# Sound Class 9 | NCERT Class 9 Chapter 12 Explanation, Summary and Question Answer

## CBSE Class 9 Science Chapter 12 Sound, Explanation, Examples, Question Answers

Sound CBSE Class 9 Science Chapter 12– Complete explanation and Notes of the chapter ‘Sound’.

Topics covered in the lesson are Introduction, Echo, Wave, and its types, Audible and inaudible sound, Characteristics of sound, Ultrasound and its applications, Sound needs a medium to travel,

SONAR, Speed of sound, Human ear, Sonic boom, Reflection of sound and its application.

Given here is the complete explanation of the chapter, along with all the important questions and NCERT solutions to book questions that have also been provided for the ease of students.

### Class 9 Science Chapter 12 – Sound

Topics to be covered

See Video Explanation of Chapter 12 sound

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### Introduction

We hear so many sounds around us in everyday life. For example, if a milkman comes to your home and the main gate is locked, he rings the doorbell. The sound of the bell is heard by us and we open the door. Let’s say if the doorbell is non functional due to any reason, then he will call your name.In doing so also, he is producing certain sound that reaches your ear and you rush to open the door. So, in the same way, we talk to our parents and siblings at home, we hear the sounds of T.V., we hear the sound of the phone, etc. Now, let us see what exactly sound is.

Sound is considered as a wave. To study wave, we need to take an example of throwing a stone into the pond .If we throw a stone in a pond, then we observe the formation of ripples.

This ripple formation is not at one place, it keeps on forming and the diameter of ripples keeps on increasing from the point of disturbance. These ripples are referred to as wave. Similarly, sound is a kind of wave.
Let us understand more about wave.

### Wave and it’s types

It is the periodic disturbance in a medium.
Types of Wave
There are two types of waves in general depending upon their propagation through a substance. They are as follows-

• Mechanical wave
• Electromagnetic wave

Mechanical Wave: It is the kind of wave which requires a medium to travel. For example: Sound Wave.
That means sound can be heard only whenever there is presence of certain substance like water, glass, air etc. It can not be heard in a vacuum no matter how loud the sound maybe.
Electromagnetic Wave: Is that which can travel through a medium as well as through vacuum. For example: Light wave.
But unlike sound, light can be seen through a substance or in vacuum. That is the reason it is referred to as electromagnetic wave.
Types of waves (depending upon the movement of particles)
We know that whenever a wave travels, the particles vibrate and they constitute the wave or we can say there periodic disturbance constitutes wave. So, depending upon the movement of particles, to produce vibrations, we have different kinds of waves as given below-
The two types of waves are as follows-

• Longitudinal wave
• Transverse wave

Longitudinal: It is the wave in which particles vibrate to and fro. This longitudinal wave consists of compression and rarefaction. The movement is shown in the following figure:

• Compression: it is the region when particles move closer to each other due to which volume decreases and pressure increases.
• Rarefaction: it is the region when particles are far apart due to which volume increases and pressure decreases.

Wave moves in an order such that each compression is followed by rarefaction.
Transverse Wave: Is that in which particles vibrate up and down. Transverse wave consists of crest and trough. This is also shown in the following figure-

Crest: it is the elevation in the wave.

Trough: it is the depression in the wave.

Sound is a kind of a longitudinal and mechanical wave.

That means, in sound, the particles vibrate to and fro consisting of compressions and rarefaction and it is a mechanical wave (needs a medium to travel).

Whereas light is an electromagnetic and transverse wave that consists of a crest and a trough and can travel through vacuum also.

Characteristics of Wave

There are certain characteristics that each wave possesses. They are as follows-

1. Oscillation: It is the complete movement of particles about their mean position.
2. Amplitude: It is the maximum displacement of a particle from its mean position in either direction (cm). It is denoted by ‘A’.
3. Frequency: It is the number of sound waves produced per second (Hertz). It is denoted by υ(nu).
4. Time Period: The time taken to complete one oscillation (sec). It is denoted by ‘T’.
5. Wavelength: The distance between two consecutive crests or troughs (cm). It is denoted by lambda
6. Velocity of Wave: The speed with which sound travels (m/sec). It is denoted by ‘v’.

Relation between time period, frequency and velocity of wave

We have: frequency =velocity /wavelength
?=c/?
The conditions necessary to produce sound
There are 3 conditions necessary to produce sound as follows-

• A vibrating body

Sound is produced only when the source of sound vibrates, i.e. oscillates at its mean position in to and fro direction. Some vibrations can be seen and sometimes they are invisible.

