Chapter 12 Sound Class 9 Notes | EduRev

Science - Short Notes Class IX

Class 9 : Chapter 12 Sound Class 9 Notes | EduRev

 Page 1


Chapter 12 Sound 
Vibration – Rapid to and from motion of an object. 
Production of sound – Sound is produced by a vibrating object. We produce sound when our vocal cords 
vibrate.  
Medium – the matter through which the sound is transmitted. For e.g. when we speak, the sound travels 
through the air and is listened or received by other person. The medium here is the air, through which the sound 
travelled. 
How does sound reaches our ear? 
The vibrating object displaces the medium around itself from its equilibrium positon. This results in the 
vibration of the particles of the medium near the object. These particles exert a force on the neighboring 
particles, which then starts vibrating. This process continues until the sound reaches our ear.  
 
 Fig. Propagation of sound through air 
 
Wave – A disturbance in a medium in which the particles of the medium propagates or transfers the disturbance 
to the neighboring particles. 
Types of waves 
1. Longitudinal waves – A type of wave in which the displacement or disturbance of the particles of the 
medium is in same or opposite direction to the propagation of the wave. 
E.g. Sound wave 
2. Transverse waves – A type of wave in which the displacement or disturbance of the wave is 
perpendicular to the direction of propagation of the wave.  
E.g. Electromagnetic waves. 
 
Sound waves are longitudinal waves so; in this chapter we will talk about longitudinal waves only. 
 
Compression – The high pressure region created in the medium when the vibrating particles pushes the 
neighboring particles is called the region of compression. 
Rarefaction – the backward moving particles during vibration creates a low pressure region in its surroundings, 
which is known as rarefaction. 
                           
Page 2


Chapter 12 Sound 
Vibration – Rapid to and from motion of an object. 
Production of sound – Sound is produced by a vibrating object. We produce sound when our vocal cords 
vibrate.  
Medium – the matter through which the sound is transmitted. For e.g. when we speak, the sound travels 
through the air and is listened or received by other person. The medium here is the air, through which the sound 
travelled. 
How does sound reaches our ear? 
The vibrating object displaces the medium around itself from its equilibrium positon. This results in the 
vibration of the particles of the medium near the object. These particles exert a force on the neighboring 
particles, which then starts vibrating. This process continues until the sound reaches our ear.  
 
 Fig. Propagation of sound through air 
 
Wave – A disturbance in a medium in which the particles of the medium propagates or transfers the disturbance 
to the neighboring particles. 
Types of waves 
1. Longitudinal waves – A type of wave in which the displacement or disturbance of the particles of the 
medium is in same or opposite direction to the propagation of the wave. 
E.g. Sound wave 
2. Transverse waves – A type of wave in which the displacement or disturbance of the wave is 
perpendicular to the direction of propagation of the wave.  
E.g. Electromagnetic waves. 
 
Sound waves are longitudinal waves so; in this chapter we will talk about longitudinal waves only. 
 
Compression – The high pressure region created in the medium when the vibrating particles pushes the 
neighboring particles is called the region of compression. 
Rarefaction – the backward moving particles during vibration creates a low pressure region in its surroundings, 
which is known as rarefaction. 
                           
                           Fig. Compression (C) and rarefaction (R) created by a vibrating tong. 
 
Sound waves can travel only through a medium. We cannot hear sounds in the outer space due to lack of a 
medium. 
The medium can be a solid, liquid or a gas. Sound cannot be heard through vacuum as vacuum is created by 
sucking of all the air is sucked out which leaves no medium inside a jar or vessel. 
 
 
Features of sound waves 
a. Propagation of sound wave through air  
b. Pressure variations showing propagation of sound wave through air 
c. Graphical representation to show propagation of sound wave through air 
A – Amplitude 
? – Wavelength 
 
 
Crest – The region of maximum compression where the particles of a medium are most compressed is known 
as compression. 
Trough – The region of minimum compression where the particles of the medium are least compressed is 
known as trough. 
Page 3


Chapter 12 Sound 
Vibration – Rapid to and from motion of an object. 
Production of sound – Sound is produced by a vibrating object. We produce sound when our vocal cords 
vibrate.  
Medium – the matter through which the sound is transmitted. For e.g. when we speak, the sound travels 
through the air and is listened or received by other person. The medium here is the air, through which the sound 
travelled. 
How does sound reaches our ear? 
The vibrating object displaces the medium around itself from its equilibrium positon. This results in the 
vibration of the particles of the medium near the object. These particles exert a force on the neighboring 
particles, which then starts vibrating. This process continues until the sound reaches our ear.  
 
