2. 1. Sound
2. Experiments with Vibrating Tuning Fork
3. Production of Sound
4. Propagation of Sound
5. Sound Waves in Air
6. Sound Needs Material Medium to Travel
– Bell Jar Experiment
1. Characteristics of a Sound Wave
2. Reflection of Sound
3. Reverberation
4. Uses of Multiple Reflection
5. Range of Hearing
6. Infrasonic and Ultrasonic Sound
7. Uses of Ultrasonic Sound
8. SONAR
Created by Shashank Garg
3. Sound is a form of energy which gives sensation of hearing to us.
Sound is produced due to vibration of a body.
It is a form of mechanical energy.
Sound is produced by:
• Vocal cords of human being
• Birds and animals
• Bell
• Vehicles
• Musical instruments like guitar, veena, violin, piano,
harmonium, flute, shehnai, tabla, drums, cymbals, etc.
SOUND
4. PRODUCTION OF SOUND
Vibrating tuning fork
splashes water.
Vibrating tuning fork makes
the pendulum bob to oscillate.
(Tuning fork vibrations were
shown in slow motion.)
9. When an object vibrates and makes sound, the air layers around it
also start vibrating in exactly the same way and carry sound waves from
the source to our ears.
The air layers (air molecules) vibrate back and forth about their mean
positions.
When the layers approach each other they form compressions and
when they move away from each other they form rarefactions.
Note that there is no actual movement of air layers from the source of
sound to the ears but the sound energy is getting transmitted from one
layer to another in succession.
The maximum distance moved by a vibrating air layer on either side
of its original position is known as the amplitude of the sound wave.
The number of complete back and forth vibrations of an air layer in
one second is known as the frequency of the sound wave.
The distance between two successive compressions or two
successive rarefactions is called wavelength.
10. Sound Waves in Air
Wavelength
Wavelength
Rarefaction Compression Air Layers
11. Sound Waves in Air
The sound waves in air are longitudinal waves.
Sound waves in air consist of compressions and rarefactions.
Air layers, though randomly distributed, are almost equally placed from each
other.
When the right prong of the tuning fork moves outwards (towards right), it
pushes the air layers to the right creating more pressure. Therefore,
compressions (regions of high pressure) are formed.
When the right prong of the tuning fork moves inwards (towards left), it creates
low pressure. Therefore, rarefactions (regions of low pressure) are formed.
These compressions and rarefactions are passed on to the next layers by the
vibrating air layers.
The process is repeated as long as the tuning fork vibrates.
The compressions and rarefactions are alternately formed.
Only the compressions and the rarefactions pass on from one place to other
but the air layers (molecules of air) vibrate back and forth about their mean
positions.
When these sound waves fall on our ears, the ear drums vibrate accordingly
and reproduce the sound.
12. Sound Waves in Air
Wavelength (λ)
Wavelength (λ)
Distance
Density
or
Pressure
R C R C R C
Average Density or Pressure
Amplitude (A)
15. S
Vacuum Pump
Bell
Air is being
evacuated
Air is completely evacuated from the jar.
i.e. vacuum is created. Test the bell now!
Bell sound is
not heard!
SOUND NEEDS A MATERIAL MEDIUM TO TRAVEL
Air particles
Vacuum
E
Sound reaches
the ears
16. Sound does not travel in vacuum.
Sound needs a material medium like solid, liquid or gas to travel and
be heard.
Therefore, sound waves are called ‘mechanical waves’.
Characteristics of a Sound Wave
1. Amplitude (A): The maximum distance moved by a vibrating air layer on
either side of its original position is known as the amplitude of the sound
wave. Its SI unit is ‘m’.
2. Wavelength (λ): The distance between two successive compressions or
two successive rarefactions is called wavelength. Its SI unit is ‘m’.
3. Time Period (T): The time taken by an air layer to complete one vibration
(to and fro motion) is known as the time period of the sound wave.
Its SI unit is ‘s’.
4. Frequency (n): The number of complete back and forth vibrations of an air
layer in one second is known as the frequency of the sound wave.
Its SI unit is ‘Hertz’.
Relation between Frequency and Time Period: n =
1
T
17. 5. Speed of Sound
The rate at which sound travels in a given medium is called speed of sound.
i) The speed of Sound depends on the nature of material (or
medium) through which it travels.
In general, sound travels slowest in gases, faster in liquids and fastest in
solids.
Speed of sound in air is 344 m/s at room temperature (20ºC).
Speed of sound in water is 1500 m/s.
Speed of sound in iron is 5130 m/s.
ii) The speed of sound depends on the temperature of the medium.
