# Why do you hear a higher pitched sound when an ambulance is moving toward you and a lower sound when an ambulance is moving away from you than you would if you and the ambulance were both at rest?

## Answer

###### March 25, 2014 11:54PM

That's called the "Doppler" effect, named for the scientist who explained it.

You're standing still, but the train or ambulance is moving toward you. The horn or siren is going. The sound waves go out in every direction, at the speed of sound (about 750 miles per hour, depending on atmospheric pressure).

As the train is moving TOWARD you, the sound waves that it generates are being pushed together, because the train is moving too. We hear the effect as if the frequency of the sound waves is pushed together, increasing the frequency. As the train passes by, we hear the "Doppler shift" to the lower frequency, because how the train is moving AWAY and the sound frequencies are stretched out. Now we hear the sound at a lower frequency.

If we carefully analyze the Doppler shift, we can determine;

1. Exactly how fast the train was moving

2. How far away the train was when it passed by.

Police use the same thing; you're driving along the highway, and a police officer points his radar gun at your car. The radar gun is precisely calibrated for frequency. The radar waves hit your car and bounce off, and the Doppler-shifted frequency can be used to calculate how fast you were going.