Page 145 - Understanding NCERT Science 09

11.3.1 ECHO What is the distance of the cliff from the
person if the speed of the sound, v is
If we shout or clap near a suitable reflecting taken as 346 m s ?
–1
object such as a tall building or a
mountain,we will hear the same sound Solution:
again a little later. This sound which we
hear is called an echo. The sensation of Given,
sound persists in our brain for about 0.1 Speed of sound, v = 346 m s –1
s. To hear a distinct echo the time interval Time taken for hearing the echo,
between the original sound and the t = 2 s
reflected one must be at least 0.1s. If we Distance travelled by the sound
take the speed of sound to be 344 m/s at a = v × t = 346 m s × 2 s = 692 m
–1
given temperature, say at 22 ºC in air, the In 2 s sound has to travel twice the
sound must go to the obstacle and reach
distance between the cliff and the
back the ear of the listener on reflection after person. Hence, the distance between the
0.1s. Hence, the total distance covered by
cliff and the person
the sound from the point of generation to
= 692 m/2 = 346 m.
the reflecting surface and back should be
at least (344 m/s) × 0.1 s = 34.4 m. Thus,
for hearing distinct echoes, the minimum uestion
distance of the obstacle from the source of
sound must be half of this distance, that Q 1. An echo is heard in 3 s. What is
is, 17.2 m. This distance will change with the distance of the reflecting
the temperature of air. Echoes may be heard surface from the source, given that
–1
more than once due to successive or the speed of sound is 342 m s ?
multiple reflections. The rolling of thunder
is due to the successive reflections of the 11.3.3 USES OF MULTIPLE REFLECTION
sound from a number of reflecting surfaces, OF SOUND
such as the clouds and the land.
1. Megaphones or loudhailers, horns,
11.3.2 REVERBERATION musical instruments such as trumpets
and shehanais, are all designed to send
A sound created in a big hall will persist
by repeated reflection from the walls until sound in a particular direction without
it is reduced to a value where it is no longer spreading it in all directions, as shown
audible. The repeated reflection that in Fig 11.10.
results in this persistence of sound is
called reverberation. In an auditorium or
big hall excessive reverberation is highly
undesirable. To reduce reverberation, the
roof and walls of the auditorium are
generally covered with sound-absorbent
materials like compressed fibreboard, Megaphone
rough plaster or draperies. The seat
materials are also selected on the basis of
their sound absorbing properties.

Horn
Example 11.2 A person clapped his hands
near a cliff and heard the echo after 2 s. Fig 11.10: A megaphone and a horn.

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