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Balloon Powered Sound! Build Your Own Balloon Amplifier


Key Concepts
Sound waves, acid-base reactions, energy
Megan Arnett, PhD, Science Buddies


Have you ever watched an old movie, or a cartoon, where a character holds a cone to their ear in order to hear what people are saying? Believe it or not, those cones (known as ear trumpets) were the very first hearing aids and were very popular for a time. Even Ludwig Van Beethoven used one! They helped people hear by acting as a sound amplifier, collecting sound waves and focusing their energy into the ear.

In today’s activity you are going to make a different type of sound amplifier – using a balloon and some kitchen chemistry. Time to turn up the volume!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


An amplifier is a device that increases the power of a signal. The signal could be anything from the sound of someone speaking, to the voltage generated by an electronic source. We are surrounded by amplifiers in our daily life, from public address systems in our schools and airports, to long distance telephone lines.

In this activity we will use chemistry to create our own sound amplifier.

The sounds we experience are actually mechanical waves traveling through the air (or whatever medium surrounds us, sound can travel through water too!). These waves are caused by the back and forth vibration of particles in the air (or other medium).

When you clap your hands, you displace the air particles between and around your hands, causing a compression wave to travel through the air. A continuous sound (like a note played on a violin) is caused by the vibrations of the violin string. The vibrations of the string repeatedly compress and displace the air particles around them, causing a repeating pattern of compressions that we hear as a single, continuous tone. When the strings move more slowly there is more time between each compression, resulting in a lower frequency sound wave.

We increase the volume of the sound we hear by increasing the energy of the sound waves. The amount of energy we can sense is known as the intensity of the sound. When thinking about energy, we have to consider the area or distance that it is traveling, and the amount of time it takes to travel. To understand why distance is important, think about someone shouting to you from 3ft away, or from 20ft away. The person standing 3ft away is going to sound louder than the same person shouting at you from 20ft away. In addition, when multiple sounds reach our ear at the same time, it sounds louder than if we hear the same sounds one at a time, because all of that energy is arriving simultaneously.

In this activity, we’ll our balloon amplifier will allow us to focus the energy of sound waves traveling to our ear.


  • 2 balloons
  • 1 clean 16oz plastic water or soda bottle
  • 4 teaspoons baking soda
  • ½ cup white vinegar
  • Small plastic funnel (if unavailable, use tinfoil or parchment paper to make a temporary funnel)
  • Sink or large casserole baking pan with an edge
  • A partner to help you with the testing phase
  • Optional: sound producing devices and instruments such as a recorder, whistle, clicker


  1. Carefully pour the vinegar into your water bottle.
  2. Use your funnel to slowly pour the 4 teaspoons of baking soda into one of the balloons. Set the balloon aside but do not tie it closed. Make sure that it’s positioned so that the baking soda will not spill out.


  1. Attach the baking soda filled balloon to the top of the water bottle. Avoid letting the baking soda fall into the bottle. To do this, hold the mouth of the balloon and shake the baking soda down to the bottom. Keep holding it this way while you gently cover the top of the bottle with the mouth of the balloon, never allowing the contents of the balloon to drop into the bottle.
  2. Make sure the mouth of the balloon covers the top of the bottle as completely and securely as possible.
  3. Place the bottle in a sink or a large baking pan. Note the size of the balloon, and the appearance of the vinegar in the bottle.
  4. Carefully tip the balloon vertical, so the baking soda spills into the bottle. What happens when the baking soda contacts the vinegar in the bottle? Is the liquid in the bottle changing? What happens to the balloon? What do you think is causing the balloon to change shape?
  5. When the reaction slows, you can gently shake the bottle and tap the balloon, to ensure no baking soda is stuck in the balloon or on the sides of the bottle.
  6. When the reaction is complete, ask your partner to help you remove the balloon from the bottle without allowing gas to escape the balloon. While the balloon is still attached, have your partner tightly squeeze the balloon closed just above where the mouth of the balloon meets the bottle. While they hold the balloon closed, you can gently remove the balloon from the bottle. Don’t be surprised if a puff of gas escapes from the bottle when you remove the balloon!
  7. Tie off the balloon. For the rest of this activity, we’ll call this balloon “Balloon 1”
  8. Take the second balloon and blow it up to the same size as Balloon 1. We’ll call the second balloon “Balloon 2”.
  9. Have your partner stand approximately 3ft away from you. Ask them to sing a song, or recite the Pledge of Allegiance. While they are singing, turn so that your right ear is pointing toward them. Listen to your friend for a few moments. Notice the volume of their voice, and anything else specific about how they sound.
  10. Hold Balloon 2 up to your right ear. Slowly move your head from side to side, and up and down. Try standing on your tiptoes, or crouching lower. Make sure to keep your ear with the balloon pointing toward your partner.  Does the balloon change the sound of their voice? If so, in what way?
  11. Place Balloon 2 somewhere safe to protect it from popping.
  12. Repeat step 9, but this time use Balloon 1. Have you partner continue to sing, and turn your body so that your right ear is pointing toward them. Pause a moment to notice their voice.
  13. Hold Balloon 1 up to your ear, and slowly move your head from side to side, and up and down. Try standing on your tiptoes, or crouching lower. Make sure to keep your ear with the balloon pointing toward your partner. Does the balloon change the sound of their voice? If so, in what way?
  14. Optional – experiment with other sounds by using musical instruments, whistles, or clickers. Explore whether the balloon changes the sounds you hear, and how the change differs with different types of sound.
Extra: Have your partner walk farther away and closer to you. Does your balloon amplifier still work when they are 5ft away? What about 10ft? Have them move to the right and left. Notice how you have to reposition your balloon in order to amplify their voice.
Extra: Do you notice any other differences between your two balloons? Do they feel different? Is one heavier or lighter than the other?
Extra: Explore other differences between your two balloons. Try holding one in each hand, and dropping them from shoulder height. Do they drop at the same rate? What if you throw the balloons?

