"See" Sound Waves, and Make Your Own Sonic Boom!

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.

Please note that this project requires adult supervision. The experiment is perfectly safe if basic safety rules are followed, but carelessness could result in serious injury, including eye damage or other severe bodily harm. Adults should demonstrate this experiment first, and all spectators should stand at a safe distance at all times.

Here's what you'll need to make your own sonic boom:

- Short, straight, sturdy stick, about eighteen inches long.
- Three pieces of strong, coarse twine, about four feet long each. "Jute" twine works well.
- Duct tape.

To start with, take your three pieces of twine, and braid them tightly together to make a rope. Leave about two inches of unbraided twine at each end. At one end of the rope, take a very narrow strip of duct tape and wrap it around the rope several turns, right at the point where the braiding ends so it can't be unbraided. At the other end of the rope, tie the unbraided ends to one end of your stick. You may also want to wrap duct tape around the stick where you tied the rope to it.

You now have a short whip. This is all you need to create a sonic boom!

Go outside, and with all spectators at least fifteen feet away, take your whip by the handle. Swing it upward, gently but swiftly. Then, suddenly reverse the motion with a quick snap of the wrist and a fast downward motion with your arm. After a few practice tries, you should be able to "crack the whip" so that it makes a sharp popping sound.
While that sound may not be all that impressive on its own merits, it is interesting to think about what is going on. When you raise the whip, the rope is traveling up at a moderate rate. When you suddenly reverse direction, the loose end of the rope reaches extremely high speeds due to the longer distance it is from your hand. When done correctly, the end of the rope will surpass the speed of sound. Since a sonic boom is created when an object exceeds the speed of sound, the crack of the whip is actually a small sonic boom!

On a side note, it might be helpful to look at why the end of the rope has to travel so fast. The reason for this can best be explained by thinking of a circle. If you swing your whip around above your head, the loose end of the rope will be traveling faster than your hand. This is because your hand is at the center of the circle, and the end of the rope is at the far edge of the circle. The end of the rope has to travel faster because it has to go farther for each revolution. When you crack your whip, you are swinging the rope in a partial circle. As you bring it up, the rope isn't going anywhere near the speed of sound. But, when you quickly reverse directions with your hand, the rope has to follow very quickly as well, and since it has farther to travel, it must reach a much higher speed than the fastest you could possibly move your hand.

To experiment further with making your own sonic booms, try braiding longer ropes for your whip. A longer rope will reach higher speeds and thus create a louder sonic boom, but will also be harder to use. You may also want to try using different materials for your rope, or try using ropes of different diameters. "Bullwhackers" of the 1800's used eighteen-foot leather whips that were about as big around as a man's little finger, and could crack them as loudly as a pistol shot.

Matthew Lewis is a 24-year old homeschool graduate. Besides being the webmaster for Home School Enrichment, he also consults for other web developers, and enjoys writing the occasional article. You can email him at matthew@HomeSchoolEnrichment.com.

This article was originally published in the Sep/Oct '05 issue of Home School Enrichment Magazine. For more details, visit http://HomeSchoolEnrichment.com To request a free sample copy, visit http://homeschoolenrichment.com/magazine/request-sample-issue.html