This issue's Unit Study provides several ways to study hearing. For Give it a Try, we'll go a little further and look at the principles behind why we hear, and what we hear. We'll start with two projects that will demonstrate the principle of sound waves, including letting you see their effects visually. After that, we'll show a quick way you can make a sonic boom right in your back yard! (But don't worry, Mom, this one won't break the windows or rattle the dishes!)

Sound is actually caused by a disturbance of some kind causing molecules in the air around us to vibrate and knock against one another. The disturbance--say, a car backfiring--causes some air molecules to be displaced. This causes the molecules to bump against one another. This effect travels in a wave-like manner, but the wave is only the result of the molecules banging together. The molecules themselves stay in about the same place as the sound wave passes.

Does that sound strange? Probably so--but our first activity will illustrate this principle. Take three quarters and lay them down on a hard, flat surface. Now, take your finger and "thunk" the first quarter in the line so that it strikes smartly against the second quarter. You will notice that, when the first quarter hits the second quarter, it stops moving. But, the energy (motion) is transferred to the second quarter, which is knocked over until it bumps the third quarter. If you had a long line of quarters, this process would go on until there was no energy to be transferred to another quarter.

In this experiment, the quarters represent air molecules, your "thunk" represents a noise-producing disturbance, and the motion of the quarters represents a sound wave. Looking at it that way, you can see how the disturbance causes air molecules to jolt against one another, which in turn bump against other molecules, with each molecule staying in approximately the same place. When the sound wave reaches your ears, your brain interprets it as a sound.

"See" Sound Waves
You can see the effects of sound waves, visually, with the following simple experiment. Here's what you'll need:

- One large tin can, the bigger the better. Make sure the lid is entirely removed and that no sharp edges are protruding.
- One plastic bag. Garbage bags work well. Dark plastic is best.
- One sturdy rubber-band.
- Scissors.
- Salt, sugar, or fine sand.

With the scissors, cut a square out of the plastic bag. The square should be large enough to entirely cover the open end of your tin can, with about three inches extra on each side. Place the square of plastic over the opening of your can, then pull the edges down over the sides of the can so that you can use the rubber-band to hold it in place. The plastic should be stretched as tightly as possible over the top of the can.

Now, sprinkle a little salt (or sugar) onto the plastic. Step back a few feet and try making a few different kinds of sounds. Talk, shout, bang pans together, or play the start of Beethoven's 5th Symphony with the volume way up. You will notice that the salt on top of the plastic "jumps" up and down. This is caused by sound waves creating a vibration on the plastic, which makes the salt hop around like jumping beans!

Try experimenting with different types and volumes of sounds, and at different distances from the can. You'll find that some sounds cause no visible vibrations at all, while other sounds will make a lot of vibrations. See how much you can make the salt "dance" on top of the can!

Make your own sonic boom!
Most people under 25 have probably never heard the "sonic booms" that are caused when an airplane reaches the speed of sound. I know I never have. But I've heard that, back in the 1970's, sonic booms were fairly common. While the sonic booms caused by airplanes breaking the sound barrier were quite loud and could sometimes break windows, shake houses and rattle dishes in the cupboard, the sonic boom we'll be making in this project will be on a much smaller scale.