Chad Walton, age 13, of Burlington, Vt., for his question:

WHAT CAUSES AN ECHO?

An echo is the sound we hear after it bounces back or is reflected after it hits some object. An echo is caused by movements of sound waves. When we shout or make a noise, we first hear that sound as waves reach our ears. If the sound waves also go out and hit some object, such as the side of a building, they are automatically bounced back and may reach our ears a second time. This second sound is called an echo.

Not all surfaces cause echoes. Some things absorb sound instead of reflecting it. Smooth surfaces such as walls, cliffs and sides of buildings often produce echoes.

There are many times when we do not hear echoes even though the reflected sound waves may reach our ears. We will not hear an echo if the original sound is too weak or if the reflecting structure is too small.

Also, we may not be able to tell the difference between the sound and its echo if the reflecting structure is less than about 30 feet away. At other times, we may very well hear more than a single echo from just one original sound. This type of repeated echoing most often happens in valleys and canyons where there are many sound reflecting surfaces. The sound waves bounce from wall to wall and may then produce a number of echoes.

Sound waves travel at the rate of about one mile in five seconds. It will take 10 seconds for sound to reach and return from an object that is one mile away.

If you want to tell how wide a canyon is, you may stand at the edge and shout. If you hear the echo five seconds later, you can make a very good assumption that the canyon is about half a mile wide.

Even clouds can reflect sounds. If a cannon is fired under a cloudless sky, the sound is heard only once. However, if there are heavy clouds overhead, the sound is echoed and re echoed much like a roll of thunder. Thunder, by the way, is sound reflections that accompany lightning.

Echoes can be produced in caves, underground tunnels and canyons. In some caves you may hear a series of echoes that are harmonious or a compound echo. There is a report that the sound of a pistol shot was repeated more than 50 times in an old palace in Milan, Italy.

Ships often use an echo to measure distances from shore in a fog. By sounding a horn and then measuring the time it takes for the echo to come back from the shore, a pilot can tell how close he is to land.

Many ships have equipment that is designed to receive sound signals under water. By sending out signals and then timing the echoes, a navigator can measure the distance from his ship to the ocean's bottom or to any nearby ship or obstacle. Sound travels in water at a speed of more than 4,700 feet per second.