Page 137 - Science Class 08

s rrup reach this coiled tube which has
Sound Waves
a volume) the sound and send it to a spiral
Bell
Speed of sound (a) depends on the type of medium and
the temperature of the medium.
The diagram given above shows how sound
waves travel from its source (bell) to the
detector (ear).
HOW SOUND IS HEARD?
Our ears carry the sense of hearing. The
following picture the three parts of the ear—
outer ear, middle ear and inner ear.
Outer Ear Malleus Incus (onvil)
(hammer)
Stapes (s rrup)
Inner Ear
Ear Canal Cochlea
Ear drum
Pinna Middle Ear
Eustachian Tube
Internal part of Ear
Let us try to understand step-by-step how we
hear a sound:
v The innermost part of the outer ear is
called ear-drum that is very sensi ve to
vibra ons. When the sound-carrying
airwaves enter the ear, they strike at the
ear drum and it begins to vibrate.
v Beyond the ear-drum is the middle ear that
has three interlocked bones hammer, anvil
and s rrup. The vibra ng ear-drum causes
these bones to vibrate.
v Beyond the s rrup, is the inner ear to
which it is linked through a window called
oval window. The inner ear has a tube
coiled like a snail. The vibra ons of the
Science-8
hair on it. They amplify (increase its
like liquid-filled tube called cochlea. From
there, the auditory nerves pick up the
sound and carry it to the brain. The brain
iden fies the sound and we hear it.
REFLECTION OF SOUND
In simple terms, the reflec on of sound is
actually similar to the reflec on of light. How? It
also abides different laws of reflec on, in which
the angle of incidence does prove equal to the
angle of reflec on. In addi on, sound rebounds
from the surface of either solid or liquid similar
to a billiard ball. For successfully experiencing
the reflec on of sound, it is important that the
surface should be polished or rough and that
too of a considerably large size.
The two laws concerned with sound reflec on
are as follows:
v The incidence angle will always be equal to
the reflec on angle.
v Moreover, the incident sound waves, the
normal at incidence point and reflected
wave, all rest in a common plane.
Take a big glass container and place a table-
clock at its bo om. Stand close to the
container and try to hear the cking noise.
You will hear it, but not very clearly.
Now, take a wooden plank and hold it in a
slan ng posi on along the mouth of the
container so that the sound of the clock
coming out of the container should strike it
and then turn to one side. As a result, the
cking sound will be heard more clearly.
This phenomenon is called reflec on of
sound and the reflected sound is called an
echo.
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