Perception of sound: Doppler effect

This phenomenon takes place when either the sound source or the listener are moving. The classic example that is always given is the ambulance siren speeding past us and disappearing into the night.

The Doppler Effect

Let's take a look at the previous figure in which the ambulance is still and the siren emits a sound that, being of a certain frequency, generates wave-fronts that are at a constant distance one from another. When, on the other-hand, the vehicle is moving and is approaching the listener, that very same siren generates wave-fronts that are closer to each other compared to when the vehicle was still. This is because motion has caused the wave-fronts to compress. Seeing that the wave-fronts are now closer to one another we perceive a higher frequency, in other words, a more acute sound. When the vehicle overtakes us (and disappears into the night), distance causes the space between the wave-fronts to increase, therefore at this point we perceive a lower sound because a lower frequency reaches our ears.

The following sound illustrates what we've been speaking about

Doppler effect (source: car with horn)  [Track 16]

The previous example reproduces one of the most typical manifestations of the doppler effect.

In our next example the sound has been obtained by taking a single sound source (of 500 Hz frequency) and simulating its movement in relation to an immobile listener, through the use of a mathematical algorithm ^{[1 ]}.

Sinusoid with 500 Hz frequency  [Track 17]

Doppler Effect (source: 500 Hz)  [Track 18]

As we can see, when the source approaches the listener, a more acute sound is perceived compared to the original one. This occurs because the wave-fronts get compressed. When the source goes beyond the listener, the wave-fronts distance themselves from one another and the sound is perceived as being less acute than the 500 Hz one.