# PSEB 8th Class Science Solutions Chapter 13 Sound

Punjab State Board PSEB 8th Class Science Book Solutions Chapter 13 Sound Textbook Exercise Questions and Answers.

## PSEB Solutions for Class 8 Science Chapter 13 Sound

PSEB 8th Class Science Guide Sound Textbook Questions and Answers

Exercises

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.
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, 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 ………………….
(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 the 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.
Number of oscillations
Solution:
Frequency =
= $$\frac{40}{4}$$
= 10 Hz.
Time period = $$\frac{1}{\text { Frequency }}$$
= $$\frac{1}{10}$$
= 0.1 s.

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 ?
Solution:
Frequency 500 vibrations = 500 Hz
Time period = ?
We know, time period = $$\frac{1}{\text { Frequency }}$$
= $$\frac{1}{500}$$
= $$\frac{2 \times 1}{2 \times 500}$$
= $$\frac{2}{1000}$$
= 2 × 10-3 s

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

 Instrument Vibrating part (а) Dholak Stretched membrane (b) Sitar String (c) Flute Air column.

Question 8.
What is difference between noise and music ? Can music become noise sometime ?
Differences between noise and music:

 Noise Music 1. It is an unpleasant sound. 1. It is a pleasant sound. 2. It causes discomfort. 2. It has a soothing effect. 3. It can lead to health problems. 3. No health problems are associated with it.

Yes, music can become noise when music is too loud, then it becomes a noise.

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

1. Sounds of vehicles.
2. Loudspeakers.
3. Working Machines.
4. Bursting of crackers.
5. Desert coolers.
6. Radios and televisions at high volumes.
7. Kitchen appliances.
8. Hawkers.

Question 10.
Explain in what ways noise pollution is harmful to humans.
Harmful effects of noise pollution:

1. Lack of sleep.
2. Hypertension.
3. Anxiety.
4. Partial deafness.

Question 11.

1. Lot of noise due to passing vehicles.
2. Smoke and dust produced by running vehicles.
3. Sound of loud horns of vehicles at the time of traffic jams.

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

Function of Larynx.
When air passes through the vocal cords, they produce sound. Vocal cords may become loose/thick or tight/thin on vibration, thus causing different types of voices.

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 speed of light is 3 × 108 m/s while that of sound is only 340 m/s. So, lightning and thunder although taking place simultaneously in the sky at the same distance will be seen and heard at different intervals of time.

PSEB Solutions for Class 8 Science Sound Important Questions and Answers

Multiple Choice Questions

Question 1.
The unit of frequency is:
(a) dB
(6) Hz
(c) dB and Hz
(d) None of these.
(b) Hz.

Question 2.
The intensity of disagreeable sound for human ear is
(a) 60 dB
(b) 10 dB
(c) 90 dB
(d) 30 dB.
(c) 90 dB.

Question 3.
Ultrasonic sound is:
(a) Sound of frequency less than 20 Hz
(b) Sound of frequency more than 20 KHz
(c) Sound of frequency 20 Hz to 20000 Hz
(d) None of these.
(b) Sound of frequency more than 20 KHz.

Question 4.
Speed of sound at 20°C is approximately:
(a) 430 m/s
(b) 304 m/s
(c) 340 m/s
(d) 3400 m/s
(c) 340 m/s

Question 5.
The intensity of sound at ordinary inhale is:
(a) 10 dB
(b) 20 dB
(c) 60 dB
(d) 70 dB.
(a) 10 dB.

Question 1.
What is sound ?
Sound. It is a form of energy which produces in us the sensation of hearing.

Question 2.
How is sound produced ?
Sound is produced by vibrations of a body.

Question 3.
Will sound travel in vacuum ?
No, it will require some medium.

Question 4.
Does sound travel in gases ?
Yes.

Question 5.
Does sound travel in liquids ?
Yes.

Question 6.
Does sound travels in solids ?
Yes.

Question 7.
On what factor does loudness of sound depend ?
Loudness of sound depends upon the amplitude of vibrating body.