 Fig. Propagation of sound through air 
 
Wave – A disturbance in a medium in which the particles of the medium propagates or transfers the disturbance 
to the neighboring particles. 
Types of waves 
1. Longitudinal waves – A type of wave in which the displacement or disturbance of the particles of the 
medium is in same or opposite direction to the propagation of the wave. 
E.g. Sound wave 
2. Transverse waves – A type of wave in which the displacement or disturbance of the wave is 
perpendicular to the direction of propagation of the wave.  
E.g. Electromagnetic waves. 
 
Sound waves are longitudinal waves so; in this chapter we will talk about longitudinal waves only. 
 
Compression – The high pressure region created in the medium when the vibrating particles pushes the 
neighboring particles is called the region of compression. 
Rarefaction – the backward moving particles during vibration creates a low pressure region in its surroundings, 
which is known as rarefaction. 
                           
                           Fig. Compression (C) and rarefaction (R) created by a vibrating tong. 
 
Sound waves can travel only through a medium. We cannot hear sounds in the outer space due to lack of a 
medium. 
The medium can be a solid, liquid or a gas. Sound cannot be heard through vacuum as vacuum is created by 
sucking of all the air is sucked out which leaves no medium inside a jar or vessel. 
 
 
Features of sound waves 
a. Propagation of sound wave through air  
b. Pressure variations showing propagation of sound wave through air 
c. Graphical representation to show propagation of sound wave through air 
A – Amplitude 
? – Wavelength 
 
 
Crest – The region of maximum compression where the particles of a medium are most compressed is known 
as compression. 
Trough – The region of minimum compression where the particles of the medium are least compressed is 
known as trough. 
Wavelength – The length between two consecutive crest or trough is known as wavelength. It is represented by 
lambda or ‘?’. 
Frequency – Number of oscillations or compressions or rarefactions per second is known as frequency of a 
sound wave. It is represented by a Greek letter ‘nu’ or ‘?’. The SI unit of frequency is Hertz of ‘H’.  
 ? = 1/T 
 where ? = frequency 
  T = time taken for one oscillation or compression or rarefaction. 
A normal person can hear sound waves from 20Hz to 20kHz or 20,000Hz. 
Infrasound – The sound waves having frequency less than 20Hz are known as infrasound. Rhinoceroses 
communicate using infrasound of wavelength 5Hz 
Ultrasound – The sound waves having a frequency higher than 20kHz are known as ultrasound. Ultrasounds 
are produced by dolphins and bats 
 
Amplitude – The maximum disturbance of the particles of a medium on either side of their mean value or 
position is known as its amplitude. It I represented by ‘A’.  
Amplitude determines the loudness or softness of the sound. Louder sounds have higher amplitude while the 
lower sounds have lower amplitude. 
Pitch – the way in which our brain interprets the sound is known as pitch of the sound. Pitch of the sound is 
directly proportional to its frequency. The higher is the pitch, the higher is its frequency. 
Tone – A sound with a single frequency is called a tone. 
Note – A sound by the mixture of several frequencies and is pleasant to listen is called a note. 
 
Speed of Sound 
Speed of sound (v) = Distance travelled (s)/Time (t) 
       v = wavelength /Time 
       v = ??     (? = 1/T) 
or speed of sound = wavelength*frequency 
The speed of sound in air at 22
0
C is 344m/s 
 
Intensity of sound – The amount of sound energy passing per unit area in a second is called the intensity of the 
sound. 
Loudness – It is the measure of the response of our ear to the sound. 
 
Speed of sound in different mediums – the speed of sound increases with increase in the density of the 
mediums. For e.g. The speed of sound is higher in solids followed by liquids and minimum in gases.  
Page 4


Chapter 12 Sound 
Vibration – Rapid to and from motion of an object. 
Production of sound – Sound is produced by a vibrating object. We produce sound when our vocal cords 
vibrate.  
Medium – the matter through which the sound is transmitted. For e.g. when we speak, the sound travels 
through the air and is listened or received by other person. The medium here is the air, through which the sound 
travelled. 
How does sound reaches our ear? 
The vibrating object displaces the medium around itself from its equilibrium positon. This results in the 
vibration of the particles of the medium near the object. These particles exert a force on the neighboring 
particles, which then starts vibrating. This process continues until the sound reaches our ear.  
 
