Science Puzzle
Why Does a Siren Change Pitch as It Passes?
An ambulance races past with its siren wailing. As it comes towards you the note sounds high; the moment it passes, the note suddenly drops lower. The driver hasn't touched the siren, and the people inside the ambulance hear the same note the whole time.
Why does the siren sound higher as it approaches and lower as it moves away?
The Answer
The motion of the ambulance squashes the sound waves together in front of it and stretches them out behind it.
A siren sends out sound as a series of pressure waves, one after another, and the pitch you hear depends on how many wave crests reach your ear each second. If the siren stood still, the crests would spread out evenly in every direction. But the ambulance is moving. It sends out each new crest from a little further along the road than the last, so in front of it the crests are crowded closer together and more of them reach your ear each second: a higher note. Behind it, each crest is sent from further away than the one before, so they're spread out and fewer arrive each second: a lower note.
This is the Doppler effect, named after the Austrian physicist Christian Doppler, who described it in 1842. Three years later it was tested in the Netherlands by putting trumpeters on a moving train and having musicians on the platform judge the notes.
The same thing happens to light. Galaxies rushing away from us have their light stretched towards the red end of the spectrum, which is how astronomers discovered that the universe is expanding. Police speed guns and weather radar use the Doppler effect too.
The principle: The Doppler effect. When a source of waves moves towards you the waves arrive closer together and the pitch rises; when it moves away they arrive further apart and the pitch falls.