You must have often shouted in an empty gallery and heard your voice shouting back at you. This is called echo. An echo can also be heard by shouting near a deep ditch or a well. The thunder of clouds is another example of echo. Have you wondered why this takes place?
We know that sound travels from one place to another in the form of waves. The velocity of sound in the air is 340 meters per second. When we speak, the sound waves emanating from our mouth spread out in all the directions. When these waves strike against wall or some other obstacle I their way, they get reflected back. These reflected waves, which we hear, are called echoes. Hence echo is produced when some obstacle reflects sound waves. But all objects do not reflect sound. There are some objects like wood, jute, cardboard, etc., which absorb sound.
Bats make high-pitched sounds while flying, which bounce off objects in the form of echoes. This gives the bats information about the distance and direction of the objects.
To hear an echo, it is essential that the obstacle reflecting the sound waves must be situated at least at a distance of more than 17 meters form us. This is because the effect of sound persists on our ears for one-tenth of a second. If one sound signal has reached the ears and within one-tenth of a second another sound signal is heard, it will not be distinguished because during this period the effect of the earlier sound is still persisting in the ear. Sound travels about 34 meters in one-tenth of a second. As such, if the object reflecting the sound waves is situated 17 meters away from the speaker, the time taken for the sound to travel this distance form the speakers mouth to the object and back to him would be one-tenth of a second and the reflected sound can be acknowledged by us as an echo.
In modern buildings, architects use methods and materials that reduce echoes and favor good sound transmission. Auditoriums are built with rounded corners and few large flat surfaces. This prevents sound waves form being reflected to a particular position. They are scattered in many directions and the only sounds heard are those sent out form the source. Some fiberboards, which have many holes, are used for making rooms sound-proof.
By the use of these materials, the sound waves are either absorbed or scattered so that production of echo is reduced. Radars and sonar’s also work on the principle of echo.
