The Doppler Effect

Question: I'm not sure if this is "on topic" but it's something I've been wondering about. I know it's called the Doppler Effect, and I've heard it dozens of times when emergency vehicles go by. What I don't get is why the sound changes. The speed of sound is a constant, right? So what am I missing? — BW, Biloxi, MS

Answer: This is actually very "on topic" since I'll be doing a post on the Doppler Effect with light waves next week. So your question is an excellent lead-up to that post, thanks. First, I think you have the speed of sound confused with the frequency of sound. The speed is more or less constant at 340 m/s (1115 ft/s), but it does depend slightly on atmospheric conditions.

The frequency is a totally different concept. This measures how many sound waves per second are arriving at your ear, and it can definitely be changed by motion of the source and/or observer. It's also related to the concept of pitch — the highness or lowness of the tone. Higher tones have higher frequencies.

Run the animation above, have your sound turned on, and you'll both hear and see the Doppler Effect. There's several important features to note:

  • Sound waves are emitted by the horn on the front of the fire truck.
  • Once emitted, they expand outward symmetrically at the speed of sound.
  • Because the vehicle is "chasing after" its sound waves, the spacing between waves is decreased ahead of it.
  • Thus, more waves arrive per second and the pitch sounds higher.
  • The opposite effect happens behind the vehicle, where the pitch sounds lower.

As the vehicle passes the observer, the pitch will audibly drop. And if it's a dual-tone horn (as in the animation) you'll hear the interval between the two tones increase. I should note that the pitch doesn't change instantaneously (as in the animation). Rather, it "slides" down from the higher pitch to the lower. That "slide" only lasts for a fraction of a second, but if you listen for it next time you'll hear it. So that's everything you need to know about the Doppler Effect — at least with sound waves. Things are a bit different with light waves, as you'll see in my Aug 8 post.

I don't normally run "ads" on my blog, but here's some shameless self-promotion. An HD version of the above animation is available on this page of my website. It includes additional sequences showing the Doppler Effect for a real vehicle, captioned comments, 3 audio clips, and 4 pages of teachers notes. Plus, you can't beat the price.

Next Week in Sky Lights ⇒ Redshift and Blueshift

2016 Perseid Meteor Shower
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