There are different types and forms of energy like mechanical energy, heat
energy, light energy etc. Sound is a form of energy that produces a
sensation of hearing in our ears. Energy can neither be created nor be
destroyed so sound energy is also not created it is converted from other
forms of energy.
Sound can be produced by plucking, scratching, rubbing, blowing or shaking
different objects. All these activities create vibration in turn produces
sound. Vibration means a kind of rapid to and fro motion of an object.
Sound of human voice is produced due to vibration in the vocal cord.
Propagation of sound
The transmitting of sound requires a medium through which it can travel. It
can be solid, liquid or gas. Sound moves from the point of generation to
the listener. When an object starts vibrating the particles do not move
towards our ear it actually makes the adjacent particle vibrate which was
in a state of equilibrium till then. This process continues till the sound
reaches our ears i.e sound is propagated from the source to the listener.
Sound needs a medium to travel
Sound is a mechanical wave and needs a material medium like air, water,
steel, wood etc to travel or propagate. Sound cannot be propagated through
vacuum. This can be seen when we put an electric bell inside a jar and make
it a vacuum space using a vacuum pump. As we start the pump to make the jar
a vacuum space while the bell continues to ring, we see that the sound of
the bell gradually decreases and becomes almost inaudible when the air
column inside the jar is totally removed. This proves that sound cannot
travel without a medium.
Sound have longitudinal waves
Sound waves are longitudinal in nature. So, sound propagates in a medium as
a series of compression and rarefaction. In this way the individual
particles of the medium move in a direction parallel to the direction of
propagation of disturbance. Particles do not move from one place to another
but they simply oscillate back and forth from its position of rest.
Transverse waves
In a transverse wave particle do not oscillate along the line of wave
propagation but oscillate up and down about their mean position as the wave
travels. So, transverse wave is one in which individual particles of the
medium move about their mean position in the direction perpendicular to the
direction of wave propagation. Light is a transverse wave and does not
require a medium to travel. It is not in mechanical waves.
Characteristic of sound
Sound can be described by three of its main characteristics
Frequency
Amplitude
Speed
As sound moves in compression and rarefaction. Compression
is the region where the density as well as pressure is high and rarefaction are the region of low pressure where particles
are separated apart and are represented by a valley in the graphical
diagram of sound waves. A peak is called the crest and valley is called the
trough of a waves.
Wavelength
Distance between two consecutive compression or two consecutive
rarefactions is called the wavelength. Wavelength is generally represented
by lambda. Its SI unit is metre.
Frequency
The number of compression or rarefaction that crosses a particular point
per unit time will give the frequency of the sound waves. The frequency of
a sound wave is generally represented by v. Its SI unit is hertz.
Amplitude
Objects of different size and conditions vibrate at different frequencies
to produce sound of different pitch. The magnitude of the maximum
disturbance in the medium or either side of the mean value is called the
amplitude of the wave. The loudness or softness of a sound is determined by
its amplitude. It is generally represented by the letter a. The unit for
amplitude is same as that of density or pressure.
Speed
Speed of sound is defined as the distance which a point on a wave such as a
compression or a rarefaction, travels per unit time.
A sound of single frequency is called a
tone. The sound
which is produced due to a mixture of several frequencies is called a
note and is pleasant to listen to.
Speed of sound in different medium
The speed of sound travelling depends on the property of the medium through
which it is travelling. The speed of sound also depends on the temperature
and pressure of the medium. Speed decreases when sound moves from solid to
gaseous state and speed increases when the temperature of the medium is
increased.
Reflection of sound
Sound bounces of a solid or liquid like a rubber ball bounces off a wall.
Sound gets reflected at the surface of a solid or liquid and follow the
same laws of reflection. The direction in which the sound is incident and
is reflected make equal angles with the normal to the reflecting surface
and all the three are on the same plane.
Reverberation
When sound is created in a big hall it will persist by repeated reflection
from the walls until it is reduced to a value when it is no longer audible.
The repeated reflection that results in the persistence of sound is called
reverberation.
Uses of multiple reflection of sound
Megaphones or loud hailers, horns are designed to channel sound in a
particular direction without spreading in all direction.
Stethoscope is a medical instrument used for listening to sound produced
within the body. In Stethoscope the sound of heartbeat which the doctor
hears by multiple reflection of sound through its tube.
The ceilings of concert halls and cinema halls are curved so that sound
after reflection reaches all corner of the hall.
Range of hearing
The audible range of sound for human being extend from 20 hertz to 20000
hertz. Sound frequencies below 20 hertz are called infrasonic sound or
infrasound. And sound above 20000 hertz or 20 khz are called ultrasonic
sound or ultrasound.
Application of ultrasound
Ultrasounds are generally used to clean parts located in hard to reach
places. Such as spiral tube, electronic components, etc.
Used to detect cracks and flaws in metal blocks. Metallic components
generally used in construction of big structures like bridge, machines,
etc.
Ultrasonic waves are used for forming the images of heart this technique is
called echocardiography.
Ultrasonic sound are used to break small stones formed inside the kidney
into fine grains.
Sonar
Sonar stands for Sound Navigation And Ranging. Sonar is a device that is
used in ships that emits ultrasonic waves to measure the distance,
direction and speed of underwater object. It is done by calculating the
time taken by the sound wave to travel once it is transmitted from the
transmitter and then it hits the target and return back to the receptors on
ship, where the speed of sound is known on that particular water. This
method of doing so is called as echo-ranging.
Structure of human ear
The outer ear is called pinna it collects the sound from the surrounding.
Then the collected sound is passed through the auditory canal, at the end
of the auditory canal there is thin membrane called the eardrum automatic
membrane is present. When a compression of the medium reaches the eardrum
the pressure on the outside of the membrane increases and forces the
eardrum inward and in case of a rarefaction the eardrum is forced outward.
The vibration is amplified several times by three bones called hammer,
anvil and stirrup.
These amplified pressure variations received from the sound wave to the
inner ear are converted into electrical signal by the cochlea. These
electrical signals are sent to the brain via auditory nerves and the brain
intercepts them as sound.