A.J. Poulin, age 11, of South Portland, Maine, for his question:

WHAT IS SOUND?

You can classify sounds as either noisy or musical. A noise is made by a vibrating object, such as a rattling window, that sends out irregular vibrations at irregular intervals. Music is made by a vibrating body that sends out regular vibrations at regular intervals, such as a violin being played by a skilled musician. Sounds can also be made by people and animals.

All sounds have one thing in common: each is caused when something moves back and forth in quivering motions called vibrations. These vibrations travel through the air to our ears and we hear them as sounds.

The definition of sound is not saying it is something you hear. Rather, sound is defined as vibrations produced by an object.

When an object vibrates, it also makes the surrounding air vibrate. This vibration has two causes: first, as the vibrating object moves outward, it compresses the surrounding air, and second, the air expands, or rushes into the space formerly occupied by the vibrating object, when the object moves inward. Each succeeding vibration of the object again causes the surrounding air to compress and expand.

Compressions and expansions are called condensations and rarefactions by scientists. The compressions and expansions of the air near the source of the sound also compress and expand the air farther away. In this way, a vibration travels through the air until it weakens and dies away. These vibrations are known as sound waves.

The air can carry many sound waves at the same time and the vibrations can travel in many different directions. You can be sitting in your living room, for example, and hear a radio playing, a bird singing outside the open window, a friend talking and an airplane passing overhead, all at the same time.

Have you heard the riddle that asks whether sound is produced when a tree crashes to earth in a distant forest and no one hears it? We know that it would indeed make a sound since any falling tree would produce vibrations, whether anyone was there to hear them or not. Sound describes events that occur in nature rather than the act of hearing these events.

The human voice is an example of sound produced by air columns vibrated by membranes. A pair of membranes called vocal cords are located on each side of the larynx, or voice box, in the throat. You make these cords vibrate and produce sounds by forcing air past them. Changing the muscular tension of the cords produces sounds of different pitches.