The Sound of Silence: How Waves Snuff a Flame

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This experiment shows us the power of a sound wave: it can put out a fire! Now, by “fire”, I mean a tiny fire from a small candle. So no, a sound wave from a yogurt container will not put out a bonfire.

This experiment shows that a sound wave, when directed at a small flame from a candle, can extinguish a fire. It showcases resonance and other sound wave properties that work together to put out a flame.

How to make the Sound Wave Candle STEM experiment

Supplies you will need

For this experiment, you will need the following:

Candle
Yogurt container with a tight-fitting lid
Scissors or box cutter
Lighter

Supplies needed for the Sound Wave Candle STEM experiment

Before you start

Please watch everyone’s fingers around the flame!

Instructions

Here is how to do this experiment with your child:

Step 1: Cut a small hole in the yogurt lid

Carefully cut out a small hole in the middle of the yogurt lid, about the size of a coin.

I found it easiest to use a box cutter, but if you don’t have one on hand, use a pair of sharp scissors.

If you removed the yogurt lid to cut the hole, be sure to replace it on the yogurt container.

Small hole cut out of the yogurt lid with a box cutter

Step 2: Light the candle and snuff it!

Light your candle and place it on a flat table.

Face the yogurt container lid-first at the candle and abruptly hit the back of the container.

The sound waves you produce will snuff the flame!

If the flame doesn’t go out right away, just keep trying. In some instances, it took a few taps on the yogurt container to put the flame out. In other instances, it went out with the first try! It’s all about how you position the yogurt container and if the bulk of the waves hit the flame.

Aiming the yogurt container lid at the candle and tapping the other end of the container

The STEM behind the Sound Wave Candle Experiment

This experiment teaches:

Sound as a vibration
Sound waves and their properties
Energy transfer

How it works

A flame needs fuel and oxygen to burn, and if that’s disrupted, the flame will vibrate or even extinguish. That’s what we’re doing in this experiment!

By beating on the end of the yogurt container, we are creating sound waves that run through the container and out of the small hole we cut out of the lid. Sound waves are vibrations that create pressure changes in the air. When powerful sound waves hit the flame, they make the air molecules around it vibrate.

The key is matching the sound wave’s frequency to the flame’s natural frequency, or the rate at which it naturally vibrates. This is resonance. When the frequencies match, the vibrations become much stronger.

These amplified vibrations shake apart the flame’s delicate balance of fuel and oxygen, breaking the continuous chemical reaction and causing the flame to go out.

Sound as a vibration

The superstar lesson of this fun STEM experiment is that sound is all about vibrations. Since sound is vibration, it’s tough to visualize it. The great thing about this experiment is that you can see the vibration from hitting the yogurt container!

When we hit the back of the yogurt container, we are making it vibrate. That vibration is basically the container rapidly moving back and forth (even though it’s a very small distance). That motion is like tiny pushes and pulls on the air molecules inside the yogurt container.

That vibration travels down the yogurt container and toward the yogurt container’s lid at the other end, then concentrates through the tiny hole we cut in the center of the lid. That’s when we see the flame go out! The sound vibration has made it all the way to the candle, extinguishing the flame!

If it’s still a challenge to visualize for your child, you can talk about dropping a pebble into a very still pond. What happens to the water? Little ripples start traveling from where you dropped the pebble. Those ripples are like what happens when sound waves travel. The vibration you produce by tapping on the yogurt container is like the pebble.

Sound waves and their properties

Sound isn’t something that we just hear; it’s a series of vibrations. When something vibrates, it pushes and pulls on the air molecules all around it, which then causes the next set of air molecules to vibrate, and so on.

This creates a chain reaction of air molecules being pushed and pulled as it moves away from the source. That’s called a sound wave!

Our ears detect these pressure changes, and our brains interpret them as sound.

There are a few properties of sound waves, some more important than others in our experiment.

The first is the amplitude and energy of the sound waves that we create by beating on the back of the yogurt container. The harder we hit the container, the larger the energy and amplitude of the sound wave. Since a lot of energy is needed to extinguish a flame, we have to tap the container pretty hard. Otherwise, the flame won’t go out!

Another property of a sound wave is the frequency and pitch of it, which is how high or low the sound is. In our experiment, this property of sound is not as important.

Resonance, another property of a sound wave, is a key ingredient in this experiment! Every system has a natural frequency at which it vibrates most efficiently. When the frequency of an external force (in this case, the sound waves) matches that natural frequency, the amplitude of the vibrations increases dramatically.

In our experiment, the system being affected is the pocket of air and fuel vapor around it. When the sound waves from beating the yogurt container hit the flame just right (a resonant frequency), the air molecules around the flame begin to vibrate at a much greater amplitude (that first property of sound waves covered above).

Once those air molecules have a greater amplitude, that vibration disrupts the fuel-oxygen mixture that’s necessary to keep a fire going. The sound waves we create essentially starve the flame of its much-needed oxygen.

Energy transfer

This experiment is also great for talking about energy transfer!

In this experiment, we are converting sound energy to mechanical energy. When we tap the yogurt container, we are causing sound vibrations to travel down the yogurt container towards the lid and out of the opening.

The air molecules that are being pushed and pulled through sound vibration is a transformation of sound energy into kinetic, or mechanical, energy.