For example: You can see vibrations in ringing bell but not in case of playing harmonium. The tightly stretched rubber string when plucked in middle, shows a fast to and fro motion.

When a tuning fork is hit on a rubber pad, a faint sound is heard but no vibrations are seen. But if you bring that tuning fork near a hanging bell then it shows oscillatory motion.

• Medium to travel

When the source of sound vibrates, it collides with the particles present in the medium like air. These particles, after further collisions, collide with other particles and this continues in the form of a chain carrying the energy until the energy of particles becomes too feeble to collide further and propagate the sound. As we move away from the sound source, the sound becomes feeble.

To show that sound needs a medium to travel, the apparatus is set as shown in figure-

When current is switched on, bell starts ringing and we can easily hear the sound of the ringing bell.
But as we start removing air by means of vacuum pump, the sound of ringing bell goes on becoming fainter and fainter and finally when there is no air left inside, the sound of ringing bell stops.

Class 9 Science Chapter wise Explanation

### Characteristics of Sound

Loudness: It is a characteristic of sound and it is the degree of sensation of sound. It depends upon amplitude. More is the amplitude, louder is the sound. It is measured in units- decibel.

Pitch: It is the property that determines the shrillness or deepness of sound. It depends upon the frequency. More is the frequency, shriller the voice will be. Less is the frequency, deep is the sound.
Quality: Is the property by which one can distinguish between two different sounds of the same pitch and loudness.
Intensity: It is the number of sound waves passing through 1 meter square area in one second. It depends upon the square of amplitude.
Out of both, which sound has a higher pitch: guitar or a car horn: Car horn is louder than guitar but guitar has a higher pitch than a car horn. The reason is because the frequency of guitar is higher than that of car horn.

### Sound needs a medium to travel

Propagation of sound through matter
Sound can travel through all 3 states of matter i.e. solid, liquid and gas. Speed of sound depends upon the density of medium (it increases with increase in density of the medium). Speed of sound is the maximum in solids and the least in gases as particles are closer in solids and so the transfer of sound energy occurs faster.

### Speed of sound

Do you know what is the speed of sound in air?
It is 340 meter per second.

You must have noticed that the speed of sound is more on a humid day as compared to that on a dry day. It is because on a humid day, the particles are comparatively closer and so, the transfer of energy occurs faster.
Let us compare the speed of sound and the speed of light in air?
We have seen that light travels faster than sound and the speed of sound is 340 m/s. This is the reason that on a stormy day, the lightning is seen first and thunder is heard later on.
You must have also heard that astronomers fail to hear the sound of each other on the surface of the moon. This is so because there is no air on the moon and sound cannot travel through a vacuum. It needs a medium to travel.

### Sonic Boom

When the speed of any object exceeds the speed of sound it is said to be travelling at supersonic speed. Bullets, jet aircrafts etc. often travel at supersonic speeds. When a sound, producing source moves with a speed higher than that of sound, it produces shock waves in air. These shock waves carry a large amount of energy. The air pressure variation associated with this type of shock waves produces a very sharp and loud sound called the “sonic boom”. The shock waves produced by a supersonic aircraft have enough energy to shatter the window glass and even damage buildings.

### Reflection of Sound and its applications

As you all know, that bouncing back of light rays on striking the surface is called reflection. Likewise, sound energy also obeys the laws of reflection. That is given below.

1. Angle of incidence is equal to the angle of reflection.
2. Incident wave, reflected wave and the normal lie in the same plane.

Let us look at certain practical examples that we see around us which are based on reflection of sound:

1. Megaphone or speaking tube

Have you ever noticed that we need to cup our hands while calling a person’s name standing far from us. It is because the hands prevent the sound energy from spreading in all directions and the person can hear the call easily.
In the same way, people use horn-shaped metal tubes, commonly called megaphones while addressing a group of people. In all such devices, the sound energy is prevented from spreading out by successive reflections from the horn-shaped tubes.

2. Ear Trumpet or Hearing Aid
You all must have noticed that there are certain people who suffer from hearing impairment. To solve this problem they make use of specific aids or hearing machines that help them hear. That is called an ear trumpet and it also works on the phenomenon of reflection.
Ear trumpet is a device which is used by the person who is hard of hearing. Its shape is like that of a trumpet. The narrow ends of it are kept in the ear hole of the person. The wider end collects the sound waves, amplifies them and makes them efficient for producing the sensation of hearing.

