# NCERT Solutions Class 8 Science Chapter 13

## NCERT Solutions for Class 8 Science Chapter 13

Science is beneficial. Science produces powerful and trustworthy knowledge. It may be used to create new technology, treat diseases, and solve various other issues. The research is ever ongoing. Science is constantly improving and increasing our understanding of the cosmos, which raises new issues for future research. However, science can never be "completed."

NCERT  Class 8 Science Chapter 13 is Sound. Students will be briefed on important issues that illustrate the relevance and role of Sound in our daily lives. The back-and-forth movement of an item is known as vibration. To travel, Sound needs a medium. As a result, it can't travel through space. Therefore, sound is very vital in our lives.

Extramarks NCERT Solutions provide detailed and authentic answers to all the textbook questions. Through those, the students can understand, remember and retain answers to NCERT questions and thus, perform well in exams.

Going through the NCERT Solutions of each Chapter is a secret passage to achieve excellent results in the examination. To help the students, Extramarks has come up with the Class 8 Science Chapter 13 Solution. These solutions are made as per the requirements of students. Class 8 NCERT Solutions of Sound are written comprehensively.

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## Key Topics Covered in NCERT Solutions for Class 8 Science Chapter 13

Mentioning below the key concepts covered in NCERT Solutions for Class 8 Science Chapter 13- Sound:

 What is Sound? Sound Waves Types of Sound Sound Properties Characteristics of Sound Sounds by Humans Amplitude, Time Period and Frequency Noise Pollution

Let us now look at Extramarks' in-depth information on each subtopic in NCERT Solutions for Class 8 Science Chapter 13- Sound.

### What is Sound?

Sound is described as vibrations that can travel through any material, such as air, and are audible when reaching an individual's or animal's ear. Musical Sound is described as a sound that is attractive to the ear, whereas Noise is defined as a jarring or unpleasant sound.

Sound is significant because it may take us in directions that vision alone cannot. It may also be significant since it can assist us in determining what we perceive. The Sound may also provide information about a character, location, and time. It facilitates communication between us. Simply hearing footsteps or hearing a voice might help us detect someone's presence. NCERT Solutions for Class 8 Science Chapter 13 lets students know how important Sound is for a living being.

### Sound Waves

NCERT Solutions for Class 8 Science Chapter 13 describes a sound wave as a pattern of disruption created by sound energy travelling away from its source. Longitudinal waves are sound waves where the direction of particle vibration is parallel to the path of energy wave transmission. The atoms vibrate back and forth when they are placed in a beat. A high-pressure and low-pressure zone in the medium occurs from this constant back and forth action. Compressions and rarefactions, respectively, are high-pressure and low-pressure zones.

Transverse Waves

• In Transverse waves, the particle's direction of motion is perpendicular to that of wave motion.
• This Mechanical wave is a transverse wave, e.g., light, or even a Mexican wave at a stadium.

Longitudinal Waves

• When the medium's particles migrate parallel to the wave motion's direction through sequential compression or rarefaction.
• It's a mechanical wave, too.
• Consider a slinky.

### Types of Sound

As per the NCERT Solutions for Class 8 Science Chapter 13, Sound can be divided into two categories:

Inaudible Sound

The frequency of inaudible noises is over 20,000 Hz or below 20 Hz. A sound at this frequency level is inaudible to the typical human ear.

• Infrasonics are low-frequency noises that are inaudible to the human ear.
• Ultrasonics are high-frequency noises that are inaudible to the human ear.

Audible Sound

The audible noises are vibrations at 20 to 20,000 hertz (20 kHz). This frequency level is audible to the human ear.

### Sound Properties

To travel, Sound needs a medium. A medium is a substance or material that allows Sound to travel.

Sound cannot travel in a vacuum.

### Characteristics of Sound

Extramarks NCERT Solutions for Class 8 Science Chapter 13 discusses the three fundamental characteristics of Sounds in the following:

• Loudness: Loudness is an acoustic feeling that allows people to discern between faint and loud sounds. The Sound generated will be more perceptible if the amplitude of vibrations is greater. This is because the cube of the amplitude of a wave is exactly proportional to its loudness.
• Pitch: Pitch is a sound property that allows people to discriminate between soft and harsh sounds. The vibration frequency is directly proportional to the pitch and shrillness and increases or decreases with it.
• Quality: Quality is a sound quality that allows people to discern between musical sounds produced by different singers or musical instruments, even if their volume and pitch are the same.

### Sounds by Humans

How do Humans Produce sounds?

To answer this question, Extramarks NCERT Solutions for Class 8 Science Chapter 13 states:

• The larynx generates the Sound, positioned at the top of the windpipe.
• In the voice box, two vocal cords span across each other and have a slot through which the lungs push air out.
• The voice cords' muscles control how tight or loose they are.

### Human Ear

Extramarks’ NCERT class 8 science chapter 13 explains that the human ear comprises three parts: External Ear, Middle Ear, and Internal Ear.

