# RBSE Solutions for Class 8 Science Chapter 10 Sound

## Rajasthan Board RBSE Class 8 Science Chapter 10 Sound

RBSE Solutions for Class 8 Science

### RBSE Class 8 Science Sound Intext Questions and Answers

Page No 108.

Question 1.
Do you know scientific cause of sound production ?
Scientific cause of sound production is the vibrations produced in objects.

Page No 109.

Question 2.
Clamp one end of rubber band on wall with nail and tense it. With the second hand, pull the rubber band from mid and release it. What does sound appear ?
Due to vibrations in the rubber band; sound is produced. When rubber is stretched and released, it vibrates up and down. This is known as Vibration.

Question 3.
If motion of rubber band Is stopped. Do you hear sound ?
When the motion of rubber band he stopped. then the vibration will also stop and hence no sound will be heard. Due to vibrations of objects sound is produced.

Question 4.
When we speak then sound generated from our vocal. How does vocal-sound generated?
Our larynx or voice box has a pair of membrane called vocal cords. Vocal-cords are stretched across the larynx in such a way that it leaves a narrow slit between them for the passage or air. When the air from the lungs is forced through the slit, it makes the vocal-cords to vibrate, and hence, generate sound,

Page No 110.

Question 5.
Does sound transmission In solids and liquids?
Yes, sound transmits in solids and liquids.

Pages No 111-112.

Question 6.
The data related to three vibrating objects are given in table. Fill in the blanks by make easy calculation.

1. Vibrations done in one second by A = $$\frac { 500 }{ 10 }$$ = 50
2. Vibrations done in one second by B = $$\frac { 400 }{ 40 }$$ = 40
3. Vibrations done in one second by C = $$\frac { 100 }{ 5 }$$ = 20

Hence, vibrations done by A are more and vibrations done by C are least.

Pages No 112-113.

Question 7.
Identify the group of soft and the loud sounds In given table.

Page No 114.

Question 8.
What do you call the sounds of frequency less than 20 Hz and more than 20,000 hertz.

1. Sound of frequency less than 20 Hz are called infrasonic sound.
2. Sound of frequency more than 20,000 Hz are called ultra sonic sound.

### RBSE Class 8 Science Sound Text Book Questions and Answers

Question 1.
In which medium, the transmission of sound is not possible’?
(a) iron rod
(b) water
(c) air
(d) vaccum
(d) vaccum

Question 2.
The up-down motion around the mean position of an object is called –
(a) vibration
(b) amplitude
(c) frequency
(d) time period
(a) vibration

Question 3.
Speed of sound at 0°C in air is –
(a) 350 m/sec
(b) 200 m/sec
(c) 400 m/sec
(d) 331 m/sec
(d) 331 m/sec

Question 4.
Time taken to complete one vibration is called –
(a) frequency
(b) time period
(c) amplitude
(d) None of these
(b) time period

In the footing statements, tick T against those which are true, and F against those which are false:

1. Sound generates due to vibration in objects. (True/False)
2. The sound waves do not need medium to Transmission. (True/False)
3. The sound has maximum velocity in solids. (True/False)
4. The Unit of loudness of sound is decibel (dB). (True/False)

1. True
2. False
3. True
4. True

Fill in the blanks with suitable words:

1. Main source of sound in human is …………………
2. The sound wave having frequency more than 20,000 hertz is called …………………
3. The international unit of frequency is …………………
4. The loudness of sound depends on …………………
5. The pitch of sound depends on …………………

1. vocal cords
2. ultrasonic
3. vibrations per second or hertz
4. Amplitude
5. frequency of sound.

Question 1.
A musical instruments takes 2 seconds to complete 200 oscillations then calculate frequency of it ?

= $$\frac { 200 }{ 2 }$$
= 100 vibrations per second
= 100 Hz

Question 2.
The ringing bell in a temple has frequency 400 vibration, then calculate the time period of it.

