The Doppler Effect

A plane flying over you, a firetruck speeding by, or an ice cream truck driving down the street - what do they all have in common? They are everyday examples of the Doppler effect.

The Doppler effect is the change in frequency of a wave in relation to the observer as the source of the wave is in motion ... say what?

So, you are standing still - you are the observer. A moving car with a siren passing by is the source of the wave in motion.

As the car approaches you, the sound waves in front have a HIGHER frequency (wavelength distance is smaller). You experience this as a HIGHER pitch.

After the car passes you, the sound waves have a LOWER frequency (bigger spaces between wavelengths). You experience this as a LOWER pitch.

Why? The sound waves in front of the moving object get compressed as the object travels and more spread out behind. (see picture above)

If an object is moving REALLY fast, the sound waves can get REALLY compressed, build up in front of the object this causes the moving object to break the sound barrier! Awesome! Another name for this is a Sonic Boom!

One more thing ... What does the siren sound like to the people in the car? Do they notice a change in frequency (pitch)? No, since they are traveling WITH the sound, the siren is the same annoying sound the whole time!

Want more? Check out this great article from MIT!