Question 8.
If an object makes 10 oscillations in a second, then what is its frequency ?
10 Hertz.

Question 9.
Will the sound travel faster, in wood or water ?
In solids, the sound travels faster than liquids. So sound will travel faster in wood than in water.

Question 10.
Sound is produced when objects …………………. .
Sound is produced when objects vibrate.

Question 11.
The number of oscillations per second is called …………………… .
The number of oscillations per second is called frequency.

Question 12.
We can respond to the frequency of sound more than ……………….. hertz and less than ……………… hertz.
We can respond to the frequency of sound more than 20 hertz and less than 20,000 hertz.

Question 13.
Name the section of throat in which the human voice is produced.
Larynx.

Question 14.
Name the characteristics of sound which help us to distinguish different sounds.
The characteristics of sound which distinguish different sounds. The pitch or loudness of the sound.

Question 15.
What is the audible range of human ear ?
Audible Range. The human ear responds to sounds having frequencies 20 hertz to 20,000 hertz.

Question 16.
Define the term pitch.
Pitch. A sensation depending upon the frequency is known as the pitch.

Question 17.
Which sound has higher frequency ? Sound produced by a buzzing mosquito or sound produced by roaring lion.
The frequency of sound produced by a buzzing mosquito will be more than the frequency of sound produced by a roaring lion.

Question 18.
Write the following frequencies in their increasing order:
(i) Voice of a child
(ii) Voice of man
(iii) Voice of a woman.
Voice in increasing order of their frequencies:
Voice of a man < Voice of a child < Voice of a woman.

Question 19.
How do we hear sound ?
When sound waves travelling through air strike our ears, diaphragm of ear starts vibrating. These vibrations reach the ear nerves by small bones and send the messages to our brain and which we hear.

Question 20.
What is length of vocal cords in man ?

Question 21.
Unpleasant sounds are called ………………. .
Noise.

Question 22.
Which sound is produced by musical instruments ?
Musical sound.

Question 23.
What is unit of loudness of sound ?
Decibel (dB).

Question 24.
Name an instrument with pleasant sound.
Harmonium/Guitar/Piano.

Question 25.
What is hearing impairment ?
The disability to hear sounds is hearing impairment.

Question 26.
What are causes of hearing impairment ?
Ear disease, injury, age and loud noise.

Question 27.
Give an example of loud noise.

1. Sound produced by machinery in a factory.
2. Loudspeakers at full volume.

Question 28.
At which unit, sound becomes harmful ?
More than 80 dB (Average factory range).

Question 29.
Which is the major cause of noise pollution ?
Vehicles.

Question 30.
Which natural organism is important to reduce noise pollution ?
Plants and trees.

Question 31.
What is noise pollution ?
Noise Pollution. Presence of excessive or unpleasant sound in the atmosphere is called, noise pollution.

Question 1.
Define the term time period, frequency and amplitude.
Time Period.
It is the time taken by a vibrating particle to complete one vibration.

Frequency.
The number of oscillations per second is called the frequency of the oscillation. Frequency is measured in hertz [Hz].

Amplitude.
The maximum distance through which a vibrating body is displaced from its central resting position, is called amplitude of oscillations.

Question 2.
One astronaut speaks with another astronaut on Moon. Can the other astronaut listen to the first astronaut ?
There is no atmosphere on the moon, thus, the astronauts cannot listen to each other. Therefore, a material medium is required for the propagation of sound.

Question 3.
Sound produced by a mosquito is quite different from the roar of a lion. Explain.
The loudness of sound depends upon the amplitude of the wave. A mosquito produces sound by the vibration of its wings, in open air while the lion roars by the vibration of its vocal chords. The amplitude of the sound produced by mosquito will be less than the sound produced by a lion. The pitch and quality of the two sounds is quite different, which makes the two sounds different and distinguishable.

Question 4.
Explain with simple experiment to show that sound propagates through solid substance.
Experiment.
Connect two empty match boxes by tying them with the two ends (15 to 20 m long). Request your friend to keep one match box close to his ear. By stretching the string speak into one match box. Sound will be heard very clearly by your friend. This shows that sound can propagate through solids.

