What it shows
A tube closed at the bottom with water gives a note when you blow across the open top, and a shorter air column gives a higher pitch. The frequency is roughly inversely proportional to the length of the air column.
The science
When you blow across the top, the air column vibrates and standing waves form. For a tube closed at the bottom and open at the top, the lowest note has a wavelength of four times the effective length of the column, so f = v divided by 4(L + 0.3d), where d is the tube diameter. The effective length is a little longer than L because the vibration extends slightly beyond the open end. A shorter column means a higher frequency, and the product f times (L + 0.3d) should be nearly constant and equal to v/4.
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.
- A tall narrow plastic bottle, jar or PVC pipe, about 40 cm long and 3 to 4 cm wide
- A tall jug or bucket of water to hold the tube upright
- A phone with a free frequency meter or tuner app
- A ruler
- A funnel and a jug for adding water
- Graph paper
Safety
For this project: Use clean tubes and wash the mouthpiece edge if shared. Do not blow hard enough to feel dizzy. Wipe spilt water.
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.
Stand the tube upright in a bucket or hold it in a stand, and pour in water until the air column is 30 cm long. Measure it with the ruler.
Blow gently across the top edge, as when blowing across a bottle, until you hear a steady note.
Hold the phone near and read the frequency from the app. Repeat three times and take the average.
Add water to shorten the air column to 25 cm, 20 cm, 15 cm and 10 cm, and repeat the measurement each time. Measure each length carefully.
Write L, L plus 0.3d, and f in a table and compute 4 times f times (L + 0.3d) for each row.
Plot a graph of f against 1/(L + 0.3d) and draw a straight line. Its slope is v/4.
Find the speed of sound from the slope and compare with about 340 m/s.
Draw the standing wave in the tube with a node at the water and an antinode at the top.
What you should see
The pitch rises as the column shortens, from about 275 Hz at 30 cm to about 770 Hz at 10 cm, and the calculated speed of sound comes near 330 to 350 m/s.
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 resonance?
The strong vibration of a system when forced at its natural frequency.
2Why is there a node at the water surface?
Air cannot move at the closed end, and moves most at the open end where the antinode is.
3Why does the pitch rise as the column shortens?
The wavelength is shorter, and v = f times the wavelength, so the frequency is higher.
4What is the end correction?
The small extra length, about 0.3 times the diameter, because the antinode is slightly beyond the open end.
Take it further
Make a set of eight tubes tuned to a musical scale using your frequency app.
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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