 Fig. Propagation of sound through air 
 
Wave – A disturbance in a medium in which the particles of the medium propagates or transfers the disturbance 
to the neighboring particles. 
Types of waves 
1. Longitudinal waves – A type of wave in which the displacement or disturbance of the particles of the 
medium is in same or opposite direction to the propagation of the wave. 
E.g. Sound wave 
2. Transverse waves – A type of wave in which the displacement or disturbance of the wave is 
perpendicular to the direction of propagation of the wave.  
E.g. Electromagnetic waves. 
 
Sound waves are longitudinal waves so; in this chapter we will talk about longitudinal waves only. 
 
Compression – The high pressure region created in the medium when the vibrating particles pushes the 
neighboring particles is called the region of compression. 
Rarefaction – the backward moving particles during vibration creates a low pressure region in its surroundings, 
which is known as rarefaction. 
                           
                           Fig. Compression (C) and rarefaction (R) created by a vibrating tong. 
 
Sound waves can travel only through a medium. We cannot hear sounds in the outer space due to lack of a 
medium. 
The medium can be a solid, liquid or a gas. Sound cannot be heard through vacuum as vacuum is created by 
sucking of all the air is sucked out which leaves no medium inside a jar or vessel. 
 
 
Features of sound waves 
a. Propagation of sound wave through air  
b. Pressure variations showing propagation of sound wave through air 
c. Graphical representation to show propagation of sound wave through air 
A – Amplitude 
? – Wavelength 
 
 
Crest – The region of maximum compression where the particles of a medium are most compressed is known 
as compression. 
Trough – The region of minimum compression where the particles of the medium are least compressed is 
known as trough. 
Wavelength – The length between two consecutive crest or trough is known as wavelength. It is represented by 
lambda or ‘?’. 
Frequency – Number of oscillations or compressions or rarefactions per second is known as frequency of a 
sound wave. It is represented by a Greek letter ‘nu’ or ‘?’. The SI unit of frequency is Hertz of ‘H’.  
 ? = 1/T 
 where ? = frequency 
  T = time taken for one oscillation or compression or rarefaction. 
A normal person can hear sound waves from 20Hz to 20kHz or 20,000Hz. 
Infrasound – The sound waves having frequency less than 20Hz are known as infrasound. Rhinoceroses 
communicate using infrasound of wavelength 5Hz 
Ultrasound – The sound waves having a frequency higher than 20kHz are known as ultrasound. Ultrasounds 
are produced by dolphins and bats 
 
Amplitude – The maximum disturbance of the particles of a medium on either side of their mean value or 
position is known as its amplitude. It I represented by ‘A’.  
Amplitude determines the loudness or softness of the sound. Louder sounds have higher amplitude while the 
lower sounds have lower amplitude. 
Pitch – the way in which our brain interprets the sound is known as pitch of the sound. Pitch of the sound is 
directly proportional to its frequency. The higher is the pitch, the higher is its frequency. 
Tone – A sound with a single frequency is called a tone. 
Note – A sound by the mixture of several frequencies and is pleasant to listen is called a note. 
 
Speed of Sound 
Speed of sound (v) = Distance travelled (s)/Time (t) 
       v = wavelength /Time 
       v = ??     (? = 1/T) 
or speed of sound = wavelength*frequency 
The speed of sound in air at 22
0
C is 344m/s 
 
Intensity of sound – The amount of sound energy passing per unit area in a second is called the intensity of the 
sound. 
Loudness – It is the measure of the response of our ear to the sound. 
 
Speed of sound in different mediums – the speed of sound increases with increase in the density of the 
mediums. For e.g. The speed of sound is higher in solids followed by liquids and minimum in gases.  
The speed of sound increases with increase in temperature due to increase in the kinetic energy of the particles 
of the medium. 
It increases on increasing the pressure because the particles of medium come closer to each other, which make 
propagation of vibration easier. 
 
Reflection of sound 
It follows the same principle as the reflection of light 
1. The angle of reflection sound wave is equal to the angle of incidence of the sound wave. 
 
Echo – the hearing of same sound again and again due to multiple reflections is known as echo. A sound persist 
in our brain for 0.1sec, so the reflected wave should reach our brain at least after 0.1sec to be heard as an echo. 
 
