WASHINGTON -- Those annoying people who unwrap candies in the theater simply cannot help making noise, researchers said Friday.
No matter how slowly a piece of wrinkled plastic is unwrapped, it will still emit sounds at a distinct frequency, they told a meeting in Atlanta of the Acoustical Society of America.Eric Kramer and colleagues at Simon's Rock College of Bard in Great Barrington, Mass., said the reason was the creases in that thin little piece of cellophane.
"You can check this for yourself," they said in a paper presented to the meeting.
They said moving an uncreased sheet of cellophane or Mylar, like that found on a cookie package, would make little or no noise. "Next, crumple the sheet into a ball and smooth it back out," they said.
"You should find that a network of sharp creases forms during the compression, and that the sheet will no longer lie flat. Repeat this procedure a few times to generate additional creases. Now you should find that even a small deformation of the material will produce the familiar rustling sound."
Working with Alexander Lobkovsky, who is now at the National Institute of Standards and Technology in Gaithersburg, Md., they made digital recordings of the rustling sound produced by heavily creased sheets of Mylar.
Small deformations put energy into the sheet, which is stored as elastic potential energy. Each time one of the deformations was manipulated, the energy erupted and the plastic sent out a click, Kramer's team said.
"This also explains a fact familiar to anyone troubled by their inability to quietly unwrap candy in a movie theater," they wrote. "You can slow down the frequency of the clicks by decreasing the rate at which you unwrap the candy, but you have no control over click loudness.