• Pinna (outer ear) collects Sound from the environment.
• The auditory canal is like a tube through which Sound travels.
• When Sound strikes the eardrum (tympanic membrane), it vibrates.
• Vibrations are sent to the inner ear, and after receiving, they send signals to the brain through the auditory nerve.

### Amplitude, Time Period and Frequency

Extramarks’ NCERT Solutions for Class 8 Science Chapter 13 explains the concept of Amplitude, Time Period and Frequency in the following points:

Amplitude

The maximum extent of vibrations produced by any vibrating body from its mean location is amplitude. The amount a radio wave goes back and forth is an example of amplitude.

Time Period

An oscillation is a full to and fro movement of the pendulum from its mean position. The Time Period is the time it takes for a vibrating particle to complete one oscillation.

Frequency

Frequency is the number of oscillations per second—Hertz unit of measurement (Hz).

### Noise Pollution

Noise pollution is often characterised as repeated exposure to high sound levels that cause harm to people or other living beings. However, according to the World Health Organization, sound levels below 70 decibels do not harm living beings, regardless of how long or constant the exposure is.

Unwanted Sound (Noise) may be harmful to one's health. Cardiovascular problems, hypertension, excessive stress levels, tinnitus, deafness, sleep difficulties, and various other detrimental and unsettling impacts are all linked to sound pollution. Environmental Noise or noise pollution is the spread of Noise that has multiple effects on human or animal life activities, most of which are damaging to some extent.

Outside Noise is mainly created by machines, transportation, and propagation systems. Poorly designed urban areas can often lead to noise disintegration or pollution; for example, sound pollution in residential neighbourhoods can be caused by industrial and residential buildings being built next to one other. Loud music, transportation (traffic, rail, aircraft, etc.), lawn care maintenance, construction, electrical generators, explosions, and other people are some of the most common noise sources in residential neighbourhoods. Noise can increase the risk of mortality in animals by disrupting predator and prey detection and avoidance, interfering with reproduction and navigation, and contributing to irreversible deafness. The water absorbs a large portion of the Noise that humans make.

Experts have focused on noise pollution in NCERT Solutions for Class 8 Science Chapter 13 straightforwardly, which will help students understand how excessive and undesired sounds the source of noise pollution may be. The noise pollution is caused mainly by human-made voices. Bursting crackers, loudspeakers, and the Sound of automobiles are the most common sources of noise pollution.

## NCERT Solutions for Class 8 Science Chapter 13 Exercise and Solutions

Extramarks' website is full of quality study material. Great resources such as previous year's question papers, revision notes, extra problems, and more are available for all the classes. Click on the below links to view NCERT Solutions for Class 8 Science Chapter 13:

1. Class 8 Science Chapter 13: Very Short Answer Type Questions
2. Class 8 Science Chapter 13: Short Answer Type Questions
3. Class 8 Science Chapter 13: Long Answer Type Questions

Extramarks NCERT solutions provide detailed and authentic answers to all the textbook questions. Through those, the students can understand, remember and retain answers to NCERT questions and thus, perform well in exams.

Students can easily access NCERT Solutions for Class 8 Science Chapter 13 and other chapters by clicking here. In addition, students can also explore NCERT Solutions for other classes below.

• NCERT Solutions Class 1
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• NCERT Solutions Class 8
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By getting access to NCERT Solutions for Class 8 Chapter 13, students can easily understand all the concepts of Sound.

### Key Features of NCERT Solutions for Class 8 Science Chapter 13

Our NCERT Solutions have proven to be a boon for students. Students tend to go through these solutions because they help them grasp all the Chapter's concepts. In addition, these solutions are made so that they meet all the requirements of the students. Don't believe us? Highlighted below are a few reasons why one should go with Extramarks:

• These solutions prove time-saving for students and also help them learn all the concepts quickly at the same time.
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• These solutions are prepared systematically in the most straightforward language possible.

Q.1 Choose the correct answer.

Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases

Ans-

The correct option is (d).

Explanation: Sound cannot travel through vacuum. Solids, liquids and gases provide the medium for sound to travel.

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

(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman

Ans-

The correct option is (c).

Explanation: The pitch of an adult man is lower as compared to baby boy, baby girl and woman. Also, frequency of a sound is directly proportional to its pitch. Hence, man’s voice is of minimum frequency in this case.

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

(a) Sound cannot travel in 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 vibration is large, 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 is 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)

Ans-

(a) Sound cannot travel in vacuum. (T)
Explanation: Sound travels in the presence of a medium.
(b) The number of oscillations per second of a vibrating object is called its time period. (F)
Explanation: The number of oscillations per second of a vibrating object is known as its frequency.
(c) If the amplitude of vibration is large, sound is feeble. (F)
Explanation: Loudness of a sound is directly proportional to the square of the amplitude of its vibration. When the amplitude of vibration of a sound is large, the sound is called very loud. The sound is feeble for small amplitude.
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
Explanation: A human can hear a sound whose frequency comes in the range of 20 Hz − 20,000 Hz.
(e) The lower the frequency of vibration, the higher is the pitch. (F)
Explanation: The pitch of a sound is proportional to its frequency.
(f) Unwanted or unpleasant sound is termed as music. (F)
Explanation: Unwanted or unpleasant sounds are known as noise.
(g) Noise pollution may cause partial hearing impairment. (T)
Explanation: When a loud and unpleasant sound is produced continuously for a long time, then it may affect temporarily our hearing impairment.

