# MP Board Class 8th Science Solutions Chapter 13 Sound

## MP Board Class 8th Science Solutions Chapter 13 Sound

### MP Board Class 8th Science Sound NCERT Textbook Exercises

Question 1.
Choose the correct answer 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 which of the following is likely to have minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman.
(a) Baby girl

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

1. Sound cannot travel in vacuum.
2. The number of oscillations per second of a vibrating object is called its time period.
3. If the amplitude of vibration is large, sound is feeble.
4. For human ears, the audible range is 20 Hz to 20,000 Hz.
5. The lower the frequency of vibration, the higher is the pitch.
6. Unwanted or unpleasant sound is termed as music.
7. Noise pollution may cause partial hearing impairment.

1. True
2. False
3. False
4. True
5. False
6. False
7. 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 (Hz)
(d) noise
(e) frequency

Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Time taken by pendulum to complete 40 oscillations = 4 seconds
Time taken by pendulum to complete 1 oscillation
= $$\frac{4}{40}$$
= $$\frac{1}{\mathrm{1} 0} = 0.1 \mathrm{sec}$$
Time period of pendulum = 0.1 sec
∴ Frequency of pendulum
$$\frac{\text { Oscillation }}{\text { Time }}$$ $$\frac{40}{4}$$
= 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?
Time period is the time taken to complete one oscillation.

Question 7.
Identify the part which vibrates to produce sound in the following
instruments:
(i) Dholak (ii) Sitar (iii) Flute.
(i) Stretched membrane vibrates to produce sound in dholak.
(ii) Stretched string vibrates to produce sound in sitar.
(iii) Air-column vibrates to produce sound in flute.

Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Regular vibrations of sound produce music. Music has pleasant sound. Irregular vibrations of sound produce noise.. Noise has unpleasant sound. Sometimes music can become noise. Loud music would become noise. Music at high volume causes noise pollution. It increases nervous tension. It can lead to high blood pressure and heart problems.

Question 9.
List sources of noise pollution in your surroundings.
Sources of noise pollution in our surroundings are:

• Barking dogs.
• Car horns.
• Loud music of transistor, T.Vs. and loudspeakers.
• Household noise and kitchen appliances.
• Desert coolers and air conditioners.
• Construction of buildings.
• Sound of machines if some industry is located near the residential area.

Question 10.
Explain in what way noise pollution is harmful to humans?
Noise pollution may cause many health related problems. For example:

1. Lack of sleep.
2. Hypertension (high blood pressure)
3. Temporary or even permanent deafness if a person is exposed to a loud sound of more than 70 decibles continuously.

Question 11.
I would suggest my parents to buy a house which is three lanes away from the roadside because it will involve less pollution of air and noise. A house near roadside will have more pollution due to vehicles. Road traffic noise is harmful for health of the people living in that area.

Question 12.
Sketch larynx and explain its function in your own words.
Larynx is also called the voice box. As the name suggests, it causes (sound) voice in humans It has vocal cords, which have air column vibrating in them, which causes sound in humans.

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 108 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.

### MP Board Class 8th Science Sound NCERT Extended Learning – Activities

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.
Some musical instruments are: tabla, guitar, harmonium, piano, banjo, mridangum, drum, manjira, sitar, etc.

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

Question 3.
Prepare a list of famous Indian musicians and the instruments they play.
Do yourself.

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 (Fig. 13.1). 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?

Do yourself.

Question 5.
Make two toy telephones. Use them as shown in Fig. 13.2. 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.
Do yourself.

Question 6.
Identify the sources of noise pollution in your locality. Discuss with your parents, friends and neighbors. Suggest how to control noise pollution. Prepare a brief report and present it in the class.

Vehicles, barking dogs, high volume television, etc. Noise pollution can be minimized by using silencing devices in transport vehicles, industrial machines and home appiliances.

### MP Board Class 8th Science Sound NCERT Additional Important Questions

Question l.
Define amplitude.
It is the maximum displacement of a vibrating body on one side.

Question 2.
Define frequency.
It is the number of vibrations produced by a vibrating body in one second.

Question 3.
What are ultrasonic waves?
It is the sound of frequencies more than 20000 hertz. We cannot hear such sounds.

B. Long Short Answer Type Questions

Question 4.
Define Hertz.
Frequency of a vibration or oscillation is measured in hertz. It is the unit of frequency.

Question 5.
Which nerve sends the signal to the brain?
The auditory nerve sends the signal to the brain.

Question 6.
What is reflection of sound?
When the sound waves strike against solid surface, they change its direction, this is called reflection of sound. in air (332 m/sec). Thus, the sound reaches through the iron rail earlier than through air. So, to hear a train approaching from far away people put their ears to the track.

Question 7.
What are the musical instruments?
Musical instruments are the devices which produce various sounds which are pleasant for our ears and produce soothing effects. The sound produced by these instruments is called music.

Question 8.
What distinguishes the sounds produced by different human beings from each other?
Vocal cords in humans are the sound producing parts. When they are vibrated they produce sound. They are held by the muscles and the tightness with which they are held determines the quality of sound. The length of the vocal cords also determines the quality of voice.

Question 9.
If you want to hear a train approaching when it is still far away why is it more successful if you pull ears close to the railway track?
As we know that the velocity of sound in iron is more (5100 m/sec) than in air (332 m/sec). Thus, the sound reaches through the iron rail earlier than through air. So, to hear a train approaching from far away people put their ears to the track.

Question 10.
What is an echo? What are conditions necessary for echo formation?
An echo is a reflected sound. The human ear can hear two sounds separately and distinctly only if there is an interval of 1/15 of a second between two. Since the speed of the sound in air is about 330 m / s, the distance traveled by sound in l/15th of a second is,
Distance = speed × time
= 330 m/s × 1/15 s
= 22 meter or m.
Therefore, the distance between the sources of a sound and the reflecting surface should be 22/2 = 11 meters.