What it shows
A vibrating plate has nodal lines where it stays still, and sprinkled salt collects on them to draw beautiful patterns. Each pattern belongs to one natural frequency of the plate, and higher frequencies give more complex figures.
The science
A plate fixed at its centre has certain natural frequencies at which it resonates in a standing-wave pattern. Some lines on the plate, the nodes, stay still, while the regions between them move up and down strongly. Grains of salt are thrown off the moving regions and collect on the nodal lines, tracing the pattern. Higher resonant frequencies have more nodal lines, and the frequencies are not simple multiples of each other.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A thin square steel or aluminium sheet about 15 cm wide (a flat biscuit tin lid works)
- A small loudspeaker of 8 ohm from an old radio
- A bolt, nut and strong glue to fix the plate at its centre above the speaker cone
- A phone with a tone generator app and a small 3 W audio amplifier board run from a 5 V USB power bank
- Table salt or fine sand
- A tray to catch spilt salt
Safety
For this project: An adult must cut the metal sheet and file the edges smooth, and must supervise the wiring of the speaker; keep volume moderate.
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.
Step 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.
Glue the bolt to the centre of the plate and fix it to the centre of the speaker cone so that the plate lies flat and level above the cone.
Connect the phone’s headphone output to the amplifier board, the amplifier to the speaker, and place the tray under the plate to catch salt.
Sprinkle an even thin layer of salt over the plate.
Start the tone generator at about 100 Hz and slowly raise the frequency; the salt will jump about when the plate resonates.
At each frequency where a clear pattern forms, hold the tone and note the frequency, then photograph the pattern.
Move through at least five different resonances up to a few kilohertz, noting that the patterns get more detailed.
Draw each pattern and label it with its frequency and its number of nodal lines.
What you should see
At particular frequencies the salt gathers into sharp lines, such as a cross, a circle or a star pattern, and the patterns have more lines as the frequency increases.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What is a nodal line?
A line on the plate that does not move during vibration.
02Why does the salt collect on the nodal lines?
It is shaken off the moving parts and settles where the plate is still.
03Why does each pattern need a particular frequency?
Only at a natural frequency does the plate resonate with a large amplitude.
04Where do musical instruments use this idea?
Violin makers tune the top and back plates by looking at their Chladni patterns.
Take it further
Use a round plate and compare its patterns with the square one.
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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