Notes:
Ch-12 Sound | Class 9th Science
Study Material and Notes
of Sound Class 9th Science
Topics in the Chapter
Introduction
• Production of Sound
• Propagation of Sound
• Sound waves as Longitudinal waves
• Characteristics of Sound wave
• Wavelength
• Frequency
• Time Period
• Amplitude
Ø Pitch
Ø Loudness
Quality or Timbre
• Velocity
• Speed of sound in various medium
• Sonic Boom
• Reflection of Sound
• Echo
• Reverberation
• Range of Hearing
• Hearing Aid
• Application of Ultrasound
• SONAR
• Structure of Human Ear
Ø Working of
Human Ear
5.
THE
FUNDAMENTAL UNIT OF LIFE
6.
TISSUES
7.
MOTION
9.
GRAVITATION
10.
WORK
AND ENERGY
11.
SOUND
12.
IMPROVEMENT
IN FOOD RESOURCES
Ø Introduction
§ The
sensation felt by our ears is called sound.
§ Sound is a
form of energy which makes us hear.
§ Law of
conservation of energy is also applicable to sound
§ Sound
travels in form of wave.
Ø Production
of Sound
o Sound is
produced when object vibrates or sound is produced by vibrating objects.
o The energy required to make an object vibrate
and produce sound is provided by some outside source (like our hand, wind
etc.).
o Example: Sound
of our voice is produced by vibration of two vocal cords in our throat.
Ø Sound of a
drum or tabla is produced by vibration of its membrane
when struck.
o In
laboratory experiments, sound is produced by vibrating tuning fork. The
vibrations of tuning fork can be shown by touching a small suspended pith ball
(cork ball) with a prong of the sounding tuning fork. The pith ball is pushed
away with a great force.
Sound
can be produced by following methods:
(i)
By vibrating string (sitar)
(ii)
By vibrating air (flute)
(iii)
By vibrating membrane (table, drum)
(iv)
By vibrating plates (bicycle bell)
(v)
By friction in objects
(vi)
By scratching or scrubbing the objects etc.
Propogation
of Sound
o The
substance through which sound travels is called a medium.
o The medium
may be solid, liquid or gas.
o When an
object vibrates, then the air particles around it also start vibrating in
exactly the same way and displaced from their stable position.
o These vibrating air particles exert a force on
nearby air particles so they are also displaced from their rest position and
start to vibrate.
o This
process is continued in the medium till sound reaches our ears.
o The
disturbance produced by sound travels through the medium (not the particles of
the medium).
o Wave is a disturbance which travels through a
medium and carries energy.
o So sound
travels in wave form known as mechanical waves.
•
When a body vibrates then it compresses the air surrounding it and form a area
of high density called compression (C).
o Compression
is the part of wave in which particles of the medium are closer to one another
forming high pressure.
o → This
compression move away from the vibrating body.
•
When vibrating body vibrates back a area of low pressure is formed
called rarefaction (R).
o →
Rarefaction is the area of wave in which particles of the medium are further
apart from one another forming a low pressure or low density area.
o → When body
vibrates back and forth, a series of compression and rarefaction is formed in
air resulting in sound wave.
o → Propagation
of sound wave is propagation of density change.
Sound
needs Medium for Propogation
•
Sound waves are mechanical waves.
o → It needs
material medium for propogation like air, water, steel etc.
o → It cannot
travel in vaccum.
o → An
electric bell is suspended in airtight bell jar connected with vacuum pump.
o → When bell
jar is full of air, we hear the sound but when air is pumped out from the bell
jar by vacuum pump and we ring the bell, no sound is heard.
o → So,
medium is necessary for propagation of sound.
Experiment
to show that sound cannot travel through vacuum
Sound
Waves as Longitudinal Waves
•
A wave in which the particles of the medium vibrate back and forth in the same
direction in which the wave is moving, is called a longitudinal wave.
o When we push and pull the slinky compression
(number of turns are more or closer) and rarefaction (number of turns are less
or farther) are formed.
o When a wave
travels along with slinky, its each turn moves back and forth by only a small
distance in the direction of wave. So the wave is longitudinal.
o The
direction of vibrations of the particles is parallel to the direction of wave.
•
When one end of a slinky is moved up and down rapidly whose other end is fixed,
it produces transverse wave.
o This wave
possess along the slinky in horizontal direction, while turns of slinky
(particles) vibrate up and down at right angle to the direction of wave.
o Thus in
transverse wave particles of the medium vibrate up and down at right angles to
the direction of wave.
o Light waves are transverse waves but they
don’t need a material
medium
for propagation.