= 0.0025 second
The Time period is 0.0025 seconds

Question 3.
What are differences in audible infra – audible and ultra-audible.

1. Infra – audible sound – Waves with frequency less than 20 Hz are called infra- audible sound.
2. Audible sound – Waves whose sound can be heard by humans are called audible waves. Their frequency is between 20-20,000 Hz.
3. Ultra-audible sound – Sounds with frequency more than 20.000 Hz. Humans can not hear ultrasonic sounds. Dog and Bat can hear ultra-audible sounds.

Question 4.
Define frequency and periodic time? Represent the relation of them by a formula.
Frequency – The number of vibrations or oscillations per second is called Frequency. The unit of frequency is vibrations or oscillations per second or hertz (Hz). Time Period – Time required to produce one complete oscillation or vibration is called Time Period. Its unit is second.

Frequency and Time Period are reciprocal of each other.

Question 1.
Draw an human vocal system diagram and explain its working principle.
The larynx or voice of humans has a pair of membranes called the vocal cords. Our vocal cords are natural musical instrument.

The two vocal cords are stretched on both sides of larynx in such a way that there is a small slit between them for the passage of air. When the lungs force air, through the slit, the vocal cords vibrate, producing sound. When the vocal cords are tight and thin. the type or quality of voice is different from when they are loose and thick.

Question 2.
What is noise pollution ? How does ¡t affect us ? How can ¡t controlled ? Explain it in detail.
(OR)
What do you understand by sound pollution? Write four measures to limit the sound (or noise) pollution.
Noise Pollution:
Any unpleasant, undesirable or loud sound is referred as noise pollution. In sound pollution, the loudness of the sound is above 80 dB. Affects of Noise Pollution – Noise pollution is caused due to the regular sound produced by motor vehicles, machines, transportation systems, aircrafts, trains. loudspeakers, etc. Noise pollution affects our health adversely. It causes hypertension high stress levels. sleep disturbances, irritability, headache, hearing problems. partial or full deafness.

Measures to Control Noise Pollution:

1. Using noise reducing devices like silencers in machines, motor vehicles and home appliances.
2. Activities involving noise should be operated away from residential areas.
3. Volume of television and loudspeakers should he kept low.
4. Plant trees near houses and on both the sides of roads to absorb sound.

Question 3.
Explain the process of receiving the sound signals to brain with labelled diagram of human ear.
The external part of human ear is funnel shaped. It is also known as Pinna. Sound funnels through the pinna into the external auditory canal, a short tube that ends at the eardrum. Due to vibrations of sound waves, the eardrum vibrates. The eardrum sends the sound towards the internal ear. From here the signal of sound are sent to the brain via sound towards the internal ear. From here the signal of sound are sent to the brain via auditory nerve. This is how sound is heard.

Question 4.
How does sound Transmission in solid, liquid and gases ? Explain it.
Propagation of Sound in Solid:
Sound travels in solids through the solid particles and reaches from one end to the other end. Propagation of Sound in Liquid : In liquids, sound travels through the liquid particles.

Propagation of Sound in Gases (Air):
In gases (air), the sound travels through vibrations. When the object vibrates, then air near it also vibrates. Every vibrating particle replaces the other particle. This is how sound travels in gases (air).

### RBSE Class 8 Science Sound Important Questions and Answers

Objective Questions:

Question 1.
The speed of sound in air at 0°C is –
(a) 0
(b) 41 m/s
(c) 331 m/s
(d) 250 m/s
(c) 331 m/s

Question 2.
The unit of Time Period is –
(a) Hour
(b) Second
(c) Minute
(d) None of these
(b) Second

Question 3.
The time to complete one vibration is called –
(a) Time Period
(b) Frequency
(c) Amplitude
(d) All of the above
(a) Time Period

Question 4.
The unit of loudness of sound is –
(a) Kilometer
(b) Decibel
(c) Meter
(d) Degree
(b) Decibel

Question 5.
Which of the following organism can hear ultrasonic sound –
(a) Dog
(b) Bat
(c) Whale
(d) All of the above
(d) All of the above

