Making Sound Waves
Have you ever heard of an “eardrum”? If the answer is yes, then you probably know that your eardrum is an essential part of your ear, allowing you to hear the world around you. But why do we call it a drum? It turns out that calling it a drum is a very accurate description of what your eardrum looks like, and what it does inside your ear. To understand how your eardrum works, imagine using a drumstick to bang on a real drum, and then touching the drum with your hand. When you do this, you can feel the vibrations moving through the drum material. Our eardrum works in a similar way, but instead of a drumstick, our eardrum vibrates in response to sound waves hitting it. We can’t see these sound waves with our eyes, but we can see how they cause vibrations in things around us, just like they do in our eardrums!
What we experience as sound is actually a mechanical wave, produced by the back-and-forth vibration of particles in the air (or whatever medium is around our ears, remember sound travels through water too!). To understand this, imagine clapping your hands in a pool of water. As your hands move towards each other they gather water, creating a space behind them that the surrounding water particles rush to fill. Once your hands meet, the water particles between your hands are squashed together. You can see the result both of these events as ripples moving away from your clapped hands through the water. Sound waves travel through air in a similar way. When you clap your hands, you displace (or move) the air particles between and around your hands. This creates a compression wave, which travels through the air (much like it did in the water). A continuous sound (like the one produced by a tuning fork) is caused by the vibrations of the fork tines. The vibrations of the tines 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 tines move faster there is less time between each compression, resulting in a higher frequency sound wave.
When this wave hits your ear, the first thing it encounters is your eardrum. Your eardrum is a very thin membrane that acts as a barrier between the outside world and the inside of your ear. While it does help protect the inside of your ear, your eardrum’s real purpose is to transmit sound. When the sound waves hit your eardrum, they cause it to vibrate, the same way that a real drum vibrates when you hit it with a drumstick. The vibrations in your eardrum are then transferred through 3 tiny bones inside your ear, into a fluid-filled chamber called the cochlea (pronounced COK-lee-uh). Vibrations in your cochlea are transformed into electrical signals that your brain interprets as sound. We hear different pitches of sound (highs and lows) based on the frequency of the sound wave. A sound with a very high frequency sounds higher in pitch.
In this activity, you will be observing the vibrations caused by sound waves as they pass through a model membrane, just like the vibrations that go through our eardrum!
- Parchment or wax paper
- A large rubber band (an elastic headband works well here, too)
- A small glass bowl, should be large enough to rest your Bluetooth speaker at the bottom (rubber band should be the correct size to wrap around the top of the glass bowl)
- Sugar or salt (to help you see the results better, you can use colored sugar sprinkles, or color the sugar or salt yourself with food dye)
- A portable Bluetooth speaker
- A phone that can connect to your speaker
- For this activity you need to play one single tone at time from your phone. There are several free tuner apps available, or YouTube videos, that you can use to play tones from your phone. Be sure to check with your parents before you put any apps on your phone
- Ear plugs (optional)
- Place the speaker in the bowl; make sure it is on and connected to your phone.
- Cover the top of the bowl with a sheet of wax paper.
- Wrap the rubber band around the edges of the bowl to secure the paper in place.
- Sprinkle a layer of sugar or salt over the paper. Make sure that the granules are spread across the paper evenly; try to avoid piles.
- Open the tuner app (or a YouTube video playing one single tone) on your phone. Start with the lowest frequency tone available. Set your volume to the lowest possible setting and hit play.
- While the tone plays, observe the sugar on the paper. What do you notice about the sugar? Are there any changes? If so, what are they?
- Slowly increase the volume on your phone. Each time you increase it, pause to observe the sugar. What do you notice? Has the sugar changed? In what way?
- Continue to increase the volume, observing any changes to the sugar on the paper. Important: Keep your speaker volume within a comfortable range. If the volume starts becoming uncomfortably loud and you still do not see any changes, see the ‘Extra’ section below for tips.
What effect does increasing the volume have on the sugar? What do you think is causing this change?
- When you see an effect on the sugar, try pausing the tone and then restarting it. When the tone stops, what happens to the sugar? What about when you restart the tone? Why do you think the tone has this effect on the sugar? Do you notice any patterns in how the sugar behaves when the tone is playing?
- Pause the tone and reset the sugar so that it is evenly spread across the paper again.
- Set your phone back to the lowest volume, and change the frequency of the tone that you are playing to a higher frequency tone.
- Repeat the experiment, slowly increasing the volume for this new tone. How is the new tone different, does it sound higher or lower? How does the new tone affect the sugar? Is the effect different than what you observed with the first tone? If so, in what way? What do you think causes the difference between the two tones?
Extra: Repeat the activity, experimenting with different tones. Try to explore a range of tones! Tip: look up a video of “Chlandi’s experiment” and use the audio to experiment with tones in your own experiment!
Extra: Try the experiment again, but this time replace the glass bowl with other household containers. Does a cake pan work? What about a vase? What about a metal or wood bowl? If you didn’t see any results the first time, try using a deeper bowl, and experiment with sizes.
Observations and Results
Did playing the tone cause the sugar to move around on the wax paper? This is what is expected.
As the sound wave travels through the wax paper, it causes the paper to vibrate. When you increase the volume of the tone, you are adding energy to the sound wave, resulting in larger vibrations. Eventually these vibrations are large enough to move the sugar on the paper.
You may have also noticed that the sugar moves in different patterns depending on the frequency of the tone. This is also expected. When the frequency of the tone changes, the vibration of the wax paper changes as well, resulting in the changing patterns of sugar.
Ask an Expert
- Do-Re-Mi with Straws, from Science Buddies
- Frequency-Dependent Sound Absorption, from Science Buddies
- What Material Makes the Most Resonant Soundboard?, from Science Buddies
- Resonance Experiment! (Full Version - With Tones), from Brusspup
- Everyday Objects Dancing on Sound, from BuzzFeed