Introduction
We hear variety of sounds in our daily
life. Thundering of clouds, chirping of birds,
mewing of cats, rustling of leaves, music on the
radio and television and noise of vehicles are
some of the sounds that all of us are familiar
with. Each sound has particular characteristics.
Sound enables us to communicate with each
other. Animals also communicate with other
members of their species with the help of
sound. Some sounds like music are pleasing
to us and we like to hear them. But some
sounds, for example noise in our surrounding
is undesired. In this lesson we will study about
the production and propagation of sound,
human voice system, hearing, noise pollution
and the ways to control it.
6.1 Production of Sound
Sound is produced when an object is set to
vibrate. Vibration means a kind of rapid to and fro
motion of a particle. This to and fro motion of the
particle causes the substances around it to vibrate.
Thus sound spreads to the surroundings. The
substance through which sound is transmitted is
called medium. Sound moves through a medium
from the point of generation to the listener. We
can understand the production of sound with the
help of some activities.
Activity 1
Take the tray of an empty match box and
stretch a rubber band around it, along its
length. Then, pluck the stretched rubber
band with your index finger. What do you
observe? Do you hear any sound?
On plucking the rubber band, it starts
vibrating. You can hear a feeble humming
sound as long as the rubber band is vibrating.
The humming sound stops as soon as the rubber
band stops vibrating. This confirms that sound
is produced by vibrating particles. You can
see this kind of vibrations in stringed musical
instruments, such as guitar and sitar also.
This activity shows that vibrating pan
produces sound. In this case vibrations can
be felt by touching the pan. But in some cases
vibrations are visible.
6.2 Propagation of Sound
When you call your friend who is standing
at a distance, your friend is able to hear your
voice. How your friend is able to hear your
voice? He is able to hear because your sound
travels from one place to another. As we saw
earlier sound is a form of energy and it needs
a medium to travel. This can be understood
from the activity given below.
It is clear from this experiment that sound
cannot travel in vacuum and it needs a medium
like air. Sound travels in water and solids also.
The speed of sound is more in solids than in
liquids and it is very less in gases.
The speed of sound is the distance travelled
by sound in one second. It is denoted by ‘v’. It
is represented by the expression, v = nλ, where
‘n’ is the frequency and ‘λ’ is the wavelength.
The speed of sound depends on the
properties of the medium through which
it travels, like temperature, pressure and
humidity. In any medium, as the temperature
increases the speed of sound also increases. For
example, the speed of sound in air is 331 ms-1
at 0°C and 344 ms-1 at 22°C. The speed of
sound at a particular temperature in various
medium are listed in Table 6.1.
Table 6.1 Speed of sound in different medium
at 25°C
We saw that sound travels in different
medium with different speed. Now let us see
how it travels in a medium. When a body
vibrates, the particle of the medium in contact
with the vibrating body is first displaced from
its equilibrium position. It then exerts a force
on the adjacent particle. This process continues
in the medium till the sound reaches the ear of
the person.
In order to understand this let us consider
a vibrating tuning fork. When a vibrating
tuning fork moves forward, it pushes and
6.3.1 Characteristic of wave
motion
1. In wave motion, only the energy is transferred
not the particles.
2. The velocity of the wave motion is
different from the velocity of the vibrating
particle.
3. For the propagation of a mechanical wave,
the medium must possess the properties
of inertia, elasticity, uniform density and
minimum friction among the particles.
6.3.2 Types of mechanical wave
There are two types of mechanical wave.
They are
1. Transverse wave
2. Longitudinal wave
Transverse wave
In a transverse wave the particles of
the medium vibrate in a direction, which is
perpendicular to the direction of propagation
of the wave. E.g. Waves in strings, light waves,
etc. Transverse waves are produced only in
solids and liquids.
Longitudinal wave
In a longitudinal wave the particles of
the medium vibrate in a direction, which is
parallel to the direction of propagation of the
wave. E.g. Waves in springs, sound waves in a
medium. Longitudinal waves are produced in
solids, liquids and also in gases.
6.4 Properties of Sound
All sounds that we hear are not the same.
There are some properties that differentiate
one kind of sound from another. We will study
about these properties now.
6.4.1 Loudness
It is defined as the characteristic of a sound
that enables us to distinguish a weak or feeble
sound from a loud sound.
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