Characteristics
of Sound Wave
•
The characteristics of sound waves are : wavelength, frequency, amplitude, time
period and velocity.
o When a wave travel in air the density and
pressure of air changes from their mean position.
o Compression is shown by crest while
rarefaction is shown by trough.
o Compression
is the region of maximum density or pressure.
o Rarefaction
is the region of minimum density or pressure.
Wavelength
o In sound
waves the combined length of a compression and an adjacent rarefaction is
called its wavelength.
o The
distance between the centres of two consecutive compressions or two consecutive
rarefactions is also called its wavelength.
o It is
denoted by the Greek letter lamda (λ). Its SI unit is metre.
Frequency
o No. of
complete waves produced in one second or number of vibrations per second is
called frequency.
o Number of
compressions or rarefactions passed in one second is also frequency.
o Frequency
of wave is same as the frequency of the vibrating body which produces the wave.
•
The SI unit of frequency is hertz (Hz). The symbol of frequency is v (nu).
• 1
Hertz: One Hz is equal to 1 vibration per second.
•
Bigger unit of frequency is kilohertz kHz = 1000 Hz.
Time
Period
o Time taken
to complete one vibration is called time period.
o Time
required to pass two consecutive compressions or rarefactions through a point
is called time period.
•
SI unit of time period is second (s). Time period is denoted by T.
•
The frequency of a wave is the reciprocal of the time period.
•
v = 1/T
Amplitude
o The maximum
displacement of the particle of the medium from their original undisturbed
position is called amplitude of the wave.
•
Amplitude is denoted by A and its SI unit is metre (m).
o Sound have
characteristics like pitch and loudness and timbre.
•
Pitch: The pitch of sound depends on the frequency of sound (vibration).
o t is
directly proportional to its frequency. Greater the frequency, higher is the
pitch and lesser the frequency, lower is the pitch.
o A woman’s
voice is shrill having a high pitch while a man’s voice is flat having low
pitch.
o High pitch
sound has large number of compressions and rarefactions passing a fixed point
per unit time.
•
Loudness: The loudness depends on the amplitude of the sound wave.
o Loudness is
the measure of the sound energy reaching the ear per sec.
o Greater the
amplitude of sound wave, greater is the energy, louder the sound; short is the
amplitude, less is the energy, soft is the sound.
o Loudness is
measured in decibel ‘dB’.
•
Quality or Timbre: The timbre of a sound depends on the shape of sound wave
produced by it. It is the characteristic of musical sound.
o It helps us
to distinguish between two sounds of same pitch & loudness.
•
Sound of single (same) frequency is called tone while a mixture of
different frequencies is called note.
o Noise is
unpleasant to hear while music is pleasant to hear and it is of good quality.
Velocity
•
The distance travelled by a wave in one second is called velocity of the wave.
•
Its SI unit is metre per second (ms-1).
Velocity
= Distance travelled/Time taken
⇒
v = λ/T (λ is the wavelength of the
waves travelled in one time time period T)
v = λv (1/T = v)
So,
Velocity = Wavelength × Frequency
This
is the wave equation.
Example: What
is the frequency of sound wave whose time period is 0.05 second ?
Solution
Frequency,
v = 1/T
Given
T = 0.05 s
v
= 1/0.005 = 100/5 = 20Hz
∴
Frequency = 20 Hz.
Speed
of Sound in Various Mediums
o Speed of
sound depends on the nature of material through which it travels. It is slowest
in gases, faster in liquids and fastest in solids.
o Speed of
sound increases with the rise in temperature.
o Speed of
sound increases as humidity of air increases.
o Speed of
light is faster than speed of sound.
o In air,
speed of sound is 344 ms-1 at 22ºC.
Sonic
Boom
o Some
aircrafts, bullets, rockets etc. have ‘supersonic speed’.
•
Supersonic refers to the speed of an object which is greater than the speed of
sound and it produces extremely loud sound waves called ‘shock waves’ in air.
o Sonic boom
is an explosive noise caused by shock waves.
o It emits
tremendous sound energy which can shatter the glass panes of windows.
o Reflection
of Sound
o Like light,
sound also bounce back when it falls on a hard surface. It is called reflection
of sound.
•
The laws of reflection of light are obeyed during reflection of sound.
(i)
The incident sound wave, the reflected sound wave and normal at the point of
incidence lie in the same plane.
(ii)
Angle of reflection of sound is always equal to the angle of incidence of
sound.
Echo
•
The repetition of sound caused by the reflection of sound waves is called an
echo.
o We can hear
echo when there is a time gap of 0.1 second in original sound and echo
(reflected sound).
o Echo is
produced when sound reflected from a hard surface (i.e. brick wall, mountain
etc.) as soft surface tends to absorb sound.
