Sound : Chapter Notes, Class 9 Science

 Page 1


 
 
SOUND
Sound 
Sound is a form of energy which makes us hear.  The nature 
of sound produced depends upon the nature of material, the 
number of vibrations the vibrating body makes in one second.  
Sound is a wave motion.  
 
Production Of Sound Waves 
Sound is produced when an object vibrates.  
 
A vibrating object, which produces sound has a certain amount 
of energy which travels in the form of sound waves.  The 
energy required to make an object vibrate and produce sound 
is provided by some outside source like our hand, wind etc.  
Sound can be produced by the following methods. 
1. By vibrating membranes (in table) 
2. By vibrating strings (in sitars) 
3. by vibrating air (in flute) 
4. by vibrating plates (in bicycle bell) 
 
Propagation  Of  Sound 
Sound waves are longitudinal waves. Sound waves in air 
consist of compressions and rarefactions. When an object 
vibrates then the air layers around it also start vibrating in 
exactly the same way and carry sound waves from the sound 
providing object to our ears. 
 
In the transmission of sound through air, there is no actual 
movement of air from the sound producing body to our ears.  
 
Nature Of Sound Waves 
Sound waves are mechanical waves i.e., they require a 
material medium to propagate. Sound waves can not travel 
through vacuum. 
 
Movement Of Sound Through Solids, Liquids And Gases 
The speed of sound is maximum in solids and minimum in 
gases and intermediate in liquids.  Actually because sound 
wave are mechanical waves.  They need material medium to 
travel.  So a substance in which there is density of particles 
the speed of sound increases due to the increase in the 
transformation of energy by a large number of particles of the 
medium. Sound travel 5 times faster in water and 15 times 
faster in solid than air. 
 
Factors On Which The Speed Of Sound Depends 
 
1. Temperature 
When the temperature increases, the speed of sound 
increases. 
 
2.. Humidity Of Air 
As the humidity of air increases, the speed of sound through it 
also increases. 
 
Frequency Range Of Hearing In Humans 
It is between 20 HZ to 20000 Hz. 
 
Infrasonic Sound  
A sound with frequency less than 20 Hz. 
 
Ultrasonic Sound 
A sound with frequency greater than 20000 Hz.  
 
Nature Of The Sound Waves: Longitudinal 
Sound waves are longitudinal waves. In the longitudinal waves 
the individual particles of the medium move in a direction 
parallel to the direction of propagation of the disturbance. The 
particles do not move from one place to another but they 
simply oscillate back and forth about their position of rest. This 
is exactly how a sound wave propagates. 
           
 
Compression  
A compression is that part of a longitudinal wave in which the 
particles of the medium are closer to one another than they 
normally are. Or the portions of medium in which the particles 
are pushed closer having a large number of particles per unit 
volume. These are the regions where density as well as 
pressure is high. 
 
Page 2


 
 
SOUND
Sound 
Sound is a form of energy which makes us hear.  The nature 
of sound produced depends upon the nature of material, the 
number of vibrations the vibrating body makes in one second.  
Sound is a wave motion.  
 
Production Of Sound Waves 
Sound is produced when an object vibrates.  
 
A vibrating object, which produces sound has a certain amount 
of energy which travels in the form of sound waves.  The 
energy required to make an object vibrate and produce sound 
is provided by some outside source like our hand, wind etc.  
Sound can be produced by the following methods. 
1. By vibrating membranes (in table) 
2. By vibrating strings (in sitars) 
3. by vibrating air (in flute) 
4. by vibrating plates (in bicycle bell) 
 
Propagation  Of  Sound 
Sound waves are longitudinal waves. Sound waves in air 
consist of compressions and rarefactions. When an object 
vibrates then the air layers around it also start vibrating in 
exactly the same way and carry sound waves from the sound 
providing object to our ears. 
 
In the transmission of sound through air, there is no actual 
movement of air from the sound producing body to our ears.  
 
Nature Of Sound Waves 
Sound waves are mechanical waves i.e., they require a 
material medium to propagate. Sound waves can not travel 
through vacuum. 
 
Movement Of Sound Through Solids, Liquids And Gases 
The speed of sound is maximum in solids and minimum in 
gases and intermediate in liquids.  Actually because sound 
wave are mechanical waves.  They need material medium to 
travel.  So a substance in which there is density of particles 
the speed of sound increases due to the increase in the 
transformation of energy by a large number of particles of the 
medium. Sound travel 5 times faster in water and 15 times 
faster in solid than air. 
 
Factors On Which The Speed Of Sound Depends 
 
1. Temperature 
When the temperature increases, the speed of sound 
increases. 
 
2.. Humidity Of Air 
As the humidity of air increases, the speed of sound through it 
also increases. 
 
Frequency Range Of Hearing In Humans 
It is between 20 HZ to 20000 Hz. 
 
Infrasonic Sound  
A sound with frequency less than 20 Hz. 
 
Ultrasonic Sound 
A sound with frequency greater than 20000 Hz.  
 
Nature Of The Sound Waves: Longitudinal 
Sound waves are longitudinal waves. In the longitudinal waves 
the individual particles of the medium move in a direction 
parallel to the direction of propagation of the disturbance. The 
particles do not move from one place to another but they 
simply oscillate back and forth about their position of rest. This 
is exactly how a sound wave propagates. 
           