Reverberation – The persistence of a sound waves in a big hall due to repeated reflections is known as 
reverberations. Excessive reverberations can be reduced by using sound absorbent materials such as rough 
plasters, fiberboard on the roof of the auditorium or hall. 
 
Applications of Multiple reflection of sound 
1. Loudspeakers or megaphones, horns are designed to send sound in a particular direction through 
multiple reflections without spreading. 
2. Stethoscope is an instrument used to hear the heartbeat of a person. The sound waves reaches to the ear 
after multiple reflections through the tube of the stethoscope. 
3. The ceiling of a concert halls is curved for repeated reflections so that it can reach all the audience 
present in the hall. 
 
Ultrasound and its Applications 
? High frequency sound waves are known as Ultrasounds.  
? Ultrasounds have frequency more than 20kHz. 
? They can travel even through obstacles 
 
Applications 
? They are used to clean parts which are difficult to reach. The object is kept in a cleaning solution, which 
is exposed to ultrasounds. The particles of grease, dust and dirt detaches due to the high frequency sound 
waves. 
? They can be used to detect cracks in buildings, bridges, machines etc. the ultrasounds are passed through 
a metal block and detected on the other side. However, they are reflected back by a crack and does not 
reach the detectors kept on the other side.  
Page 5


Chapter 12 Sound 
Vibration – Rapid to and from motion of an object. 
Production of sound – Sound is produced by a vibrating object. We produce sound when our vocal cords 
vibrate.  
Medium – the matter through which the sound is transmitted. For e.g. when we speak, the sound travels 
through the air and is listened or received by other person. The medium here is the air, through which the sound 
travelled. 
How does sound reaches our ear? 
The vibrating object displaces the medium around itself from its equilibrium positon. This results in the 
vibration of the particles of the medium near the object. These particles exert a force on the neighboring 
particles, which then starts vibrating. This process continues until the sound reaches our ear.  
 
 Fig. Propagation of sound through air 
 
Wave – A disturbance in a medium in which the particles of the medium propagates or transfers the disturbance 
to the neighboring particles. 
Types of waves 
1. Longitudinal waves – A type of wave in which the displacement or disturbance of the particles of the 
medium is in same or opposite direction to the propagation of the wave. 
E.g. Sound wave 
2. Transverse waves – A type of wave in which the displacement or disturbance of the wave is 
perpendicular to the direction of propagation of the wave.  
E.g. Electromagnetic waves. 
 
Sound waves are longitudinal waves so; in this chapter we will talk about longitudinal waves only. 
 
Compression – The high pressure region created in the medium when the vibrating particles pushes the 
neighboring particles is called the region of compression. 
Rarefaction – the backward moving particles during vibration creates a low pressure region in its surroundings, 
which is known as rarefaction. 
                           
                           Fig. Compression (C) and rarefaction (R) created by a vibrating tong. 
 
Sound waves can travel only through a medium. We cannot hear sounds in the outer space due to lack of a 
medium. 
The medium can be a solid, liquid or a gas. Sound cannot be heard through vacuum as vacuum is created by 
sucking of all the air is sucked out which leaves no medium inside a jar or vessel. 
 
 
Features of sound waves 
a. Propagation of sound wave through air  
b. Pressure variations showing propagation of sound wave through air 
c. Graphical representation to show propagation of sound wave through air 
A – Amplitude 
? – Wavelength 
 
 
Crest – The region of maximum compression where the particles of a medium are most compressed is known 
as compression. 
Trough – The region of minimum compression where the particles of the medium are least compressed is 
known as trough. 
Wavelength – The length between two consecutive crest or trough is known as wavelength. It is represented by 
lambda or ‘?’. 
Frequency – Number of oscillations or compressions or rarefactions per second is known as frequency of a 
sound wave. It is represented by a Greek letter ‘nu’ or ‘?’. The SI unit of frequency is Hertz of ‘H’.  
 ? = 1/T 
 where ? = frequency 
  T = time taken for one oscillation or compression or rarefaction. 
A normal person can hear sound waves from 20Hz to 20kHz or 20,000Hz. 
Infrasound – The sound waves having frequency less than 20Hz are known as infrasound. Rhinoceroses 
communicate using infrasound of wavelength 5Hz 
Ultrasound – The sound waves having a frequency higher than 20kHz are known as ultrasound. Ultrasounds 
are produced by dolphins and bats 
 
Amplitude – The maximum disturbance of the particles of a medium on either side of their mean value or 
position is known as its amplitude. It I represented by ‘A’.  
Amplitude determines the loudness or softness of the sound. Louder sounds have higher amplitude while the 
lower sounds have lower amplitude. 
Pitch – the way in which our brain interprets the sound is known as pitch of the sound. Pitch of the sound is 
directly proportional to its frequency. The higher is the pitch, the higher is its frequency. 
Tone – A sound with a single frequency is called a tone. 
Note – A sound by the mixture of several frequencies and is pleasant to listen is called a note. 
 
