Seeing Sound Vibrations with Rice

No adult supervision needed. Do not bang the tray close to anyone's ears, and keep cling film away from babies and toddlers.

Protective equipment and “do not substitute” warnings have not been recorded for this experiment yet. They are added when its safety review is completed; until then, follow the supervision notes above.

Disposal: Put the rice in food waste. Reuse or recycle the cling film where your local collection accepts soft plastics; otherwise general waste.

Materials

• A bowl
• Cling film
• A pinch of uncooked rice
• A metal baking tray
• A wooden spoon

Steps

1. Stretch cling film tightly over the top of the bowl so it is flat like a drum skin.
2. Sprinkle a few grains of rice on the cling film.
3. Hold the baking tray about 30 cm from the bowl, without touching the bowl or the table it is on, and hit the tray with the spoon.
4. Try a gentle tap and then a harder hit, and watch the rice each time.
5. Move the tray further away, about 1 m, and hit it as hard as before.

What you should see

• The rice grains jump when the tray is hit, even though nothing touches the bowl.
• A harder hit makes the rice jump more.
• With the tray further away the rice moves less.

Why it works

Hitting the tray makes it vibrate. The vibrating metal pushes and pulls on the air next to it, making regions where the air is squeezed together and regions where it is spread out. These travel outwards through the air as a sound wave.

When the wave reaches the stretched cling film, the changing air pressure makes the film vibrate too, and the vibrating film throws the rice grains up. Sound is a disturbance of matter travelling out from its source; you are seeing it arrive.

Learn the ideas behind it

• Energy
• Waves

Explore it further

The same ideas, as something you can change and watch on screen:

• Waves & Interference (simulation)

Sources

• College Physics 2e, 17.1 Sound — OpenStax