# Class 8 Science NCERT Solutions for Chapter – 13 Sound

## Sound

Question 1.

Sound can travel through
(a) Gases only
(b) Solids only
(c) Liquids only
(d) Solids, liquid, and gases

(d) solids, liquid, and gases

Question 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

(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 a vacuum.
True

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

(c) If the amplitude of vibration is large, the sound is feeble.
False

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

(e) The lower the frequency of vibration, the higher is the pitch.
False

(f) Unwanted or unpleasant sound is termed as music.
False

(g) Noise pollution may cause partial hearing impairment.
True

Question 4.
Fill in the blanks with suitable words :

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

(b) Loudness is determined by the ______ of vibration.
Amplitude

(c) The unit of frequency is ______.
Hertz

(d) Unwanted sound is called ______.
Noise

(e) Shrillness of a sound is determined by the ______ of vibration.
Frequency

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

Time period: 40 oscillations take 4 sec.
∴ 1 oscillation will take =$\frac{4}{40} \mathrm{~s}$
=$\frac{1}{10} \mathrm{~s}$
= 0.1 s
Time period (T) = 0.1 s

$=\frac{40}{4}=10 \mathrm{~Hz}$

Question 6.
The sound from the 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?

In 1 second 500 vibrations are made. i. e., 500 vibrations take 1 second
∴ 1 vibration will take $\frac{1}{500} \mathrm{~s}$
$\left(\frac{1}{500}\right)$s is the time period of vibration as the time period is defined as the time taken by one vibration.

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

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

The unpleasant sound is called noise, whereas a pleasant sound is called music. Noise can produce so many health hazards, whereas music brings about a soothing effect. Yes, music can become noise sometimes when the musical instruments produce very high volume sounds.

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

The sources of noise pollution are :

1. The sound produced by buses and trucks.
2. The sound produced at the construction site.
3. The sound produced by the playing of T.V., radios, and loudspeakers at very high volume.
4. Bursting of crackers, and the sound of big machines in the factories.

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

The noise pollution may cause many health-related problems.

(1) Lack of sleep, hypertension (high blood pressure), anxiety, etc. are some of the problems that may be caused due to noise pollution.

(2) A person who is exposed to a loud sound continuously, may get temporary or even permanent deafness.

Question 11.

I would suggest my parents buy the house which is three lanes away from the roadside. This would protect us from noise pollution which is maximum on the roadside.

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

Explanation: Larynx is present at the upper end of the windpipe, two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound.

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?

The light travels at the speed of 3 x 10s ms”1, which is very large in comparison to the speed of sound which travels at the rate of 330 ms-1 in the air. That is why lightning is seen earlier and thunder is heard later.

Question 1.
How do you come to know that a ‘period’ is over in your school?

When the bell rings, we understand that period is over.

Question 2.
How is the blindfolded person able to guess which player is closest to her?

By hearing the sound made by the person, the blind-folded person becomes able to guess which player is closest to her/him.

Question 3.
Make a list of sounds you hear in your surroundings.

The sounds I hear in my surroundings are as follows:

1. Sounds (noise) made by street vendors.
2. Horns of the rickshaw, bus, tempo, trains, etc.
3. Sounds of loudspeakers playing in the nearby temple, mosque, and gurudwara, etc.
4. Sounds of music systems, T.V., etc.

Question 4.
How is the sound produced? How does it travel from one place to another? How do we hear the sound? Why are some sounds louder than others?

(1) The vibrating object produces sound.

(2) When vibration is produced in the air, sound travels from one place to another in the form of disturbance of the air particles. For propagation from one place to another, sound needs a medium (gas, liquid or solid). It cannot travel in a vacuum.

(3) The eardrums of our ears sense the vibrations of a vibrating body producing sound and send them to the brain. Then the brain interprets it. This process is called hearing.

(4) Some sounds are louder than others because this sound has a larger amplitude of vibration than that of others. So, some sounds are louder than others.

13.1 Sound is Produced by a Vibrating Body

Question 5.
Touch the school bell when not in use. What do you feel?

We do not feel anything in the bell.

Question 6.
Again touch it (bell) when producing sound. Can’ you feel it vibrating?

Yes, the bell is vibrating this time.

Activity 13.1

Question 1.
Do you feel the vibrations?

Yes, I feel the vibrations.

Question 2.
Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Do you still hear the sound?

No, now I don’t hear the sound.

Question 3.
Touch the plate after it stops producing sound. Can you feel the vibrations now?

No, there are no vibrations in the plate after it stops producing sound.

Activity 13.2

Question 1.
Do you hear any sound? Does the band vibrate?

Yes, there is a sound produced on plucking the rubber band. Yes, the band is vibrating.

Activity 13.3

Question 1.
Do you hear a sound?

Yes, very little sound (but not metallic) is produced.

Question 2.
Again strike the dish and then touch it. Can you feel the dish vibrating?

Yes, the dish is vibrating.

Question 3.
Do you see any waves there?

Yes, there are vibrations at the surface of water and waves are (circular) produced there.

Question 4.
What change do you observe on the surface of the water? Can you explain the change?

Now, there is no wave on the surface of the water. The waves disappeared because the disturbances produced were stopped by holding the dish.

Question 5.
Is there a hint to connect sound with the vibrations of a body?

Yes, vibrations in the body produce sound.

Activity 13.4

Question 6.
Play this instrument and identify its vibrating part.

The vibrating part of the instrument is stretched string.

Question 7.
Make a list of familiar musical instruments and identify their vibrating parts. A few examples are given in Table 13.1 of the Textbook, Page 159. Complete the rest of the Table.