3. Sounding board

The sound waves obey the laws of reflection on plane as well as curved reflecting surfaces. In order to spread the sound evenly, big halls or auditoriums keep the speaker at the principal focus of the concave reflector. This concave reflector is commonly called sounding board. The sound waves, on striking the sounding board, get reflected parallel to the principal axis and spread the sound evenly in the hall.

4. Stethoscope

It is an instrument used by the doctors for listening to sounds produced within the body, especially in the heart and lungs. In a stethoscope, the sound produced within the body of a patient is picked up by a sensitive diaphragm and then it reaches the doctor’s ears by multiple reflections.

### Echo

If we stand in one corner of a big empty hall and shout, we will hear the word again. It is heard because the sound gets reflected from the walls of the empty hall. This repetition of sound is called echo.
The phenomenon due to which repetition of sound is heard after reflection from a distant object (such as a high building or a hillock), after the original sound from a given source dies off, is called an echo.
It has been found that the sensation of any sound persists for 1/10 second in our ear, after the exciting sound dies off. This time is called persistence of audibility. If the repeating sound comes before this 1/10th of a second that is before the original sound dies off, then we are not able to hear it. But if it comes after 1/10th of second then we are able to hear echo.
Mathematically

Let’s say a person stands at one corner and shouts the word ‘hello’. This sound travels a distance let’s say ‘x’, suffers reflection through a wall and returns the same distance ‘x’ towards the speaker. Then the time taken to hear an echo (that includes the travelling time towards wall and reflecting time towards speaker) should be minimum 0.1sec.Then ,
Total distance travelled by sound =x +x=2x
Time interval=0.1sec
Velocity = 340m/sec
Speed =distance /time
V= 2x/t
x= v. t/ 2
So, x=340 .0.1/2
=17m (minimum distance required to hear an echo)
To Hear an Echo:

1. Minimum distance between listener & obstacle must be 17m.
2. Time interval must be 0.1 sec.
3. Intensity of sound should be sufficient.

Reverberation
If a sound is made in a big hall, the sound waves are reflected repeatedly through the walls, ceiling and floor of the hall and produce many echoes.The echo time is so short that many echoes overlap with the original sound. Due to this, the original sound lasts for a longer time.

Reverberation: Is the persistence of sound after the source has stopped producing it. Due to reverberation, the sound becomes difficult to hear.

To avoid this the following are observed-

1. Walls are made of sound absorbing materials.
2. Carpets are put on the floor to absorb sound
3. Heavy curtains are put on doors and windows to absorb sound.
4. Panels with sound absorbing quality are used in halls.

### Audible and Inaudible Sound

The sounds produced in our environment have many different frequencies. Some sounds can be heard by us whereas some sounds can not be heard by us but can be heard by other animals. The reason being that there is a certain frequency range of sound that can be heard by any species. That is, some sounds can be heard (audible) and some can not be heard (inaudible).

Audible Sound: Sound that can be heard by humans.
Inaudible: Sound that can not be heard by humans.
Audible frequency range for humans: 20 to 20000 hertz

• Frequency less than 20hz ultrasonic sound
• Frequency more than 20000 hertz infrasonic sound

### Ultrasound and it’s applications

There are certain applications of ultrasonic waves as follows-
1. It helps in cleaning.

These are used to clean those parts of appliances that are not in reach otherwise like electronic components, etc. Appliances to be cleaned are placed in cleaning solutions and ultrasonic waves are sent through the cleaning solution.
Due to high frequency of ultrasounds, the dust, oil, etc. gets detached. The object thus, gets cleaned.
2. Ultrasound are used to detect cracks in the metal block
They are used to detect minor cracks in metal blocks. Ultrasounds are allowed to pass through metal blocks and detectors are used to detect the transmitted waves. If there is a crack in a block, the ultrasound gets reflected back, indicating the presence of a crack in the metal block.

### SONAR (Sound Navigation & Ranging)

This is very helpful and works on the property of reflection of sound.

• It is a technique to detect the depth of water.
• It is used to locate submarines.

