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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Activity 15. Speed of sound


More details on this experiment are provided in the Teachers section.


Equipment needed:

  • 2 Vivitar 283 flash units
  • 8 AA batteries or 2 SB-4 AC adapters
  • 2 cords to connect flash units to sound triggers
  • 2 flash clamps
  • 2 piezoelectric sound triggers
  • High-frequency clock (such as the clip clock)
  • Electronic stroboscope
  • Meter stick and protractor
  • 2 wooden blocks
  • Note: Before doing this activity, try Problem 1 in Appendix F for practice.

    1. For this and the next activity, you’ll be using two sound triggers and two flash units. Set up the high-speed clock as you did in Activity 3 and measure its frequency. In that activity, you were measuring the duration of a single flash. In this activity, you’ll measure the time interval between two flash discharges to use in determining the speed of sound.
    2. Connect a sound trigger to each flash unit (in yellow automatic mode) and arrange them so that they illuminate the clock disc. Adjust each trigger for maximum sensitivity. (Note that if the sensitivity is too high, the flash will not discharge at all.) Place both triggers next to each other and stand about a meter away. Clap the blocks together, watching the rotating clock disc while you are doing this. If the two triggers have identical sensitivities, the flash units will discharge simultaneously, giving a single image of the radial line on the disc. If you see two images separated by more than a few degrees, adjust the sensitivity of one trigger until it matches that of the other. (In order to determine which flash discharged first, put a piece of colored acetate film--such as a report cover--over one of the flash units.)

    1. Now place the sound triggers about a meter apart as shown in the diagram above. Clap the wooden blocks next to one of the triggers while watching the clock disc. You should see two images of the hand of the clock. You can either estimate the angle between the images or take a measurement from a photograph of the clock disc. Once you know the angle, calculate the time between the flash discharges. Dividing that into the distance between the sound triggers will give a value for the speed of sound. (If you’re making visual estimates, move the triggers far enough apart to give a 90° angle between the images of the clock hand.)



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