Key concepts
Physics
Sound
Frequency
Vibrations
Introduction
You might know sound is caused by vibrations, but did you know sound can also make things vibrate? You might have noticed this if you’ve ever been at a concert or in a car with the stereo volume turned up. Sometimes the sound is so loud that you can feel its vibrations! Sounds that loud, however, can damage your hearing—so in this project you’ll investigate quieter noises with something a little more fun!
Background
Have you ever heard the age-old riddle, “If a tree falls in a forest and there is no one there to hear it, does it make a noise?” If you know how sound waves work, the answer is pretty simple. Sound waves are created by vibrating objects that bump into adjacent air molecules. These air molecules bump into other nearby air molecules and so on, transmitting the vibration through the air to our ears. Sometimes these vibrations are easy to see (for example, when you pluck a rubber band) but most times the vibrations are too small or too fast for us to see (when you knock on a door it makes a sound even though you can’t see the door vibrate). In the case of the falling tree, the tree will still cause vibrations when it hits the ground—even if there is no one there to hear it.
So if vibrations cause sounds, can sounds also cause vibrations? It turns out that it works both ways, and this is what allows us to hear. Your eardrums are tiny membranes inside your ears (and maybe an adult told you not to poke anything tiny or sharp into your ears because you could damage them). When vibrating air molecules hit the membrane they cause it to vibrate. These vibrations are converted into electrical signals that are sent to your brain. In this activity you will make a model of your eardrum, and watch how sounds can make it vibrate!
Materials
- Cup or bowl
- Rubber band that fits around the cup or bowl
- Plastic wrap
- Colored sprinkles (You can also use salt, but the sprinkles are easier to see.)
- Large plate or tray
Preparation
- Stretch a piece of plastic wrap across the top of the bowl.
- Use a rubber band to hold the plastic wrap in place.
- Adjust the plastic wrap so it is as tight and as flat as possible with no wrinkles.
- Put the bowl onto a plate or tray to catch any sprinkles that fall off.
Procedure
- Bring your lips very close to the edge of the bowl without touching it.
- Try humming loudly, and watch the plastic wrap closely. What happens? Can you see anything?
- Add some sprinkles to the top of the plastic wrap.
- Try humming again. Watch the sprinkles closely. What happens?
- If nothing happens, try humming louder. What happens?
- If nothing, try varying the pitch of your humming (higher or lower). What happens?
- If the sprinkles fall off the bowl, add more of them as needed.
- What happens after you stop humming?
- Extra: Try the activity with different-size granular materials. For example, what happens if you use tiny, spherical sprinkles versus bigger oblong ones, or “jimmies”? What about salt or grains of rice?
- Extra: Try the activity with different size, shape and material containers. Can you find the best material or shape that carries sound waves at different pitches?
- Extra: Try putting the bowl in front of a speaker and playing music. What happens?
- Extra: Search for a “tone generator” app or Web site on your phone, and try playing a continuous tone near the bowl. Some tones can hurt your ears, so start with the volume very low.
Observations and results
Did you make the sprinkles dance? When you hummed loudly enough and at the right pitch, the sound waves generated by your voice should have made the membrane vibrate. The vibrations of the clear plastic wrap, however, are hard to see. After you added sprinkles the vibrating membrane made them bounce up and down so the vibrations were much easier to see.
Depending on the size, shape and material of the container you used, you might have had to adjust the pitch of your humming to get the sprinkles to dance. If your pitch was too high or too low, the sprinkles might not have moved at all; if you got the pitch “just right,” they may have bounced around like crazy and even fallen off the bowl. The way an object responds to sounds of different pitches is called its frequency response. Frequency is measured in hertz (Hz), or the number of sound waves per second. Human hearing typically has a range from about 20 to 20,000 Hz, meaning frequencies in that range will cause your eardrum to vibrate. Some animals, such as dogs, can hear much higher frequencies, up to about 45,000 Hz. That means, unlike humans, their eardrums are sensitive to vibrations at higher frequencies. That’s why dogs can hear “dog whistles” but we can’t!
More to explore
Sound and Vibrations 1: Rubber Band Guitar, from Science Buddies
Sound and Vibrations 2: Make Sprinkles Dance, from Science Buddies
Talk through a String Telephone, from Scientific American
Sound Waves and the Eardrum, from The Physics Classroom
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