At the bottom of the ship, transmitters and receivers are fitted. Transmitters emit ultrasonic waves which on striking at the bottom of the water, return, and are received by the receiver. The time taken by a wave to travel down and get received back by the receiver is calculated and using the velocity of ultrasonic waves in water, the time interval and the distance of submarine from surface or depth can be calculated.
Mathematically:

Let’s say the distance travelled is =x +x=2x
Time interval=t
Velocity of ultrasonic wave in water=v
We know, speed =distance /time
V=2x/t
Or, x=v .t/2

### Human ear

As we have so many organs in our body specialized in different functions. Among them, the ear also plays an important role that is of hearing.

Ear: It is an organ that helps us in hearing. It is divided into 3 parts:
1. Outer ear: It consists of ear pinna, auditory canal and ear drum.
2. Middle ear: It consists of 3 interlocked bones- malleus, incus and stapes.
3. Inner ear: It has a semicircular canal and a coiled organ filled with a fluid called cochlea.

The sound produced by a vibrating body travels in the form of vibrations and is collected by ear pinna and it directs into the ear canal where it travels and hits the eardrum. On receiving vibrations, eardrum starts vibrating. Once the eardrum vibrates, it passes the vibrations to the 3 interlocked bones of the middle ear which further increase the amplitude of vibrations.

In inner ear the vibrations are received by a coiled organ. On receiving the vibrations its fluid also starts vibrating and sensory cells present in them transform the vibrations into electrical signals which are carried by the auditory nerve to the brain for interpretation and understanding.

Sound Produced by Man and animals

In man, a voice box called larynx is present. It consists of two flap – like ligaments called vocal cords with a narrow slit spacing them.

When we speak, air is forced out. This exhaled air forces the vocal cords to vibrate due to which sound is produced.

• Length of vocal cord in males:20 mm
• Length of vocal chords in  females: 15 mm
• In males it is a secondary sexual character and is enlarged. It is called Adam’s apple.

Sound Produced by Animals: Animals produce a wide range of sounds. Many animals use their lungs to blow air that makes their vocal cords vibrate and sound is produced. Maximum lower organisms use their wings to produce buzzing sound. Fish use the air bladder to produce sound and so on.

### Sound Class 9 NCERT Book solutions

1. How does the sound produced by vibrations reach our ears?

Ans: Sound in air gets propagated in the form of longitudinal waves consisting of compressions and rarefactions. For example: if a tuning fork is struck against the rubber pad, it sets it to vibrate. As the prong moves towards right, it compresses the layer of air in contact with it. As air has elasticity, the compressed air tends to relieve itself from the strain and moves forward to compress the next layer and so on. When the prong moves towards left, a region of reduced pressure or rarefaction is produced and so each compression is followed by rarefaction.

2. Explain how sound is produced by school bell?

Ans: when the school bell is struck by a hammer, it starts vibrating due to elasticity and inertia. During the forward motion, compression is caused and due to backward motion rarefaction is produced.

3. Why are sound waves called mechanical waves?

Ans: Sound waves are called mechanical waves because they need a medium to travel.

4. Suppose you and your friends are on the moon. Will you hear each other?

Ans: No, we will not hear each other on the moon as there is no medium present for sound to travel.

5. Which property determines (a) loudness (b)pitch?

Ans: (a) amplitude (b) frequency.

6. Which sound has a higher pitch: guitar or horn?

Ans: The frequency of guitar is higher so, it has a higher pitch.

7. Define wavelength, frequency, timeperiod and amplitude of sound wave?

Ans: Time Period: The time taken to complete one oscillation (sec). It is denoted by ‘T’.
Wavelength: The distance between two consecutive crests or troughs (cm). It is denoted by lambda
Amplitude: It is the maximum displacement of particle from its mean position in either direction (cm). It is denoted by ‘A’.
Frequency: It is the number of sound waves produced per second (Hertz). It is denoted by υ(nu).

8. How wavelength, frequency and velocity are related?
Ans: frequency = velocity/wavelength

9. Calculate the wavelength of a wave whose frequency is 220hz and speed is 440m/sec in medium?
Ans: frequency=velocity/wavelength
So, 220=440/wavelength
wavelength =2m.

10. A person is listening to a tone of 500Hz sitting at a distance of 450m from the source of sound. What is the time interval between successive compressions from the source?

Ans: T=1/f
T=0.002 sec

11. Distinguish between loudness and intensity of sound?

Ans: Loudness: It is the characteristic of sound and is the degree of sensation of sound. It depends upon amplitude. More is the amplitude, louder is the sound. It is measured in unit- decibel.
Intensity: It is the number of sound waves passing through 1 meter square area in one second. It depends upon the square of amplitude.