 
Compression  
A compression is that part of a longitudinal wave in which the 
particles of the medium are closer to one another than they 
normally are. Or the portions of medium in which the particles 
are pushed closer having a large number of particles per unit 
volume. These are the regions where density as well as 
pressure is high. 
 
 
 
Rarefaction 
A rarefaction is that part of longitudinal wave in which the 
particles of the medium are farther apart than normal.  Those 
portions in which the density of the particles and the pressure 
is lesser than normal. 
          
Transverse Waves 
There is another type of wave called transverse wave. In a 
transverse wave the particles do not oscillate along the line of 
wave propagation but oscillate up and down about their mean 
position as the wave travels. Light is an example of transverse 
wave in which the particles of the wave move in a direction 
perpendicular to the direction of propagation of wave. 
 
Characteristics Of The Sound Waves 
The characteristics of sound waves are: - 
(i) Frequency 
(ii) Amplitude 
(iii) Velocity Of Wave 
(iv) Wavelength 
(v) Time period 
 
1. Wavelength 
The distance between two nearest points in a wave which are 
in the same phase of vibration is called wavelength.  In other 
words we can say that the distance between two nearest 
crests of a wave or the distance between two nearest troughs 
of a wave is called wavelength.  Wavelength is denoted by 
(Lambda). S.I. unit of wavelength is m. 
                
         
2. Amplitude 
The maximum displacement of the particles of the medium 
from their original undisturbed positions, when a waves passes 
through the medium is called amplitude of the wave.  This 
describe the size of the wave.  The S.I. unit is metre.  It is 
denoted by A. 
 
 
3. Frequency 
The number of waves produced per second.  It is denoted by 
? (nu).  The S.I. unit of frequency is Hertz (Hz). The frequency 
of a work remains same in air, water, oil etc. Hertz is equal to 
vibrations per second. 
            
4. Time Period 
The time taken to complete are vibration is called time period.  
Time period is denoted by T. 
 
Relation Between Time Period And Frequency 
The time required to produce one complete wave is called time 
period of the wave. 
Suppose time period of a wave is T seconds 
Now,  
In T Seconds, number of waves produced = 1 
So, 
In 1 second, number of waves produced = 
T
1
 
But the number of waves produced in 1 second is called 
frequency.  This means that frequency of wave of time period 
T will be 
T
1
. 
So, we can say that the frequency of a wave is the reciprocal 
of its time-period, i.e 
f = 
T
1
     Or 
Frequency = 
Period Time
1
 
 
5. Velocity Of Wave 
The distance traveled by a wave in one second is called 
velocity of wave. It is denoted by ?. 
 
S.I. Unit 
m/s      Or     
1
ms
-
 
 
Pitch, Loudness And Quality 
Pitch 
Pitch is the characteristic of sound, which distinguishes 
between a shrill sound and a grave sound. Pitch is the 
interpretation of frequency of the emitted sound by the brain. 
Higher the pitch, higher is the frequency. A high pitch sound 
corresponds to more number of compressions and rarefactions 
passing through a point per unit time. 
 
Page 3


 
 
SOUND
Sound 
Sound is a form of energy which makes us hear.  The nature 
of sound produced depends upon the nature of material, the 
number of vibrations the vibrating body makes in one second.  
Sound is a wave motion.  
 
Production Of Sound Waves 
Sound is produced when an object vibrates.  
 
A vibrating object, which produces sound has a certain amount 
of energy which travels in the form of sound waves.  The 
energy required to make an object vibrate and produce sound 
is provided by some outside source like our hand, wind etc.  
Sound can be produced by the following methods. 
1. By vibrating membranes (in table) 
2. By vibrating strings (in sitars) 
3. by vibrating air (in flute) 
4. by vibrating plates (in bicycle bell) 
 
Propagation  Of  Sound 
Sound waves are longitudinal waves. Sound waves in air 
consist of compressions and rarefactions. When an object 
vibrates then the air layers around it also start vibrating in 
exactly the same way and carry sound waves from the sound 
providing object to our ears. 
 
In the transmission of sound through air, there is no actual 
movement of air from the sound producing body to our ears.  
 
Nature Of Sound Waves 
Sound waves are mechanical waves i.e., they require a 
material medium to propagate. Sound waves can not travel 
through vacuum. 
 
Movement Of Sound Through Solids, Liquids And Gases 
The speed of sound is maximum in solids and minimum in 
gases and intermediate in liquids.  Actually because sound 
wave are mechanical waves.  They need material medium to 
travel.  So a substance in which there is density of particles 
the speed of sound increases due to the increase in the 
transformation of energy by a large number of particles of the 
medium. Sound travel 5 times faster in water and 15 times 
faster in solid than air. 
 
Factors On Which The Speed Of Sound Depends 
 
1. Temperature 
When the temperature increases, the speed of sound 
increases. 
 
2.. Humidity Of Air 
As the humidity of air increases, the speed of sound through it 
also increases. 
 
Frequency Range Of Hearing In Humans 
It is between 20 HZ to 20000 Hz. 
 
Infrasonic Sound  
A sound with frequency less than 20 Hz. 
 
Ultrasonic Sound 
A sound with frequency greater than 20000 Hz.  
 
