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Name: Mariyam
Status: student
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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
Physics Instructor
Illinois Central College

Dear Marilyn,

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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