NCERT Class 8 Science Chapter 10 ‘Sound’: Notes and Solutions (Free PDF)

Chapters of NCERT become easier to understand when we have well-written notes. Especially when the exam season comes, the value of these notes becomes so much more. Hence, keeping the same in mind, we have prepared NCERT Solutions and Notes for Class 8 Science Chapter 10: Sound. These notes are in a point-wise manner to cover the entire chapter easily and serve as a method of quick revision. After the notes, solutions to the NCERT textbook questions are also available. The PDF of the same is available for you to download. So, without any further delay, let’s explore this blog. 

Download the NCERT Class 8 Science Chapter 10 Solutions PDF Here!

Explore Notes of all the Chapters of Class 8 Science:

Chapter 1	Chapter 2	Chapter 3
Chapter 4	Chapter 5	Chapter 6
Chapter 7	Chapter 8	Chapter 9
Chapter 10	Chapter 11	Chapter 12
Chapter 13

Class 8 Science Chapter 10 Sound: Notes

In this section of Class 8 Science Chapter 10 notes, we will define sound, how it is formed, and how it propagates, we will learn about the human ear as well, as other related concepts of sound. Let’s start.

Sound plays an important role in our lives. It helps us to communicate with one another.

Sound is Produced by a Vibrating Body

• The to-and-fro motion of an object is known as vibration.
• For example:- When a tightly stretched band is plucked, it vibrates and produces sound.
• Vibrating objects produce sound. 
• In most cases, the amplitude of vibrations is so small that we cannot see them. However, we can feel them.

Sound Produced by Humans

• In humans, the voice box i.e. Larynx produces the sound.
• The voice box is located at the upper end of the windpipe.
• Two vocal cords stretch across the voice box leaving a narrow slit for the passage of air between them.
• When the lungs force air through the slit, the vocal cords vibrate, producing sound.
• There is a set of muscles that are attached to the vocal cords. These muscles can make the vocal cords tight or loose and based on that, the sound changes.

Sound Needs a Medium for Propagation

• To travel, sound needs a medium. It can be anything, air, water, glass, etc. 
• When air is completely removed from the vessel, it is called a vacuum. And since there is no medium in a vacuum, sound doesn’t travel in a vacuum. 
• Also, in different mediums, the travelling speed of sound is different. 

We Hear Sound through Our Ears

• The shape of the outer part of the ear is like a funnel.
• When sound enters it, it travels down a canal.
• At the end of the canal, there is a thin stretched membrane. It is called the eardrum.
• This eardrum is stretched like a membrane, and when sound enters our ear, this eardrum vibrates.
• This eardrum sends the vibrations to the inner ear and from there, they go to the brain where sound is interpreted. Hence, in this way, we hear. 

You can also check out: How to Become a Sound Engineer? or an Audio Engineer

Class 8 Science Chapter 10 Sound: Characteristics of Sound Notes

In the previous sections of Class 8 Science Chapter 10, we have learnt what sound is and how it is produced, now let us learn about its characteristics such as amplitude, frequency, time period, audible and inaudible sound, etc.

Amplitude, Time Period and Frequency of a Vibration

• The vibration is also known as oscillatory motion.
• The number of oscillations per second is called the frequency of oscillation. It is expressed in ‘hertz’ (Hz). 
• 1 Hz is defined as 1 oscillation per second
• The frequency determines the shrillness or pitch of a sound
• Higher frequency means the sound is shrill and has a higher pitch for example: a whistle has a high frequency, and lower frequency means the sound has a lower pitch for example: a drum vibrates with a low frequency.
• The time period is defined as the time taken to complete one oscillation. It is also the inverse of frequency.
• The loudness of sound depends on its amplitude. 
• When the amplitude of the vibration is large, the sound produced is loud. 
• When the amplitude is small, the sound produced is low.
• Amplitude and frequency are two important properties of any sound.

Audible and Inaudible Sounds

• The sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible sounds.
• The sounds of frequencies higher than about 20,000 vibrations per second (20 kHz) are also not audible to the human ear.
•  For the human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz.

Noise and Music

• The unpleasant sounds are called noise. For example: the sound of horns of buses and trucks, sounds coming from a construction site, etc.
• We enjoy sounds from musical instruments. These are Musical sounds. For example: sound produced by a harmonium, sound produced by the string of a sitar, etc.

Noise Pollution

• The presence of excessive or unwanted sounds in the environment is called noise pollution. 
• Major causes of noise pollution are the sounds of vehicles and, explosions including the bursting of crackers, machines, loudspeakers etc. 

What are the Harms of Noise Pollution?

• Noise pollution may cause many health-related problems such as:-

1. Lack of sleep
2. Hypertension (high blood pressure)
3. Anxiety
4. A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.

Measures to Limit Noise Pollution

• To control noise, we must control the sources of noise.
• Silencing devices can be installed in aircraft engines, transport vehicles, industrial machines and home appliances.
• All noisy operations must be conducted away from any residential area.
• Noise-producing industries should be set up away from residential areas.
• The use of automobile horns should be minimised.
• TV and music systems should be run at low volumes.
• Trees must be planted along the roads and around buildings to reduce the sounds reaching the residents, thus reducing the harmful effects of noise pollution.

