Doppler Effect and Ice Cream

Posted by Kelly Whitt May 22, 2007

This weekend we were out working in the yard when we heard the ice cream truck a couple blocks away. I can't recall what song he played, but it was a familiar one in that pinging sound these trucks make. Something like "The Candy Man" or "Pop Goes the Weasel". The street we live on is a quiet street and has no outlet. So when he came to us he was moving at quite a clip, seeing that none of us were paying any attention to him. As the truck went by I noticed a change in the sound of the music, and suddenly realized that I could use it as a teaching tool for my son, who was next to me digging in the dirt.

"Okay honey, when the truck goes by again, listen very closely. Let me know if you hear a difference."

As the truck turned around at the end of the road and came whizzing by again, we stopped and stood still to listen. The ice cream truck's song seemed to strike some off-key notes as it pulled even with us and then pulled away. I realized at this point that a song is not nearly as good a teaching instrument as an actual siren that follows a very predictable pattern, but it was too late. I had already set him up for it.

I asked him if he noticed the song sounded a little funny as the truck reached us and then passed us. Sound travels to us in the air as waves. As something is headed toward us, the waves coming from it are compressed, or "smooshed together" as you will, and we hear a higher frequency sound. As the truck moves away from us, the waves stretch out or lengthen, and the sound will change to a lower pitch.

(Try this link to hear the Doppler Effect in a siren: http://www.arachnoid.com/sky/doppler.wav )

The Doppler Effect (which you now know is better heard with sirens than ice cream trucks) can be seen instead of heard in the universe. The vast majority of the galaxies in the universe are moving away from us. When the light spectrums of galaxies are measured, their light is shifted to the red end of the spectrum. Red wavelengths of light are longer while blue is shorter, therefore a redshift in the target source is moving away from us, while a blueshift shows the item is moving toward us.

Listen for it yourself the next time an emergency vehicle or car with its horn blaring is passing you.