Sound chapter 12 class-8

Saturday, December 16, 2017 1
CREATED BY – RAVI PRAKASH SINGH
FOR CLASS – VIII
SUBJECT – PHYSICS (SCIENCE)

Sound is a form of energy produce by vibrations.
Sound plays an important part in our daily lives. It helps us to
communicate with each other. We hear a wide variety of sounds in our
surroundings.
Sound is produced by a vibrating body.
Vibration is the to and fro or back and forth motion of an object.
Eg :- If you strike a school bell, it vibrates and produces sound.
If you pluck a stretched rubber band, it vibrates and produces
sound.
If you beat a drum, its stretched membrane vibrates and produces sound.
If you blow a bugle, the air column vibrates and produces sound.
Saturday, December 16, 2017 RAVI PRAKASH SINGH 2

 Production of sound
musical instruments
depends on the type of
instrument.
 There are four types.
1) Woodwind
2) String
3) Percussion
4) Brass

 Woodwinds Instruments make music by blowing
on the top (or on a reed) of the instrument.
Flute Oboe
Clarinet
Bassoon
Piccolo

 Strings Instruments make music by plucking or
strumming (झनकार) the strings.
Violin
Cello
Electric
GuitarGuitar Harp

 Percussion instruments make music by
striking, shaking or scraping (खुरचना) them.
Drum
Cymbals
Tambourine
Xylophone
Piano
Maracas

 Brass instruments make music by buzzing (गूंज,
भिनभिनाहट) lips while blowing.
French Horn
Trumpet
Trombone
Tuba

Sl.No. Musical instrument Vibrating part producing sound
1 Veena Stretched string
2 Sitar Stretched string
3 Violin Stretched string
4 Guitar Stretched string
4 Tabala Stretched membrane
5 Mridangam Stretched membrane
6 Drum Stretched membrane
7 Shehnai Air column
8 Flute Air column
9 Mouth organ Air column

Some musical instruments produce sound
when they are beaten or struck.
Eg:- bell, ghatam, manjira, jaltarang etc.

In humans sound is produced by the vioce box or larynx. It is the
upper part of the wind pipe. Two vocal cords are stretched across the
voice box leaving a narrow slit. When the lungs force air through the
slit, it vibrates and produce sound.

1) Animal like dogs, cats, horses, lion cows also
produced Sound by their vocal chords like
human beings.
2) Birds produce sound with ring of cartilage
called syrinx in their wind pipe.
3) Insects like mosquitoes and bees etc.
produced sound by vibrating their wings
rapidly.
4) Frogs produce sound by puffing their vocal
sacs up their throat like balloon while croaking

sound requires a medium to propagate.Sound travels through
solids, liquids and gases. Sound does not travel in vacuum.
Eg :- Sound travels through the solid thread of a can telephone.
A whale listens to the reflected sound (echo) in water to locate
its prey.
We are able to listen to sounds because sound travels in air.
(a mixture of gases)

Speed of a sound depends on the
1) ELASTICITY-The ability of an object or material to resume
its normal shape after being stretched or compressed
2) The Density Of The Medium through which it
propagates
In case of air, it also depends upon the temperature of air.
The speed of sound through air is about 330 m/s.
The speed of sound through water is about 1500 m/s.
The speed of sound in
Solid > liquid > gases
The speed of sound in gas is least.
Relationship between Speed (v), frequency (٧) and wave length (λ)
Speed = wave length x frequency
v = λ x ٧

State Substance Speed in m/s
Solid Aluminium 6420
Steel 5960
Iron 5950
Brass 4700
Glass 3980
Liquid Water (Sea) 1531
Water (Distilled) 1498
Ethanol 1207
Methanol 1103
Gas Hydrogen 1284
Helium 965
Air 346
Oxygen 316
Sulphur dioxide 213

The speed of sound in air is about 330 m/s.
The speed of light in air is about 3 * 108 m/s.
Speed of light is faster than speed of sound.
That is why the thundering sounds are always heard a
little after seeing the flash of lightning.
Both lightening & thundering occur together.
Delay in hearing is due to the difference in the speeds of
sound and light.

