RBSE Class 8 Science Important Questions Chapter 13 Sound

Rajasthan Board RBSE Class 8 Science Important Questions Chapter 13 Sound Questions and Answers.

RBSE Class 8 Science Chapter 13 Important Questions Sound

Objective Questions

Question 1.    
The "to and fro" or "back and forth" motion of an object is termed as
(a) amplitude    
(b) frequency
(c) vibration    
(d) wave
Answer:
(c) vibration

Question 2.    
In humans, the sound is produced by
(a) tongue    
(b) food pipe
(c) larynx    
(d) hands
Answer:
(c) larynx

Question 3.    
The length of larynx in men is approximately
(a) 5 mm    
(b) 10 mm
(c) 15 mm    
(d) 20 mm
Answer:
(d) 20 mm

Question 4.    
Which of the following is not the medium for soimd travel
(a) air    
(b) water
(c) vacuum    
(d) solids
Answer:
(c) vacuum 

Question 5.    
The loudness of sound in a busy traffic is
(a) 30 dB    
(b) 60 dB
(c) 70 dB    
(d) 90 dB
Answer:
(c) 70 dB    

Fill in the blanks

1. Sound is produced by a ...................... object.
Answer:
vibrating

2. If frequency of vibration is more, ...................... is more.
Answer:
pitch

3. No. of ...................... per second is called oscillation frequency.
Answer:
oscillations / vibrations

4. More the amplitude of vibration, more is ...................... of sound.
Answer:
the intensity

5. Absence of air is called ......................
Answer:
vacuum.

True/False

Read the following statements carefully and write 'T' for true and 'F' for false.

1. The sound cannot travel through a vacuum.
Answer:
True

2. Bird produces a sound with low pitch while lion's roar has high pitch.
Answer:
False

3. Frequency is measured in hertz.
Answer:
True

4. Noise pollution cannot be reduced by planting trees.
Answer:
False

5. Humans produce sound by vibrating objects.
Answer:
True

Match the words given in 'Column-A' with 'Column-B'

Question 1. 

Column-A	Column-B
(i) Normal breathing	(a) 10 dB
(ii) Normal Conservation	(b) 80 dB
(iii) Average factory	(c) 60 dB
(iv) Jet plane	(d) 150dB

Answer:

Column-A	Column-B
(i) Normal breathing	(a) 10 dB
(ii) Normal Conservation	(c) 60 dB
(iii) Average factory	(b) 80 dB
(iv) Jet plane	(d) 150dB

Question 2.    

Column-A	Column-B
(i) High amplitude	(a) Low sound
(ii) High frequency	(b) Strong sound
(iii) Low amplitude	(c) Sharp sound
(iv) Low frequency	(d) Weak sound

Answer:

Column-A	Column-B
(i) High amplitude	(d) Weak sound
(ii) High frequency	(c) Sharp sound
(iii) Low amplitude	(a) Low sound
(iv) Low frequency	(b) Strong sound

Question 3.    

Column-A	Column-B
(i) Noise	(a) Sound pleasing to ears
(ii) Ultrasonic sound	(b) Number of oscillations per second
(iii) Frequency	(c) Sound of frequency greater than 20 K Hz
(iv) Musical sound	(d) Unpleasant sound

Answer:

Column-A	Column-B
(i) Noise	(d) Unpleasant sound
(ii) Ultrasonic sound	(c) Sound of frequency greater than 20 K Hz
(iii) Frequency	(b) Number of oscillations per second
(iv) Musical sound	(a) Sound pleasing to ears

Very Short Answer Type Questions

Question 1. 
How is sound produced?
Answer:
Sound is produced due to vibration of objects.

Question 2. 
What is the unit of measurement of frequency?
Answer:
Unit of measurement of frequency is Hertz.

Question 3. 
What is the relation between time period and frequency?
Answer:
Time period x frequency = 1 

Question 4. 
Sound waves of frequency less than 20 Hz and frequency more than 20,000 Hz is called?
Answer:

• Infrasonic - less than 20 Hz 
• Ultrasonic - more than 20,000 Hz.

Question 5. 
Name the two animals that can respond to waves with frequency more than 20,000 Hz. 
Answer:
Dogs and Bats.

Question 6. 
What do you understand by amplitude of a wave?
Answer:
Due to travelling of wave, the maximum displacement of particles from its main position is called amplitude of wave.

Question 7. 
What do you understand by frequency of wave?
Answer:
Number of vibrations made by source in one second is called frequency.

Question 8.
What do you understand by loudness of sound?
Answer:
Loudness is that quality of sound which is proportional to the square of amplitude of sound. When amplitude is large, sound is said to be loud. When amplitude is small, the sound produced is feeble.

Question 9. 
What do you understand by pitch?
Answer:
Pitch is that property of sound due to which sound is either shrill or heavy.

Question 10. 
How do we determine the pitch of sound?
Answer:
Frequency decides the pitch of sound. If frequency of vibration is more, sound is shrill. If less, sound is heavy.

Short Answer Type Questions

Question 1. 
How does the bell of your school produce sound?
Answer:
When the bell is hit with a hammer, it vibrates. Sound is produced due to this vibration. We can feel this vibration by touching the bell. Due to the small amplitude, we are not able to see the vibration.

