» MAKE: NOISE — Discuss this article

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• Resonance
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  The jello in each straw is resonant at about 600hz due to the size of the straw. Since jello is the definition of dampened oscillation, the energy of those waves at the resonant frequency is absorbed by the jello and converted to heat.

  Posted by rmd6502 on May 07, 2012 at 21:46:15 Pacific Time

• Sound sucker
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  I'll agree with "resonance" as the root of the phenomenon, but I don't think absorption by the jello is part of the solution.
  The wavelength of 660 Hz sound in free air is 1100(fps)*12(in/ft)/660(Hz) = 20 inches.
  If 5 inches of straw extends above the jello "stop", the tube is exactly a quarter wave long, and the round trip for sound from the open end to the stop and back is 1/2 wave.
  This makes the reflected sound (at 660 Hz) exactly 180° out of phase with the incoming, and they cancel out.

  Posted by Dave_Bell on May 07, 2012 at 23:21:50 Pacific Time

• LC Oscillator
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  I'm too tired to figure out how to model the jello, which I suspect is important, but this noise cancelation effect is because closed-ended tubes act like series LC oscillators.
  
  The inductance of a tube is equal to the density of the fluid flowing through it, times the length of the tube, divided by the area of the tube.
  
  The compliance of the closed end is the enclosed volume divided by the bulk modulus of elasticity, if memory serves, but I think that's wrong.
  
  Granted, with fluids, you get funky units like kg/m^4 for inductance, instead of Henrys for electricity, but the math all works out the same.
  
  Of course, how much of the closed tube is an inductive tube, and how much is a compliant chamber? Tough question, and if I hadn't happened to leave a specific textbook at my parent's house, I could probably tell you how to really model that. The jello probably adds an appreciable amount of compliance.
  
  Fun fact: This is the same principle used in those ceiling tiles with all the funky holes poked in them. Different lengths and areas cause all the holes to resonate (and cancel out) different frequencies, causing those ceiling tiles to be broad spectrum audio dampers; a desired property in the hallways, classrooms, offices, and most other institutional establishments in which you see them.
  
  Someone with a fluids text at hand, feel free to jump in with the actual math and claim all the glory.

  Posted by PhirePhly on May 08, 2012 at 00:24:37 Pacific Time

• Very simple explanation.
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  Standing waves creating cancelling resonance.
  
  http://en.wikipedia.org/wiki/Null_(physics)#Standing_waves

  Posted by BBQ Steve on May 08, 2012 at 04:48:45 Pacific Time

• From my dad..
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  (Posted for my father - who was having problems sending)
  
  The tone pressure wave travels down the straw and is reflected at the bottom of the cup. It is reflected at the bottom in the opposite sense from the incoming wave and travels back up the tube. The reflected wave is 180 deg out of phase from the incoming wave and cancels the incoming wave in a destructive cancellation in a region near the end of the straw.
  
  In 1968 I worked at Boeing Aircraft Company on a project to reduce engine noise inside the B-737 fuselage. We tested a honeycomb structure that was cemented to an aluminum plate at one end. The other end was covered by a burlap type cloth. The honeycomb thickness was 1/4 the length of the primary jet engine frequency. Such a configuration was very effective at reducing the dB level of the primary jet engine noise. [He included a graph and how to calculate the proper length, I've scanned and uploaded here: startup-something.com/files/wave.pdf )

  Posted by bigattichouse on May 13, 2012 at 16:08:11 Pacific Time