Why Does a Train Whistle Change Its Pitch When It Passes?
You may have noticed that the pitch of a train whistle seems to rise when the train is approaching and drop as the train passes and begins to move away. This apparent change in frequency is caused by motion, and is known as the Doppler Effect.
The Doppler effect, which occurs both in sound and electromagnetic waves—including light waves—has a number of applications. Astronomers use it, for instance, to gauge the movement of stars relative to Earth. Closer to home, principles relating to the Doppler effect finds application in radar technology.
As the train approaches, more sound waves strike your ears each second because the sound waves are crowded together. The waves are compressed and the frequency of the sound waves is higher. This results in a higher pitch.
You hear fewer sound waves each second after the train passes, because the sound waves are stretched apart. The frequency decreases, resulting in a lower pitch. Actually, the sound of a train whistle has only one pitch.