Also Read: Sound Engineering: Courses, Universities & Careers

Important Definitions in NCERT Class 8 Science Chapter 10: Sound

Some important definitions covered in Class 8 Science Chapter 10 are mentioned below:

• The to-and-fro motion of an object is known as vibration. The vibration is also known as oscillatory motion.
• When air is completely removed from the vessel, it is called a vacuum.
• The number of oscillations per second is called the frequency of oscillation. It is expressed in ‘hertz’ (Hz).
•  The sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible sounds.
• The unpleasant sounds are called noise. For example: the sound of horns of buses and trucks, sounds coming from a construction site, etc.
• The presence of excessive or unwanted sounds in the environment is called noise pollution.

Download the NCERT Class 8 Science Chapter 10 Solutions PDF Here!

Explore NCERT Solutions of all the Chapters of Class 8 Science:

Chapter 1	Chapter 2	Chapter 3
Chapter 4	Chapter 5	Chapter 6
Chapter 7	Chapter 8	Chapter 9
Chapter 10	Chapter 11	Chapter 12
Chapter 13

NCERT Solutions of Class 8 Science Chapter 10: Sound

Let us now explore NCERT Solutions of Class 8 Science Chapter 10 to get answers to all writing questions in the lesson Sound.

Exercise Questions

1. Choose the correct answer.

Sound can travel through

(a) gases only

(b) solids only

(c) liquids only

(d) solids, liquids and gases

Soln:

(d) solids, liquids and gases

2. Voice of which of the following is likely to have a minimum frequency?

(a) Baby girl

(b) Baby boy

(c) A man

(d) A woman

Soln:

(c) A man

3. In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.

(a) Sound cannot travel in a vacuum. (T/F)

(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)

(c) If the amplitude of the vibration is large, the sound is feeble. (T/F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

(e) The lower the frequency of vibration, the higher the pitch. (T/F)

(f) Unwanted or unpleasant sound is termed as music. (T/F)

(g) Noise pollution may cause partial hearing impairment. (T/F)

Soln:

a) True 

b) False 

c) False

d) True 

e) False

f) False

g) True 

4. Fill in the blanks with suitable words.

(a) Time taken by an object to complete one oscillation is called __________.

(b) Loudness is determined by the __________ of vibration.

(c) The unit of frequency is __________

(d) Unwanted sound is called __________.

(e) The shrillness of a sound is determined by the __________ of vibration.

Soln:

(a) Time taken by an object to complete one oscillation is called the time period.

(b) Loudness is determined by the amplitude of vibration.

(c) The unit of frequency is hertz.

(d) Unwanted sound is called noise.

(e) The shrillness of a sound is determined by the frequency of vibration.

5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Soln:

The number of oscillations per second is called the frequency of oscillation. It is expressed in ‘hertz’ (Hz).

Frequency = Total number of oscillations

Total time taken

= 50/5

= 10 Hz

The time period is the inverse of frequency.

Time period =  1/Oscillating frequency

= 1/10

= 0.1 s

Hence, frequency = 10 Hz

Time period = 0.1 s

6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Soln:

The time period is defined as the time taken to complete one oscillation. It is also the inverse of frequency.

Time period =  1

Oscillation frequency = 500 Hz

The time period of the vibration = 1/500 = 0.002 s

7. Identify the part which vibrates to produce sound in the following instruments.

(a) Dholak

(b) Sitar

(c) Flute

Soln:

a) Dholak – It has a head on which there is a stretched membrane. This membrane vibrates to produce sound.

b) Sitar – A Sitar consists of many strings stretched across it. These strings, when plucked, create vibrations and hence, sound.

c) Flute – A flute is a hollow pipe. When air is blown through its mouthpiece, it vibrates thus creating music.

8. What is the difference between noise and music? Can music become noise sometimes?

Soln:

The unpleasant sounds are called noise. For example: the sound of horns of buses and trucks, sounds coming from a construction site, etc.

The pleasant sounds that we enjoy such as sounds from musical instruments are music. These are Musical sounds. For example: sound produced by a harmonium, sound produced by the string of a sitar, etc.

Yes, when played at loud volume, music can become noise.

9. List sources of noise pollution in your surroundings.

Soln:

Sources of noise pollution in our neighbourhood are mentioned below:-

(a) Automobile horn

(b) Firecrackers

(c) High volumes of televisions 

(d) Appliances at home such as mixtures and grinders

e) Sirens from factories

f) Loud volume of stereo systems

10. Explain in what way noise pollution is harmful to humans.

Soln:

Noise pollution may cause many health-related problems such as:-

• Lack of sleep
• Hypertension (high blood pressure)
• Anxiety
• Headache
• A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.

11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Soln:

I would suggest the one three lanes away from the road because there, most of the noise, such as vehicles and their horns, road construction, etc., will be minimal. Hence, it will be better for us to live there.

13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier, and thunder is heard later. Can you explain why?

Soln:

The speed of light is much more as compared to that of the speed of sound. Hence, lightning is seen first, which is then accompanied by thunder later.

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