NCERT Solutions for Class 8 Science Chapter 13 Sound

These NCERT Solutions for Class 8 Science Chapter 13 Sound Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Sound NCERT Solutions for Class 8 Science Chapter 13

Class 8 Science Chapter 13 Sound Textbook Exercise Questions and Answers

Page 168-169

Question 1.
Sound can travel through
a. gases only
b. solids only
c. liquids only
d. solids, liquids and gases
d. solids, liquids and gases

Question 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
c. A man

Question 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)
a. True
b. False
c. False
d. True
e. False
f. False
g. True

Question 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.
a. time period,
b. amplitude,
c. hertz,
d. noise,
e. frequency

Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Number of oscillations = 40
Total time taken = 4 seconds
Time period = Time taken for one oscillation
= $$\frac{\text { Total time }}{\text { Total number of oscillations }}$$
= $$\frac{4 \text { seconds }}{40}$$ = $$\frac {1}{10}$$second = 0.1 second
Frequency of oscillations is defined as the number of oscillations of a vibrating body per second.
Frequency = $$\frac{\text { Number of oscillations }}{\text { Total time }}$$ = $$\frac {40}{4}$$ = 10 Hz.

Question 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?
The time required to complete on oscillation is known as time period. It is given by the inverse of the frequency.
Time period = $$\frac{1}{\text { Frequency of oscillations }}$$
Frequency of oscillations = 500 Hz
Time period = $$\frac {1}{500}$$ = 0.002 s

Question 7.
Identify the part which vibrates to produce sound in the following instruments:
a. Dholak
b. Sitar
c. Flute
a. Stretched membrane,
b. String,
c. Air column

Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Unwanted sound that is unpleasant to ear is called noise. A pleasant sound is called music. Music can become noise at many instances. When someone tries to enjoy very loud music, it can become noise for someone else. When loud music is played during religious celebrations or marriages, it can be annoying for many people.

Question 9.
List sources of noise pollution in your surroundings.
Some sources of noise pollution are as follows:

• Television and transistors running at high volumes
• Loudspeakers and crackers
• Homs of buses, cars and trucks
• Home appliances such as mixer, desert cooler, etc.

Question 10.
Explain in what way noise pollution is harmful to human.
Noise pollution is harmful to humans in many ways. Constant exposure to noise pollution can create many health related problems like insomnia, hypertension and may even lead to loss of hearing.

Question 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.
I will suggest my parents to buy a house which is away from the roadside, because house at the roadside would be noisy due to running vehicles. On the other hand, the house which is three lanes away from the roadside would be quieter and healthier to live in.

Question 12.
Sketch larynx and explain its function in your own words.
Larynx is a part of throat responsible for the production of sound. A sketch of a human larynx is shown as follows:

Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.

Question 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?
We know that the speed of light is much more than the speed of sound. Due to this, light reaches us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.

NCERT Extended Learning Activities and Projects

Question 1.
Visit the music room of your school. You may also visit musicians in your locality. Make a list of musical instruments. Note down the parts of these instruments that vibrate to produce sound.
Hint.
There are membrane instruments like tabla, mridangam and drums which produce sound when the membrane is struck. There are wind instruments like flute, trumpet and saxophone which produce sound by the vibrating air columns. There are string instruments like violin, veena and guitar which produce sound when the string is struck.

Question 2.
If you play a musical instrument, bring it to the class and demonstrate how you play it.
Hint.
Do it yourself.

Question 3.
Prepare a list of famous Indian musicians and the instruments they play.
Hint:

 Indian musicians Instruments they play Hari Prasad Flute Zakir Hussain Tabla U. Srinivas Mandolin Shiv Kumar Sharma Santoor Garib Das Dhol Adil Hussaini Harmonium Vishwa Mohan Bhatt Guitar TJstad Bendu Khan Sarangi

Question 4.
Take a long thread. Place your hands over your ears and get someone to place this thread round your head and hands. Ask her to make the thread taut and hold its ends in one hand. Now ask her to draw her finger and thumb tightly along the thread. Can you hear a rolling sound like that of a thunder? Now repeat the activity while another friend stands near both of you. Can he hear any sound?

Hint.
Do it yourself.

Question 5.
Make two toy telephones. Use them as shown in the given figure. Make sure that the two strings are taut and touch each other. Let one of you speak. Can the remaining three persons hear? See how many more friends you can engage in this way. Explain your observations.

Hint.
Do it yourself.

Question 6.
Identify the sources of noise pollution in your locality. Discuss with your parents, friends and neighbours. Suggest how to control noise pollution. Prepare a brief report and present it in the class.
Hint.
The various sources of noise pollution are television, transistor, loudspeakers, crackers, horns of vehicles, home appliances, etc. Noise pollution can be controlled by planting tree belts, using silencers in automobile engines, sound barriers, etc.

