Sound surrounds us all the time. You may awaken in the morning to the sound of an alarm clock or Dad snoring. During the day you may hear the cacophony of auto traffic or the hustle and bustle of city life. But all sounds have one thing in common. They are formed by the movement or vibration of an object.
The sounds we hear every day are formed by the vibration or movement of air. Basically what happens is sound makes tiny particles in the air, called molecules, bump into each other. The molecules bump into each other compressing and then expanding to cause the wave to move like a falling column of dominos. This vibration of molecules is passed from molecule to molecule until it reaches our ears where we then 'hear' the vibrating air.
In this experiment, we'll demonstrate how this phenomena occurs. Using nothing more than marbles, we'll see how a sound wave can pass from molecule to molecule...
Take 4 marbles and line them up in a straight line on a table or other flat surface. Make sure the marbles are all touching.
Take a 5th marble and thump it so it hits one of the end marbles.
One by one, each marble will hit the other marble passing its energy down the line. When the last marble receives this energy, it will roll away leaving all the other marbles in their original positions. This passing of energy from marble to marble closely mimics the way a sound wave is produced.
Interesting Note. Since sound is caused by the vibration of an object it stands to reason that there must be some object for sound to exist. Hence, in the empty realm of outer space - there is no sound.
Parent's Note. The number of condensations or rarefactions produced by a vibrating object each second is called the frequency of the sound waves. The more rapidly an object vibrates, the higher will be the frequency. Scientists use a unit called the hertz to measure frequency. One hertz equals one cycle (vibration) per second. As the frequency of sound waves increases, the wavelength decreases. Wavelength is the distance between any point on one wave and the corresponding point on the next one.
Most people can hear sounds with frequencies from about 20 to 20,000 hertz. Bats, dogs, and many other kinds of animals can hear sounds with frequencies far above 20,000 hertz. Different sounds have different frequencies. For example, the sound of jingling keys ranges from 700 to 15,000 hertz. A person's voice can produce frequencies from 85 to 1,100 hertz. The tones of a piano have frequencies ranging from about 30 to 15,000 hertz.
taken from here