1312 Sound Produced by Humans

Question 8.
Speak loudly for a while or sing a song or buzz like a bee. Put your hand on your throat as shown in Fig. 13.8, Textbook Page. 160. Do you feel any vibrations?

Yes, there are vibrations as long as the sound is produced.

13.3 Sound Needs a Medium for Propagation

Question 9.
When you call up your friend who is standing at a distance, your friend is able to hear your voice. How does the sound propagate or travel to her?

Sound travels through air (gas medium) from me to my friend.

Activity 13.7

Question 1.
Does the sound become fainter as you suck air?

Yes, it does.

Question 2.
Does the sound become loud again?

Yes, the sound becomes louder again.

Based on Activity 13.7

Question 10.
Can you think of an explanation? Is it possible that the decreasing amount of air in the tumbler had some¬thing to do with decreasing the loudness of the ring?

The sound needs a medium (gas, liquid, or solid) to travel. In the first case, the sound became fainter because the air (medium) is sucked up.
In the second case, when the sucking of air was stopped, the sound became louder again, as the air remained intact this time. Yes, the decrease in the amount of air (medium) leads to a decrease in the loudness of the sound.

Question 11.
Does sound travel in liquids?

Yes, sound travels in liquids also.

Activity 13.8

Question 1.
Can you hear the sound of the bell? Does it indicate that sound can travel through liquids?

Yes, I can hear the sound of the bell. Yes, this activity indicates that sound travels through liquids.

Activity 13.9

Question 1.
Can you hear the sound of the scratching? Ask your friends around you if they were able to hear the same sound?

Yes, I can hear the sound of the scratch. But, the friends around me. are not able to hear the sound.

Question 12.
Can you say that the sound can travel through strings?

Yes, sound can travel through metallic string as it is a solid medium.

Question 13.
We have learned so far that vibrating objects produce sound and it is carried in all directions in a medium. How do we hear it?

The eardrums of our ears sense the vibrations of a vibrating object producing sound and send them to the inner ear. From there, the signal goes to the brain. This process is called hearing. This is how we hear the sound.

13.4 We Hear Sound through Our Ears Activity 13.10

Question 1.
Observe what happens to the grains. Why do the grains jump up and down?

The grains jump on the stretched. rubber up and down repeatedly. The grains are made to jump up and down because of the sound vibrations caused underneath the stretched rubber.

13.5 Amplitude, Time Period, and Frequency of a Vibration

Activity 13.11

Question 1.
Is the sound louder when the tumbler is struck hard?

Yes, the sound is louder, when the tumbler is struck hard.

Question 2.
In which case is the amplitude larger?

The amplitude of the sound is larger in the first case.

Question 14.
I wonder why my voice is different from that of my teacher.

The voice of a human being is determined by the amplitude of vibration and frequency of the voice. Amplitude determines loudness and frequency determines shrillness. Considering the two factors every voice differs in some way. That’s why Boojho’s voice is different from that of his teacher.

Question 15.
Every day you hear the voices of children and adults. Do you find any difference in their voices? Can you say that the frequency of the voice of a child is higher than that of an adult?

Yes, the sound of a child is different from that of an adult. Differences in the voice of a child of an adult :
(1) The child’s voice produces the sound of higher pitch, whereas an adult voice produces the sound of low pitch.

13.7 Noise and Music

Question 16.
We hear different types of sounds around us. Is the sound always pleasing? Does a sound sometimes cause discomfort to you?

No, sounds are not always pleasing. Some sounds are pleasant to ears, whereas some are not. Yes, the sound, sometimes causes discomfort, if it is not pleasing to the ear.

Question 17.
Are the sounds coming from the construction site pleasing?

No, the sounds coming from a construction site, are not pleasing. In fact, these sounds are very harsh to the ears.

Question 18.
Do you enjoy the sounds produced by the horns of buses and trucks?

No, the sound produced by buses and trucks is very unpleasant. No one can enjoy the sounds produced by these vehicles.

Question 19.
In a classroom, if all students speak together, what would the sound produced be called?

The sound produced in the classroom, when all students speak together, will be called noise.

Question 20.
But if the musical sound becomes too loud, would it remain melodious?

No, not at all.

13.8 Noise Pollution

Question 21.
Can you list some sources of noise pollution?

Some of the sources of noise pollution are :

1. The sound produced by the horn of buses and trucks.
2. The sound produced at the construction site.
3. The sound produced by the playing of T.V., radios, and loudspeakers at very high volume.
4. Bursting of crackers, and the sound of big machines in the factories, etc.

Question 22.
What sources in the home may lead to noise?

In-home, television, transistors, and music systems played at very high volume are some of the sources of noise. At the same time, some kitchen appliances, desert coolers, air conditioners, also contribute to the noise pollution in the homes.

Question 23.
What are the harms of noise pollution?

The noise pollution in the surroundings may cause many health problems. Lack of sleep, hypertension (high blood pressure), anxiety, and many more health disorders may be caused due to. noise pollution. A person, who is exposed to a loud sound continuously, may get temporary or even permanent deafness.

Question 24.
How can noise pollution be controlled in a residential area?

The following are the ways in which noise pollution can be controlled in a residential area :

1. The noisy operation must be conducted away from any residential area.
2. Noise producing industries should be set up away from residential areas.
3. The use of automobile horns should be minimized.
4. T.V. and music systems should be. played at lower volumes.
5. Plants and trees must be planted along the roads and around buildings to minimize the harmful effects of noise pollution.

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