Activity 1

Objective: To show that a vibrating body produces sound.
Materials Required: A metal plate or a frying pan and a stick.
Procedure:

• Take a frying pan.
• Hang it at a convenient place in such a way that it does not touch any wall.
• Now strike it with a stick.
• Touch the frying pan gently with your finger, and try to feel the vibrations.
• Touch the frying pan after it stops producing sound. Try to feel the vibrations again.

Observation: On striking the frying pan, it produces sound and we feel vibrations on touching it. But when it stops producing sound, there are no vibrations at all.
Conclusion: Sound is produced by a vibrating object. If an object does not vibrate, it cannot produce sound.

Sound Produced by Humans: Sound is produced by the voice box or larynx in human beings. The voice box or larynx is situated in the upper part of windpipe. There are two vocal cords stretched across a small gap between them. When air is forced through the cords, they begin to vibrate and sound is produced. Muscles which are attached to the vo’cal cords enable us to make the vocal cords tight or loose as per need. Sound quality varies according to the tension or slack in the vocal cords. The vocal cords in males are of length 20 mm, and females have 15 mm long vocal cords. Children, on the other hand, have very short lengthed vocal cords. Hence, the voices, their quality and their type are always different in women, men and children.

Propagation of sound: The travelling of sound from source of receiver is called propagation of sound. Sound is propagated by the to and fro motion of particles of the medium.

Sound needs a medium to propagate: A medium is necessary for the propagation of sound waves. The matter or substance through which sound is transmitted is called a medium. The medium can be solid, liquid or gas. Sound cannot travel in vacuum. A true vacuum refers to the complete absence of matter.

Activity 2

Objective: To show that sound travels through liquids.
Materials Required: A bucket or a bathtub, clean water, a small bell.
Procedure:

• Take a bucket or a bathtub.
• Fill it with clean water.
• Take a small bell in one hand.
• Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub.
• Place your ear gently on the water surface.
• Try to hear the sound of the bell.

Observation: On shaking the bell in water and placing the ear on water surface, the sound of bell is heard.

Conclusion: Sound travels through liquids.

We hear sound through our ears: Sounds are produced as waves in the air or any other medium. These” waves are converted into electrical signals or messages that our brain can understand. Our ears have a special structure that allows this function. There are three major parts of the human ear:

i. Outer ear: It consists of ear pinna and a canal at the end of which is a stretched membrane, called eardrum. Pinna collects the sound waves which then pass through the ear canal and causes vibrations in eardrum.
ii. Middle ear: It has a set of three bones which are linked together and attached to the eardrum. These bones receive vibrations from the eardrum, amplify them and pass on to the inner ear.
iii. Inner ear: The inner ear receives vibrations from the middle ear and changes them into electrical signals. These signals are carried by the auditory nerves to the brain. The brain interprets these signals as sound.

Amplitude, Time Period and Frequency of a Vibration:
A repeated ‘back and forth’ motion is called vibration (or oscillation). When an object moves back and forth continuously, we say that it is under oscillatory motion. For example, when a swing moves back and forth repeatedly, we say that the swing is making oscillations.

i. Amplitude: The amplitude of a sound wave can be defined as the maximum displacement of the particles from their mean position due to the vibration.

ii. Time period: The time taken for one complete oscillation of a sound wave is called the time period of the sound wave.
iii. Frequency of oscillation: The number of oscillations an object completes per second is called its frequency. The SI unit of frequency is Hertz (Hz).
1 Hz = 1 oscillation per second
20 Hz = 20 oscillations per second

Loudness of Sound: Loudness of sound depends on the amplitude of vibration. Loudness of sound is directly proportional to the square of amplitude of vibration. Louder sound has higher amplitude while quieter sound has lower amplitude. Loudness is expressed in terms of decibel.

The following table gives the loudness of sound from various sources:

 Source of sound Loudness Normal breathing 10 dB Soft whisper (at 5 m) 30 dB Normal conversation 60 dB Busy traffic 70 dB Average factory 80 dB

Activity 3

Objective: To show that the loudness of sound depends upon the amplitude of vibration.
Materials Required: A metallic tumbler, a table spoon, a small thermocol ball.
Procedure:

• Take a metallic tumbler and a table spoon.
• Now suspend a small thermocol ball touching the rim of the tumbler.
• Vibrate the tumbler by striking it with spoon.
• See how far the ball is displaced. The displacement of the ball is a measure of the amplitude of vibration of the tumbler.
• Now, strike the tumbler (a) gently and then (b) with some force.
• Compare the amplitudes of vibrations of the tumbler in the two cases.

Observation: The amplitude of sound is larger in the first case (a) as compared to the second case (b) as the ball was displaced more in case (a).
Conclusion: The loudness of sound depends upon the amplitude of vibration.