Q.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) Shrillness of a sound is determined by the __________ of vibration.

Ans-

(a) Time taken by an object to complete one oscillation is called time period.
(b) Loudness is determined by the amplitude of vibration.
(c) The unit of frequency is hertz (Hz).
(d) Unwanted sound is called noise.
(e) Shrillness of a sound is determined by the frequency of vibration.

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

Ans-

$\begin{array}{l}\text{Number of oscillations = 40}\\ \text{Total time taken = 4 s}\\ \text{The number of oscillations per second is called frequency}\text{.}\\ \therefore \text{\hspace{0.17em}}\text{Frequency =}\frac{\text{Number}\text{\hspace{0.17em}}\text{of}\text{\hspace{0.17em}}\text{oscillations}}{\text{Total}\text{\hspace{0.17em}}\text{time}\text{\hspace{0.17em}}\text{taken}}\text{=}\frac{\text{40}}{\text{4}\text{\hspace{0.17em}}\text{s}}\text{= 10}\text{\hspace{0.17em}}\text{Hz}\\ ⇒\text{\hspace{0.17em}}\text{Time}\text{\hspace{0.17em}}\text{period}\text{\hspace{0.17em}}\text{=}\text{\hspace{0.17em}}\frac{\text{1}}{\text{Frequency}}\text{=}\frac{\text{1}}{\text{10}\text{\hspace{0.17em}}\text{Hz}}\text{= 0}\text{.1}\text{\hspace{0.17em}}\text{s}\end{array}$

Q.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?

Ans-

$\begin{array}{l}\text{Frequency of sound produced by mosquito = 500 vibrations per second = 500 Hz}\\ \text{Time period =}\frac{1}{\text{Frequency}}\\ \text{On putting the given value,}\\ \text{Time period =}\frac{1}{\text{Frequency}}=\frac{1}{500\text{Hz}}=0.002\text{s}\end{array}$

Q.7 Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute

Ans-

(a) Dholak: It is a musical instrument which consists of a stretched membrane known as its head. When the head is beaten softly, the stretched membrane sets into vibration. Hence, the dholak produces a sound when it is beaten.
(b) Sitar: It is also a musical instrument which consists of stretched strings. When a string is plucked, it sets into vibration. Hence, the sitar produces sound when it is beaten.
(c) Flute: It is a hollow pipe. When air is blown over its mouth, the air inside the pipe is set into vibration. Thus, a pleasant sound is produced.

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

Ans-

Music: The sound which seems pleasant to ear is called music. For example, the sound produced by violins, pianos, flutes, etc.

Noise: The sound which seems unpleasant to ear is called noise. For example, sound produced by horns of buses and trucks, sound of electrical generators, sound of a gunshot, etc.

Yes. When music is played at very high volume, it becomes noisy.

Q.9 List sources of noise pollution in your surroundings.

Ans-

The sources of some noise pollution are as follows:

1. Horns of vehicles
2. Loudspeakers
3. Crackers.

Q.10 Explain in what way noise pollution is harmful to human.

Ans-

Noise pollution can cause stress, hypertension, insomnia, hearing loss, etc.

Q.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.

Ans-

There will be more noise around the house which is located along the roadside. The effect of noise decreases with the distance between the source and the listener. Thus, it is better to choose the house that is three lanes away from the roadside.

Q.12 Sketch larynx and explain its function in your own words.

Ans-

In human beings, sound is produced by the voice box or larynx ,which is at the upper end of the windpipe. Two vocal cords are stretched across the larynx. These vocal cords leave a narrow slit between them for the passage of air. The air from the lungs passes through the slit due to which vocal cords starts vibrating. As we know that vibration of an object is the necessary condition for the production of sound, so the sound is produced due to vibrations of the vocal cords.

Muscles attached to the vocal cords can make the cords tight or loose and can change the type or quality of the voice produced.

Q.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?

Ans-

The speed of light is more than the speed of sound. Therefore, light reaches us before thundering.

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##### FAQs (Frequently Asked Questions)
1. What is the central idea of the 13th Chapter of Class 8 Science?

Chapter 13 of the NCERT Solutions for Class 8 Science Chapter 13 helps to teach students about the process that causes the Sound to be produced. Anything that vibrates causes disruptions in the medium around it. This surrounding medium carries the disruptions or waves to the human ear. Therefore, the Sound generated will be perceptible if the frequency of the waves is within the human hearing range. This is the central idea of this Chapter.

2. How do we tell the difference between music and Noise?

Music is an aesthetically pleasing sound. It is made up of uniform spacing and rhythmic essential overtones and frequencies. Noise is sound that has no discernible structure. It’s inconsistent and irregular, and it’s unpleasant to listen to. The Sound generated by musical instruments, for example, is music, but the Sound created by vehicles is Noise.