Sound Waves Longitudinal, Traces Transverse
If sound waves are longitudinal, why are transverse
(up-and-down) 'waves' seen on the screen of the oscilloscope above
when someone whistles into the microphone?
The oscilloscope is plotting voltages produced by the microphone
sensor, not the actual displacement of air molecules in the
sound. The oscilloscope plots two variables: voltage in the
vertical axis, and time in the horizontal axis. It is not showing
the motion of air molecules through space. To plot that properly,
you would need a plot with rather more information on it that a
simple oscilloscope squiggle. You would need to show the positions
of multiple air molecules at different times.
When I think about it, that might make a very interesting plot. I
need to consider how to actually make one...
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
A sound wave would be quite difficult to actually see on a screen. You
could not view the forward and backward motion of the air. The
oscilloscope shows something that can be measured and pictured, the
electrical signal that the microphone emits. A microphone usually has a
thin surface that gets pushed in and out by the sound waves. When the
surface is pushed inward, the signal is positive. When the surface is
pulled outward, the signal is negative. Really, this would just be a
motion up and down on the screen. Because an oscilloscope "remembers"
the signal, the horizontal axis can be used as time. To imagine what
the sound wave really is, hold the microphone vertically and speak into
it from directly above. Forget about the time axis. Imagine the
up/down motion always lined up with the microphone. This is the sound
wave pushing and pulling on the microphone's surface. This is the
moving back and forth that the air molecules do when a sound wave passes
through the air.
Dr. Ken Mellendorf
Illinois Central College
This has to do with the nature of the instrument itself. An
oscilloscope simply picks up the fluctuations of pressure in the
longitudinal wave and transforms them into electrical impulses. It
uses these impulses to "draw" a picture of the wave. It can only
draw a sine wave or a square wave or a saw tooth wave. This is the
best representation of an invisible phenomena that we can produce at this time.
Do not get the "model" confused with the thing it is trying to represent.
Thanks for your question,
Mrs. Martha Croll
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Update: June 2012