Pitch or Shrillness: Pitch of sound depends on the frequency of vibration. A high pitched sound has high frequency, while a low pitched sound has low frequency. Children and women generally produce sound with a high pitch.

Audible and Inaudible Sounds:
Human beings can hear sounds between frequencies 20 Hz and 20,000 Hz. This range of frequencies is called the audible range for humans. The sound with frequency below 20 Hz is called infrasound. On the other hand, the sound with frequency above 20,000 Hz is called ultrasound.

Noise and Music: A sound which is pleasant to ears is called music. However, any unpleasant sound is called noise.

Noise Pollution: Presence of excess noise in the environment is called noise pollution. Automobiles, factories, loud music, construction works, firecrackers, stone quarry, etc., are all sources of noise pollution.

Effects of Noise Pollution: Continuous exposure to noise pollution can result in lack of sleep (insomnia), hypertension (high blood pressure), anxiety and many other disorders. Noise pollution can also lead to partial loss of hearing in some cases.

Measures to Limit Noise Pollution:

• Aircraft engines and automobile engines should be fitted with silencing devices. The muffler (or silencer) in a motorcycle is an example of such device.
• Factories should be relocated far away from the residential areas. Many factories from Delhi had been shifted to outskirts in the 1990s.
• Trees should be planted along the roads because trees absorb sound waves or noise.
• Sound barriers should be installed along flyovers.

Hearing Impairment: Loss of hearing is called hearing impairment. It can be total or partial, but total hearing impairment is rare and is usually congenital, i.e., by birth. A person with hearing impairment can learn sign language to communicate with others. Hearing aids can be used by people who are suffering from partial hearing impairment.

Class 8 Science Chapter 13 Sound Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is sound?
Sound is a form of energy which gives the sensation of hearing.

Question 2.
How does sound travel from one place to another?
Sound travels through a medium, like solid, liquid or gas by the vibrations of the particles of medium. It cannot travel through vacuum.

Question 3.
Define frequency.
The number of oscillations per second is called frequency of oscillation.

Question 4.
What is the unit of frequency?
Hertz.

Question 5.
What are the two important properties of sound?
Amplitude and frequency.

Question 6.
Name the factor on which the loudness of the sound depends.
Amplitude.

Question 7.
How can we minimise the noise made by transport vehicles?
By installing silencing device in transport vehicles. Also, the use of automobile horns should be minimised for this purpose.

Question 8.
Name some health disorders caused by noise pollution.
Hypertension, lack of sleep, anxiety, etc.

Question 9.
Name some musical instruments.
Tabla, sitar, harmonium, flute, etc.

Question 10.
What is the frequency limit upto which bats can hear?
Bats can hear upto a frequency of 100 kHz.

Question 11.
Define loudness. Name its unit.
Loudness is the degree of sensation of sound produced by an object and is usually a perception of sound pressure. It is measured in a unit called decibel (dB).

Question 12.
Name the three parts of the human ear.
Outer ear, middle ear and inner ear.

Question 13.
Name some sources of sound.
Human vocal cords, tuning fork, stringed instruments, drums, bells, etc.

Question 14.
What do you understand by the term quality of sound?
Factors which distinguish between various sounds are called the quality or timber of sound.

Question 15.
Define hertz.
A frequency of one hertz is defined as one oscillation per second.

Question 16.
Define vacuum.
Vacuum is the space that is completely devoid of matter.

Question 17.
What are the sound waves having frequency greater than 20,000 Hz called?
Ultrasonic waves.

Question 18.
Define noise.
Unpleasant or unwanted sounds are called noise.

Question 19.
Name the voice producing organ of the humans.
Voice box.

Question 20.
Define oscillatory motion.
The to and fro motion of an object is known as oscillatory motion.

Question 21.
What do you mean by hearing impairment?
Hearing impairment refers to the hearing loss that prevents a person from receiving and processing sounds properly through the ear. If the loss is mild, the person has difficulty in hearing.

Short Answer Type Questions

Question 1.
How is sound produced?
Sound is produced by the vibrations of a body. Vibration is the to and fro motion of a particle. These vibrations are passed on from one particle of the medium to another in the form of sound wave.

Question 2.
Which property of vibration determines the pitch of the sound? Explain giving some examples.
Frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher, the sound has a higher pitch. If the frequency of vibration is lower, the sound has a lower pitch. Sound of women is shriller than that of men due to high frequency of sound in women.

Question 3.
What is noise pollution? What are its major causes?
Presence of excessive or unwanted sounds due to irregular frequencies in the atmosphere is called noise pollution. Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers, etc.

Question 4.
Why is there a difference in the sound of a baby and an adult?
The loudness of sound depends upon the amplitude of vibration. When the amplitude of vibration is large, sound produced is loud and when the amplitude of vibration is low, sound produced is feeble. A baby has comparatively shorter vocal cords and hence it produces a feeble sound with low amplitude whereas longer vocal cords of adults produce a louder voice.

