Overview

This project lets students see sound.
By stretching a balloon over a container and sprinkling salt on top, you create a simple device that turns sound vibrations into moving, dancing patterns. These shapes are known as Chladni patterns, and they reveal how materials vibrate at different frequencies.

It’s hands-on, visual, and mesmerizing.

Materials

You will need:

• An empty coffee can or similar round container

• Tape (strong masking tape or duct tape)

• A large balloon (14–16 inches works well)

• Scissors

• Table salt

• A long rope (6 ft or more) for a demonstration

• Optional: a musical instrument or speaker to create sound vibrations

Procedure

1. Build the Vibration Drum

1. Cut the neck off the balloon.

2. Stretch the balloon tightly over the open end of the can.

   • You may need a helper to hold it in place.

3. Secure the balloon with tape all the way around.
   Now you have a drum-like surface.

2. Add the Salt

Sprinkle a light, even layer of salt over the stretched balloon.
You don’t want thick piles—just enough to look like a dusting.

3. Create Vibrations

Choose a spot near the edge of the balloon surface.
Using your fingertip, gently rub the balloon surface as if you were slowly scratching it.

You’ll see:

• The salt begin to hop

• Grains move around

• Lines and shapes forming

These shapes appear because different parts of the balloon vibrate differently.

Discovering Patterns

Try rubbing in a different spot now.

You’ll notice:

• The pattern changes

• The “quiet” areas (places where salt settles) move

• Some spots produce stronger movement than others

This happens because you’re exciting different vibration modes each time.

The Science Behind It

1. Vibrations Create Waves

When you rub the balloon:

• The balloon vibrates

• Waves spread out from the spot where you’re touching

• Some areas vibrate strongly

• Some areas barely move

The places that hardly move are called nodes.

2. Salt Moves Away From Motion

Salt bounces around wherever the balloon vibrates the most.
But the grains rest in the quiet zones.

Over time, the salt collects in intricate shapes—those shapes map out the nodes.

These are called Chladni patterns (pronounced klad-nee), named after the scientist who first studied them using metal plates and sand.

Rope Demonstration (Optional but Excellent)

To help students picture nodes and waves:

1. Have one student hold one end of a long rope.

2. You hold the other end.

3. Shake your hand back and forth.

4. A wavelike pattern appears in the rope.

Some points wiggle a lot.
Others hardly move.
The unmoving points are nodes—exactly like the salt’s resting places.

Extension: Use Real Sound

Your balloon drum responds to actual sound frequencies too!

Try:

• Playing different notes on a musical instrument

• Speaking or humming near the surface

• Holding the drum near a speaker

• Playing a movie sound effect with deep bass

You may see:

• Patterns shift

• Salt hop higher

• Certain notes “match” the vibration of the balloon

• Other notes barely move anything

When a note matches the balloon’s natural vibration rate, the salt dances dramatically. This is resonance.

Why This Matters

These patterns help scientists and instrument makers:

• Design violins, guitars, and drums

• Understand how sound waves behave

• Prevent unwanted buzzing or weak tones

• Improve the richness and clarity of musical instruments

Nodes show where parts should be attached so the instrument vibrates freely.

Discussion Questions

• Why does salt gather in some areas but not others?

• What happens when you rub closer to the center vs the edge?

• Do certain musical notes cause stronger patterns? Why?

• How does the rope demonstration relate to vibrating surfaces?

Teacher Tips

• Use a dark balloon so the salt stands out clearly.

• Remind students not to push down too hard—gentle rubbing works best.

• This activity connects beautifully to units on sound, waves, musical instruments, and physics.