Nature Of The Sound Waves: Longitudinal 
Sound waves are longitudinal waves. In the longitudinal waves 
the individual particles of the medium move in a direction 
parallel to the direction of propagation of the disturbance. The 
particles do not move from one place to another but they 
simply oscillate back and forth about their position of rest. This 
is exactly how a sound wave propagates. 
           
 
Compression  
A compression is that part of a longitudinal wave in which the 
particles of the medium are closer to one another than they 
normally are. Or the portions of medium in which the particles 
are pushed closer having a large number of particles per unit 
volume. These are the regions where density as well as 
pressure is high. 
 
 
 
Rarefaction 
A rarefaction is that part of longitudinal wave in which the 
particles of the medium are farther apart than normal.  Those 
portions in which the density of the particles and the pressure 
is lesser than normal. 
          
Transverse Waves 
There is another type of wave called transverse wave. In a 
transverse wave the particles do not oscillate along the line of 
wave propagation but oscillate up and down about their mean 
position as the wave travels. Light is an example of transverse 
wave in which the particles of the wave move in a direction 
perpendicular to the direction of propagation of wave. 
 
Characteristics Of The Sound Waves 
The characteristics of sound waves are: - 
(i) Frequency 
(ii) Amplitude 
(iii) Velocity Of Wave 
(iv) Wavelength 
(v) Time period 
 
1. Wavelength 
The distance between two nearest points in a wave which are 
in the same phase of vibration is called wavelength.  In other 
words we can say that the distance between two nearest 
crests of a wave or the distance between two nearest troughs 
of a wave is called wavelength.  Wavelength is denoted by 
(Lambda). S.I. unit of wavelength is m. 
                
         
2. Amplitude 
The maximum displacement of the particles of the medium 
from their original undisturbed positions, when a waves passes 
through the medium is called amplitude of the wave.  This 
describe the size of the wave.  The S.I. unit is metre.  It is 
denoted by A. 
 
 
3. Frequency 
The number of waves produced per second.  It is denoted by 
? (nu).  The S.I. unit of frequency is Hertz (Hz). The frequency 
of a work remains same in air, water, oil etc. Hertz is equal to 
vibrations per second. 
            
4. Time Period 
The time taken to complete are vibration is called time period.  
Time period is denoted by T. 
 
Relation Between Time Period And Frequency 
The time required to produce one complete wave is called time 
period of the wave. 
Suppose time period of a wave is T seconds 
Now,  
In T Seconds, number of waves produced = 1 
So, 
In 1 second, number of waves produced = 
T
1
 
But the number of waves produced in 1 second is called 
frequency.  This means that frequency of wave of time period 
T will be 
T
1
. 
So, we can say that the frequency of a wave is the reciprocal 
of its time-period, i.e 
f = 
T
1
     Or 
Frequency = 
Period Time
1
 
 
5. Velocity Of Wave 
The distance traveled by a wave in one second is called 
velocity of wave. It is denoted by ?. 
 
S.I. Unit 
m/s      Or     
1
ms
-
 
 
Pitch, Loudness And Quality 
Pitch 
Pitch is the characteristic of sound, which distinguishes 
between a shrill sound and a grave sound. Pitch is the 
interpretation of frequency of the emitted sound by the brain. 
Higher the pitch, higher is the frequency. A high pitch sound 
corresponds to more number of compressions and rarefactions 
passing through a point per unit time. 
 
 
              
Example 
Sound from a flute has high pitch whereas sound from a violin 
is of low pitch. 
 
Loudness  
Loudness of the sound is defined as the degree of the 
sensation produced on the ear.     
                    
The loudness or softness of a sound depends upon amplitude 
of the sound. Amplitude of sound wave depends upon the 
force with which an object is made to vibrate. When the table 
is hit lightly soft sound is produced. When a table is hit hardly 
a loud sound is produced. A loud sound can travel a larger 
distance as they have more energy. As sound waves spread 
out from its source its amplitude as well as its loudness 
decreases. 
 
Quality 
The quality or timber of sound is that characteristic which 
enables us to distinguish one sound from another having the 
same pitch and loudness. The more the sound is pleasant, the 
rich is its quality. 
 
Tone 
A sound of single frequency is called a tone. 
 
Note  
The sound which is produced due to the mixture of several 
frequencies is called a note and is pleasant to listen to. 
 
Intensity 
The amount of sound energy passing each second through 
unit area is called intensity of sound. Sometimes the terms 
intensity and loudness is used interchangeably but they are 
not same. Loudness is a measure of the response of the ear to 
sound. Even when two sounds are of equal intensity, we may 
hear one sound louder than other simply because our ear 
detects it better. 
 
Reflection Of Sound Waves 
The returning back of sound waves after striking a hard 
surface is called reflection of sound.  Sound can be reflected 
from any hard surface whether smooth or rough. The 
reflection of sound causes echoes.  
 
Echoes 
The repetition of sound caused by the reflection of sound 
wave is called on echo. The sensation of sound persists in our 
brain for about .1 s. To hear a distinct echo the time interval 
between the original sound and the reflected one must be at 
least .1 s. For hearing distinct echoes the minimum distance of 
the obstacle from the source of sound must be 17.2 m. This 
distance will change with the temperature of air. Echoes may 
be heard more than once due to successive or multiple 
reflections. 
 
