Experiment with the way vibrations make sound

Sacred Hearts fifth-grader Samantha Tang, 10, experiments with a balloon whistle.

But stop and think: What causes sound?

To make sound, you must have a "thing" vibrating. Vibrating means the "thing" is moving back and forth. Energy has caused this to happen.

Then, once you have a thing vibrating, you need a medium to carry the sound. The medium can be any of the kinds of matter: a solid, liquid or gas. Mostly you hear sound that travels through the air.

Here are some experiments to test out these ideas:

• Let's make a thing vibrate. Find a ruler (or a flexible slat of wood). Hold it at least halfway out, long ways, over the edge of a table, with your hand on the ruler about an inch back from the table edge.

Make the end of the ruler vibrate by bending it down with your other hand and letting it go. Did you hear a sound? Why? Because the ruler (or slat of wood) was moving and regularly striking the edge of the table top. The ruler and the table top caused the air in the room to be set into motion and carry the energy of the vibrations to your ear.

• Let's try something else. Get a rubber band and stretch it between the last three fingers of both hands. Pull it tight. Use your thumb on either hand to pluck one side of the rubber band. Did you hear a sound? If not, hold the apparatus closer to your ear and repeat the plucking. Did you feel the vibrations in your fingers?

If you look closely, you can even see the rubber band vibrating. But it was suspended between your hands. How did it produce the sound? The sound was produced because the vibrating rubber band was able to set a small amount of air into motion at the same rate it was vibrating. The surrounding air carried the vibrational energy to your ear.

• Put your fingers up to your neck and talk. Do you feel the vibrations? How does your voice produce sound?

Get a balloon and blow it up. Hold the balloon between your fingers on each side of its neck, near the edge. Stretch the edges away from each other and allow the air to escape from the balloon. Did you hear a shrieking sound? Did the tone of the sound change as the air rushed out?

Try this again and change the tension, the amount of pulling apart that you apply to the edges. Did the tone get higher or lower as you changed the amount of stretching? How is the balloon like your voice?

To make your voice work, you must have air in your lungs. The balloon is like a lung. It is just a bag of air. Pressure from the walls of the balloon causes the air to escape. Air is pushed out of your lungs by your diaphragm, a muscle just below your lungs that separates them from your stomach. Your diaphragm also causes air to be pulled into the lungs.

In your throat, you have a larynx or "voice box." Within the larynx are two vocal cords. As the air from your lungs rushes past them, they are set into a vibration that causes the following air to be set into specific vibration patterns. You recognize the patterns that reach your ears as words or other sounds.

In the same way, stretching the neck of the balloon isolated the walls into two "vocal cords." They vibrate as the air from the balloon escapes. You can probably feel the vibrations through your fingers that are doing the stretching just like the vibrations you felt on your neck. Sound energy travels to your ears, and you hear it.

In future columns I'll be presenting gadgets you can make and test that show more ideas about sound. Until then . . . a hui hou!

"Dr. Gadget's Science Machine" is written by Joe Laszlo, a retired science teacher and winner of a Presidential Award for Excellence in Science Teaching. His column alternates in this spot with "Hawai'i Nature Squad." Write: Dr. Gadget, c/o The Honolulu Advertiser, P.O. Box 3110, Honolulu, HI 96802, or fax 535-8170. You may send e-mail to ohana@honoluluadvertiser.com only if you're 13 or older.