Question 5.
Define the amplitude of a vibration with the help of a diagram.
The maximum distance moved by a particle from the mean or central position in a vibrational motion is called the amplitude of that particle.

Question 6.
What are the two main properties of sound?
The two main properties of sound are as follows:

1. Amplitude: The maximum displacement of an oscillatory body from the mean position is called amplitude. It controls the loudness of a sound; more the amplitude, more loud the sound is.
2. Frequency: The number of oscillations in one second is called frequency. It controls the pitch of the sounds. High pitched sound means higher frequency.

Question 7.
What are the characteristics of sound?
Following are some of the characteristics of sound:

• Sound travels in longitudinal waves.
• The amplitude of sound determines its loudness.
• Tone is a measure of the quality of a sound wave.
• Sound travels faster in hot medium, and in solids.

Question 8.
Why do astronomers fail to hear the sound of each other on the surface of the moon?
Astronomers fail to hear sound of each other while they are on the surface of the moon because there is no air or atmosphere present on the moon and we know that sound cannot travel through vacuum. It needs a medium to travel. Hence, sound cannot propagate on the surface of the moon.

Question 9.
Who has a higher pitched sound, a man or a woman? What is the reason for the difference between the two?
Men normally have longer vocal cords than those of women. Thus, men have lower pitched voice while women have higher pitched voice.

Question 10.
What is SONAR? What is the basic principle of its working?
SONAR refers to Sound Navigation and Ranging. The principle of reflection of sound is used in SONAR. SONAR is used to measure the depth of the ocean. Ultrasonic waves are sent from the ship down into the sea. They are received back after reflection from the sea bed. The depth is calculated by noting the time period.

Question 11.
Distinguish between high pitch sound and low pitch sound.

 High pitch sound Low pitch sound 1. If the frequency of vibration is higher, we say that the sound is shrill and has a higher pitch. 1. If the frequency of vibration is lower, we say that the sound has a lower pitch. 2. High pitch sounds are called treble. 2. Low pitch sounds are called bass. 3. Sound produced by a bird is a high ^ pitch sound. 3. Sound produced by a lion is a low pitch sound.

Question 12.
State the conditions necessary for sound to be heard.

1. There must be a vibrating body which should be capable of transferring its energy to its surroundings.
2. There must be a material medium to pick up the energy and then propagate it in forward direction.
3. There must be a receiver, so as to receive the sound vibration and then transmit it to the brain for final interpretation.

Question 13.
It is not advisable to construct houses near airports. In spite of that many new residential apartments are constructed near airports. Raman files RTI and also complains the municipal office about the same.
a. Why should one not reside near an airport?
b. Name other two places where there is noise pollution.
a. Airplanes cause noise pollution while their landing and taking off. Due to this, many health problems occur like deafness, high blood pressure, etc.
b. Near the railway station and near the heavy traffic area.

Long Answer Type Questions

Question 1.
List some methods of controlling noise pollution.
Methods of controlling noise pollution are as follows:

• Planning land use to reduce noise; for example, making tree-lined buffer zones between residential colonies and roads with heavy traffic.
• Reducing noise emissions by developing low-noise products, for example, better silencers for automobiles.
• Screens and enclosures can be made around machinery to obstruct the path of noise. This will help the people working in and living near the factories.
• Using double glazed glass windows at home to keep out noise.
• Control over recreational noise, such as use of loudspeakers.
• Increasing public awareness by providing factual information of the harmful effects of noise from blowing horns, loud music and TV, so that people themselves reduce noise generation.

However, all these measures can succeed only by the participation of public.

Question 2.
Explain the working of human ear with the help of a labelled diagram.
The ear can be divided into outer ear, middle ear and inner ear. The outer ear consists of pinna which collects sound waves and the ear canal which conducts these waves to the eardrum. The eardrum, or tympanic membrane, is a thin, cone-shaped membrane that separates the outer ear from the middle ear in humans. It is like a stretched rubber sheet and sound vibrations make the eardrum vibrate. The eardrum sends these vibrations to the middle ear where the three bones, named malleus, incus and stapes, transmit and amplify these sound waves. From here the wave enters the inner ear which converts sound waves into electrical signals and send them to the brain. In this way, we are able to hear the sound around us.

Question 3.
Sound cannot travel in vacuum. Justify the statement with the help of an activity.
Take a metal or glass tumbler. Make sure that it is dry. Place a cell phone in it. Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands. Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler. The sound becomes fainter as you suck the air. Now, remove the tumbler from your mouth. The sound becomes loud again. This shows that sound can travel in any medium (solid, liquid or gas) but not is vacuum.

Picture-Based Questions

Question 1.
Observe the diagram and answer the following questions.

a. Name the figure shown here.
b. Why are the different bowls tilled with different amounts of water?