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Magnifying sound

By David, 3 October 2019 Activity

A man holdiing a pink baloon near his face and a phone at arms length.

Move the balloon closer. Does it get louder or softer?

A magnifying glass can bend light waves to make an object look bigger. Can you make a magnifying glass for sound waves too?

You will need

  • Bottle with a small neck, e.g. soft drink bottle
  • Funnel
  • Balloons
  • Vinegar
  • Bicarbonate of soda (bicarb)
  • Teaspoon
  • Smartphone or tablet

What to do

  1. Someone has put a funnel into the mouth of a balloon.Put the funnel into the balloon’s mouth. And put four teaspoons of bicarb into the balloon.
  2. Someone is using a funnel to pour vinegar into a plastic bottle.Put the funnel into the bottle’s mouth and pour about 3/4 cup of vinegar into the bottle.
  3. The mouth of the balloon has been palced over the top of the bottle.Being careful to keep the bicarb in the balloon and the vinegar in the bottle, put the mouth of the balloon over the top of the bottle.
  4. A plastic bottle is full of bubbles and a balloon is inflating over the mouth of the bottle.Lift the balloon up and let the bicarb fall into the vinegar.
  5. When the balloon is full of gas, take it off the bottle. Tie a knot so the gas doesn’t escape. This is your sound magnifying glass.
  6. Try throwing or bouncing you balloon around. Does it seem a bit heavy?
  7. You’re going to need a sound to magnify. Grab a smartphone or tablet and search the internet to find a test tone of 3000 hertz or 3 kilohertz.
  8. Adjust the volume so it’s very quiet.
  9. Stand about a metre away from the sound source.
  10. A man holding an inflated baloon next to his ear with a mobile phone held at arms length.Put the balloon between your ears and the sound source. Move the balloon closer to you or closer to the sound and listen. Does it get louder or softer?
  11. Try Steps 7 to 10 again with a standard balloon, filled with your breath. Which balloon magnifies sound more?
  12. Can you adjust this activity to make a balloon that magnifies sound even better? Post your findings in the comments section below!

What’s happening?

Magnifying glasses are a type of lens. They bend light, focusing it to a point, which makes things appear larger than they really are. This bending happens because light travels more slowly through glass – an effect known as refraction.

Back to your balloon. The chemical reaction between bicarb and vinegar makes carbon dioxide gas to fill your balloon.

Sound moves more slowly through carbon dioxide because it is denser than ordinary air. This denser gas refracts and bends sound waves. That’s why you can use a carbon dioxide heavy balloon as a lens to focus sound!

If you’ve ever used a magnifying glass, you’ll know you need to move it back and forth to focus the image. It’s the same with your carbon dioxide heavy balloon. You’ll need to get it in just the right spot for the greatest magnification of sound. So if it doesn’t work straight away, keep trying!

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