Question 6.
Among the following sound can not propagate through –
(a) Iron rod
(b) Water
(c) Air
(d) Vacuum
(d) Vacuum

Question 7.
To and fro motion from mean position of any object or particle is called as—
(a) Vibration
(b) Amplitude
(c) frequency
(d) Time period
(a) Vibration

Question 8.
In humans, the sound is produced by –
(a) longue
(b) food pipe
(c) larynx
(d) hands
(c) larynx

Question 9.
The length of larynx in men is approximately –
(a) 5 mm
(b) 10 mm
(c) 15 mm
(d) 20 mm
(d) 20 mm

Question 10.
Which of the following is not the medium for sound travel –
(a) air
(b) water
(c) vacuum
(d) solids
(c) vacuum

Question 11.
The loudness of sound in a busy traffic is –
(a) 30 dB
(b) 60 dB
(c) 70 dB
(d) 90 dB
(c) 70 dB

Question 12.
In which medium highest –
(a) In liquid
(b) In vacuum
(c) In solid
(d) In gas
(c) In solid

Fill In the blanks:

1. For the propagation of sound …………………… is required.
2. Sound can not travel in ……………………
3. Time period and frequency are …………………… to each other.
4. Pitch of sound depends upon …………………… of sound.
5. Our vocal cords are natural ……………………
6. The unit of frequency is ……………………
7. The unwanted sound is called ……………………
8. Shrillness of a sound is determined by the …………………… of vibrations.

1. medium
2. vaccum
3. Reciprocal
4. frequency
5. musical instruments
6. Hertz
7. noise
8. frequency.

Math the Column –

Question 1.
Match the following correctly:

1. (ii)
2. (i)
3. (iii)
4. (iv)

In the following statements, tick T against those which are true, and F against those which are false:

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

1. T
2. F
3. F
4. T
5. F
6. F
7. T

Question 1.
What is vibration motion ?
The movement of any particle in to and fro or up and down from its mean position is called vibration. The propagation of sound in object is called vibration motion.

Question 2.
How is sound produced?
Sound is produced due to the vibration in objects.

Question 3.
What is meant by vocal cords?
A pair of membranes present in the larynx or voice box are called vocal cords.

Question 4.
What are natural music instrument?
Our vocal cords are the natural music instruments.

Question 5.
In which medium does sound propagates?
Sound propagates in all the three medium i.e. solid, liquid and gas.

Question 6.
Why the astronauts are not able to talk with each other on moon ?
Due to the absence of air (atmosphere) on the moon, the astronauts are unable to talk on moon.

Question 7.
What is amplitude ?
The maximum displacement of the vibrating object from the equilibrium position.

Question 8.
What is Frequency ?
Frequency:
The number of vibrations or oscillations per second is called Frequency.

Question 9.
Name the unit of frequency.
Vibrations per second or Hertz (Hz).

Question 10.
What is Time Period ? Write its unit.
Time Period:
Time required to complete one vibration or oscillation is called Time Period. The unit of time period is ‘‘seconds”.

Question 11.
What is the relationship between frequency and time period ?
Time period and frequency are inversely proportional to each other.

Question 12.
The loudness of sound depends on what?
Loudness of the sound depends upon its amplitude.

Question 13.
What is the unit to measure loudness of sound?
Loudness of the sound is measured in decibel (dB).

Question 14.
What is meant by pitch of sound?
The characteristic of shrillness or flatness of sound is called the pitch. Pitch depends upon the frequency of sound.

Question 15.
Why the voice of women and children is shrill and fine ?
Due to more pitch their voice is shrill and fine.

Question 16.
Which pitched sound has minimum frequency?
The sound of lower pitch has minimum frequency.

Question 17.
What is range of human hearing?
Our ear can hear sounds of frequencies between 20 Hz to 20,000 Hz. This is called audible range.

Question 18.
What is audible sound?
We can hear sound of frequency 20 Hz to 20,001 Hz which is known as audible sound.

