Measuring the Speed of Sound with a Drum

One student beats a drum at a steady rate while others back away until the strike is seen at the same instant the previous beat is heard. Using the beat period and the measured distance at that point, students estimate the speed of sound.

1. Choose an open, quiet area with clear line of sight (e.g. school oval). Assign one drummer and at least two observers with a tape or measuring wheel.
2. Set a steady beat rate (e.g., 2 beats per second, so the period T = 0.5 s) with a drum (e.g. snare drum) that can be heard crisply from far away.
3. Observers walk straight away from the drummer, watching the stick strike while listening to the sound.
4. Stop when the observers see a strike at the exact moment they hear the previous beat (the sound is one beat behind).
5. Measure the straight-line distance d from drummer to observers.
6. Compute speed of sound v = d / T, where T is the beat period. Repeat and average.

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• Repeat with different beat rates (e.g., 1 Hz, 1.5 Hz, 2.5 Hz) and compare v from each trial.
• Try early morning vs midday to discuss temperature effects on sound speed.

• Conduct the activity in a safe, traffic-free area with clear footing.
• Keep volume moderate and consider hearing protection for repeated loud hits.
• Maintain clear communication so no one backs into obstacles while moving away.
• Do not perform near roads or reflective walls that could cause confusing echoes.

• Why does the method use the moment when the sound is one beat behind what you see? (Because the sound delay equals one full period T of the beats, so v ≈ distance/period.)
• If T = 0.5 s and the distance measured is 170 m, what speed do you get? (v ≈ 170 / 0.5 = 340 m/s, close to the typical value at room temperature.)
• How would temperature changes affect your result? (Warmer air increases sound speed; cooler air decreases it, so trials at different times may shift v.)
• What systematic errors might make v too high or too low? (Imprecise alignment, slanted distance, uneven beat timing, wind, or echoes can bias the measurement.)
• How could you improve precision without changing the basic idea? (Use a metronome, longer distances, multiple trials, and average results.)