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17 October 2012

Doppler demo -- with an onion bag

photo credit Graham McBride
Doppler effect problems in textbooks often involve trains and trumpeters.  That's all great, but I want to demonstrate the Doppler effect in my classroom, in which I have neither train nor trumpet.  Furthermore, when I've actually used a trumpeter playing a note out the window of my car, it's been hard to hear the effect -- I can't go much faster on the road here than 20 mph, and I'm not confident that my student trumpeter can maintain a constant pitch while turned sideways in the front seat.*  I need something better.

* Band veterans probably are ready to interject here the ol' wisecrack about the definition of the "minor second" being two high school trumpet players attempting to play the same note.  

My favorite Doppler effect demonstration involves twirling a speaker in a circle.  You get the speaker to play a constant frequency, attach it to a string... then when the speaker is briefly traveling toward the listeners, the pitch increases noticeably, especially compared with the decreased pitch that happens less than a second later.  The "wahh-wahh-wahh"   two-toned pitch is easy for everyone to hear.

The problem I've always had with that demo is the physical setup.  I attach a rather bulky frequency generator to a cannibalized speaker using alligator clips.  If I'm not extra careful, the alligator clips fail, and the speaker goes flying.

Nowadays, I use my iphone or ipad as a frequency generator with either the "freqgen" app or the "tone generator" app.  My question this morning was, how do I whirl my phone in a reasonably high-speed circle without the risk of breaking the phone?  Tying string to the iphone wasn't getting me anywhere.

My Chinese-teaching colleague Scott Navitsky gave me the key suggestion:  use an onion bag instead of string!  I asked our dining services for an onion bag, and I thank Jim Robertson and Aimee Carver for providing me with one.  I told the app to play a 600 Hz note, stuck the phone in the bag, twirled... and the warbling frequency was apparent to everyone nearby.



  1. I have a frsibee with a buzzer soldered on, and we go outside and throw it around. I'm not sure how much they 'hear', but I can notice it.

  2. I use a tuning fork on a string to get a similar effect.

  3. I've seen the frisbee -- awesome. Michael, I've never tried a tuning fork, because I can barely hear a tuning fork when I'm standing a meter away. Is that just because (as Burrito Girl tells me all the time) my hearing sucks? Or do you get enough volume that your whole class can hear?

  4. Ooh, and a student correctly used the word "ululate" to describe the effect of the demonstration. Wow.

  5. Greg you came through with a great idea as usual, thank you! Still, I think I'll use an old phone just to be sure.