Observations and Results

In the first part of this activity you combined baking soda and vinegar to create the gas carbon dioxide or CO2. You may be familiar with this reaction, as it is the same one used to create the popular science fair volcano project. In this reaction, baking soda and vinegar are known as the reactants, because they take part and undergo a change during the reaction.  Baking soda is a bicarbonate and vinegar is a weak acid, and combining them results in a 2-step reaction. Step 1 is an acid-base reaction and step 2 is a decomposition reaction. The final products of this 2 step process are carbon dioxide (CO2) and water (H2O). The CO2 gas caused the bubbling and foaming you observed when you added the baking soda to the vinegar. This reaction produced so much CO2, the bottle couldn’t hold all of it. Therefore, the CO2 gas moved into Balloon 1, causing the balloon to expand. The reaction ended because all of the baking soda mixed with the vinegar and was changed to form carbon dioxide (CO2) and water (H2O), so there was nothing left to react.  

In the second part of this activity you compared whether Balloon 1 or Balloon 2 could act as an amplifier. Balloon 1 contained carbon dioxide (CO2), whereas Balloon 2 contained the air you exhaled while blowing it up. The air we exhale is mostly nitrogen and oxygen, and only about 4-5% carbon dioxide. Therefore, in this part of the activity you were exploring whether the mostly pure carbon dioxide in Balloon 1 was a better amplifier than the mostly nitrogen and oxygen gas in Balloon 2.

When you held Balloon 1 up to your ear, you should have noticed that the volume of the sound increased. You may have needed to move your head around to find the right position, but when you found it, your partner’s voice should have sounded noticeably louder. Amazing! Carbon dioxide acts as an amplifier because CO2 molecules are larger than the molecules found in our air. Similar to how walking through water forces you to walk more slowly than you walk on land, these larger molecules slow the movement of sound waves moving through them. As the sound waves entered the balloon on their way to your ear, the CO2 gas caused them to slow and bend slightly. Similar to the way in which a magnifying glass magnifies light by causing it to slow and bend to come together at a certain point, the CO2 gas in your balloon slows and bends sound waves to create a focal point on the other side of your balloon. As a result, holding the balloon in a certain position creates a sound spot where your partner’s voice sounds significantly louder.

When you held Balloon 2 up to your ear, you should not have noticed a significant difference in the quality of the sound, or the volume. This is because the gas in Balloon 2 was similar to the air in our environment, and did not significantly change the shape or speed of sound waves passing through it.

You may have also noticed other differences between Balloon 1 and Balloon 2. For example, when you dropped them at the same time, Balloon 1 should have fallen faster than Balloon 2. It also may have been slightly easier to throw Balloon 1 than Balloon 2. In both cases, this is because the CO2 gas in Balloon 1 is denser than the mixed gas in Balloon 2. As a result, it falls faster and it’s easier to control its direction when you throw it.

Next time someone tells you to turn up the volume, just hand them a CO2 filled balloon!

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Additional Resources

For more fun science activities with sound, check out:

Watch how your mouth movements change to create different types of sound waves in The Diva and the Emcee from Dr. Krishna Nayak at University of Southern California

Sound Localization from Science Buddies

Measuring Your Threshold of Hearing for Sounds of Different Pitch from Science Buddies


Science Activities for All Ages! from Science Buddies


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