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Reflection of Sound
Reflection of Sound
Sound gets reflected from hills, buildings etc. just as light get reflected from polished surface. The sound waves reflected from the sides of a forest or mountain or a high building come to our ears as echo. When the voices uttered by a person are heard two-three times after the reflection from some object, they are called echoes.
If the area of the reflecting surface is large, the words spoken are heard as clear echo because the words become smaller and of larger frequency.
The effect of any voice, clapping or bullet firing sound persists on car drum for a period f about 1/10th of a sound. Any sound which comes as reflection of ears after 1/10th of a second travels a distance of about 34 meters. Therefore to hear echo, the barrier reflecting the sound should be at least at a distance of 17 meters.
Multiple echoes are heard when the sound waves get reflected several times between the two parallel hills or high buildings. Such multiple echoes produce thundering sounds as produced by the clouds.
Principle of echo is used in determining the depth of ocean bed, in radars and for locating the submarines in oceans.
If you speak in an open it is very difficult to hear the sound because the sound energy gets scattered widely but in large buildings the walls behave as sound reflectors. To make the sound clearly audible concave or parabolic reflectors are mount4ed at the back of stage and speaker delivers the talk from the focus of the reflector. The reflected voice reaches to the audience clearly.
The whispering galleries also work on the principle of reflection of sound. Normally the walls of dome shaped buildings and churches are made in round shape and on the sides of these walls are the places for sitting. In these galleries th4e sound after several reflections reaches to the audience. That is why these are called whispering galleries. In these galleries if a person speaks near the wall, the sound after many reflections can reach another person who after putting his ears near the wall rises to listen the voice.
The reflection of sound or echoes is used by fishermen to detect the shoals of fish, position of submarines and by geologists to detect the minerals under the ground. Sonar (Sound Navigation and Ranging) also work on the principle of reflection of sound waves. In this instrument sound waves from a transmitter at the ship are transmitted into the sea water. These waves get reflected from the ocean bed. The reflected waves are picked by the receiver of sonar and the time taken from the transmitter to the bed and back to the receiver is measured. By using the speed of sound in the sea water and time, and depth of the bottom of the sea is measured.
Bats, whales, porpoises etc. make use of echoes to communicate with each other and can locate the position of their pray. These animals transmit waves of certain frequencies in all directions and receive the reflected waves and locate the pray on the basis of the direction of these waves.