People use sound for things other than talking and making music

November 24, 2012 2:14 pm
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Your alarm clock goes off with a loud ring or buzz. You sing to yourself in the shower. You listen to your favorite music as you get dressed. You hear the wail of a siren on the way to school. You talk to some friends before class begins. You live in a world filled with sounds.
All of these sounds seem to be so different. Yet all sounds share one thing—vibrations. All sounds come from something that vibrates. If you bang on a drum, the top of the drum vibrates. When you talk, the vocal chords in your throat vibrate.

VIBRATIONS AND WAVES
You can see how vibrations make sound by plucking a guitar string. The string vibrates back and forth. The vibrating string makes the air around it vibrate. The vibrations make sound waves in the air.
The sound waves travel through the air to your ears. They make your eardrum and the inside of your ears vibrate. Your ears send a signal to your brain. Your brain tells you that you are hearing a guitar string.
You cannot see sound waves, but you can see water waves. If you drop a pebble in a still pond or a big tub of water, the pebble will make waves. You will see the waves go outward through the water in circles. Sound moves through the air in similar waves.

HIGH AND LOW
The sound of a whistle is different from the sound of a drum. The whistle makes a high sound. The drum makes a low sound. The highness or lowness of a sound is called its pitch.
Whether a sound is high-pitched or low-pitched depends on how fast something vibrates. Fast vibrations make high-pitched sounds. Slow vibrations make low-pitched sounds.
Fast vibrations make sound waves that are close together. Slow vibrations make sound waves that are farther apart. The spacing between sound waves is called the frequency. Scientists measure frequency with a unit called the hertz. One hertz is one vibration or sound wave per second.
High-frequency sound waves make high-pitched sounds. Low-frequency sound waves make low-pitched sounds. Young people with normal hearing can hear sounds that are between 15 and 20,000 hertz.

LOUD AND SOFT
A police siren makes a loud sound. Whispering makes a soft sound. Whether a sound is loud or soft depends on the force or power of the sound wave. Powerful sound waves travel farther than weak sound waves. To talk to a friend across the street you have to shout and send out powerful sound waves. Your friend would never hear you if you whispered.
A unit called the decibel measures the power of sound waves. The sound waves of a whisper are about 10 decibels. Loud music can have a level of 120 decibels or more. Sounds above 140 decibels can actually make your ears hurt.

MUSIC OR NOISE?
When you sing or talk, you send out sound waves with all kinds of frequencies. These sound waves mix together. How they mix makes music or noise.
A chorus or choir sings in parts. The sounds of the parts mix well together. This kind of mixing is called harmony. Harmony makes the beautiful sounds of music.
Honking horns and the motors of cars, buses, and trucks send out sound waves that do not mix well together. That is why traffic on a busy street makes awful-sounding noise.

THE SPEED OF SOUND
The speed of sound is how fast the sound wave travels. There is not just one speed of sound. At sea level sound travels through cool, dry air at about about 1,088 feet per second (about 332 meters per second). In warmer air, the speed of sound increases. In cooler air it slows down.
Sound waves can travel through liquids, solids, and air and other gases. Sound moves faster through water and other liquids than it does through air. Sound moves fastest through solids. The speed of sound in steel is about 16,000 feet per second (about 4,880 meters per second).

ECHOES
Did you ever stand and shout in a cave, canyon, or big empty room? You would hear your voice twice. First you would hear yourself talk or shout. Then you would hear the same sound coming from a distance.
The sound waves of your voice go out until they hit a wall or other surface. Then they bounce back toward you and make an echo. Bats use echoes when they fly around on dark nights. The echoes help keep the bats from flying into trees, houses, and other objects.

HOW WE USE SOUND
People use sound for things other than talking and making music. Doctors use ultrasound to see inside the body. Ultrasound has frequencies too high for you to hear. Echoes from ultrasound waves can show what is inside the body.
The Navy uses sound to “see” underwater. A detection device on ships called sonar sends out sound waves. The waves bounce back when they hit an object. Sonar helps sailors find submarines and other things underwater. Sonar also tells how fast and what direction things are moving. Scientists use sonar to make maps of the bottom of the sea.

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