Physics
The Doppler effect
Why a siren drops in pitch as it passes you, even though nobody touched the horn.
What you need first
An ambulance comes down the street, its siren growing louder and louder. The instant it shoots past you the tone audibly drops: a bright wail turns into a deeper moan. Yet nobody touched the siren, it sounds the same the whole time. What changed is only the way the arrive at your ear. This effect is called the Doppler effect, after the Austrian physicist Christian Doppler, who described it in 1842.
Crests bunch up
A siren sends out sound waves like a stone dropped into water: ring after ring, always the same distance apart. While the siren stands still those distances are the same in every direction. When it drives towards you, each new crest starts a little closer to you than the one before. In front of the vehicle the crests therefore bunch up, behind it they stretch apart. That means more crests reach you every second, and more crests per second mean a higher , so a higher note. Important: the sound itself keeps the same speed, about 343 m/s in air. Only the spacing between the crests changes.
Higher frequency = higher pitch.
Larger amplitude = louder.
What happens as it passes
As long as the vehicle is approaching you hear a steady note that is too high, not one that slowly climbs. Only as it passes does the pitch slide down, running briefly through the true note on the way. After that it stays too low until the vehicle is out of earshot. An example: a siren really produces 800 Hz. As it approaches you hear 880 Hz, once it has passed only 730 Hz. The driver inside hears 800 Hz for the whole trip, because the siren does not move relative to them. And it makes no difference who is moving: ride a train past a ringing level crossing bell and you hear exactly the same change in pitch.
Not only with sound
Waves are waves, so the Doppler effect holds for light and radio waves too. A speed camera sends radio waves at your car and measures how far the returning wave has shifted in frequency, which gives your speed. Weather radar uses the same trick to tell whether raindrops are flying towards the station or away from it, and in hospitals Doppler ultrasound shows how fast blood is flowing through a vessel. The longest view of all belongs to astronomy: almost all distant are moving away from us, so their light arrives shifted towards longer , towards the red. This is how we know that the universe is expanding.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
An ambulance with its siren on drives past you. How does the note change for you?
What actually changes in the Doppler effect?
A siren really produces 800 Hz. As it approaches you hear 880 Hz, after it has passed 730 Hz. By how many hertz does the note drop as it passes?
When a sound source moves away from you, the crests stretch apart and you hear a … note.
A speed camera measures how fast a car is going. How does it do that?
Match each situation to what you hear or see.