Page 144 - Understanding NCERT Science 09

Q uestion Q uestion

1. Distinguish between loudness
1. In which of the three media, air,
and intensity of sound.
water or iron, does sound travel
11.2.3 SPEED OF SOUND IN DIFFERENT the fastest at a particular
temperature?
MEDIA
Sound propagates through a medium at a finite 11.3 Reflection of Sound
speed. The sound of a thunder is heard a little
later than the flash of light is seen. So, we can Sound bounces off a solid or a liquid like a
make out that sound travels with a speed rubber ball bounces off a wall. Like light, sound
which is much less than the speed of light. The gets reflected at the surface of a solid or liquid
speed of sound depends on the properties of and follows the same laws of reflection as you
have studied in earlier classes. The directions
the medium through which it travels. You will
learn about this dependence in higher classes. in which the sound is incident and is reflected
The speed of sound in a medium depends on make equal angles with the normal to the
temperature of the medium. The speed of reflecting surface at the point of incidence, and
sound decreases when we go from solid to the three are in the same plane. An obstacle of
large size which may be polished or rough is
gaseous state. In any medium as we increase needed for the reflection of sound waves.
the temperature, the speed of sound increases.
For example, the speed of sound in air is 331 Activity _____________11.5
–1

–1
m s at 0ºC and 344 m s at 22 ºC. The speeds
of sound at a particular temperature in various • Take two identical pipes, as shown in
media are listed in Table 11.1. You need not Fig. 11.9. You can make the pipes
memorise the values. using chart paper. The length of the
pipes should be sufficiently long
Table 11.1: Speed of sound in as shown.
• Arrange them on a table near a wall.
different media at 25 ºC • Keep a clock near the open end of one
of the pipes and try to hear the sound
State Substance Speed in m/s of the clock through the other pipe.
• Adjust the position of the pipes so
Solids Aluminium 6420
that you can best hear the sound of
Nickel 6040 the clock.
Steel 5960 • Now, measure the angles of incidence
and reflection and see the
Iron 5950
relationship between the angles.
Brass 4700 • Lift the pipe on the right vertically
to a small height and observe
Glass (Flint) 3980
what happens.
Liquids Water (Sea) 1531 (In place of a clock, a mobile phone
on vibrating mode may also be used.)
Water (distilled) 1498
Ethanol 1207
Methanol 1103
Gases Hydrogen 1284
Helium 965
Air 346
Oxygen 316
Sulphur dioxide 213
Fig. 11.9: Reflection of sound
SOUND 133

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