What it shows
Sound is collected by the outer ear and makes the eardrum vibrate. A balloon stretched over a tube acts as the eardrum, and grains of salt dance on it when you speak into the funnel.
The science
The outer ear funnel gathers sound waves and sends them down the ear canal to the eardrum, a thin membrane that vibrates. Behind it three tiny bones, the hammer, anvil and stirrup, pass and strengthen the vibrations to the fluid-filled cochlea. Hair cells inside the cochlea turn the movement into nerve signals that the brain reads as sound. The model shows only the first step: the vibrating membrane, made visible by dancing salt.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A plastic funnel or a cone of thick paper (the outer ear)
- A cardboard tube or a cut plastic bottle 6 cm wide (the ear canal)
- A balloon, cut open and stretched flat (the eardrum)
- A rubber band
- A pinch of salt or sugar
- Three small cardboard strips and a pin for the bone chain
- Card, a coiled pipe cleaner or a spiral of wire for the cochlea
- Glue and markers
Safety
For this project: An adult must cut the balloon and bottle. Never put anything into a real ear, and do not shout loudly close to anyone’s ears.
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.
Stretch the cut balloon tightly over one end of the tube and fix it with a rubber band, so it is flat like a drum skin.
Sprinkle a pinch of salt on the balloon surface.
Fit the funnel into the other end of the tube, wide side facing out, and tape the joint.
Hold the model upright and say a long, steady "aaa" into the funnel, close to it but not shouting; watch the salt move.
Try a low voice and a high voice, then a soft and a loud sound, and note which makes the salt jump higher.
Make a chain of three cardboard strips pinned together to show the hammer, anvil and stirrup behind the eardrum on a card drawing.
Coil the pipe cleaner into a snail shape beside the bones and label it cochlea, with a card arrow marking the nerve to the brain.
Label all parts on a base card: outer ear, ear canal, eardrum, bones, cochlea and auditory nerve.
What you should see
The salt grains hop and dance on the balloon when you speak, more strongly with louder and lower sounds. The labelled model shows how sound travels through the ear.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes the salt jump?
The vibrating balloon, which stands for the eardrum, set moving by sound waves.
2Name the three bones of the middle ear.
The hammer, anvil and stirrup.
3What does the cochlea do?
It turns vibrations into nerve signals using hair cells.
4Why should we never push objects into the ear?
They can damage the eardrum and the canal.
Take it further
Compare the salt dance for different distances from the funnel and make a graph of the distance against the height of the jumps.
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.
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