12. In which of three media, air, water or iron does sound travel the fastest at a particular temperature?

Ans: Sound travels faster in iron followed by water. It is the slowest in air as particles of air are far away from each other.

13. An echo returned in 3 seconds. What is the distance of the reflecting surface from the source, given that the speed of sound is 342m/sec?

14. Why are ceilings of concrete halls curved?

Ans: The ceilings of concrete halls are curved to reduce reverberations.

15. What is the audible range of average human ear ?

Ans: 20-20,000 Hz.

16. What are the frequencies of infrasonic and ultrasonic sound ?

Ans: Infrasonic –less than 20hz.
Ultrasonic – more than 20,000hz.

17. A submarine emits a SONAR pulse, which returns from underwater cliff in 1.02 sec. If the speed of sound in salt water is 1.531 m/sec, how far away is the cliff?

Ans: let the cliff at distance ‘x ‘meters.
Total distance travelled = 2x
S= v. t
X= 1531. 0.51=780.81 m

18. What is sound and how is it produced?

Ans: Sound is an energy that produces the sensation of hearing. It is produced by vibrations in a body.

19. Prove with an experiment that sound needs a medium to travel?

Ans: The apparatus is set as shown in figure.

When current is switched on, bell starts ringing and we can easily hear the sound of a ringing bell.
But as we start removing air by means of a vacuum pump, the sound of the ringing bell goes on becoming fainter and fainter and finally when there is no air left inside, the sound of ringing bell stops.

20. Explain how human ear works?

Ans: Ear: It is an organ that helps in hearing. It is divided into 3 parts:

1. Outer ear: It consists of ear pinna, auditory canal and ear drum
2. Middle ear: It consists of 3 interlocked bones- malleus, incus and stapes.
3. Inner ear: It has a semicircular canal and a coiled organ filled with a fluid called cochlea.

The sound produced by the vibrating body travels in the form of vibrations and is collected by ear pinna and it directs into ear canal where it travels and hits the eardrum. On receiving vibrations, the eardrum starts vibrating. Once the eardrum vibrates, it passes the vibrations to the 3 interlocked bones of middle ear which further increase the amplitude of vibrations.
In inner ear vibrations are received by coiled organ. On receiving the vibrations its fluid also starts vibrating and sensory cells present in them transforms the vibrations into electrical signals which are carried by the auditory nerve to the brain for interpretation and understanding.

21. Explain the working of SONAR?

Ans:

• It is a technique to detect the depth of water.
• It is used to locate submarines.

At the bottom of a ship, transmitters & receivers are fitted. “Transmitters emit ultrasonic waves which on striking the bottom of water return and are received by the receiver. The time taken by a wave to travel down and be received back by receiver is calculated and using the velocity of ultrasonic waves in water, time interval, the distance of submarine from surface or depth can be calculated.
Mathematically:
Let’s say the distance travelled is =x +x=2x
Time interval=t
Velocity of ultrasonic wave in water=v
We know, speed =distance /time
V=2x/t
Or, x=v .t/2

22. How is ultrasound used for cleaning?

Ans: These are used to clean parts that are not in reach otherwise like electronic components, etc. Appliances to be cleaned are placed in cleaning solutions and ultrasonic waves are sent through the cleaning solution. Due to high frequency of ultrasounds, the dust, oil, etc get detached. The object thus, gets cleaned.

23. How cracks in metal block can be detected?

Ans: Ultrasound waves are used to detect minor cracks in metal blocks. Ultrasounds are allowed to pass through metal blocks and detectors are used to detect the transmitted waves. If there is a crack in a block, the ultrasounds get reflected back, indicating the presence of a crack in the metal block.

24. What is reverberation. How can it be reduced?

Ans: Reverbration is the persistence of sound after the source has stopped producing it. Due to reverberation, sound becomes difficult to hear.
To avoid this, the following measures are adopted-

1. Walls should be made of sound absorbing materials.
2. Carpets are put on floor to absorb sound
3. Heavy curtains are put on doors and windows to absorb sound.
4. Panels with sound absorbing quality are used in halls.

25. Frequency of sound is 100Hz. How many times does it vibrates in a minute?

Ans: Vibration in one second=frequency=100
In one minute =100 x 60=6000 per minute

26. A stone is dropped from the top of tower 500m high into a pond of water at the base of tower .When is the splash heard at the top?