Now that we know something about sound, let’s find out what makes an echo and whether an echo can travel through water.

Sound travels outward from its source at a speed of about 375 meters per second in open air. Sound travels in waves much like the ripples made by a pebble thrown into the water. However, sound waves go out in all directions like the light from an electric bulb.

A sound wave may meet an obstacle and bounce back, or be reflected just as light is reflected. When a sound wave is thus reflected, it is heard as an echo. Therefore, an echo is sound repeated by reflection.

Not all obstacles can cause echoes; there are some objects which absorb the sound instead of reflecting it. If a sound is reflected by some obstacle, only one echo is heard. This is called a simple echo. If the sound is reflected by two or more obstacles, the echo may be repeated many times. The echo, however, becomes fainter each time until it dies away altogether. When repeated more than once, it is known as a compound echo, or reverberation.

An echo cannot be heard as a separate sound unless the sound is made some distance away from the reflecting surface. This allows time between the sound and echo. At a distance of 167 meters from a wall, for example, the echo returns in just one second.

Whether an echo can travel through water depends on whether sound can travel through water—and we know that it can. In fact, sound travels through water at a speed of more than 1,430 meters a second! This ability of sound to produce an echo through water has proven very useful.

Ships are often equipped with devices for sending and receiving sound signals under water. By sending out sharp signals and timing the echoes, a navigator can measure the distance from his vessel to the ocean bottom or to any nearby vessel or obstacle!