Minimum
distance to hear an echo
Speed
= Distance/Time
Here,
Speed of sound in air = 344 ms-1 at 22ºC
Time
= 0.1 second
344
= Distance/0.1 sec
⇒
Distance = 344 × 0.1 = 34.4 m
So,
distance between reflecting surface and audience = 34.4/2 = 17.2 (at 22ºC).
o Rolling of
thunder is due to multiple reflection of sound of thunder from a number of
reflecting surfaces such as clouds and the earth.
Reverberation
•
The persistence of sound in a big hall due to repeated reflection of sound from
the walls, ceiling and floor of the hall is called reverberation.
o If
reverberation is too long, sound becomes blurred, distorted and confusing due
to overlapping of different sound.
Methods
to reduce reverberation in big halls or auditoriums
o Panels made
of felt or compressed fibre board are put on walls and ceiling to absorb sound.
o Heavy
curtains are put on doors and windows.
o Carpets are
put on the floor.
o Seats are made
of material having sound absorbing properties.
Applications
of Reflection of Sound
(i)
Megaphone, loudspeakers, bulb horns and trumpets, shehnai etc. are designed to
send sound in a particular direction without spreading all around.
o All these
instruments have funnel tube which reflects sound waves repeatedly towards
audience. In this amplitude of sound waves adds up to increase loudness of
sound.
(ii)
Stethoscope: It is a medical instrument used for listening the sounds produced
in human body mainly in heart and lungs. The sound of the heartbeats reaches
the doctor’s ears by the multiple reflection of the sound waves in the rubber
tube of stethoscope.
(iii)
Sound Board: In big halls or auditoriums sound is absorbed by walls, ceiling,
seats etc. So a curved board (sound board) is placed behind the speakers so
that his speech can be heard easily by audiences. The soundboard works on the
multiple reflection of sound.
(iv)
The ceiling of concert halls are made curved, so that sound after reflection
from ceiling, reaches all the parts of the hall.
Range
of Hearing
(i)
Range of hearing in human is 20 Hz to 20000 Hz.
o Children
younger than 5 years and dogs can hear upto 25 KHz.
(ii)
The sounds of frequencies lower than 20 Hz are known as ‘infrasonic sounds’.
o A vibrating
simple pendulum produces infrasonic sounds.
o Rhinoceroses
communicate each other using frequencies as low as 5 Hz.
o Elephants
and whales produces infrasonic waves.
o Earthquakes
produces infrasonic waves (before shock waves)
which
some animals can hear and get disturbed.
(iii)
The sounds of frequencies higher than 20 KHz are known as ‘ultrasonic waves’.
o Dogs,
parpoises, dolphins, bats and rats can hear ultrasonic sounds.
o Bats and
rats can produce ultrasonic sounds.
Hearing
Aid
o It is
battery operated electronic device used by persons who are hard of hearing.
o Microphone
convert sound into electrical signals, than those are amplified by amplifier.
Amplified signals are send to the speaker of hearing aid. The speaker converts
the amplified signal to sound and sends to ear for clear hearing.
Applications
of Ultrasound
(i)
It is used to detect cracks in metal blocks in industries without damaging
them.
(ii)
It is used in industries to clean ‘hard to reach’ parts of objects such as
spiral tubes, odd shaped machines etc.
(iii)
It is used to investigate the internal organs of human body such as liver, gall
bladder, kidneys, uterus and heart.
(iv) Ecocardiography:
These waves are used to reflect the action of heart and its images are formed.
This technique is called echocardiography.
(v) Ultrasonography:
The technique of obtaining pictures of internal organs of the body by using
echoes of ultrasound waves is called ultrasonography.
(vi)
Ultrasound is used to split tiny stones in kidneys into fine grains.
SONAR
o The word
‘SONAR’ stands for ‘Sound Navigation And Ranging’.
SONAR
is a device which is used to find distance, direction and speed of underwater
objects.
o SONAR
consists of a transmitter and a receptor or detector and installed at the
bottom of a ship.
o The
transmitter produces and transmits ultrasonic waves.
o These waves
travel through water and after striking the objects on the bottom of sea, are
reflected back and received by detector.
o These
reflected waves are converted into electric signals by detector.
o The sonar
device measures the time taken by ultrasound waves to travel from ship to
bottom of sea and back to ship.
o Half of
this time gives the time taken by the ultrasound waves from ship to bottom.
•
Let the time interval between transmission and reception of ultrasound signal
is t.
Speed
of sound through sea water is v
Total
distance travelled by waves = 2d.
Then,
2d = v × t.
This
is called echo ranging.
o The sonar
is used to find the depth of sea, to locate underwater hills, valleys, submarines,
icebergs and sunken ships etc.
o Bats fly in
the dark night by emitting high pitched ultrasound waves which are reflected
from the obstacle or prey and returned to bats ear.
o The nature
of reflection tells the bat where the obstacle or prey is and what it is like.