Multiple Echoes 
When the sound is reflected repeatedly from a number of 
obstacles, more than one echoes are produced called multiple 
echoes.  
 
Minimum Distance For An Echo To Be Heard  
We know, 
Distance = Speed × Time 
Now, 
Speed of sound in air = 344 m/s 
Time taken = 0.1 s (
10
1
 s) 
Because 0.1 is the minimum time interval between two sounds 
to be heard clearly. Therefore, 
Distance traveled = 344 × 0.1  
= 34.4 m  
This distance is the total distance traveled by sound. But, our 
distance from the sound reflecting surface such as a wall to 
hear an echo should be half of the total distance traveled by 
sound, i.e.  
2
4 . 34
 = 17.2 m.  
From this we conclude that the minimum distance from a 
sound reflecting surface to hear an echo is 17.2 m. 
 
Reverberation 
The repeated reflection that results in the persistence of sound 
is called reverberation. In an auditorium and a big hall 
excessive reverberation is highly undesirable, therefore to 
reduce it the walls of the auditorium are generally covered 
with sound absorbing materials like compressed fireboard, 
rough plaster or draperies. The seat materials are also 
selected on the basis of their sound absorbing properties. 
        
 
Page 4


 
 
SOUND
Sound 
Sound is a form of energy which makes us hear.  The nature 
of sound produced depends upon the nature of material, the 
number of vibrations the vibrating body makes in one second.  
Sound is a wave motion.  
 
Production Of Sound Waves 
Sound is produced when an object vibrates.  
 
A vibrating object, which produces sound has a certain amount 
of energy which travels in the form of sound waves.  The 
energy required to make an object vibrate and produce sound 
is provided by some outside source like our hand, wind etc.  
Sound can be produced by the following methods. 
1. By vibrating membranes (in table) 
2. By vibrating strings (in sitars) 
3. by vibrating air (in flute) 
4. by vibrating plates (in bicycle bell) 
 
Propagation  Of  Sound 
Sound waves are longitudinal waves. Sound waves in air 
consist of compressions and rarefactions. When an object 
vibrates then the air layers around it also start vibrating in 
exactly the same way and carry sound waves from the sound 
providing object to our ears. 
 
In the transmission of sound through air, there is no actual 
movement of air from the sound producing body to our ears.  
 
Nature Of Sound Waves 
Sound waves are mechanical waves i.e., they require a 
material medium to propagate. Sound waves can not travel 
through vacuum. 
 
Movement Of Sound Through Solids, Liquids And Gases 
The speed of sound is maximum in solids and minimum in 
gases and intermediate in liquids.  Actually because sound 
wave are mechanical waves.  They need material medium to 
travel.  So a substance in which there is density of particles 
the speed of sound increases due to the increase in the 
transformation of energy by a large number of particles of the 
medium. Sound travel 5 times faster in water and 15 times 
faster in solid than air. 
 
Factors On Which The Speed Of Sound Depends 
 
1. Temperature 
When the temperature increases, the speed of sound 
increases. 
 
2.. Humidity Of Air 
As the humidity of air increases, the speed of sound through it 
also increases. 
 
Frequency Range Of Hearing In Humans 
It is between 20 HZ to 20000 Hz. 
 
Infrasonic Sound  
A sound with frequency less than 20 Hz. 
 
Ultrasonic Sound 
A sound with frequency greater than 20000 Hz.  
 
Nature Of The Sound Waves: Longitudinal 
Sound waves are longitudinal waves. In the longitudinal waves 
the individual particles of the medium move in a direction 
parallel to the direction of propagation of the disturbance. The 
particles do not move from one place to another but they 
simply oscillate back and forth about their position of rest. This 
is exactly how a sound wave propagates. 
           
 
Compression  
A compression is that part of a longitudinal wave in which the 
particles of the medium are closer to one another than they 
normally are. Or the portions of medium in which the particles 
are pushed closer having a large number of particles per unit 
volume. These are the regions where density as well as 
pressure is high. 
 
 
 
Rarefaction 
A rarefaction is that part of longitudinal wave in which the 
particles of the medium are farther apart than normal.  Those 
portions in which the density of the particles and the pressure 
is lesser than normal. 
          
Transverse Waves 
There is another type of wave called transverse wave. In a 
transverse wave the particles do not oscillate along the line of 
wave propagation but oscillate up and down about their mean 
position as the wave travels. Light is an example of transverse 
wave in which the particles of the wave move in a direction 
perpendicular to the direction of propagation of wave. 
 
Characteristics Of The Sound Waves 
The characteristics of sound waves are: - 
(i) Frequency 
(ii) Amplitude 
(iii) Velocity Of Wave 
(iv) Wavelength 
(v) Time period 
 
1. Wavelength 
The distance between two nearest points in a wave which are 
in the same phase of vibration is called wavelength.  In other 
words we can say that the distance between two nearest 
crests of a wave or the distance between two nearest troughs 
of a wave is called wavelength.  Wavelength is denoted by 
(Lambda). S.I. unit of wavelength is m. 
                
         
2. Amplitude 
The maximum displacement of the particles of the medium 
from their original undisturbed positions, when a waves passes 
through the medium is called amplitude of the wave.  This 
describe the size of the wave.  The S.I. unit is metre.  It is 
denoted by A. 
 
