Sound and Vibrations 2: Make Sprinkles Dance

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

The background section of the first lesson plan in this series explains how vibrating materials create sounds. However, the reverse is also true: sound can make materials vibrate. You might have noticed this if you've ever been to a concert, or in a car with the stereo cranked up all the way. The music can get so loud that you can actually feel the ground or your seat vibrating. Vibrations from the air molecules are transferred to the solid objects strongly enough that we can feel them. Of course, loud sounds like that can damage your hearing, so in this project your students will explore something a little safer.

The fact that sound can make materials vibrate also explains how our ears work. Vibrations from air molecules are transferred to our eardrum, a thin membrane inside our ear (Figure 1). The vibrations of the eardrum are converted into nerve signals, which are sent to the brain, allowing us to hear the sound. Our ears are very sensitive, so even soft sounds like a whisper still cause the eardrum to vibrate.

In this project, your students will use a thin membrane made from plastic wrap stretched over the opening of a cup. If you put your face very close to the membrane and hum, you can make it vibrate. These vibrations can be hard to see on their own, however. To make them easier to visualize, you can sprinkle some granular material (like sugar, salt, or sprinkles) on top of the membrane first. Then you will see them jump and "dance" around as the membrane vibrates.

Additional Background Links

• Video Describing the Process of Hearing and How it Works, MED-EL
• Sound Waves and the Eardrum, The Physics Classroom

Prep Work (10 minutes)

Prepare one membrane for each group of students, as shown in Figure 2.

1. Tear a piece of plastic wrap that is larger than the opening of the container.
2. Stretch the plastic wrap tightly across the top of the container.
3. Wrap a rubber band around the edge of the container to hold the plastic wrap.
4. Adjust the plastic wrap to make sure it is tight and flat with no wrinkles.

5. Make sure the activity works. Sprinkle some sprinkles on top of the membrane, hold your mouth close to it, and try humming loudly. Vary the pitch of your hum until you see the sprinkles vibrate. If it doesn't work, try humming louder, holding your mouth closer, or changing the pitch even more. If it still doesn't work, you may need to use different materials. Remember that your students' voices are probably higher-pitched than yours, so if you need to hum very deeply, it might not work for your students. Here are some things you can try:
   1. Use smaller granular materials (e.g. fine grain instead of coarse-grain salt; tiny spherical sprinkles instead of oblong sprinkles or "jimmies")
   2. Try a different size container.
   3. Try a container made from a different material (ceramic, metal, plastic, etc.).
   4. Make sure the plastic wrap is pulled tight.

Engage (5 minutes)

Sprinkle some sprinkles onto one of the membranes you made. Tell your students you have a challenge for them: you want them to make the sprinkles move only by using sound. They can't touch the sprinkles (or the container, or the table it's sitting on, etc.), blow on it, wave their hands near it to "fan" it, etc. Do they think it's possible?

Give the students a few minutes to talk to their group and brainstorm ideas. You can ask them to write their ideas down. Now explain that they will do a simple experiment to test their ideas.

Explore (20 minutes)

1. Give each group one of the containers you prepared, placed on a cafeteria tray (to catch any sprinkles that fall off). Tell the students not to poke the plastic wrap; the activity will not work if the plastic wrap is torn or stretched.
2. Sprinkle a small amount of salt, sugar, or sprinkles onto the plastic wrap, as shown in Figure 3.

3. Give students 5–10 minutes to test out their ideas.
   1. Tell students that they should plan out, and agree on, what to do before they start. Don't just start making random noises at the sprinkles. How will they make sounds? How will they observe if the sprinkles move? You can ask them to make a list of things they will try, record whether or not they worked.
   2. If needed, encourage them to work as a team. For example, instead of two people making sounds at once, one person can watch the sprinkles closely while the other person tries to make it move with sound. That way they each only need to concentrate on one thing.
   3. If necessary, remind them of the rules, and watch out for loopholes or unintentional violations of the rules. For example, if you talk or sing with your mouth very close to the sprinkles, you may exhale and blow on them, which doesn't count as moving the sprinkles only with sound. A student might try jumping up and down to shake the floor (and shake the desk as a result). Again, this might make the sprinkles move, but not as a result of sound.
4. Ask the class if anyone was successful in getting the sprinkles to move. If so, ask them to explain how they did it to the rest of the class. If no one could figure it out, give them a hint to try humming (for example, "how can you make sound without opening your mouth?"), and give them another 5–10 minutes to explore. Monitor their progress as they do this and provide guidance if they're still having trouble. It works best if you hold your mouth very close to the container, and they may need to try humming louder or at different pitches (higher or lower) to find what works best. They should be able to make the sprinkles "dance" (bounce up and down on the membrane) by humming just right!

Reflect (5 minutes)

Ask your students to explain their observations.

Assess

Ask each group to demonstrate making the sprinkles dance by humming (or any other sounds they found that work; as long as they follow the other rules and don't touch/blow on the sprinkles or objects touching the sprinkles).

Lesson Plan Variations

• Search for a "tone generator" app or website. These will allow you to play sounds at a constant pitch and can be easier to control than humming. Use one with computer speakers or a phone/tablet to make the sprinkles vibrate. What frequency (pitch) makes it vibrate the most? Important: turn your speaker volume all the way down before you start playing the tone, as they can be very loud and unpleasant.
• Put more salt/sugar/sprinkles on the plastic wrap, so the entire surface is covered in a very thin coating. Do the grains form a distinct pattern or shape when you hum? This may be easier to observe with a tone generator instead of humming (see previous point). If you don't have a tone generator available, try connecting a cardboard tube to the container as shown in this video to concentrate the sound of your voice.
• Let your students experiment with different size granular materials. Is it easier to get tiny sprinkles to jump up and down, or big grains of rice? What about even bigger objects like pebbles?
• Can sounds make things vibrate so much that they break? Search online for a video of "wine glass breaking with sound" or "breaking wine glass with sound slow motion." Explain that this also applies to our ears and is why very loud sounds (like rocket engines) are dangerous without ear protection!