May. 26, 2010

Sound! It's in the Air

by Lynn Brunelle

Click to enlarge images

Good Vibrations

Did you know that every time you make a sound you’re causing a major collision? Yup. Every sound you make or hear is an actual chain reaction of vibrating molecules crashing into each other until they bump into the tiny hairs and bones and membranes inside your ear. It’s sound. And it’s very dramatic. In this experiment, you can see the vibrations that you’re making every time you make a noise. Here’s how.

What You Need

  • A big bowl
  • Plastic wrap
  • Rubber band
  • Uncooked rice


What You Do

  • Take a square of plastic wrap and secure it around the opening of the bowl with the rubber band. Stretch the plastic wrap until it is as tight as a drum.
    • Set your drum on a table and sprinkle a little uncooked rice on the plastic wrap.
    • Get about five inches away and talk to your drum. Sing to it. Hum. Yell. Whisper. What happens? Can you make your rice dance?

What’s Going On?

Whenever we make a noise—squawk, sing, speak or peep—we’re making vibrations. We make sounds by forcing air from our lungs past our vocal cords. The vocal cords waggle in the breeze and vibrate the air molecules around them and making a sound. We shape sounds by loosening and tightening the cords. Tighter cords make faster vibrations and higher sounds while looser cords make slower vibrations and deeper sounds. See if you can feel the difference when you sing high and low notes. We also change sounds by changing the shape of our mouths and by controlling the amount of air that rushes past them. Whatever we do, we’re changing the vibrations of the cords and sending those vibrations out into the environment as sound waves.

Those sound waves come out of us and bump into the air molecules around us. They bump into more air molecules, causing what’s called a chain reaction. That’s kind of like what happens when you line up dominoes in a row, and then knock the first one over: the first domino bumps into the next one, which bumps into the next one, all the way down the line until the whole row collapses.

When you aim sounds at your drum, the vibrating air molecules bounces off the plastic wrap, and in turn, makes the rice start to dance.

This isn’t unlike how your own ear drum works--without the rice, of course. Sound waves bounce off your ear drums and bump into little hairs and bones inside your ear. That sets off nerve endings, which then send electrical messages to your brain that your brain reads as sound.


Music to the Ear

Organized vibrations come across as music. Different instruments vibrate the air around them in different ways. Music is all vibrations, although to play music, you vibrate different parts of different instruments in different ways.

So go ahead and vibrate!

You can make your own music by vibrating a few air molecules. Here are three different ways to vibrate.

REED this!

Some instruments make sounds when you blow into them and vibrate the column of air inside. You vibrate your lips when you blow into a brass instrument, like a trumpet. You vibrate a reed when you blow into a woodwind instrument, like an oboe. You vibrate the mouthpiece when you blow across the opening at the top end of a flute. With all these instruments, you change the sound by changing the shape of the vibrating air column inside by using slides, or placing your fingers on or off valves or holes.

To make a Straw Reed Instrument:

What You Need

  • Plastic straws
  • Scissors

What You Do

  1. Flatten the end of a straw with your fingers and cut the corners off.
  2. Put the cut end in your mouth and press your lips together to keep the straw sides close together.
  3. Blow until you start to get vibrations. You’ll hear them and feel them. If you don’t, then re-flatten the end and try again.
  4. Keep blowing and making different sounds. Try cutting the straw length to see how the sound changes. How does changing the length of the straw affect the sound?
  5. Can you use two straws (one slightly larger in diameter than the other) to create a trombone-like instrument?

What’s Going On?

When you blow, you are vibrating the straw and setting off collisions of air molecules down inside the straw tube. When the tube is longer, the vibrations are slower because they had a longer journey to take through the long tube. Then when you cut the tube, the vibrations are faster. The shorter the tube, the faster the vibrations. The faster the vibrations, the higher the sound. The number of times that a sound wave vibrates in a second is called its frequency. Each sound produced in an instrument relates directly to the frequency of the vibrations. (High frequency or pitch = fast vibrations; low = slow vibrations.)



Stringy Thingy

Stringed instruments make sounds when you pluck or rub one or more of the strings. The strings vibrate, which makes the instrument vibrate, which causes the air around the instrument to vibrate and . . . you get the picture. The shape of the instrument has a lot to do with the kind of sound it makes. Bigger shapes like a bass fiddle have longer, slower waves and make deeper sounds, while violins create faster vibrations and higher sounds.

To Make a Guitar

What You Need

  • A shoe box with a lid
  • Six rubber bands (try using different bands of different widths to see what sounds they will make)
  • An index card or a piece of cardboard about the size of an index card
  • Scissors
  • Pencil
  • Coffee mug

What You Do:

  1. Use the pencil to trace around the mouth of your coffee mug in the middle of the box lid. Cut this circle out. This is the sound hole, that allows sound waves to come out.
  2. Place the lid back on your box and stretch six rubber bands around the box the long way. Make sure the rubber bands stretch over the sound hole and that they don’t touch each other. The bands are the strings on your guitar.
  3. Fold your index card, end to end. Then fold it again and then once more -- three times in all. You should end up with a sturdy piece about three inches long. This will be your bridge. Slide it in under all the strings on one end of the hole to lift the strings away from the hole.
  4. Pluck the rubber bands where they pass over the hole. What kind of sound do you hear? Can you see the vibrations of the rubber bands? What happens when you strum them?



It’s a Hit!

You hit percussion instruments and that vibrates the air. This is a real countertop crowd pleaser in my family.

To Make a Juice Glass Xylophone

What You Need

  • 8 glasses (or 8 glass bottles)
  • Pitcher of water
  • Chopsticks (a pencil will work, too)

What You Do

  1. Place 8 glasses on the counter.
  2. GENTLY tap a glass with a chopstick to hear the sound.
  3. As you tap, fill the glass. what happens? (The more water you put in a glass, the higher the sound will be.)
  4. Create a scale by filling each glass at different levels.
  5. When you’re done, tap each glass and compose a song.
About Lynn Brunelle

Lynn Brunelle is a four-time Emmy Award-winning writer for the television series Bill Nye the Science Guy. An editor, illustrator, and award-winning author, Lynn has created, developed, and written projects for PBS, NPR, and Disney, among others.

The views expressed are those of the author and are not necessarily those of Science Friday.

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