this is a notes of sound for class 8 students. this will help you to revise the chapter very quickly. recap is also included in this. with the help of pictures you will be able to understand it quickly.

1. Sound is a form of energy that enables us to hear. It plays an important role in
our life. Every day we come across many types of sound. It can sound of our
parents, friends, telephone, television, radio, machines and many more. Each
sound is special to the object which produces it. Due to this, we can distinguish
different types of sound. Sound helps us to communicate with others or others
to communicate with us. But have you ever thought how sound is produced?
SOUND IS PRODUCED BY A VIBRATING BODY
What happens when you beat a table or a drum? It produces sound. If you touch
the table or drum while producing sound you will feel that it is vibrating. Thus,
sound is produced because of vibrations of an object. In other words, a
vibrating object produces sound.In some cases, the vibrations are easily visible
to our naked eyes but in most cases, they can only be felt and not seen.
Have you ever thought how musical instruments like cymbals (Manjiro), ghatam,
veena, tabla, etc., produce sound? Different musical instruments produce
different types of sound because of vibration. Stringed instruments such as
violin have a series of stretched strings, which vibrate when they are plucked or
stroked. Wind instruments (such as flutes) vibrate across their air columns when
blown, percussion instruments (such as drum) vibrate when they are struck. For
example, a flute produces sound because of vibrations in its air column,
Similarly, a guitar produces sound because of vibrations of its stretched string or
a drum produces sound because of vibrations of its diaphragm.

2. A human being produces sound because of
vibration of his voice box. The voice box is also
known as larynx. Larynx is situated at the upper
end of windpipe. There are two stretched
membranes or vocal cords attached in larynx;
with a narrow slit between them. When air
passes through those stretched membranes
they vibrate and the vibrating vocal cords
produce different types of sound with different combinations of stretching.
For example, when the muscles attached to the vocal cords make them tight and
thin, the type or quality of voice is different from that when they are loose and
thick.
Do you know why there is a difference in the voices of men, women and
children? It is due to difference in the length of their vocal cords. The vocal cords
in men are longer about 20 mm long, while in women these are shorter about
15 mm and children have very short vocal cords.
The travelling of sound is called propagation of sound. Sound can travel through
different media like solid, liquid and gas. But sound cannot propagate in the
absence of a medium or in vacuum. The place where there is no air or air is
removed, is said to has vacuum. Sound does not propagate (travels) through
vacuum.
This is the reason that one cannot hear his/her clapping or any other sound on
the moon or outer space, because there is no atmosphere or medium. We
usually hear sound which comes to us through air. Aquatic animals communicate
as sound travels through water also.

3. WE HEAR SOUND THROUGH OUR EARS
Our external ear called pinna shapes like a funnel. Due to its unique shape,
sound waves are concentrated once they enter external ear. Then the sound
waves reach the eardrum or
tympanum through a 2-3 cm
long passage called ear canal.
The eardrum is like a stretched
membrane which vibrates when
sound waves strike it. From
eardrum, sound waves reach
inner ear, through middle ear. In
the middle, there are three small
interconnected bones namely
hammer, anvil and stirrup. The inner part of ear has a coiled tube called
'cochlea'. It’s one end is connected to middle ear and another is connected to
auditory nerves which go to brain. Cochlea is filled with a liquid containing nerve
cells sensitive to sound. From inner ear, sound waves are sent to the brain via
auditory nerves. The brain interprets these signals and we hear the sound.
Note: Our ears are delicate and sensitive organ. We should never put a sham
pointed or hard thing into our ears. This can damage the eardrum. The damaged
eardrum can make a person hearing impaired.
Fast Fact
How do Astronauts Communicate in Space?
In the outer space there is no air or atmosphere. Sound cannot travel through the vacuum
or space, but radio waves can. Radio waves unlike sound waves can travel through vacuum.
Astronauts have devices in their helmets which transform the sound waves from the voices
into radio waves and transmit them to the other astronaut in space or to the ground. This is
exactly how a radio at our home works.

4. Since sound is produced by vibrations, amplitude and frequency are two
important characteristics of sound. Sound produced by different objects is
differentiated by the amplitude and frequency of sound.
Amplitude: It is defined as the maximum distance of the vibrating object from
its mean position. It is denoted by A. In SI units, it is measured in metre(m).
The amplitude indicates
how far the object is
displaced from its central
position.
Greater the amplitude,
louder is the sound. For
example, if the string of
a guitar is pulled gently, a soft (feeble) sound is heard. On the other hand, if it is
pulled hard, i.e., its amplitude is increased a louder sound is heard.
Frequency: The number of vibrations or oscillations per second is called
frequency.
𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑂𝑠𝑐𝑖𝑙𝑙𝑎𝑡𝑖𝑜𝑛𝑠/𝑉𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑠
𝑇𝑖𝑚𝑒
Frequency is expressed in hertz (Hz). It is denoted by f or v. If an object
oscillates or vibrates 400 times in 1 second, then its frequency will be equal to
400 hertz.
Time period: The time required to produce one complete oscillation is called
time period. It is measured in second(s). It is denoted by T.
𝑇𝑖𝑚𝑒 𝑃𝑒𝑟𝑖𝑜𝑑 =
𝑇𝑖𝑚𝑒
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑂𝑠𝑐𝑖𝑙𝑙𝑎𝑡𝑖𝑜𝑛𝑠/𝑉𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛𝑠
Thus, time period and frequency are reciprocal to each other.

