Notes

PROPAGATION OF SOUND

Sound and music are parts of our everyday sensory experience. Sound is a wave that is created by
vibrating objects and propagated through a medium from one location to another.

Sound is a form of energy that produces the sensation of hearing in our ears. It is produced by a vibrating
body.

Mechanical energy is required to start vibrations in a body producing the sound. The vibrations of the
body are transmitted into the medium in the form of waves from one point to another. These waves on
reaching our ears, produce vibrations in the ear drum which are perceived as sound by us. Thus, sound
is likened to form of mechanical energy.

Sound requires a material medium for propagation. The medium should have the following properties

1. Elasticity: The medium must be elastic so that its particles may come back to their initial
positions after displacement on either side.
2. Inertia: The medium must have inertia so that its particles may store mechanical energy.
3. No Friction: The medium should be frictionless so that there is NO loss of energy during sound
propagation.

Sound propagates in all 3 states of matter – Solid, Liquid and Gas. Sound cannot travel in vacuum. This
is proved by the Bell Jar experiment. Therefore, on the moon where there is no atmosphere or in space
one cannot hear the sound produced by another person. Thus, astronomers require special devices to talk
with one another

TYPE OF WAVES

There are two types of waves –

• Longitudinal waves: The type of wave where the particles vibrate in the same direction as the
propagation of the wave. E.g. Sound waves
Longitudinal waves have alternating regions of high pressure and low pressure called
compression and rarefaction respectively.
 • Transverse waves: The type of wave where the particles vibrate perpendicular to the direction
of the propagation of the wave. E.g. Light waves
Transverse waves are composed of crest and trough, the position of maximum upward
displacement is called the crest whereas the position of maximum downward displacement is
called a trough.

CHARACTERISTICS OF WAVE MOTION

1. A wave is produced by the periodic disturbance at a point in the medium

2. Due to propagation of the wave in the medium, the particles in the medium vibrate about their
mean positions (without leaving their position) and energy is transferred at a constant speed
from one point in the medium to another.

RELATED TERMS

TERM DEFINITION S.I. UNIT

Amplitude
Maximum displacement from the mean position Metre (m)
(a)
Time Period
Time taken by a particle of the medium to complete one vibration Second (s)
(T)
Number of vibrations in one second
Frequency The wave frequency is equal to the frequency of vibration of the source.
Hertz (Hz) Or s-1
(f, n or ν) Frequency is the inverse of time period (T) i.e.
f = 1/T
Distance travelled by the wave in one time period of vibrations
In a longitudinal wave, the distance between two consecutive compressions or
Wavelength
two consecutive rarefactions is equal to one wavelength. Metre (m)
(λ)
For a transverse wave, one wavelength corresponds to the distance between two
consecutive crests or two consecutive troughs.
Distance travelled by the wave in one second. Also known as wave speed.
It is the speed with which energy is transferred from one place to another in the
Wave velocity Meter/Second (m
medium.
(V) s-1)
It is constant for a given medium and changes when the wave passes from one
medium to another.
RELATIONSHIP BETWEEN WAVELENGTH, WAVE VELOCITY AND FREQUENCY

Distance = Speed x Time

∴ Wavelength = Wave velocity x Time period

 λ=VxT
Since T = 1/f

λ = V x (1/f) = V/f

OR

V=fλ
i.e.

Wave velocity = Frequency x Wavelength

SPEED OF SOUND

The speed of sound in the medium is

1. Directly proportional to the elasticity of the medium (E)

2. Inversely proportional to the density of the medium (ρ)

The speed of sound is more in solids, less in liquids and least in gases. E.g. Speed of sound is nearly 5100
m/s in steel, 1450 m/s in water and 330 m/s in air at 0oC.

The speed of sound in a gas increases with rise in temperature and increase in humidity. If the sound
propagation is in the direction of the wind, speed of sound increases whereas it decreases when
propagating against the direction of the wind.

Speed of sound DOES NOT depend on the pressure in the medium, amplitude, wavelength or frequency
of the wave.

Note: If the sound is produced at one end of a very long steel bar two distinct sounds are heard at the other
end. One through the steel (faster) and the other through air (slower). Hence people living near railway
tracks often check for an incoming train by keep their ear against the steel rail. THIS IS AN EXTREMELY
DANGEROUS ACT; PLEASE DO NOT TRY EVER!

The speed of sound in air (330 m/s) is much lesser than the speed of light in air (3 x 108 m/s). Thus,
we see lighting before we hear thunder. The athlete observes the smoke from the gun shot fired which
is instantaneously seen before he can hear the sound of the gun shot. Similarly, the spectator in a cricket
field hear the sound of the stroke much after they seen the batsmen actually making it.
INFRASONIC, SONIC AND ULTRASONIC FREQUENCIES

The human ear is able to hear the sound in the frequency range 20 Hz to 20,000 Hz which is known as
the audible range of frequency or sonic sound. It is most sensitive in the range 2000 Hz to 3000 Hz.
Children can hear sounds upto 30,000 Hz whereas older people can hear only upto 12,000 Hz.

Sound of Frequency less than 20 Hz is known as Infrasonic sounds whereas Sounds about 20,000 Hz
are known as Ultrasonic sound.

Animals can produce and hear sounds in Infrasonic and Ultrasonic range. Different animals have different
ranges of audible sound to them.

Elephants and Whales produce infrasonic waves that enable them to communicate with one another.

Dogs, Bats and Dolphins can hear high frequency Ultrasonic waves.

Ultrasound can travel quite freely in solids and liquids. It can carry high energy and can travel along a
well-defined straight path without bending, as it has low wavelength. This makes it useful in many ways
as outlines below

1. Bats avoid obstacles in their path by producing and hearing the reflection of the ultra sound.
2. Ultrasound is used for drilling holes or making cuts of desired shapes in materials like glass.
3. Ultrasound is used for cleaning minute objects such as part of watches and electronic
components.
4. Ultrasound also helps in detecting cracks or flaws in metals.
5. It is used in the imaging or human organs
6. It is used in surgery to remove cataract and kidney stones.
7. It is used in SONAR (Sonographic Navigation and Ranging) to detect and find distance of object
under water.

Note: Supersonic is a term used for objects that travel faster than the speed of sound in air.

LIGHT Vs SOUND

LIGHT SOUND
Cannot travel in vacuum. Needs a medium for
Can travel in vacuum
propagation.
Speed of light in air / vacuum: 3 x 108 m/s Speed of sound in air: 330 m/s
Speed of light is the most in air, less in liquids Speed of sound increases is most in solids, less in
and least in solids liquids and least in air
Transverse wave Longitudinal wave