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Conversation Piece
A balloon acts as a lens to focus sound.
Conversation Piece
A balloon acts as a lens to focus sound.

Sound energy spreads out as it travels away from a source, but a balloon filled with carbon dioxide gas can focus sound, acting like a lens to create a loud spot.

Tools and Materials
  • Sturdy balloon, 12 to 16 inches (30 to 40 centimeters) in diameter
  • A way to fill the balloon with carbon dioxide (CO2) just before use, such as a tank of carbon dioxide or with dry ice. If using dry ice, you'll need:
    • Crushed dry ice (available at many supermarkets, fishing/bait shops and some liquor stores)
    • Thick, lined leather gloves (not shown)
    • Tongs (not shown)
    • A flask or 1- or 2-liter plastic soda bottle
  • Optional: Toy metal clicker or other noisemaker such as a radio or cell phone
  1. Carbon dioxide leaks rather rapidly through rubber, so inflate the balloon with carbon dioxide just before you use it. If using a tank of carbon dioxide, fill the balloon until it is full size. If using dry ice as a source for your CO2, put about 1/4 cup (60 mL) ice into the plastic bottle or flask. Caution: Be sure you don’t touch dry ice with your bare skin! Then fit the balloon securely over the top of the bottle. The balloon will slowly fill with carbon dioxide gas as the dry ice warms. Be patient: It usually takes 10 to 15 minutes. To inflate the balloon more quickly, put the bottle in some warm water while the balloon is filling.
  2. When the balloon is full, remove it from the bottle or flask and tie it off. You now have a “lens” that will focus sound.
To Do and Notice

Have one person stand in place and hold the balloon up to his or her ear. Have a second person stand on the other side of the balloon, about 3 feet (1 meter) away from the center of the balloon, and make noise: encourage him or her to talk, sing, click the clicker, or use some other noisemaker to produce sounds. The first person should then move the balloon around to find the position in which the sound is loudest.

The point where the sound is the loudest should be on a straight line from the sound source through the center of the balloon to about 18 in (46 cm) away from the center. The exact distance will depend on the size of your balloon and the position of the sound source. When you find the loudest location, remove the balloon and listen to the noise again. Notice that the sound is now quieter.

What happens if you change where you’re standing? Have the noisy person move to another position and have the first person find the loudest point again. Notice that the loud point moves in a direction opposite to the motion of the sound source. When the sound moves up, the loud point moves down. When the sound moves closer, the loud point moves farther away. (Note that if you move the sound source too close, there will be no loud point.)

Try moving farther and farther away. Find the loudest point for a distant sound (one that’s more than 10 feet [3 meters] away). The distance from the center of the balloon to this point is the focal length of the balloon.

What’s Going On?

Carbon dioxide (CO2) molecules are more massive than the nitrogen, oxygen, and argon molecules that make up air (N2, O2, Ar), so the speed of sound in the carbon dioxide gas is slower than the speed of sound in air.

As sound waves enter the carbon dioxide-filled balloon, they slow and bend, just as light waves slow down and bend when they pass from air into a glass lens. The sound waves that pass through different parts of the balloon bend by different amounts and then come together at one point on the other side of the balloon, creating a loud spot. The balloon focuses sound waves to create a loud spot much the way a magnifying glass focuses the sun’s rays to create a hot spot.

Going Further

A balloon full of helium does not focus sound. Helium is lighter than air, so the speed of sound in helium is faster than the speed of sound in air. A helium balloon causes sound waves to diverge, making sound spread out more quickly than it would in air.

When you’re done using the balloon as a lens for sound, try inflating another balloon to the same size just with air and observe what happens when you drop both at once. You'll notice the carbon dioxide-filled balloon falls faster. That's because carbon dioxide is denser than air.