ESA Science & Technology - A novel experiment to demonstrate doppler shifting

A novel experiment to demonstrate doppler shifting

There are many everyday examples of the Doppler effect - the changing pitch of police and ambulance sirens, train whistles and racing car engines as they pass by.

In each case, there is an audible change in pitch as the source approaches and then passes an observer. The effect arises because sound waves arrive at the listener's ear closer together as the source approaches, and further apart as the sound waves recede. The faster the object is moving, the greater the effect.

To illustrate the Doppler shift and the effect that the distance to the source has on it, Jean-Pierre Lebreton performed the following simple experiment:

A video camera recorded a scooter, blaring its horn, approaching and passing by the cameraman. Two recordings were made; the first one while the cameraman was standing close to the road on which the scooter was driving, the second one some distance away from the road. The video clips and associated sound files are featured below. The sound files were analysed with an audio analysis programme (see the related links) and the result of the analysis is shown in the graph to the right of each video clip.

The graphs show the spectral analysis of the signal. The main horn frequency is just below 4000 Hz, but you can see that the horn spectrum is rich in harmonics. Note the fast change in tone (frequency) at around 7 seconds in the top graph - this is around the point when the scooter passes the cameraman. Before that point, the scooter was approaching the cameraman (the frequency increases); after that, the scooter was moving away (the frequency decreases).

The bottom graph shows a much smoother frequency change. This is because the Doppler effect depends on the relative movement toward or away from the observer, not on absolute movement. With increasing distance between the source (the scooter) and the observer (the cameraman), the relative movement towards or away from the observer becomes smaller and thus the Doppler shift diminishes.

The experiments

Experiment 1

Experiment 2