2006-12-08 / Columnists

Drawing On Science

Sound
Commentary by Stephen Yaeger

You're sitting in your car listening to your favorite music when suddenly you're being attacked by a loud, low decibel boom-boom-boom...no music, just a boom-boom-boom. It's coming from the car behind you. The sound is so overwhelming that it literally vibrates throughout your car...even within your chest. You look in your rear-view mirror and, as you glare angrily at the driver, you realize that the mirror is vibrating too...just like the Tyrannosaurus' heavy footsteps vibrated the glass of water in "Jurassic Park."

When a musician draws a bow along the strings of a violin, plucks a guitar, or plays a piano, vibrations of the strings result in sound being produced. Even the sound of your voice is produced by vibrations of the larynx or voice box. But no matter how sound is produced it must have some medium in which it can travel. The medium may be a gas, liquid, or solid. The speed at which the sound travels depends on the distances between the molecules of the medium; that is, the denser the material, the faster the sound will travel.

As we live in an atmosphere the sounds we hear travel through air. Air being a mixture of gases is the least dense of the mediums in which sound energy travels. When you speak your vibrating larynx creates sound in the form of your voice. Air molecules in contact with your larynx begin to vibrate. These molecules then transmit the vibrations to molecules nearby causing them to vibrate and they, in turn, pass vibrations on to other molecules. When the sound energy is transmitted to molecules in contact with your friend's eardrums they also vibrate.

His brain then identifies the sound as your voice, since it is the brain that interprets sound.

Sound travels faster in a liquid since the molecules are closer together. Sound energy will travel faster and with a greater force because the distance between molecules is shortened. Therefore, liquid is a better conductor of sound than a gas.

A solid is the best conductor of sound since it is the densest of the three phases of matter. The molecules of a solid are so close to each other that the sound energy travels with the greatest of speed and force. Now, contrary to Hollywood, when a space ship blows up or a planet is zapped by some super cannon, no sound is produced. Space is a vacuum; there are no molecules in a vacuum to vibrate.

Try this: Place a ticking clock...you know a regular wind-up type that really tick-tocks...at one end of a wooden (or metal) table. You stand at the other end and listen to the ticking of the clock. Now place one ear on the table surface and listen to the difference in the clock's ticking...is it more distinct.

So, you ask, what exactly is sound? Sound is a form of energy produced, as you now know, when a body vibrates. The vibrations are in the form of sound waves much like the ripples that spread out in a quiet pond when a stone is dropped in the water. In the case of the horizontal movement of ripples in water the molecules are traveling up and down.

These are known as transverse waves . Sound waves travel away from the source of the sound in what are known as longitudinal waves. The molecules are moving back, closer to each other ( condensation ) and forth, away from each other ( rarefaction) in the direction of the wave itself.

The louder the sound the more vibrations it causes. Loudness is measured in decibels (dB). A decibel scale may be used to see how sound affects your hearing. A near in-audible sound is 0 dB. Sound 10 times louder is 10 dB, 100 times louder is 20 dB, 1,000 times louder is 30 dB, etc. A sound 85 dB or higher may or will result in hearing loss, depending on how long one's ears are subjected to it. For example normal conversation is done in 50 dB, listening to a Walkman or similar device is the same as listening to a motorcycle giving off 95 dB, while a rock concert at 4 - 6' distant subjects your ears to decibels ranging from 120 to 150 depending on amplification (a rock musician's hearing loss is estimated to be 13 - 30%.

You may have experienced the following: some distance away you see a man hammering away on something. You watch as he brings the hammer down and some time later you hear the sound. Or you see a lightning bolt and some seconds later you hear the sound it makes in the form of thunder. You can determine the distance from the source of the sound to where you are standing. For example when you see a lightning bolt begin counting "One-one thousand, Two-one thousand, Three one-thousand, etc." until you hear the thunder. Since sound energy travels at about 186,000 feet per second just multiple the numbers of seconds by that number and you know the approximate distance.

Keep in mind, however, that the more humidity in the air, the faster sound will travel, since the medium is now denser. Of course if you see lightning and never get a chance to count, well, unfortunately, the lightning bolt struck in the same location you were standing. As for the 'einstein' who has a car attached to his speakers and sub-woofers, he's probably subjecting his ears to 120 dB or higher and should be saving money for a good hearing aid when he's older and, hopefully, wiser.

Stephen Yaeger is available for school presentations through The Rockaway Museum.

Questions/comments? E-mail Steve: Drawingonscience@aol.com

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