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Measurement at home

# Measuring the speed of sound using toilet rolls

## How fast does sound travel?

• Can you measure the speed of sound?
• If you measure it twice, can you get the same value?
• NPL uses this technique to measure temperature very accurately

Estimated time: 20 minutes
No experience needed

### Equipment required

• 3 un-used toilet rolls
• Blue Tack
• Smartphone or tablet for YouTube
• Ruler or tape measure
• Optional pen (non-permanent)

### Risks

• For hygiene reasons please use unused toilet rolls (you'll need them to be full size anyway)
• The sounds you play are quite pure and loud if listened to directly. It’s best not to put earphones in ear when playing them

Measuring the speed of sound report

## Steps

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)

## Thoughts, tips and information

### 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.

## Enter your results

Please enter your results in the table below. You may enter as many results as you like THOUGH PLEASE REFRESH THIS PAGE BEFORE EACH NEW ENTRY. Each time you click Submit, you will send us the information written in the cells below.

 Frequency (Hz): Wavelength (mm): Speed of sound = frequency x wavelength divided by 1000 (m/s): Comments: Your first name*: The first half of your post code*: Age*: Please select Under 11 11 - 18 Over 18 * These are optional – the name is so we could mention you in the end of week report and the postcode is to see where people are taking part. We won’t use this information for anything else. Verification: