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									                                          Poor Man’s Oscilloscope

Sound waves are the result of a vibration. This image from the Physics Classroom shows that sound is a
longitudinal wave. The region where the particles are pushed together is called a compression. These
are areas of high pressure. Where the particles are spread apart, this is known as a rarefaction.
Rarefactions denote low pressure.


An oscilloscope is used to visualize sound waves. Such devices can show the frequency of the wave and
the amplitude or volume of the sounds. Oscilloscopes use microphones to capture the sound. The
microphone converts the sound waves into electrical signals. The longitudinal wave information is
graphed by the oscilloscope and displayed as a transverse wave. The graph or line displayed on an
oscilloscope shows changes in air pressure. The equilibrium line (or horizontal line) shows normal air
pressure. The compressions, or higher than average pressure areas, will be displayed above the
equilibrium line on the oscilloscope. The rarefactions indicate areas where air pressure is lower than
normal and will be below the line. In a transverse wave, the high points of the wave are referred to as
crests. The low points are called troughs. The amplitude is measured from the baseline to the crest or
baseline to the trough. Large amplitudes are the result of greater changes in air pressure or louder
sounds. The frequency of a wave is the number of complete wave cycles (crest to crest or trough to
trough) completed per second. High frequency waves are heard as high pitches. Low frequency waves
are heard as low pitches.

The directions for building a “cheapie oscilloscope” were first seen in Al Guenther’s Science Solutions
and Practical Activities for Strengthening Your Teaching of Physical Science Concepts. I’ve modified
those instructions. The beauty of this demonstration is the massless pointer, or beam of light. This lever
allows you to magnify the small vibrations on the head of the balloon.

· Removed both ends of a steel pineapple can.
· Cut the bottom neck from a 11” sized latex balloon.
· Stretch the balloon over one end of the can and fasten with a large, wide rubber band.
· Cut a 12” piece of ¾” corner molding. Sand ends.
· Cut two pieces of ¾” corner molding 1” and 1 ¼” long. Sand.

Physics : Nickey Walker
· Fasten the can to the inside track of the molding with the edge of the can and edge of the molding
  coinciding. The opened (non balloon) end of the can is flush with the edge of the molding.
· Glue the two smaller pieces of molding one on top of the other. Do not glue to the track yet.
· If you plan on using this demonstration with a laser pen pointer, leave an appropriate amount of space
   to accommodate the length of the pen. You can use either a pen or key chain laser with this device.
· Test laser location before gluing. The beam should strike towards the center of the balloon.
· Press the button on the laser to see where the beam strikes the balloon. Using a disco ball type of
  Christmas ornament, peel one small piece of mirror from the ball.
· Use 3-M’s sticky tac and place a very small amount on the back side of the mirror. Do not buy the
  Elmer’s variety of sticky tac. It does not hold for long. Press mirror onto the balloon. Slide as necessary
  to adjust mirror location with the laser beam. Be careful NOT to look directly into the original or
  reflected path of the laser beam. When the correct location is found, rubber band the laser to the ¾”
· Variations in this can be accomplished by using different sized cans. Ask students if this will affect the
  resulting waves. Will crumpling up a clean kleenix and stuffing it in the can have any affect? Does it
  matter how long you hold a note or speak a certain syllable? Does one letter of the alphabet yield a
  different pattern than another?
· You can also look for cans with plastic lids on the end, like baking powder cans. Cut a small hole in the
  center of the can. Does this make a difference in the resulting patterns?

Physics : Nickey Walker

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