Page 132 - Science Class 08

trained and used to detect crime. Ultrasonic
hear the sound of scratching? Yes, you can.
technology is now used by doctors to detect as
Note the me carefully that the sound takes
well as treat body disorders.
to reach your ear.
PROPAGATION OF SOUND Now, repeat the above

We already know that air carries sound and experiment with a
enables us to talk to each other. This journey of balloon filled with air
sound is called propaga on of sound. But, instead of water. You
sound can travel through a medium, not in will again be able to
vacuum. hear the sound of
scratching, but it will
reach the ear with a
slower speed.
Take a thick pad of rubber and place a table-
clock on it. Cover it with a bell-jar. You will be So, this experiment
proves two facts:
able to hear the cking of the clock.
(a) Sound can travel
Now, start pumping all the air out of the bell-
through liquids.
jar. The cking sound will go on becoming
lower and lower. When all the air has been (b) Sound can travel faster through liquids
pumped out, the cking will not be heard at than through the air.
all.
But, as you disconnect the exhaust pump,
the air will again start entering the bell-jar.
Along with it, the cking of sound will also Take a spoon and e it from the middle with
start again. Soon, you will be able to hear it a string. Let it strike against a metallic vessel.
as before. You will hear a sound.

Two things are clear from the experiment: Now, hold the ends of the string close to
(a) Sound needs a medium to travel. your ears. Let the spoon strike against the
same vessel again. Do you hear the sound?
(b) Sound cannot travel in vacuum.
Yes, you will hear a far louder sound.

SOUND AND VARIOUS MEDIUMS So, two facts about sound become evident:

We have proved that sound can travel through (a) Sound can travel through solids.
air (a gas)? Can sound travel through liquids (b) Sound travels much faster through solids
and solids too? than through the air and liquids.

Let us discover these two facts by means of
experiments: SPEED OF SOUND
The speed of sound is a term used to describe
the speed of sound waves passing through an

Fill a large balloon with water and bring it elas c medium.
close to one of your ears. Now, scratch it with
your finger- ps on the outer side. Can you

132 Science-8