Dancing sprinkles

This activity shows that a loud sound is capable of making small grains jump. You can use it to introduce the idea that sound is a vibration of the air.

Equipment

Each group of students will need:

• Bowl
• Cling film
• Hundreds and thousands sprinkles of the type used for cake decorations
• Large spoon or drumstick
• Metal baking tray, drum or similar to hit to make a loud noise

Procedure

Ask the students to:

1. Cover the top of the bowl with cling film. Stretch it tightly.
2. Scatter some of the hundreds and thousands sprinkles on the cling film.
3. Hold the baking tray close to – but not touching - the cling film and strike it sharply with the spoon.

Discussion prompts

• Why does the baking tray make a sound?
• How do the sprinkles move when they haven’t been touched by anything?

Teaching notes

Students will probably have heard of a ‘sound wave’ but, based on everyday experience (e.g. shouting or whistling), think it involves air travelling en masse from source to detector. In this activity there is no obvious source of moving air. Identify the source, medium and detector in your explanation and introduce the idea that a sound is a vibration of the air.

The baking tray is a sound source because it vibrates when it’s struck. The vibrations are transmitted through the air (the medium) to the bowl and cling film (the detector). The incoming sound wave makes the surface of the cling film move up and down and the sprinkles on its surface dance in response.

Learning outcome

Students describe sound waves as vibrations of the air, initiated by the vibrating source of the sound.

This experiment was safety-checked in March 2020.