It is hard for most of us to imagine a world without sound. Sounds are all around us. There is no time during the day when we don't hear sounds. We hear traffic on  a busy street, the roar of a school bus engine, horns blowing, brakes screeching and sirens blaring from emergency vehicles. In school we hear the ticking of the clock on the wall, footsteps in the hall, children and teachers talking, desks and chairs moving. At home you hear many more sounds, many coming from televisions, VCRs, radios and computers.
    We often hear peaceful sounds such as the leaves rustling in the trees due to a cool breeze passing through. We hear birds chirping and dogs barking. Sounds such as music and singing can give us pleasure. Sometimes our safety depends on sounds. At traffic crossings a train whistle signals the danger of a train approaching. Ships in a fog blast their fog horns to warn others of their whereabouts. A babies cries when they need an adult. Everybody recognizes the call for "Help!" as a signal for danger.
    One sound that you hear most often is talking. This is the most common way we communicate our thoughts. Wouldn't our lives be very different if we could not talk. Of course we could still communicate through sign language like people who are hearing impaired, but we would always have to directly face the person we were communicating with. We would be unable to yell to a friend in another room or call to someone across the street. There would be forms of communicating long distance.
    Without sound our world would be quite different. It would surely lose some of its beauty, be a much more dangerous place and difficult for us to communicate.
                                      What is Sound?
    Sound is a form of energy that is produced by a vibrating object. Energy is the ability to do work-to move or push something-to give it a push or a pull. To vibrate means to move back and forth. The hum we hear from a plucked rubber band is due to the back-and-forth motions of the rubber band. You can see the band move and it looks blurry as it moves.
Try this: Hold a ruler, or some other object the same length, firmly so that most of it hangs over a table or desk top. Push down on the end of the ruler that is hanging off then quickly let go. You can see the ruler vibrate up and down and hear the humming sound it makes as it does so.

        How Does Sound Travel?

    In order to hear sounds we need to have a sender and a receiver. The sender is the object making the sound or vibration and the receiver is the object that the sound or vibration travels to. For example, the ruler , in the experiment above, was the object making the sound (vibration). Your ear was the receiver (object) that received the sound (vibration). 
    Sound  travels through a medium (solid, liquid, or gas) in the form of  waves. Medium comes from the Latin word medius which means "middle". When someone speaks you can hear the vibrations from their voice because the air is a conductor (helps to carry, directs) the vibrations to your ears. Air is a gas and serves as the middle that brings the vibrations to your ear.
    You have learned that sound travels best through a solid because molecules in a solid are close together and vibrate rapidly. Next to a solid, sound travels better through liquids than gases because the molecules in a liquid are still relatively close together and are very far apart in a gas. This means that the molecules vibrate quicker creating waves to carry sound. It takes longer for the molecules in a gas to vibrate causing the sound waves to be sent more slowly.

	Roll your mouse over a term below to see the molecule arrangement.

    In space there is no air. Space is a vacuum. A vacuum is a space that is empty of all matter. It is an area where all gases are totally removed.

   Knowing that space is a vacuum, do you think sound can travel in space?
 

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