Measuring the speed of sound using toilet rolls

First, watch the video.

1. Place toilet roll on a flat surface with one headphone at the bottom of the tube
2. Stack two more rolls on top to make a tunnel for the sound and help isolate experimental sound from other noise
3. Stick some Blue Tack on the cable near the other headphone to make it heavier and keep the cable taut
4. Search YouTube for ‘3 kHz test tone’ or find similar elsewhere
5. Whilst playing the sound, carefully lower the second headphone into the tube as far as it will go. Slowly raise the headphone. Listen carefully. At one point the sound should almost disappear. Make tiny adjustments to find the quietest point
6. Mark the cable at the top of the tube with the pen (or Blue Tack)
7. Move the cable up higher to find the next quietest point, and mark that too
8. Remove the headphone and measure the distance between the marks in millimetres. This distance is the sound’s wavelength
9. Multiply your frequency by the distance (in mm) divided by 1000 (to convert mm to metres). This is your answer speed of sound in metres per second
10. You can check by repeating. You could also try other frequencies which will give different wavelengths, but the calculation should give the same speed (Try 2 kHz and 4 kHz)

SI measurement units

• metre (m) for length
• second (s) for frequency, actually 1/s which is written as hertz (Hz)

Challenge Topics

Measurement Science, Maths, Physics

• Do you get the same speed result for different pitched sounds?
• What happens if you change the type of headphone?
• Because the speed of light is about a million times faster than that of sound, you can use the time difference between seeing and hearing lightning to indicate distance. Each second represents a distance of about 3 km.