3B55.55 - Diffraction Around an Obstacle - Through A Fan

Code Number:
3B55.55
Demo Title:
Diffraction Around an Obstacle - Through A Fan
Condition:
Excellent
Principle:
Diffraction of Sound
Area of Study:
Acoustics
Equipment:
PVC Tube, Buzzer and Battery (Doppler App.), and Fan.
Procedure:

Put the buzzer and battery assembly in the end of the tube and then point the other end of the tube either above or below a table. This will show some properties of the diffraction of sound around an object.

Hold the buzzer and battery assembly on the back side of a running fan while the class listens to the sound coming through the fan. Properties of the dependence of diffraction on wavelength may be explored with this demonstration.

References:
  • Joe L. Ferguson, "Why Can We Hear but Not See Around a Corner?", AJP, Vol. 54, #7, July 1986, p. 661.
  • Bertram Schwarzschild, "Diffraction Around the Head Makes Hearers Mislocate Sound Sources", Physics Today, March 2010, p. 16.
  • Robert Ehrlich, "Q.4, Sound Through a Fan", Turning the World Inside Out, p. 195.
  • Robert Ehrlich, "Q.6, Moving a Beeper Below Desk Level", Turning the World Inside Out, p. 197.
  • Robert Ehrlich, "Q.5, Diffraction with a Rolled-Up Carpet", Turning the World Inside Out, p. 196.
  • Jearl Walker, "3.21, Cheerleader's Horn, Foghorns", The Flying Circus of Physics Ed. 2, p. 154.




Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.