Multiple choiceQuestion 1 · Recall
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By the end you can explain why a moving source is heard at a different frequency from the one it emits, and describe what happens to the sound in front of and behind it.
An ambulance comes towards you with its siren blaring. As it draws level, the pitch drops — often with a distinct, slightly cartoonish "nee-naw" slide.
Nothing has changed inside the siren. The same note is being produced. What has changed is how often the wavefronts arrive at your ear.
This shift in the detected frequency, caused by relative motion between a source and an observer, is called the Doppler effect.
Doppler effect
The change in the frequency detected by an observer when a source of waves moves relative to that observer.
For a source moving towards a stationary observer, the detected frequency is higher than the emitted frequency. For a source moving away, it is lower.
The frequency emitted by the source does not change, and neither does the speed of sound in the air.
Think of the source as releasing one wavefront every seconds, where is the period of the note.
If the source stays still, each wavefront is a circle centred on the same point, and the circles are evenly spaced by one wavelength.
Now let the source move forwards. Each new wavefront is released from a point slightly ahead of the last one, so the circles are packed closer together in front and stretched further apart behind.
Closer wavefronts mean a shorter wavelength in front of the source and a longer wavelength behind it.
The wave still travels at the same speed through the air. Since , a shorter wavelength must arrive with a higher frequency, and a longer wavelength with a lower frequency. That is exactly what your ear reports as pitch.
Imagine throwing one ball per second forwards from a moving bike.
When you pedal towards a friend, each ball is thrown from a little closer, so the balls arrive more often than once a second. When you cycle away, they arrive less often.
Sound does the same thing with wavefronts: the source keeps "throwing" one per period, so a moving source delivers them more often ahead and less often behind.
Where the analogy stops: the balls are separate objects, while a sound wave is a continuous pattern of pressure. The rate at which wavefronts arrive is what your ear turns into pitch, so the bunching is heard as a change of frequency.