A Staff Report from the Straight Dope Science Advisory Board

Is the sound of a snapping towel a sonic boom?

June 20, 2001

Dear Straight Dope:

My uncle told me that the sound of a towel snapping was the end of it breaking the sound barrier. My first thought was no way--if a towel went the speed of sound it would have to cut you when it hit you. So I told him that I would just have to ask Cecil what the sound really was when you snap a towel.

Hope you didn't put serious money on this, Jerry. If your uncle had been talking about a bullwhip, he definitely would have been right. Back in the fifties (yes, that's last century), three scientists at the Naval Research Laboratory measured the speed of the tip of a bullwhip by shooting the action with a high-speed camera at 4000 frames per second. The maximum speed turned out to be about 1400 feet per second, about 25% faster than the speed of sound in air. So the crack of a whip is, in fact, a miniature version of the sonic boom created by jets traveling faster than the speed of sound. This experiment was written up in the Journal of the Acoustical Society of America, vol. 30, p. 1112 (1958). If this stuff thrills you, this might be the society for you. 

But you and your uncle weren't talking about bullwhips, you were talking about towels. When I started looking into this, I was prepared to tell you that your uncle was full of bull and should be whipped. Little did I know how creative physics students can be when trying to come up with a unique project.

Back in the early nineties (still last century, not the one before), three students at the North Carolina School of Science and Mathematics, apparently anticipating your discussion with your uncle, set out to prove that the speed of the tip of a snapped towel can exceed the speed of sound, just like a bullwhip. The students, not having the resources of the Naval Research Laboratory, couldn't afford a high-speed camera, so they had to rig up something using some high-speed photographic equipment that had been developed for experiments by former students. 

Getting a picture of a snapping towel wasn't easy. The students had to learn proper towel snapping technique, no small feat. In addition, the towel tip had to be within a very small area at the time of snappage to be in camera range, the time window for greatest speed was less than a millisecond, the experiment had to be performed in near darkness, etc. But the students needed a grade, so they persevered.

After a month (!!) of attempts, the trio finally achieved a decent snap, and it appeared that the tip of the towel had gone supersonic. But the result was so close to the actual speed of sound that they didn't feel it provided unequivocal evidence, so they kept at it, making minor adjustments in their photographic technique. More failures led to more changes, and finally, three months (!!!) after starting, they had proof positive that the end of their towel did indeed break the speed of sound. The details of their experiments, complete with photos, can be found at www.hiviz.com/PROJECTS/towel/to wel.htm and in Physics Teacher, vol. 31, p. 376 (1993).

Unfortunately, I have to point out one midstream modification of the experimental procedure that makes me skeptical of the results. After many failures, the website mentioned above states, "They also made a new, longer towel from a piece of a cotton bed sheet." I don't know about you or your uncle, Jerry, but the last time I dried off with a cotton bed sheet was never. Which puts a bit of a pall over the claim that a snapped towel exceeds the speed of sound--I mean, why not just substitute a bullwhip for the towel and free up a couple of months for parties? I leave it to you to decide whether your uncle is all wet or really snaps a supersonic towel.

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Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.

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