Page 138 - Science Class 08

In nature, the echo is used by bats to avoid
The reflec on of sound obeys all the laws of
reflec on of light.
1. Solid materials are good reflectors of
sound; as metal, plywood, etc.
2. Porous materials are bad reflectors of
sound as they absorb most of it: as cork,
cloth, etc.
ECHOES
The phenomenon of echo takes place when
sound undergoes reflec on due to some
obstacle that stands in its way. Generally, an
echo is said to have been there if the speaker
hears his or her own words back a er they have
reflected from a well, dome, valley, hill, etc.
An echo is a sound that travels back to its source
a er it has rebounded from a reflec ng surface.
Distance
senor
Sender
Object
Original wave
Reflec on of Sound (Echo)
Echo is used to determine the depth a water-
body— sea, ocean, etc. For it, a short sound is
sent to the floor of the sea from a ship
sta oned at the surface of the sea. The sound
goes down and comes back a er being
reflec ng from the sea-floor. Then, using the
speed of sound in sea-water, the me taken by
the sound to reach the bo om and then be
back, the depth of the sea can be calculated by
mul plying the speed of sound in sea-water
and half the me men oned above.
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obstacles while flying and also to locate their
prey. They eject high frequency sounds-signals
and listen carefully to their echoes. The sound
signals get reflected from the bodies of moths
and the bats know the loca on of the prey. But,
if the prey is shrewd, it spots the sound-signals
of the bat in me and leaves the place at once.
Or even it squeaks back to confuse the bat.
ABSORPTION OF SOUND
Most of the metals reflect sound, however,
there are so many other materials that observe
sound and do not reflect it. Curtains, thermocol,
straw, co on, carpets, etc. are good examples of
such materials that absorb sound. Absorp on of
sound is a phenomena in which most of the
sound is absorbed by some materials.
To avoid the echoing effect of sound in audito-
riums or cinema halls, their walls are coated
with some rough materials besides long and
heavy curtains are also hung around the walls.
Rough materials used in walls and curtains
absorb most of the sound and do not reflect any
sound. In this way, we enjoy a good quality of
sound in an auditorium or cinema hall.
Have you ever thought, why do we not hear
echoes in a small room? It is because in a small
room the original sound and reflected sound
reach our ears at almost the same me and no
echo is formed.
Noise
What is meant by noise?
A noise is a sound that is unpleasant due its
random and irregular vibra ons.
Science-8