What it shows
A sound source moving towards you sounds higher in pitch, and one moving away sounds lower. Astronomers use the same idea with light to find that distant galaxies are moving away from us.
The science
When a source moves towards you, each sound wave starts a little closer, so the waves bunch up and the pitch rises. When it moves away the waves stretch and the pitch falls. The change in frequency depends on the ratio of the source speed to the speed of sound. Light shows a similar shift: stretched light looks redder.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small battery buzzer or a phone playing a steady tone inside a padded box
- A strong string 1 m long
- A second phone with a free tuner app
- A tape measure
- A stopwatch
- Soft padding and a cloth bag
Safety
For this project: Whirl only a light device in a soft padded bag, at a safe distance from people. An adult must supervise, wear a hat, and keep others far away.
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.
Put the buzzer or phone in the cloth bag and tie it to the string securely.
Stand 5 m away from the whirling area with the second phone ready to listen.
Whirl the source in a horizontal circle about 1 m wide at a steady speed.
Record the sound for 20 seconds with the tuner app running.
Read the highest and lowest pitch values seen, and subtract.
Time ten turns to find the speed of the source as circumference divided by time.
Compare the measured frequency shift with the value predicted by speed divided by 343 m per second.
Repeat at a faster speed and compare.
What you should see
A pitch that rises and falls in a regular pattern, with a larger shift at a higher whirling speed. The shift matches the prediction within about 30 per cent.
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 the Doppler effect?
A change in observed frequency due to motion of the source or the observer.
2Why does an ambulance siren drop in pitch as it passes?
It moves from towards you to away from you.
3What does redshift of a galaxy tell us?
The galaxy is moving away from us.
4Which speed is needed to predict the shift?
The speed of the source compared with the speed of sound.
Take it further
Use a spectrum app to record a passing train or bus horn and measure the shift.
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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