Page 139 - Understanding NCERT Science 09
P. 139
the vibrating object to the ear. A particle of
the medium in contact with the vibrating
object is first displaced from its equilibrium
position. It then exerts a force on the adjacent
particle. As a result of which the adjacent
particle gets displaced from its position of
rest. After displacing the adjacent particle the
first particle comes back to its original
position. This process continues in the
medium till the sound reaches your ear. The
disturbance created by a source of sound in
Fig. 11.3: Both the prongs of the vibrating tuning the medium travels through the medium and
fork dipped in water not the particles of the medium.
A wave is a disturbance that moves
From the above activities what do you through a medium when the particles of the
conclude? Can you produce sound without medium set neighbouring particles into
a vibrating object? motion. They in turn produce similar motion
In the above activities we have produced in others. The particles of the medium do not
sound by striking the tuning fork. We can move forward themselves, but the
also produce sound by plucking, scratching, disturbance is carried forward. This is what
rubbing, blowing or shaking different objects. happens during propagation of sound in a
As per the above activities what do we do to medium, hence sound can be visualised as a
the objects? We set the objects vibrating and wave. Sound waves are characterised by the
produce sound. Vibration means a kind of motion of particles in the medium and are
rapid to and fro motion of an object. The called mechanical waves.
sound of the human voice is produced due Air is the most common medium through
to vibrations in the vocal cords. When a bird which sound travels. When a vibrating object
flaps its wings, do you hear any sound? Think moves forward, it pushes and compresses the
how the buzzing sound accompanying a bee air in front of it creating a region of high
pressure. This region is called a compression
is produced. A stretched rubber band when
plucked vibrates and produces sound. If you (C), as shown in Fig. 11.4. This compression
have never done this, then do it and observe starts to move away from the vibrating object.
the vibration of the stretched rubber band. When the vibrating object moves backwards,
it creates a region of low pressure called
Activity _____________11.3 rarefaction (R), as shown in Fig. 11.4. As the
object moves back and forth rapidly, a series
• Make a list of different types of of compressions and rarefactions is created in
musical instruments and discuss the air. These make the sound wave that
with your friends which part of the
instrument vibrates to produce
sound.
11.2 Propagation of Sound
Sound is produced by vibrating objects. The
matter or substance through which sound
is transmitted is called a medium. It can be
solid, liquid or gas. Sound moves through a
medium from the point of generation to the
listener. When an object vibrates, it sets the
Fig. 11.4: A vibrating object creating a series of
particles of the medium around it vibrating. compressions (C) and rarefactions (R) in
The particles do not travel all the way from the medium.
128 SCIENCE
Rationalised 2023-24