Question 5.
How is the human voice produced ?
Production of Human voice.
The human voice is the result of vibrations. It is produced in the larynx, a section of the throat. Muscles in the larynx tighten the vocal cords. Air from the lungs rushes past the tight stretched cords and causes the vocal cords to vibrate. The vibrations produced in the vocal cords produce the sound or the voice.

Question 6.
What are ultrasounds ?
Ultrasound.
Our ear does not respond to sounds of frequencies less than 20 hertz or greater than 20,000 hertz. Sound of frequency greater than 20,000 hertz is called ultrasonic. The instrument used for producing ultrasonic sound is ultrasonic CT.

Question 7.
What are the uses of ultrasonic sound ?
Uses of ultrasonic sound:

1. Dogs can listen ultrasonic sounds. So to call their dogs people use ultrasonic sounds.
2. In medical science, ultrasonics are used for forming the images of internal organs of human body.

Question 8.
A boy claps his hands in front of stair-case and hears a musical sound. Explain.
The distance of each step of stair case increases from the boy. When a boy claps, sound will not strike all the steps simultaneously but it will strike in short and regular intervals. The reflected sound from them will be received by the ear in the form of a number of waves at regular intervals. Thus a periodic disturbance produces a musical sound.

Question 9.
Give an activity to prove that sound travels faster in water than in air.
Sound travels faster in water (liquids) than in air, we can prove this fact by the following activity.
Activity. Take a longer balloon and fill it with water. Hold it close to your ear and scratch it gently with index finger on the farther side of the balloon. A sound will be heard. Repeat this experiment with air-filled balloon. On comparing the two sounds, it is proved that sound travels faster in water than in air.

Question 10.
What is noise ? What is its unit ?
Noise.
The unpleasant sounds which are not soft and sweet are known as noise such as sound of machines, automobiles, crackers, etc.
The unit of voice is Decibel (dB).
Noise level is 0-120 dB.

Question 11.
State one difference between noise and a musical sound.
Noise is a sound, which produces disagreeable (jarring) effect on the ear. On the other hand, musical sound produces pleasing effect on the ear of the listener.

Question 12.
How do children with impaired hearing communicate ?
Children with impaired hearing communicate using sign language and with technically developed devices.

Question 13.
Identify the part which vibrates to produce sound in the following instruments ?

 Sr. No. Musical Instrument Sound producing part 1. Flute 2. Dholak

 Sr. No. Musical Instrument Sound producing part 1. Flute Air column 2. Dholak Stretched membrane

Question 1.
Give an activity to show that sound needs medium to be heard.
Sound needs medium.
Sound produced by a vibrating object reaches our ears due to vibrations of the molecules of the medium (air) in succession. If there is no air between the vibrating object and our ear, we would not hear any sound at all. We can study this by the following activity.

Activity.
Take a wooden stick and hold one end close to your ear. Ask your friend to scratch the other end gently, you will hear the sound.

This activity shows that sound can travel through wood. Sound can travel through liquids as well. You can check this by filling a balloon with water. Hold a water filled balloon close to you ear and scratch the opposite surface of the balloons with your finger.You will again hear sound. Now do the same experiment with an air-filled balloon. This time the sound heard is very feeble in comparison.
Thus, we conclude that sound requires a medium to be heard.

Question 2.
What is noise pollution ? What are its causes and its effects ?
Noise pollution.
The unwanted sound which is not soft and is disagreeable to the ear, is called noise. The presence of loud sound in atmosphere is only noise pollution.

Causes of noise pollution.

1. The loud sounds produced by machines in factories.
2. Loud speakers.
3. Generators.
4. Railway stations.
5. Air ports.
6. Music programmes.
7. Crackers.

Effects of noise pollution:

1. The worst effect of noise pollution is deafness.
2. It raises the heart beat.
3. It also effects the pupil of eyes, thus causing night blindness or colour blindness.

Question 3.
What is music? Name the various types of vibrating objects used in
different kinds of musical instruments.