What it shows
Sound travels along a taut string as vibrations. A loose string cannot carry the vibration well, so the voice becomes faint.
The science
When you speak into a can, the bottom vibrates and these vibrations travel along the string as a wave. At the far end they shake the other can and you hear them as sound. If the string is loose, it does not carry vibrations well and touching it with a finger stops the sound. Solids carry sound better than air because their particles are closer.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two clean empty tin cans or paper cups
- About 5 metres of strong string or fishing line
- A nail to make holes (adult only)
- Two buttons
- Tape
- A measuring tape
Safety
For this project: An adult must make the holes and cover the sharp edges of the cans with tape.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
An adult makes a small hole in the bottom of each can with a nail.
Thread the string through each hole and tie a button inside as a knot.
Move apart until the string is straight and tight.
One person speaks quietly and the other listens at the can.
Let the string go slack and try again.
Touch the string with a finger while speaking and see what happens.
Try longer and shorter strings and note the greatest distance for clear sound.
Record in a table: tight, loose, touched, long and short.
What you should see
Clear voices with a tight string, nearly nothing with a slack or touched string.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How does sound travel in this phone?
As vibrations along the string.
2Why does a finger on the string stop the sound?
It damps the vibrations.
3Does sound travel better in solid or air?
In solids.
4Why is the string kept tight?
A tight string carries vibrations better.
Take it further
Try wire, wool thread and cotton string and compare how loud the voice sounds.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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