by reasoning through the situation in detail.

Case 1: moving source, stationary observer (ESCMP)

Let us consider a resource of sound waves with a continuous frequency and also amplitude. The sound waves have the right to be stood for as concentric circles wherein each circle represents a crest or top as the wavefronts radiate away from the source. This is since the waves travel away native the source in every directions and the distance in between consecutive crests or continually troughs in a wave is continuous (the wavelength as we learnt in great 10). In this number the crests are represented by by the black lines and the troughs through the orange lines.


Figure 6.1: Stationary sound resource as much more wavefronts room emmitted.

The sound source is the police auto in the middle and is stationary. For the Doppler result to take location (manifest), the source must be moving relative to the observer.

Let"s think about the complying with situation: The resource (represented by the black dot) emits one tide (the black circles represent the crests that the sound wave) that moves far from the resource at the same rate in all directions. The distance in between the crests represents the wavelength (\(\lambda\)) of the sound. The closer together the crests, the greater the frequency (or pitch) that the sound follow to \(f=\fracv\lambda\), where \(v\) (speed the sound) is constant.


As this crest move away, the resource also moves and also then emits more crests. Currently the 2 circles are not concentric any more, however on the one side they room closer together and also on the other side lock are further apart. This is shown in the next diagram.


If the source continues relocating at the very same speed in the same direction, climate the distance in between crests ~ above the best of the resource is constant. The distance in between crests on the left is likewise constant. The distance between successive crests top top the left is continuous but larger than the distance in between successive crests on the right.


When a vehicle approaches you, the sound waves the reach you have actually a much shorter wavelength and a higher frequency. You hear a sound with a greater pitch. When the vehicle moves away from you, the sound waves that reach you have a longer wavelength and lower frequency. Friend hear a sound through a reduced pitch.


Figure 6.2: relocating sound source as much more wavefronts are emmitted.

Case 2: moving observer, stationary resource (ESCMQ)

Just as we walk before, permit us consider a resource (a police car) that sound waves v a continuous frequency and also amplitude. There room two observers, one on the left that will move away from the resource and one on the right that will relocate towards the source. We have actually three diagrams:

shows the in its entirety situation v the siren beginning at time \(t_1\); reflects the case at time \(t_2\) when the observers space moving; and also shows the case at \(t_3\) after the observers have been relocating for a time interval, \(\Delta t=t_3-t_2\).

The crests and also troughs are numbered therefore you have the right to see exactly how they move additional away and also so that we deserve to track which persons an observer has measured.


The observers can hear the sound tide emitted by the police car and they begin to relocate (we disregard the time the takes them to accelerate).


The frequency the the wave that an observer actions is the number of complete tide cycles every unit time. By numbering the crests and troughs we have the right to see which finish wave cycles have been measured by each of the observers in time, \(\Delta t\). To find the frequency we divide the number of wave cycles by \(\Delta t\).

In the time interval that passed, the observer relocating towards the police vehicle observed the crests and also troughs numbered 1 v 5 (the section of the tide is emphasize below). The observer relocating away encountered a smaller portion of the wavefront, comb 3 and also trough 4. The time interval because that each of lock is the same. Come the observers this will mean that the frequency castle measured is different.

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The movement of the observer will alter the frequency the the measure sound from a stationary source:

one observer moving towards the source measures a higher frequency. one observer moving away from the source measures a lower frequency.

It is necessary to note that we have only looked in ~ the situations where the resource and observer room moving directly towards or far from every other and these room the only instances we will certainly consider.