What it shows
The pitch of a note from a tapped bottle depends on the amount of water in it: more water gives a higher note when tapped. You tune bottles into a scale and learn how vibration speed sets pitch.
The science
Tapping a bottle makes the glass and the water vibrate together, and the heavier the vibrating water, the slower the vibration and the lower the note. Fewer vibrations per second means a lower pitch. Blowing across the mouth of a bottle is different, because then the air column vibrates and more water makes the note higher.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 8 identical glass bottles or tumblers
- Water and a jug
- A spoon of steel or a wooden stick
- Food colour, optional
- A ruler
- Tape and a marker
- A phone with a tuner app, optional
Safety
For this project: Use unchipped glass and tap gently. An adult must handle any broken glass, and keep the bottles away from table edges.
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
7 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 eight bottles and label them 1 to 8.
Leave bottle 1 empty and add a growing amount of water to the others, a little more each time.
Tap each bottle with the spoon and listen to the note.
Pour in or take out water until the notes rise like a scale: do, re, mi, fa, so, la, ti, do.
Measure the height of the water in each bottle and write it in a table.
Plot the water height against note number.
Play a simple tune such as a national song and note which bottles you used.
What you should see
A graph of water height against note shows a steady rise with the water level, and you can play a tune on the tuned set. The notes get lower as the water gets deeper in a tapped bottle.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is pitch?
How high or low a note sounds.
2What changes pitch when you tap?
The amount of water, which changes the vibrating mass.
3What is frequency?
Vibrations per second, measured in hertz.
4What happens if you blow across the top instead?
More water gives a higher pitch because the air column is shorter.
Take it further
Use a tuner app to measure the frequency of each bottle and graph frequency 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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