Vibration is the to and fro or back
and forth motion of an object.
Vibration is also called oscillatory
motion.
The motion of the pendulum from
one extreme position to the other
extreme position and back is
called an oscillation. (A to B and
back to A)
A B
O
Motion of a simple
pendulum

The number of oscillations per second is called
frequency.
It is represented by the symbol ٧ (Greek letter nu).
The unit of frequency is hertz (Hz).
A B
O
Motion of a simple
pendulum

The time taken for one oscillation is called time period.
It is represented by the letter T.
The SI unit is second (s).
Frequency and time are represented as follows
T =
𝟏
٧
٧ for one oscillation
٧ =
𝟏
𝑻
A B
O
Motion of a simple
pendulum

The maximum displacement of an oscillating body from its central
position is called its amplitude. (OB or OA)
OR
The amplitude of sound wave is the height of the crest or tough.
It is represented by the letter A.
The SI unit is the same as that of density or pressure.
Wavedisturbance
A B
O
Motion of a simple
pendulum

i)The loudness of sound depends upon the amplitude of vibration.
If the amplitude is less the sound is feeble.
If the amplitude is more the sound is loud.
The unit of loudness is called decibel (dB).
ii)The pitch of sound (shrillness or flatness) depends upon the
frequency of vibration.
If the frequency is less the sound has low pitch.
Eg :- sound of a drum, sound of an adult etc.
If the frequency is more the sound has a high pitch.
Eg :- sound of a whistle, sound of a baby etc.

HUMAN EARS
HOW WE HEAR SOUND THROUGH OUR EARS ?
Sound enters the ear through the ear canal. It makes the
eardrum to vibrate. The ear drum sends the vibrations to
the inner ear. From there the signals go to the brain and
then we hear the sound.

Hammer
Cochlea
Pinna
Anvil
Stirrup
The outer ear called pinna collects the sound waves. The sound
waves passes through the ear canal to a thin membrane called eardrum.
The eardrum vibrates. The vibrations are amplified by the three bones of
the middle ear called hammer, anvil and stirrup. The middle ear then
transmits the sound waves to the inner ear. In the inner ear the sound
waves are converted into electrical signals by the cochlea and sent to
the brain through the auditory nerves. The brain then interprets the
signals as sound.

The sensation of hearing of any sound persists in
our brain for 0.1s.This is called the persistence of
hearing.
Persistence of hearing helps us to get a feeling of
continuity while hearing one sound after another.
Due to Persistence of hearing the direct original
sound and reflected sound reaching ear in lesser
times than 0.1 sec mingle together & do not produce
any separate sound of their own. This saves us from
confusion & difficulty

A repetition of sound produced by the reflection of sound
waves from a wall, mountain, or other obstructing surface is
called ECHO.
If we shout or clap near a reflecting surface like tall building or a
mountain, we hear the same sound again. This sound which we
hear.
Echo is caused due to the reflection of sound.
To hear an echo clearly, the time interval between the
original sound and the echo must be at least 0.1 s.
Time period of sound = 0.1 s
Speed of sound in air = 330 m/s,
Distance travelled by sound in 0.1 s = 330 m/s x 0.1 s
= 33 m
So to hear an echo clearly,
the minimum distance of the reflecting surface should be half
this distance, that is
33
2
m = 17 m (approx.)

Echoes may be heard more than once due to repeated or
multiple reflections of sound from several reflecting surfaces.
This causes persistence of sound called reverberation.
In big halls or auditoriums to reduce reverberation, the roofs
and walls are covered by sound absorbing materials like
compressed fiber boards, rough plaster or draperies.
A reverberation is perceived when the reflected sound wave reaches
your ear in less than 0.1 second after the original sound wave. Since
the original sound wave is still held in memory, there is no time
delay between the perception of the reflected sound wave and the
original sound wave. The two sound waves tend to combine as one
very prolonged sound wave. Ex- If you have ever sung in the
shower (and we know that you have), then you have probably
experienced a reverberation.

1) Audible sound is sound which we can hear.
It has frequencies between 20 Hz and 20000 Hz.
2) Inaudible sound is sound which we cannot hear.
It has frequencies less than 20Hz and more than 20000 Hz.
Sound whose frequencies are more than 20000 Hz is called
ultrasonic sound.
Some animals like dogs can hear ultrasonic sound.
Bats produce ultrasonic sound.