 
3. Frequency 
The number of waves produced per second.  It is denoted by 
? (nu).  The S.I. unit of frequency is Hertz (Hz). The frequency 
of a work remains same in air, water, oil etc. Hertz is equal to 
vibrations per second. 
            
4. Time Period 
The time taken to complete are vibration is called time period.  
Time period is denoted by T. 
 
Relation Between Time Period And Frequency 
The time required to produce one complete wave is called time 
period of the wave. 
Suppose time period of a wave is T seconds 
Now,  
In T Seconds, number of waves produced = 1 
So, 
In 1 second, number of waves produced = 
T
1
 
But the number of waves produced in 1 second is called 
frequency.  This means that frequency of wave of time period 
T will be 
T
1
. 
So, we can say that the frequency of a wave is the reciprocal 
of its time-period, i.e 
f = 
T
1
     Or 
Frequency = 
Period Time
1
 
 
5. Velocity Of Wave 
The distance traveled by a wave in one second is called 
velocity of wave. It is denoted by ?. 
 
S.I. Unit 
m/s      Or     
1
ms
-
 
 
Pitch, Loudness And Quality 
Pitch 
Pitch is the characteristic of sound, which distinguishes 
between a shrill sound and a grave sound. Pitch is the 
interpretation of frequency of the emitted sound by the brain. 
Higher the pitch, higher is the frequency. A high pitch sound 
corresponds to more number of compressions and rarefactions 
passing through a point per unit time. 
 
 
              
Example 
Sound from a flute has high pitch whereas sound from a violin 
is of low pitch. 
 
Loudness  
Loudness of the sound is defined as the degree of the 
sensation produced on the ear.     
                    
The loudness or softness of a sound depends upon amplitude 
of the sound. Amplitude of sound wave depends upon the 
force with which an object is made to vibrate. When the table 
is hit lightly soft sound is produced. When a table is hit hardly 
a loud sound is produced. A loud sound can travel a larger 
distance as they have more energy. As sound waves spread 
out from its source its amplitude as well as its loudness 
decreases. 
 
Quality 
The quality or timber of sound is that characteristic which 
enables us to distinguish one sound from another having the 
same pitch and loudness. The more the sound is pleasant, the 
rich is its quality. 
 
Tone 
A sound of single frequency is called a tone. 
 
Note  
The sound which is produced due to the mixture of several 
frequencies is called a note and is pleasant to listen to. 
 
Intensity 
The amount of sound energy passing each second through 
unit area is called intensity of sound. Sometimes the terms 
intensity and loudness is used interchangeably but they are 
not same. Loudness is a measure of the response of the ear to 
sound. Even when two sounds are of equal intensity, we may 
hear one sound louder than other simply because our ear 
detects it better. 
 
Reflection Of Sound Waves 
The returning back of sound waves after striking a hard 
surface is called reflection of sound.  Sound can be reflected 
from any hard surface whether smooth or rough. The 
reflection of sound causes echoes.  
 
Echoes 
The repetition of sound caused by the reflection of sound 
wave is called on echo. The sensation of sound persists in our 
brain for about .1 s. To hear a distinct echo the time interval 
between the original sound and the reflected one must be at 
least .1 s. For hearing distinct echoes the minimum distance of 
the obstacle from the source of sound must be 17.2 m. This 
distance will change with the temperature of air. Echoes may 
be heard more than once due to successive or multiple 
reflections. 
 
Multiple Echoes 
When the sound is reflected repeatedly from a number of 
obstacles, more than one echoes are produced called multiple 
echoes.  
 
Minimum Distance For An Echo To Be Heard  
We know, 
Distance = Speed × Time 
Now, 
Speed of sound in air = 344 m/s 
Time taken = 0.1 s (
10
1
 s) 
Because 0.1 is the minimum time interval between two sounds 
to be heard clearly. Therefore, 
Distance traveled = 344 × 0.1  
= 34.4 m  
This distance is the total distance traveled by sound. But, our 
distance from the sound reflecting surface such as a wall to 
hear an echo should be half of the total distance traveled by 
sound, i.e.  
2
4 . 34
 = 17.2 m.  
From this we conclude that the minimum distance from a 
sound reflecting surface to hear an echo is 17.2 m. 
 
Reverberation 
The repeated reflection that results in the persistence of sound 
is called reverberation. In an auditorium and a big hall 
excessive reverberation is highly undesirable, therefore to 
reduce it the walls of the auditorium are generally covered 
with sound absorbing materials like compressed fireboard, 
rough plaster or draperies. The seat materials are also 
selected on the basis of their sound absorbing properties. 
        
 
 
Uses Of Multiple reflection Of Sound 
1. Megaphones and musical instruments like trumpet and 
shehnais have a tube followed by conical opening which 
reflects sound successively. 
                             
2. In stethoscope the sound of the patient’s heart beat 
reaches the doctor’s ears by multiple reflections. 
             
3. Generally the ceilings of the concert halls, conference 
halls and cinema halls are curved so that the sound after 
reflection reaches all corners of the hall. 
                  