Speed of Sound 
Speed of sound (v) = Distance travelled (s)/Time (t) 
       v = wavelength /Time 
       v = ??     (? = 1/T) 
or speed of sound = wavelength*frequency 
The speed of sound in air at 22
0
C is 344m/s 
 
Intensity of sound – The amount of sound energy passing per unit area in a second is called the intensity of the 
sound. 
Loudness – It is the measure of the response of our ear to the sound. 
 
Speed of sound in different mediums – the speed of sound increases with increase in the density of the 
mediums. For e.g. The speed of sound is higher in solids followed by liquids and minimum in gases.  
The speed of sound increases with increase in temperature due to increase in the kinetic energy of the particles 
of the medium. 
It increases on increasing the pressure because the particles of medium come closer to each other, which make 
propagation of vibration easier. 
 
Reflection of sound 
It follows the same principle as the reflection of light 
1. The angle of reflection sound wave is equal to the angle of incidence of the sound wave. 
 
Echo – the hearing of same sound again and again due to multiple reflections is known as echo. A sound persist 
in our brain for 0.1sec, so the reflected wave should reach our brain at least after 0.1sec to be heard as an echo. 
 
Reverberation – The persistence of a sound waves in a big hall due to repeated reflections is known as 
reverberations. Excessive reverberations can be reduced by using sound absorbent materials such as rough 
plasters, fiberboard on the roof of the auditorium or hall. 
 
Applications of Multiple reflection of sound 
1. Loudspeakers or megaphones, horns are designed to send sound in a particular direction through 
multiple reflections without spreading. 
2. Stethoscope is an instrument used to hear the heartbeat of a person. The sound waves reaches to the ear 
after multiple reflections through the tube of the stethoscope. 
3. The ceiling of a concert halls is curved for repeated reflections so that it can reach all the audience 
present in the hall. 
 
Ultrasound and its Applications 
? High frequency sound waves are known as Ultrasounds.  
? Ultrasounds have frequency more than 20kHz. 
? They can travel even through obstacles 
 
Applications 
? They are used to clean parts which are difficult to reach. The object is kept in a cleaning solution, which 
is exposed to ultrasounds. The particles of grease, dust and dirt detaches due to the high frequency sound 
waves. 
? They can be used to detect cracks in buildings, bridges, machines etc. the ultrasounds are passed through 
a metal block and detected on the other side. However, they are reflected back by a crack and does not 
reach the detectors kept on the other side.  
                                                     Fig. Ultrasounds are used to detect cracks in metal pieces 
 
? Echocardiography is a technique used to force an image of the heart by reflecting ultrasonic sounds by 
various parts of the heart. 
? Ultrasound scanner is used to get the image of internal organs of the human body. It can be used to 
detect stones or tumors in liver, kidneys, uterus, gall bladder etc. Ultrasonography is the technique to get 
the images of various organs of human body by using high frequency sound waves. It is used in 
examining the fetus during pregnancy. 
? Ultrasounds are also used to break small stones formed in kidneys so that the smaller pieces can be 
flushed out with urine. 
? Bats detect their pray using sound waves in the dark. Bats produces high frequency sound waves, which 
are reflected by the pray and received by bat’s ears. The nature of the reflected waves given an idea of 
the size, shape and speed of the prey to the bat. 
 Fig. Bats emit ultrasounds, which are reflected by an obstacle or prey 
 
SONAR or Sound Navigation And Ranging 
SONAR is a device that is used to measure the distance and speed of an object underwater.  It consists of a 
transmitter which sends ultrasonic sound waves into the water and a detector which detects the reflected sound 
waves by some object in the water. 
The detector converts the sound waves into electrical signals, which are detected accordingly. 
Knowing the speed of sound waves in water and time taken by them to return (or detection time after reflection) 
can be used calculate the distance of the object from the ship. 
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