Beats (constructive and destructive interference) Demo
F. Lee 12/20/09

This demo uses a computer to play two close but different tones on two separate speakers, creating beats (wooawooawooawooa...). Students can put their ears to each speaker to hear flat tones from each to verify the beats are a result of the two tones combined.

Materials
  • PC with Audacity installed
  • A pair of stereo speakers

Directions for creating a 1 Hz beat
  1. Download and install the free sound editing program Audacity (for PC, Windows): http://audacity.sourceforge.net/
  2. Generate a 440 Hz tone with amplitude 0.5 (Generate => Tone...). We half the amplitude so if you decide to add the two tones later, the constructive interference will result in an amplitude of 1 instead of being clipped off.
    beats01.png
    After you click Generate Tone, a track will appear and the waveform will be automatically selected.
  3. Click in a blank area beneath the track to unselect the waveform (otherwise it will be overwritten when you generate the next tone).
  4. Generate a 441 Hz tone with amplitude 0.5 (Generate => Tone...)
    beats03.png
  5. Change one of the tracks so it plays only on the left channel (speaker) (Audio Track => Left Channel)
    beats04.png
  6. Change the other tone so it plays only on the right channel. (Audio Track => Right Channel)
    beats05.png
  7. Each tone should show they’re playing on opposite channels. You can first show what each channel sounds like individually by pressing the Mute button on the other track and pressing the green Play triangle button at the top. Play both tones together by making sure both channels are not muted, then press Play.
    beats06.png
    You should hear a satisfying “wooawooawooa” sound, a product you can explain to your kids as “magic”.

To see what the waveform looks like if they’re added
  1. Make both tracks mono so they’re both on the same single channel. (Audio Track => Mono... Do this to both tracks)
    beats07.png
  2. Select both tracks by going to Edit => Select... => All
  3. Combine the two tracks by going to Project => Quick Mix

Tips
  • Zoom by clicking the Zoom in button so you can see the actual peaks and troughs of the wave
  • Compare all three waveforms (440 Hz, 441 Hz, and the combined waveform) by regenerating the 440 Hz and 441 Hz tracks so three tracks are displayed (stacked above each other). It’s about impossible to fit one beat on the screen and still be able to see the individual peaks on the 440 Hz and 441 Hz tracks, so if you want to see the constructive interference, do this using tones that result in a higher frequency beat, such as 440 Hz and 450 Hz.
    beats08.png
  • Misconception alert: Students will see the sounds represented on these graphs and may think they are looking at the actual sounds... which may lead them to believe sounds are transverse waves. Sounds are longitudinal waves, not transverse waves. What the students see here are visual representations of waves... graphs of the amplitude of the wave versus time.