Speed of sound in air at 0ºC is 332 m/s and at 20ºC, it is 344 m/s.
iii) The speed of sound depends on the humidity of air.
Speed of sound in dry air is less but more in humid
air.
18. S.
No.
Medium Temperature
in ºC
Speed of sound in
m/s
1 Dry Air 0 332
2 Dry Air 20 344
3 Hydrogen 0 1284
4 Water (Distilled) 20 1498
5 Sea-water 0 1531
6 Blood 37 1570
7 Copper 20 3750
8 Aluminium 20 5100
9 Iron 20 5130
10 Glass 20 5170
Speed of Sound in Different Media
19. Comparison of Speed of Sound and Light
Sound travels in air at 20 ºC at the speed of 344 m/s.
Light travels at the speed of 300,000,000 m/s. (3 x 108
m/s)
Examples:
1. In the rainy season, the flash of lightning is seen first and the sound of
thunder is heard a little later though both are produced at the same time.
Package
2. In the game of cricket, the ball is seen to hit the bat first and the sound of
hitting is heard a little later.
3. If a gun is fired from a distance, we see the flash of gun first and the sound
of gun shot is heard a little later.
4. In villages, when clothes are washed by beating them on the stone, the
clothes hitting the stone is seen first and the sound of hitting is heard a
little later.
Click the Package to see the effect!
21. Reverberation
A sound created in a big hall will persist by repeated
reflection from the walls until it is reduced to a value
where it is no longer audible.
The repeated reflection that results in this persistence of
sound is called reverberation.
In an auditorium or big hall excessive reverberation is
highly undesirable.
To reduce reverberation, the roof and walls of the
auditorium are generally covered with sound-absorbent
materials like compressed fibreboard, rough plaster or
draperies. The seat materials are also selected on the
basis of their sound absorbing properties.
22. USES OF MULTIPLE REFLECTION OF SOUND
1. Megaphones, horns, musical instruments such as trumpets and shenais
are designed to send in a particular direction with multiple reflections.
23. In these instruments, a tube followed by a conical opening reflects sound
successively to guide most of the sound waves from the source in the
forward direction towards the audience.
24. 2. A stethoscope is a
medical instrument used
for listening to sounds
produced within the
body, mainly in the heart
or lungs.
In stethoscopes the
sound of the patient’s
heartbeat reaches the
doctor’s ears my
multiple reflection of
sound.
25. 3. The ceilings of concert halls and
cinema halls are curved so that
sound after reflection reaches all
corners of the hall.
Sometimes, a curved soundboard may be placed behind the stage so that
the sound, after reflecting from the sound board, spreads evenly across
the width of the hall.
27. Range of Hearing
The audible range of sound for human beings is from about 20 Hz to
20000 Hz.
Children under the age of five and some animals like dogs can hear upto
25000 Hz.
Sounds of frequencies below 20 Hz are called infrasonic sound.
Rhinoceroses, whales and elephants produce infrasonic sound.
Sounds of frequencies above 20000 Hz are called ultrasonic sound.
Dolphins, bats, porpoises, moths of certain families and rats can produce
ultrasonic sound.
1. Ultrasound is used to clean parts located in hard-to-reach places.
2. Ultrasound can be used to detect cracks and flaws in metal blocks.
3. Ultrasonic waves are made to reflect from various parts of the heart
and form the image of the heart. This technique is called
‘echocardiography’ or ECG.
Applications of Ultrasound
28. 4. Ultrasound scanner is used for imaging patient’s organs such as the
liver, gall bladder, uterus, kidney, etc. This technique is called
‘ultrasonography’. It is also used for examination of the foetus during
the pregnancy.
5. Ultrasound may be employed to break small ‘stones’ formed in the
kidneys into fine grains. These grains later get flushed out with urine.
SONAR
SONAR stands for SOund Navigation And Ranging.
SONAR uses ultrasonic waves to measure distance, direction and speed of
underwater objects.
SONAR consists of a transmitter and a detector and is installed in a boat or a
ship.
The transmitter produces and transmits ultrasonic waves. These waves
travel through water and get reflected from the ocean bed.
By measuring the time taken between the sending and receiving of the waves
and knowing the speed of sound in water, the distance can be calculated. If
the total distance travelled is 2d (to and fro), ‘v’ is the speed of sound in
water and ‘t’ is the time taken then 2d = v x t.
The above method is called ‘echo-ranging’. This technique is also used to
locate underwater hills, valleys, submarines, icebergs, sunken ship, etc.
32. Acknowledgemen
t
Reference Material
• IX – Science & Technology by NCERT
• Science for Ninth Class (Part-1: Physics) by S. Chand
Many objects & sounds used in this PPT are copied from various sites.
