What it shows
Tapping a glass bottle makes the glass and the water vibrate, and more water makes the vibration slower and the note lower. Blowing across the top vibrates the air instead, so the bottle with the most water gives the highest note.
The science
Every sound comes from something vibrating. When you tap the bottle, the glass and the water inside shake together, and the extra water makes the whole thing heavier and slower to vibrate, so the pitch drops. When you blow across the mouth, it is the column of air above the water that vibrates, and a short column of air vibrates faster than a long one. So the same bottle can give opposite results depending on whether you tap it or blow across it.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Six identical glass bottles or glasses
- Water and a jug
- Food colour to make the levels easy to see
- A metal spoon or a pencil to tap with
- A ruler to measure the water height
Safety
For this project: Tap glass gently; an adult must clear up if any bottle breaks.
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.
Line up the six bottles and pour different amounts of water into them, from almost empty to almost full.
Add a drop of colour to each and measure the water height with the ruler, writing it on a label.
Tap each bottle on the side with the spoon and listen: the fullest bottle gives the lowest note.
Arrange them in order of pitch and play a simple tune like a scale.
Now blow gently across the mouth of each bottle until it hums; the fullest bottle now gives the highest note.
Make a table with water height, pitch when tapped and pitch when blown.
Adjust the water in two bottles until they sound the same note, and check that their water levels match.
Explain in your report why tapping and blowing give opposite orders.
What you should see
A set of bottles that play a scale. When tapped, the pitch falls as water is added; when blown across, the pitch rises as water is added, because a different thing is vibrating each time.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What vibrates when you tap the bottle?
The glass and the water inside it.
2What vibrates when you blow across the top?
The column of air above the water.
3Why does more water give a lower note when tapped?
The extra water makes the bottle heavier and slower to vibrate.
4How would you tune two bottles to the same note?
Make their water levels the same.
Take it further
Use a free tuner app on a phone to find the frequency of each bottle and plot it against water height.
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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