Question 19.
What is infrasonic sound?
Sounds with frequency less than 20 Hz are called infrasonic sound.

Question 20.
What is ultrasonic sound ?
Sounds with frequency more than 20,000 Hz are called ultrasonic sound.

Question 21.
Name the instrument used to measure the depth of sea and to know the position of submarine.
The instrument named ‘SONAR’ is used.

Question 22.
How do our ears recognized a sound?
Sound recognition is done due to loudness, pitch and quality of sound.

Question 23.
How does pitch affect sound ?
Due to high pitch. females and children have sharp and thin voice in comparison to males.

Question 24.
What type of sound waves does a bat produce ?
Ultrasonic.

Question 1.
Separate the following sounds ¡n strong and weak sounds and tabulate them – Ghungharu, Flute (Bansuri), Drum, Roar of lion

Question 2.
Observe the given table and complete

Question 3.
How sound is produce? Explain with the help of an activity.
When any object vibrates then due to this vibration sound is produce. It can be explained from the following activity.

Activity –
Cut out two rectangular rubber strips of size 4 cm long x 3 cm breadth of balloon. Now placed them one over the other and stretch with your both hands. Now try to produce sound by bIowing – air between them with your mouth, On blowing, the strips open and close repeatedly and a sound is produced.

Question 4.
Draw the diagram of closed vocal system and open ocal system.

Question 5.
Explain the propagation of sound in solids with an activity.
Make a hole in the middle of two blank match-boxes internal parts. Pass the cotton thread through each of the holes and tie a knot at its end by sticks. Two friends. holding each match-box and move away from each other. Ask YOU friend to put the match-box on his ear. Speak in the match box you are holding. On speaking the sound is heard. This proves that sound travels in solid.

Question 6.

1. The speed of sound is maximum and minimum ¡n which medium?
2. What is the speed of sound at 0°C in air?
3. An astronaut is unable to talk to another astronaut on moon. Why?

1. The speed of sound is maximum in solids, then in liquids and minimum in gases.
2. At 0°C, the spread of sound in air is 331 m/s.
3. Due lo the absence of air on moon, one astronaut is unable lo talk to the other astronaut.

Question 7.
Explain the completion of one vibration by an object diagrammatically.
According to figure, an object vibrating from its mean position O goes to point A of maximum displacement position, again it comes to O and goes downward points B of maximum displacement position, again comes upward up to point O. Thus it completes one vibration. We can express it as

Question 8.
Give reasons for the following –

1. Sound of sitar feels more musical than sound of dholak.
2. Sound of kuckoo is sweet than a crow.

1. The pitch of sitar is more than dholak due to which it appeals more musical.
2. The pitch of kuckoo is more than that of a crow that is why sound of kuckoo is more pleasant than that of a crow.

Question 9.
Write two important uses of ultrasonic sound.

1. Ultrasonic sound is used in diagnosis of diseases through ultrasonically.
2. Ultrasonic sound Is used In an equipment named SONAR which measures the depth of the sea and the status of submarines.

Question 10.
Prove through an experiment that sound travels faster in liquids than In air.
Sound travels faster pin liquids than In air. It can be proved by the following experiment. Experiment – like a long balloon fill It with water. Put It near your ear and scratch It with your finger on the other side. A sound will be produced. Repeat the same activity with an air filled balloon. Comparison of both sounds will decide that sound travels faster In liquids.

Question 11.
Identify the part which vibrates to produce sound In the following Instruments:
(a) Dhalak
(b) Sitar
(c) flute.

Question 12.
What is the difference between noise and music ? Can music become noise sometimes?
Yes, if the intensity of music becomes very high, then music also appears to be noise.

Question 13.
List sources of noise pollution in your surroundings.