Human beings can hear sound frequencies between 20
Hz and 2000 Hz.
Sound whose frequency is less than 20 Hz is called
infrasonic sound. Animals like dogs, elephants, rhinoceros,
whales etc. produce and hear infrasonic sound.
Sound whose frequency is more than 2000 Hz is called
ultrasonic sound. Animals like dolphins, bats, rats
porpoises etc. produce and hear ultrasonic sound.
Bats use reflection of ultrasonic sound waves to detect an
obstacle or its prey.

1) Noise - Unpleasant sounds are called noise.
It is produced by irregular or non-periodic vibrations.
Eg :- If all the students in a classroom speak together, a
noise is produced.
Sounds produced by horns of busses and trucks.
2) Musical sound - Sound which is pleasing to the ears is
called musical sound.
It is produced by regular or periodic vibrations.
Eg :- Sounds produced by musical instruments.
Sound of a person singing a song.
Nightingale

The presence of excessive or unwanted (undesirable) sound
in the environment is called noise pollution.
a) Causes of noise pollution :-
Noise pollution is caused by sounds of vehicles, explosions
including bursting of crackers, machines, loudspeakers etc. In the home
noise pollution is caused by television radio and music systems at high
volume, some kitchen appliances, desert coolers, air conditioners etc.
b) Harmful effects of noise pollution :-
Noise pollution causes several health related problems like lack of
sleep, hypertension, high blood pressure, anxiety etc. A person
exposed to loud sound continuously may get temporary or permanent
impairment of hearing.
c) Measures to limit noise pollution :-
Noise pollution can be reduced by using silencers in vehicles,
industrial machines, and home appliances reducing use of vehicle
horns, running TV, radio and music systems at low volumes. Planting of
trees along roads and buildings also help to reduce noise pollution.

Hearing loss, also known as hearing impairment, is a
partial or total inability to hear.
A deaf person has little to no hearing.
Partial disability is generally the result of a disease
injury or age.

Sign language (also signed language) is a language that
uses manual communication to convey meaning. This can
include simultaneously employing hand gestures, movement,
orientation of the fingers, arms or body, and facial
expressions to convey a speaker's ideas.

Hearing aid is an electronic battery operated
device. It receives sound through its microphone,
which convert these sound waves to electrical
signals, these signal are amplified & given to the
speaker of the hearing aid. The speaker converts
this amplified signals back into sound. This
amplified sound is then sent to the ear & this
helps the affected person to hear it much more
clearly.

Like light, sound gets reflected at the surface of a solid or liquid and
follows the laws of reflection.
i) The angle of incidence is equal to the angle of reflection.
ii) The incident ray, the reflected ray and normal at the point of
incidence all lie in the same plane.
Activity :- Take two pipes of the same length and arrange them on a
table near a wall or metal plate. Keep a clock near the open end of one
pipe and try to hear the sound of the clock through the other pipe by
adjusting the position of the pipe.
Now measure the angles of incidence
and reflection. Then lift the second
pipe and try to hear the sound.
It will be seen that the angle of
incidence is equal to the angle of
reflection. The incident ray, the
reflected ray and normal all lie in
the same plane.

i) Megaphones, horns, musical instruments like trumpets, shehnais
etc. are deigned to send sound by multiple reflection in a particular
direction without spreading in all directions.
ii) Doctors listen to sounds from the human body through a
stethoscope. The sound of heartbeat reaches the doctor’s ears by
multiple reflection.
iii) Generally the ceilings of cinema halls and auditoriums are curved
so that sound after multiple reflection reaches all parts of the hall.
Sometimes a curved sound board is placed behind the stage so that
sound after multiple reflection spreads evenly across the hall.

Sonar stands for Sound Navigation And Ranging. It is a device which
uses ultrasonic waves to measure distance, direction and speed of
underwater objects.
Sonar has a transmitter and a detector installed in ships. The
transmitter produces ultrasonic sound waves which travel through the
water and after striking the object in the sea bed is reflected back to the
detector.
The distance of the object can be calculated by knowing the speed of
sound in water and the time taken between the transmission and
reception of ultrasound.
If the time taken for the transmission and reception of ultra sound is t
and the distance travelled is 2d by the ultra sound, then 2d = v x t
or d =
v x t
2

Sound chapter 12 class-8

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Sound chapter 12 class-8