Wherever there is a sound, something is moving to make that sound. It is a to-and-fro movement that makes sound. Scientists have a name for this kind of movement. It is called vibration. Things can vibrate without making a sound (such as vibrating your hand like a fan). In order for you to hear a sound, something must vibrate between about sixteen times a second and about twenty thousand times a second. The number of times which an object vibrates in a second is called its frequency. A frequency of twenty-five means that something is vibrating twenty-five times a second.

Sometimes an object vibrates so fast that you cannot hear it. If an object has a frequency of over 20,000, you will not hear it. Such very fast vibrations are called ultrasonic vibrations. A dog is able to hear frequencies up to 40,000. Some people can hear better than others.

Your vocal chords vibrate inside your throat when you hum, speak, or sing. Sounds have different pitches, ranging from high to low (such as humming a high note or low note). The higher the pitch of the sound, the faster the vibrations that make the sound. The lower the pitch, the slower the vibrations. In order to change the pitch, the speed of the vibrations must change. A thicker object will vibrate slower, or less frequently, than a thinner object of the same length and the same material. The thicker object produces a lower sound than the thinner object. When two objects are of the same thickness, the shorter object has the higher pitch. The tightness of a vibrating object helps determine its pitch. This tightness or looseness is called tension.

When something vibrates in the air, it makes waves. The waves travel through the air. Sound waves are carried by the air. No air means no sound waves. Sound waves must travel through some substance. The vibrations are passed along through the air from molecule to molecule. This movement of molecules, to and fro, is a sound wave.

Sound travels better through a solid. The molecules of a solid are close together. So molecules can bump each other more easily. If molecules bump more easily, they can carry sound better.

It is the molecules in air that pass the vibrations along. Molecules next to the vibrating object are pushed by the object as it vibrates. Those molecules push the molecules next to them. So the push is passed along from one molecule to the next. Each molecule moves back and forth in its place as it passes the vibration along. What if the molecules could not move? The sound would not be passed along. There would be no sound waves.

An echo happens when sound waves bounce. One thing that is needed for an echo is something for sound to bounce off. If molecules did not bounce, there would be no echoes. If molecules were not able to move fast or slowly, there would be no difference in pitch.

### How Fast is Sound?

It takes time for sound to travel. After many investigations and experiments, scientists have measured the speed of sound. Sound travels about 1,100 feet in one second of time. That’s about as long as four football fields. If you say “hello” to a wall close to you, the sound does bounce from the wall. But it goes to the wall and returns to you in so little time that you cannot make it out. You cannot hear an echo from a close wall, even though sound waves bounce from it.

### Controlling Noise

Now that we know what sound is and how it behaves, we can control it better. If we do not want the molecules of a sound wave to bounce off a wall, we hang soft curtains on the wall. The moving molecules in the air sink in among the molecules of cloth and do not bounce. We say that the cloth absorbs the sound. The cloths are porous. They contain many dead air spaces which stop or absorb sound waves. Materials are different in their ability to absorb sound waves. The more porous the material, the better the sound waves are absorbed.

There are other ways of controlling noise. Planning the location of a building is one way. Sometimes bushes are planted to act as sound-wave absorbers. Floors of rubber or cork tile absorb sound waves. A door with a control on it closes the door quietly and slowly. Manufacturers of heating systems find it easier to sell heating systems that work quietly.

### Hearing Sound

When sound waves reach your ear, they push against the eardrum and make it vibrate. The eardrum is made of thin material called a membrane. When the eardrum vibrates, the vibration passes along three tiny bones into a special fluid in your ear. The waves in this fluid travel along to nerve endings. Then the nerve endings send messages to the brain. Your brain receives these messages and gives meaning to them. You actually hear sounds with your brain. Your ear and nerves act as a road to the brain to make hearing possible. The louder the sound is, the stronger the sound waves were produced in the air. The stronger the sound waves in the air, the harder they push against your eardrums. The harder they push, the louder the sound you hear. Strong sound waves can be heard farther away than weak ones.

### Measuring the Volume of Sound

The unit used to measure the loudness of sound is a decibel. The number of decibels depends on how loud a sound is. If the sound is loud, the decibel number is high.

Here are some common sounds with their approximate decibel measurements:

 Rustle of leaves 9 decibels Whisper 15 decibels Quiet home 25 decibels Passing train 90 decibels Airplane engine 110 decibels Thunder 118 decibels

Would you say that a passing train is only six times louder than a whisper? Common sense will tell you that the train is many times louder than that. You cannot compare decibels as you would some other units of measurement. Decibels have a different method of comparison.