 
Range Of Hearing 
The audible range of sound for human beings extends from 
about 20 Hz to 20,000 Hz. Children under the age of five and 
some animals such as dogs can hear upto 25,000 Hz. 
 
Ultrasound 
The sound wave having frequency greater than 20000 Hz 
which can not be heard by human beings are called ultrasonic 
sound or ultrasound.  Dogs, bats and dolphins can hear 
ultrasound.  Due to its very high frequency ultrasound has a 
greater penetrating power.  
 
Applications Of Ultrasound 
(i) Ultrasound is used to investigate inside the human body. 
(ii) Ultrasound is used in sonar to measure the depth of sea 
or ocean and to locate under water objects like shoals of 
fish, ship works, submarines, sea rocks and hidden ice 
bergs in sea. 
(iii) Ultrasound is used for finding the level of a liquid in a 
metal tank.  
(iv) Ultrasound is used in the industry for detecting flaws 
(faults) in metal blocks or sheets. 
(v) Ultrasound is used in the treatment of muscular pain and 
in treatment of disease called arthritis.  
(vi) Ultrasound may be employed to break small stones 
formed in the kidneys into fine grains. These grains later 
get flushed out with urine. 
(vii) Ultrasonography is used for the examination of the foetus 
during pregnancy to detect congenial defects and growth 
abnormalities. 
(viii) Ultrasound is generally used to clean parts located in hard 
to reach places. For example spiral tube, odd shaped 
parts, electronic components etc. 
(ix) Ultrasonic waves are used for ‘echocardiography’.  
(x) Ultrasound is used by bats to find their prey. 
            
SONAR : Sound Navigation And Ranging  
Sonar is a device which is used to find the depth of a sea or to 
locate the under water things.  
 
Working Of SONAR 
In SONAR ultrasonic waves are sent out in all directions from 
the ship and reflected waves are received.  By measuring the 
time between sending the sound waves and receiving the 
reflected waves the distance of the under water object from 
the ship can be calculated. 
 
Apparatus Of SONAR 
The SONAR apparatus consists of two parts. 
1. A transmitter which emits ultrasonic waves. 
2. A receiver which revives the reflected ultrasonic waves. 
 
           
 
 Supersonic 
Supersonic refers to sped of an object which is grater than 
speed of sound. Nowadays so many jet aeroplanes have been 
developed which have the speed 5 to 7 times greater than 
sound. 
 
Infrasound 
The sound frequencies below 20 Hz are called infrasonic 
sounds or infrasound. For example the vibrations of a 
pendulum. 
 
Structure And Working Of Human Ear 
The ear allows us to convert pressure variations in air with 
audible frequencies into electric signals that travel to brain via 
the auditory nerve. Human ear converts sound energy to 
mechanical energy. It allows us to perceive the loudness of 
sound by detection of the wave’s amplitude. 
 
The human ear consists of three basic parts: - 
1. The outer ear 
2. The middle ear 
3. The inner ear 
 
Page 5


 
 
SOUND
Sound 
Sound is a form of energy which makes us hear.  The nature 
of sound produced depends upon the nature of material, the 
number of vibrations the vibrating body makes in one second.  
Sound is a wave motion.  
 
Production Of Sound Waves 
Sound is produced when an object vibrates.  
 
A vibrating object, which produces sound has a certain amount 
of energy which travels in the form of sound waves.  The 
energy required to make an object vibrate and produce sound 
is provided by some outside source like our hand, wind etc.  
Sound can be produced by the following methods. 
1. By vibrating membranes (in table) 
2. By vibrating strings (in sitars) 
3. by vibrating air (in flute) 
4. by vibrating plates (in bicycle bell) 
 
Propagation  Of  Sound 
Sound waves are longitudinal waves. Sound waves in air 
consist of compressions and rarefactions. When an object 
vibrates then the air layers around it also start vibrating in 
exactly the same way and carry sound waves from the sound 
providing object to our ears. 
 
In the transmission of sound through air, there is no actual 
movement of air from the sound producing body to our ears.  
 
Nature Of Sound Waves 
Sound waves are mechanical waves i.e., they require a 
material medium to propagate. Sound waves can not travel 
through vacuum. 
 
Movement Of Sound Through Solids, Liquids And Gases 
The speed of sound is maximum in solids and minimum in 
gases and intermediate in liquids.  Actually because sound 
wave are mechanical waves.  They need material medium to 
travel.  So a substance in which there is density of particles 
the speed of sound increases due to the increase in the 
transformation of energy by a large number of particles of the 
medium. Sound travel 5 times faster in water and 15 times 
faster in solid than air. 
 
Factors On Which The Speed Of Sound Depends 
 
1. Temperature 
When the temperature increases, the speed of sound 
increases. 
 
2.. Humidity Of Air 
As the humidity of air increases, the speed of sound through it 
also increases. 
 
Frequency Range Of Hearing In Humans 
It is between 20 HZ to 20000 Hz. 
 
Infrasonic Sound  
A sound with frequency less than 20 Hz. 
 
Ultrasonic Sound 
A sound with frequency greater than 20000 Hz.  
 
Nature Of The Sound Waves: Longitudinal 
Sound waves are longitudinal waves. In the longitudinal waves 
the individual particles of the medium move in a direction 
parallel to the direction of propagation of the disturbance. The 
particles do not move from one place to another but they 
simply oscillate back and forth about their position of rest. This 
is exactly how a sound wave propagates. 
           