Question 2. 
Prove through an experiment that sound travels faster in liquids than in air. 
Answer:
Sound travels faster in liquids than in air. It can be proved by the following experiment. 
Experiment: Take 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 3. 
How does a disturbance produced due to sound, reaches our ears, through a medium?
Answer:
When a disturbance is produced in a medium due to sound, it produces some movement in the medium particles. This particles in turn produce movement in the adjacent particles. This activity is spread to all particles. These disturbances reach our ears.

Question 4. 
Explain oscillatory motion and frequency.
Answer:
Oscillatory motion: Moving an object to and fro repeatedly is called vibration. This movement is called oscillatory motion.

Frequency: Number of oscillations per second is called frequency. Its measured in hertz.

Question 5. 
What do you mean by loudness of sound? Explain.
Answer:
The loudness of sound is proportional to the square of amplitude of vibration, of the sound producing source. For example, if amplitude becomes double, loudness increases by four times. Loudness is measured in decibel (dB). Loudness of sound more than 80 dB is harmful for the body. When amplitude of a sound is more, sound is louder. If amplitude is small, sound is weaker.

Question 6. 
How does frequency affect sound? Write.
Answer:
Frequency is number of vibrations per second. It decides the pitch of the sound. If frequency of vibration is more, we say the sound is shrill. If frequency of vibration is less, the pitch is less. For instance, a drum has low frequency of vibration, therefore produces a low pitch sound. Whereas, a whistle's frequency is more and it thereby produces a high pitch sound. In the same way, birds produce a shrill sound whereas lion produces a low pitch sound. Although, a lion's roar is more intense than a bird's voice. Similarly, a woman's voice is more shrill and with more frequency than that of a male.

Question 7. 
Sound produced by a mosquito is different from that produced by a lion. Why? 
Answer:
Loudness of sound depends on amplitude of the source. Mosquito produces sound due to flapping of its wings, whereas lion's roar is the result of sound produced due to larynx present in its neck. Amplitude of mosquito's sound is small. Therefore, pitch and quality of both sounds are different, due to which, both sounds are different and differentiable.

Question 8. 
Sound is produced due to vibration, then why does an oscillating swing or pendulum not produce sound?
Answer:
We can only hear sound with frequency between 20 Hz - 20,000 Hz. This frequency range is called Audible Frequency Range. Frequency of oscillating swing or pendulum is much smaller than this and does not come under this range. Therefore, they do not produce sound waves.

Question 9. 
Explain the following:
(1) Infrasonic waves 
(2) Audible waves 
(3) Ultrasonic waves.
Answer:
(1) Infrasonic waves: Waves with frequency less than 20 Hz are called infrasonic waves.

(2) Audible waves: Waves whose sound can be heard by humans are called audible waves. Their frequency is between 20 - 20,000 Hz.

(3) Ultrasonic waves: Waves with frequency more than 20,000 Hz. Humans can not hear ultrasonic waves. Dog and Bat can hear ultrasonic sounds.

Question 10. 
Frequency of a tuning fock is 512 Hz. How many oscillations will it perform in 10 seconds and what is its time period?
Answer:
Frequency = 512 Hz.
No. of oscillations in 10s = 512 x 10 = 5120
Time period (T) = \(\frac{1}{512}\) second.

Question 11. 
Time period of a tuning fock is\( \frac{1}{40}\) s. Find the frequency of tuning fock.
Answer:
Frequency of tuning fock = \(\frac{1}{\text { Time period of tuning fock }}\)
= \(\frac{1}{1 / 40}\) = 40 Hz

Essay Type Questions

Question 1. 
Prove through an experiment that for transfer of sound, medium is required. 
Answer:
Sound needs a medium for propagation:
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 your friend to give a ring again. Listen to the ring while sucking air from the tumbler.

This shows that decreasing amount of air in the tumbler had something to do with the decreasing loudness of the ring. Indeed, if you had been able to suck all the air in the tumbler, you will not listen any sound. Actually, sound need a medium to travel. When air has been removed completely from a vessel, it is said that there is a vacuum in the vessel. The sound cannot travel through a vacuum. 

Question 2. 
Draw an structure of human ear and explain the process of hearing.
Or
Explain the process of hearing of sound with diagram of human ear.
Answer:
The outer part of our ear is like a funnel. When sound enters it, it passes through a tube whose one end has a thin tightly stretched membrane. This is called ear drum. When vibrations of sound, vibrates the ear drum, the ear drum sends the vibrations to the internal ear. From there, signals are sent to the brain. This way we hear sound. We should not put any pointed or sharp object in our ears. This can harm the ear drum.

Question 3. 
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.
Answer:
Noise pollution:
Excess or unwanted noise in the environment is called noise pollution. 

Harmful effects:
Noise pollution can produce many health related problems. Sleeplessness, hypertension (high B.P.), tension and many health related disorders can be produced due to noise pollution. Person constantly living in loud noises may lose hearing power temporarily or permanently. 

Methods to prevent noise pollution:

1. Radio, T.V., Taperecorder etc. should be kept at low volume.
2. Trees should be planted around houses and on streets.
3. Home appliances should have silencers.
4. One should be made aware of harmful effects of noise pollution.
5. Use of automobile horns should be minimised.