1. Sounds of vehicles
2. Loudspeaker
3. Machines
4. Firecrackers
6. Few divices in the kitchen
7. Cooler
8. Air Conditioner
9. Washing Machine
10. Vacuum Cleaner
11. Telephone
12. Typewriter
13. Vendors
14. Rallies
15. Festivals
16. Social Programmes like Marriages
17. Religious programmes
18. Aeroplanes takings off at the airport.

Question 14.
Explain in what way noise pollution is harmful to humans.
Noise pollution is harmful to humans in the following ways –

1. Noise pollution causes lack of sleep.
2. It can cause high blood pressure.
3. The capacity to hear can be temporarily or permanently reduced.
4. It can cause tension like problems in brain.

Question 15.
If a Ghunghuru vibrate 400 times in 10 seconds, find out its frequency.
Given
1. Total number of vibrations = 400 vibrations
2. Time taken = 10 seconds

Frequency = 40 oscillations per second.
(or)
40 hertz.

Question 16.
A musical instrument takes 4 secs. to complete 400 oscillations. Calculate the frequency of it.
Given, No. of oscillations = 400
Time taken = 4 secs.

Frequency = oscillations per sec.
Frequency = 100 oscillations per second
(or)
100 hertz.

Question 17.
If the sound produced by an school bell has frequency 500 Hz, then calculate Its time period.
Given,
Frequency = 500

time period = $$\frac { 1 }{ 500 }$$
= 0:002 secs.

Question 18.
A sitar takes 2 second to complete 300 oscillations then calculate its frequency.
Given,
No. of Oscillations = 300
Time taken = 2 secs.

Frequency = $$\frac { 300 }{ 2 }$$
Frequency = 150 Vibrations per
sec. or hertz

Question 1.
Define amplitude, frequency and time period. Give relation between frequency and Time Period.
Amplitude:
The maximum displacement of the vibrating object from the equilibrium position.

Frequency:
The number of vibrations per second is known as frequency. The unit of frequency is vibrations per second or hertz (Hz).

Time Period:
Time required to complete one vibration is called Time Period. The unit of time period is second.

Relation between Frequency and Time Period:
Frequency and Time Period are reciprocal of each other.

Question 2.
What is the pitch of a sound ?
The sensation of a frequency is commonly deterred to as the pitch of a sound. The texture of sound whether it is thin or heavy is known as Pitch of the Sound. The pitch of the sound depends upon the frequency of sound wave.

Question 3.
How is the loudness of sound related to the amplitude?
Loudness of the sound depends on the amplitude of the sound wave. This means greater amplitude produces louder sound. When we speak softly, then less or low intensity sound is produced but when we speak loudly, then high intensity sound is generated.

Question 4.
How does a bat fly in night?
Bats produce ultrasonic sounds and can listen the sound returning back after hftting the objects; duc lo which they know about the hindrances. That is the reason that why bats can fly in night.

Question 5.
What is the unit to measure loudness of sound ? Write the loudness of various sounds.
Loudness of a sound is measured in decibel (dB). Normal talking sound is approximately 60 dB. Scooter, buses, trucks, etc. produce 90 dB sound, whereas jets produce 150 dB sound. Rockets produce sound of 180 dB. Sounds above 80 dB can harm humans,

Question 6.
What do you understand by intensity of sound ? What does it depend on ?
Intensity of sound is the approximate sensation of our ears. Intensity of sound and energy of a sound wave depend on the sensation of our ears. Same sound can be more intensified for a person and can be less for another person.

Question 7.
Prove through an experiment that for transfer of sound, medium is required.
An electric bell, with the help of a rubber band is hanged in an air tight glass bell jar. The bell jar is connectèd to a vacuum pump to pump out air. Initially bell jar is filled with air. When bell is switched on, the bell rings and we can hear its sound. We now switch on vacuum pump and pump all air out slowly. As more and more air is pumped out, sound starts getting diminished. When all air is pumped out, we can not hear sound al all. Therefore, for sound waves to travel, medium is required.

Question 8.
What is noise pollution ? What are its harms ? What are the ways to prevent it?
(OR)
What do we call noise pollution ? Write down harms caused by it