 
Compression  
A compression is that part of a longitudinal wave in which the 
particles of the medium are closer to one another than they 
normally are. Or the portions of medium in which the particles 
are pushed closer having a large number of particles per unit 
volume. These are the regions where density as well as 
pressure is high. 
 
 
 
Rarefaction 
A rarefaction is that part of longitudinal wave in which the 
particles of the medium are farther apart than normal.  Those 
portions in which the density of the particles and the pressure 
is lesser than normal. 
          
Transverse Waves 
There is another type of wave called transverse wave. In a 
transverse wave the particles do not oscillate along the line of 
wave propagation but oscillate up and down about their mean 
position as the wave travels. Light is an example of transverse 
wave in which the particles of the wave move in a direction 
perpendicular to the direction of propagation of wave. 
 
Characteristics Of The Sound Waves 
The characteristics of sound waves are: - 
(i) Frequency 
(ii) Amplitude 
(iii) Velocity Of Wave 
(iv) Wavelength 
(v) Time period 
 
1. Wavelength 
The distance between two nearest points in a wave which are 
in the same phase of vibration is called wavelength.  In other 
words we can say that the distance between two nearest 
crests of a wave or the distance between two nearest troughs 
of a wave is called wavelength.  Wavelength is denoted by 
(Lambda). S.I. unit of wavelength is m. 
                
         
2. Amplitude 
The maximum displacement of the particles of the medium 
from their original undisturbed positions, when a waves passes 
through the medium is called amplitude of the wave.  This 
describe the size of the wave.  The S.I. unit is metre.  It is 
denoted by A. 
 
 
3. Frequency 
The number of waves produced per second.  It is denoted by 
? (nu).  The S.I. unit of frequency is Hertz (Hz). The frequency 
of a work remains same in air, water, oil etc. Hertz is equal to 
vibrations per second. 
            
4. Time Period 
The time taken to complete are vibration is called time period.  
Time period is denoted by T. 
 
Relation Between Time Period And Frequency 
The time required to produce one complete wave is called time 
period of the wave. 
Suppose time period of a wave is T seconds 
Now,  
In T Seconds, number of waves produced = 1 
So, 
In 1 second, number of waves produced = 
T
1
 
But the number of waves produced in 1 second is called 
frequency.  This means that frequency of wave of time period 
T will be 
T
1
. 
So, we can say that the frequency of a wave is the reciprocal 
of its time-period, i.e 
f = 
T
1
     Or 
Frequency = 
Period Time
1
 
 
5. Velocity Of Wave 
The distance traveled by a wave in one second is called 
velocity of wave. It is denoted by ?. 
 
S.I. Unit 
m/s      Or     
1
ms
-
 
 
Pitch, Loudness And Quality 
Pitch 
Pitch is the characteristic of sound, which distinguishes 
between a shrill sound and a grave sound. Pitch is the 
interpretation of frequency of the emitted sound by the brain. 
Higher the pitch, higher is the frequency. A high pitch sound 
corresponds to more number of compressions and rarefactions 
passing through a point per unit time. 
 
 
              
Example 
Sound from a flute has high pitch whereas sound from a violin 
is of low pitch. 
 
Loudness  
Loudness of the sound is defined as the degree of the 
sensation produced on the ear.     
                    
The loudness or softness of a sound depends upon amplitude 
of the sound. Amplitude of sound wave depends upon the 
force with which an object is made to vibrate. When the table 
is hit lightly soft sound is produced. When a table is hit hardly 
a loud sound is produced. A loud sound can travel a larger 
distance as they have more energy. As sound waves spread 
out from its source its amplitude as well as its loudness 
decreases. 
 
Quality 
The quality or timber of sound is that characteristic which 
enables us to distinguish one sound from another having the 
same pitch and loudness. The more the sound is pleasant, the 
rich is its quality. 
 
Tone 
A sound of single frequency is called a tone. 
 
Note  
The sound which is produced due to the mixture of several 
frequencies is called a note and is pleasant to listen to. 
 
Intensity 
The amount of sound energy passing each second through 
unit area is called intensity of sound. Sometimes the terms 
intensity and loudness is used interchangeably but they are 
not same. Loudness is a measure of the response of the ear to 
sound. Even when two sounds are of equal intensity, we may 
hear one sound louder than other simply because our ear 
detects it better. 
 
Reflection Of Sound Waves 
The returning back of sound waves after striking a hard 
surface is called reflection of sound.  Sound can be reflected 
from any hard surface whether smooth or rough. The 
reflection of sound causes echoes.  
 
Echoes 
The repetition of sound caused by the reflection of sound 
wave is called on echo. The sensation of sound persists in our 
brain for about .1 s. To hear a distinct echo the time interval 
between the original sound and the reflected one must be at 
least .1 s. For hearing distinct echoes the minimum distance of 
the obstacle from the source of sound must be 17.2 m. This 
distance will change with the temperature of air. Echoes may 
be heard more than once due to successive or multiple 
reflections. 
 
Multiple Echoes 
When the sound is reflected repeatedly from a number of 
obstacles, more than one echoes are produced called multiple 
echoes.  
 
