For a school science experiment, we have stripped an audio cable and hooked the right channel and ground wires up to a voltage graphing device. The audio cable is plugged into the audio output of our computer. The computer is playing a constant sine wave tone at 880Hz, and so we'd expect the voltage graph to have a period of 1/880 seconds. However, the voltage graph is instead showing a very slow sine wave with a period of 12 seconds. When doubling the frequency of the sound we were playing, the period halved to 6 seconds. Why would the computer be outputting a different frequency voltage than we were playing?

  • \$\begingroup\$ Are you also listening to the audio? Is it the right pitch? \$\endgroup\$ – Pete Becker Aug 24 '15 at 16:15
  • \$\begingroup\$ @Pete Becker Yes. When we played it on a regular speaker it sounded like an 880Hz sine wave should \$\endgroup\$ – Parker Hoyes Aug 24 '15 at 16:17
  • \$\begingroup\$ What exactly is the audio source? What level? What is the sources target impedance? Is it terminated properly? How are you coupling to the 'graphing device'? For that matter, what is the 'graphing device' ? I would normally think oscilloscope, but don't want to make any assumptions. \$\endgroup\$ – R Drast Aug 24 '15 at 16:25
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    \$\begingroup\$ What kind of "voltage graphing device" are you using? If its sampling frequency is below 1760 Hz (2*880Hz) you can't expect to plot correctly the sine wave. That's called "aliasing", google for details. \$\endgroup\$ – Nicolas D Aug 24 '15 at 16:29

The graphing device in the question was a Vernier LabPro with a voltage sensor. Silly me didn't realize the sampling frequency was 20Hz. That means that the sine wave from the computer could complete 44 times in between samples from the sensor. Credits to @Nicolas D for pointing this out. Will leave this question up for anyone else with a similar confusion.


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