5. The sound that we hear around us is the type of energy made by the vibration
that travels through the air or any other medium and can be heard when it
reaches a person's ear. So, basically sound helps us to communicate with the
other person. For example, we can hear the musical instruments like tabla, flute,
a guitar due to vibration. The characteristics of sound are as follows:
• Pitch
• Loudness
• Quality
Shrillness or pitch
Frequency of a sound determines the shrillness or pitch of the sound. Shrillness
or pitch indicates how high or low a sound is. It increases with increase in
frequency of sound. Sound with greater frequency is shriller and has higher
pitch. Sound with lower frequency is less shrill and has lower pitch.
Loudness
Loudness of sound depends on the amplitude of the sound waves. Greater
amplitude produces louder sound and smaller amplitude produces feeble
sound. The loudness of sound is measured in decibel (dB) unit. The sound above
80 decibel becomes physically painful. Loudness of some types of sound is given
here in decibel.
Loudness of sound decibel (dB) Type of sound
Normal Breathing 10 Very Faint
Soft whisper 30 Faint
Normal conversation 60 Moderate
Busy traffic (Inside car) 70 Loud
Telephone dial tone 80 Loud
Train whistle 90 Very Loud
Hand drill 98 Very loud
Jet engine 140 Painfully loud

6. Quality
One can distinguish between the sound of clapping and that of singing or the
sound of a guitar and that of piano without actually seeing them. This is because
their difference in quality. Quality of a sound enables us to differentiate between
the sound of a guitar and that of a piano. It is also called timber. It is due to the
timber that we can a recognise the voice of different playback singers without
actually seeing them.
Although all vibrating objects produce sound, but we cannot hear all of them.
Our ears are sensitive only to a fixed range of frequencies called audible range.
Sounds with frequency between 20 Hz to 20,000 Hz are audible sound. The
hearing range of human beings is between 20 hertz to 20,000 hertz. It indicates
that we can only hear the sound if a vibrating object oscillates at least 20 times
per second. That's why we cannot hear the sound of a vibrating simple
pendulum because its frequency is very low, i.e., it oscillates less than 20 times
per second.
Sound with frequency below 20 hertz is
called infrasonic and above 20,000 hertz is
called ultrasonic sound of the inaudible
range. Humans cannot hear the sound of
inaudible range.
Some animals, such as dogs, cats, bats etc.
can hear the sound with frequency above
20,000 hertz. We cannot hear sound of this
frequency, but this sound is applied in
various technologies such as SONAR system,
surgical tools and cleaning system. The
ultrasound equipment used to diagnose
many internal ailments works on this
frequency range.
Fast Fact
Dogs can be easily trained to
respond to ultrasonic
sound. For this purpose, an
ultrasonic vibration
generating whistle called
Galton’s whistle is used. It is
within the range of 16,000
Hz to 22,000 Hz which is
beyond the human audible
range.

7. Every day we hear different types of sound. Do all types of sound please or
irritate us?
Sound that appears unpleasant to us is called noise, such as sound of horn,
sound near the site of construction work, sound of aeroplane, etc. The word
noise has been derived from Latin word noxia means I do harm. Noise is
produced by the irregular vibrations of the sound producing source.
Sound that appears pleasant to our ear is called musical sound, such as sound of
musical instruments, song of a good singer, etc. A musical sound is produced by
the regular vibrations of the sound producing source. Thus, noise and music
both are mixtures of sound of different frequencies.
Musical Instruments
We all hear music. Music is the systematic arrangement of sounds of different
frequencies (or notes) supported by musical instruments that is pleasant to hear.
Musical instruments also produce sound by vibration. The type of instrument is
decided by what part of the instrument vibrates.
Wind Instruments Percussion Instruments String Instruments
Wind instruments are
required to blow into the
instrument in order to
produce the desired
sound. On blowing into
the instrument, the pitch
of the sound note
produced, varies
depending on the length
of the internal air
column in which the
sound waves vibrate.
The shorter the air
column, the higher is the
Percussion instruments are
required to strike or beat
the instrument surface in
order to generate
vibrations, which then
produce the desired sound
note. Depending on the
type of surface, one can
use one's hands, sticks or
other similar objects for
striking the instrument
surface. Drum, Tabla, Dhol,
Nagara, Cymbals, Bells are
String instruments
work on the basis of
sound wave vibrations
that are created with
the help of strings. The
pitch of the sound
note produced
depends on the length
of the air column as
well as the type and
thickness of the string
involved. Guitar, Piano,
Violin, Sitar, Sarod,
Santoor are some

8. pitch of the note
produced and vice-
versa. Flute, Shehnai,
Trumpet are some
examples of wind
instruments.
some examples of
percussion instruments.
examples of string
instruments.
How do you feel when suddenly someone starts singing loudly on loudspeaker during your
examination?
Loud and excessive sound is unbearable to our ears, and is called noise. Unwanted and
excessive sound in our environment creates noise pollution. Sounds of crackers, factories,
vehicles, desert coolers, air conditioners. aeroplane, transistors or television with high
volume, loudspeakers, etc., create sound pollution.
Harms of Noise Pollution
Noise pollution can create many types of health-related problems, such as lack of sleep
(insomnia), hypertension, loss of hearing, anxiety, fatigue etc. A person who is exposed to
loud sound continuously may get permanent or temporary loss of hearing.
Measures to Limit Noise Pollution
Noise pollution can be limited or controlled by controlling the noise coming from source.
Noise pollution can be limited by taking following measures:
• TV or radio should be played at low volume.
• By installing high quality silencers in vehicles and Minimum use of vehicle horns.
• Misuse of loudspeakers, worshippers, outdoor parties and public announcement system
should be checked.
• Plantation of trees along the road sides and around buildings. Trees absorb sound.
• Noisy operations should be conducted away from residential area.
• Awareness campaign to make people aware about the harmful effects of noise pollution
and measures to control noise pollution.

9. Recap:Sound