Minimum Distance For An Echo To Be Heard  
We know, 
Distance = Speed × Time 
Now, 
Speed of sound in air = 344 m/s 
Time taken = 0.1 s (
10
1
 s) 
Because 0.1 is the minimum time interval between two sounds 
to be heard clearly. Therefore, 
Distance traveled = 344 × 0.1  
= 34.4 m  
This distance is the total distance traveled by sound. But, our 
distance from the sound reflecting surface such as a wall to 
hear an echo should be half of the total distance traveled by 
sound, i.e.  
2
4 . 34
 = 17.2 m.  
From this we conclude that the minimum distance from a 
sound reflecting surface to hear an echo is 17.2 m. 
 
Reverberation 
The repeated reflection that results in the persistence of sound 
is called reverberation. In an auditorium and a big hall 
excessive reverberation is highly undesirable, therefore to 
reduce it the walls of the auditorium are generally covered 
with sound absorbing materials like compressed fireboard, 
rough plaster or draperies. The seat materials are also 
selected on the basis of their sound absorbing properties. 
        
 
 
Uses Of Multiple reflection Of Sound 
1. Megaphones and musical instruments like trumpet and 
shehnais have a tube followed by conical opening which 
reflects sound successively. 
                             
2. In stethoscope the sound of the patient’s heart beat 
reaches the doctor’s ears by multiple reflections. 
             
3. Generally the ceilings of the concert halls, conference 
halls and cinema halls are curved so that the sound after 
reflection reaches all corners of the hall. 
                  
 
Range Of Hearing 
The audible range of sound for human beings extends from 
about 20 Hz to 20,000 Hz. Children under the age of five and 
some animals such as dogs can hear upto 25,000 Hz. 
 
Ultrasound 
The sound wave having frequency greater than 20000 Hz 
which can not be heard by human beings are called ultrasonic 
sound or ultrasound.  Dogs, bats and dolphins can hear 
ultrasound.  Due to its very high frequency ultrasound has a 
greater penetrating power.  
 
Applications Of Ultrasound 
(i) Ultrasound is used to investigate inside the human body. 
(ii) Ultrasound is used in sonar to measure the depth of sea 
or ocean and to locate under water objects like shoals of 
fish, ship works, submarines, sea rocks and hidden ice 
bergs in sea. 
(iii) Ultrasound is used for finding the level of a liquid in a 
metal tank.  
(iv) Ultrasound is used in the industry for detecting flaws 
(faults) in metal blocks or sheets. 
(v) Ultrasound is used in the treatment of muscular pain and 
in treatment of disease called arthritis.  
(vi) Ultrasound may be employed to break small stones 
formed in the kidneys into fine grains. These grains later 
get flushed out with urine. 
(vii) Ultrasonography is used for the examination of the foetus 
during pregnancy to detect congenial defects and growth 
abnormalities. 
(viii) Ultrasound is generally used to clean parts located in hard 
to reach places. For example spiral tube, odd shaped 
parts, electronic components etc. 
(ix) Ultrasonic waves are used for ‘echocardiography’.  
(x) Ultrasound is used by bats to find their prey. 
            
SONAR : Sound Navigation And Ranging  
Sonar is a device which is used to find the depth of a sea or to 
locate the under water things.  
 
Working Of SONAR 
In SONAR ultrasonic waves are sent out in all directions from 
the ship and reflected waves are received.  By measuring the 
time between sending the sound waves and receiving the 
reflected waves the distance of the under water object from 
the ship can be calculated. 
 
Apparatus Of SONAR 
The SONAR apparatus consists of two parts. 
1. A transmitter which emits ultrasonic waves. 
2. A receiver which revives the reflected ultrasonic waves. 
 
           
 
 Supersonic 
Supersonic refers to sped of an object which is grater than 
speed of sound. Nowadays so many jet aeroplanes have been 
developed which have the speed 5 to 7 times greater than 
sound. 
 
Infrasound 
The sound frequencies below 20 Hz are called infrasonic 
sounds or infrasound. For example the vibrations of a 
pendulum. 
 
Structure And Working Of Human Ear 
The ear allows us to convert pressure variations in air with 
audible frequencies into electric signals that travel to brain via 
the auditory nerve. Human ear converts sound energy to 
mechanical energy. It allows us to perceive the loudness of 
sound by detection of the wave’s amplitude. 
 
The human ear consists of three basic parts: - 
1. The outer ear 
2. The middle ear 
3. The inner ear 
 
 
 
The outer ear is called ‘pinna’. It collects the sound from the 
surroundings. The collected sound passes through the auditory 
canal. At the end of the auditory canal there is a thin 
membrane called the ear drum or tympanic membrane. When 
a compression of the medium reaches the ear drum the 
pressure on the outside of the membrane increases and forces 
the eardrum inward. Similarly an eardrum moves outward 
when a rarefaction reaches. In this way the eardrum vibrates. 
The middle ear consists of three bones the hammer, anvil and 
stirrup. These three bones amplify the vibrations several 
times.  
The inner ear consists of cochlea. The middle ear transmits the 
amplified pressure variations received from the sound waves 
to the inner ear. In the inner ear the pressure variations are 
turned into electrical signals by the cochlea. These electr8ical 
signals are sent to the brain via the auditory nerve, and the 
brain interprets them as sound.