Doppler Effect
Why does an approaching ambulance sound higher?
Summary
When a sound source moves, the waves in front of it bunch up and the ones behind it spread out. That is why the siren of an approaching ambulance sounds high and a receding one sounds low. Turn on the sound and listen as the source goes past; raise its speed above the speed of sound and watch the shock cone form.
In depth
In 1842 the Austrian physicist Christian Doppler proposed that the frequency we perceive changes when the source of a wave and the observer move relative to each other. For a source approaching a stationary observer at speed v, the heard frequency is f′ = f · c / (c − v) (c is the speed of sound); for a receding source it is f′ = f · c / (c + v). When the source exceeds the speed of sound, the wavefronts pile up into a shock cone, and when the cone reaches the ground a sonic boom is heard. The effect appears in all waves: light from approaching objects shifts toward blue, and from receding objects toward red. Doppler ultrasound, police radar, weather radar and the search for exoplanets all rely on this principle. In the interactive model the speed of sound is 343 m/s (air at 20 °C); the motion is shown in slow motion.
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