Structure
of Human Ear
o The ear
consists of three parts: outer ear, middle ear and inner ear.
o The ears
are the sense organs which help us in hearing sound.
o The outer
ear is called pinna. It collects the sound from surroundings.
o This sound
passes through the auditory canal.
o At the end
of auditory canal, is a thin elastic membrane called ear drum or tympanic
membrane.
o The middle
ear contains of three bones: hammer, anvil and stirrup linked with one another.
Free end of hammer touches ear drum and that of stirrup linked with membrane of
oval window of inner ear.
o The lower
part of middle ear has a narrow ‘Eustachian tube’.
o The inner
ear has a coiled tube called cochlea, which is connected with oval window.
Cochlea is filled with a liquid containing nerve cells.
o Other side
of cochlea is connected to auditory nerve which goes to brain.
Working
of Human ear
Pinna
→ Ear canal → Ear drum → Hammer → Anvil → Stirrup → Oval window → Cochlea →
Auditory nerve → Brain
→
When compression of sound wave strikes the ear drum, the pressure on the
outside of ear drum increases and pushes the ear drum inwards.
→
While during rarefaction ear drum moves outwards. Thus, ear drum starts
vibrating back and forth.
→
These vibrations are increased by three bones and middle ear transmits these
amplified pressure variations received from sound waves to inner ear.
→
In the inner ear the pressure variations are turned into electric signals by
the cochlea.
→
These electric signals are sent to the brain via auditory nerve and the brain
interprets them as sound.
===========================
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Questions
1. How does the
sound produced by a vibrating object in a medium reach your ear?
1. Explain how
sound is produced by your school bell.
2. Why are sound
waves called mechanical waves?
3. Suppose you and
your friend are on the moon. Will you be able to hear any sound produced by
your friend?
1. Which wave
property determines (a) loudness, (b) pitch?
2. Guess which
sound has a higher pitch: guitar or car horn?
1. What are
wavelength, frequency, time period and amplitude of a sound wave?
2. How are the
wavelength and frequency of a sound wave related to its speed?
3. Calculate the
wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a
given medium.
4. A person is
listening to a tone of 500 Hz sitting at a distance of 450 m from the source of
the sound. What is the time interval between successive compressions from the
source?
1. Distinguish
between loudness and intensity of sound.
1. In which of the
three media, air, water or iron, does sound travel the fastest at a particular
temperature?
1. An echo is heard
in 3 s. What is the distance of the reflecting surface from the source, given
that the speed of sound is 342 m s–1?
1. What is the
audible range of the average human ear?
2. What is the
range of frequencies associated with (a) Infrasound? (b) Ultrasound?
1. A submarine
emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the
speed of sound in salt water is 1531 m/s, how far away is the cliff?
Exercises
1. What is sound
and how is it produced?
2. Describe with
the help of a diagram, how compressions and rarefactions are produced in air
near a source of sound.
3. Cite an
experiment to show that sound needs a material medium for its propagation.
4. Why is sound
wave called a longitudinal wave?
5. Which
characteristic of the sound helps you to identify your friend by his voice
while sitting with others in a dark room?
6. Flash and
thunder are produced simultaneously. But thunder is heard a few seconds after
the flash is seen, why?
7. A person has a
hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound
waves in air corresponding to these two frequencies? Take the speed of sound in
air as 344 m s–1 .
8. Two children are
at opposite ends of an aluminium rod. One strikes the end of the rod with a
stone. Find the ratio of times taken by the sound wave in air and in aluminium
to reach the second child.
9. The frequency of
a source of sound is 100 Hz. How many times does it vibrate in a minute?
10. Does sound
follow the same laws of reflection as light does? Explain.
11. When a sound is
reflected from a distant object, an echo is produced. Let the distance between
the reflecting surface and the source of sound production remains the same. Do
you hear echo sound on a hotter day?
12. Give two
practical applications of reflection of sound waves.
13. A stone is
dropped from the top of a tower 500 m high into a pond of water at the base of
the tower. When is the splash heard at the top? Given, g = 10 m s–2 and speed
of sound = 340 m s–1 .
14. A sound wave
travels at a speed of 339 m s–1. If its wavelength is 1.5 cm, what is the
frequency of the wave? Will it be audible? 2020-21 SOUND 175
15. What is
reverberation? How can it be reduced?
16. What is
loudness of sound? What factors does it depend on?
17. Explain how
bats use ultrasound to catch a prey.
18. How is
ultrasound used for cleaning?
19. Explain the
working and application of a sonar.
20. A sonar device
on a submarine sends out a signal and receives an echo 5 s later. Calculate the
speed of sound in water if the distance of the object from the submarine is
3625 m.
21. Explain how
defects in a metal block can be detected using ultrasound.
22. Explain how the
human ear works.
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