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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.
- 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.
- 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.)
- 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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