Page 136 - Science Class 08

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Reflection of Sound
• Sound Waves • How Sound is Heard?
• Echoes • Absorp on of Sound
• Musical Instruments • Human Voice
We have studied a number of basic facts about
sound— its defini on, its produc on, its media
of travel, its speed, its loudness, its pitch and
its audibility. In this chapter, we shall learn
more facts about sound— its reflec on, its
mode of travel, its hearing by the ear and also
its various types— music, noise, echo, etc.
SOUND WAVES
In the previous chapter, we have studied that
sound needs a medium to travel and that it
cannot travel through vacuum. Let us now
study the mode in which sound travels. We can
easily follow this mode with the help of a
tuning fork.
We know that air is all around us in the form of
layers. When the tuning fork is at rest, the
layers of the air around it are undisturbed. But
when the fork vibrates, it disturbs the layers of
the air. How a er all ?
Let us follow this process step-by-step:
v The prongs of the fork first of all move right
and they push the layers of the air on this
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• Reflec on of Sound
• Musical Sounds
• Animal Sound
side. As a result, the layers get a jerk and
get compressed, i.e., the distance between
each two layers decreases. This is called a
compression of the air.
v Then, the prongs move le releasing their
push on, the air to their right. As a result,
the compressed layers of the air tend to
come back to their previous relaxed
posi ons, i.e., the distance between each
two layers of the air increases. This process
is known as a rarefac on of the air.
v As the fork vibrates con nuously, the layers
of the air undergo a system of comp-
ressions and rarefac ons. A set of one
compression and one rarefac on forms
one sound-wave.
v Thus, a system of sound-waves is set up.
These waves carry sound through the
medium (air, water, etc.) that they travel
in. In this way, sound travels from its
source to the detector (ear) to be heard.
Science-8