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VIII SCIENCE SOUND EVALUATION QUESTION ANSWERS
SOUND
TEXTBOOK EXERCISES:
I. Choose the best answer.
1. Sound waves travel very fast in
a) air
b) metals
c) vacuum
d) liquids
Ans: b) metals
2. Which of the following are the characteristics of vibrations?
i. Frequency ii. Time period iii. Pitch iv. Loudness
a) i and ii
b) ii and iii
c) iii and iv
d) i and iv
Ans: a) i and ii
3. The amplitude of the sound wave decides its
a) speed
b) pitch
c) loudness
d) frequency
Ans: c) loudness
4. What kind of musical instrument is a sitar?
a) String instrument
b) Percussion instrument
c) Wind instrument
d) None of these
Ans: a) String instrument
5. Find the odd one out.
a) Harmonium
b) Flute
c) Nadaswaram
d) Violin
Ans: d) Violin
6. Noise is produced by
a) vibrations with high frequency.
b) regular vibrations.
c) regular and periodic vibrations.
d) irregular and non-periodic vibrations.
Ans: d) irregular and non-periodic vibrations.
7. The range of audible frequency for the human ear is
a) 2 Hz to 2000 Hz
b) 20 Hz to 2000 Hz
c) 20 Hz to 20000 Hz
d) 200 Hz to 20000 Hz
Ans: c) 20 Hz to 20000 Hz
8. If the amplitude and frequency of a sound wave are increased, which of the following is true?
a) Loudness increases and pitch is higher.
b) Loudness increases and pitch is unchanged.
c) Loudness increases and pitch is lower.
d) Loudness decreases and pitch is lower.
Ans: a) Loudness increases and pitch is higher.
9. Which of the following may be caused by noise?
a) Irritation
b) Stress
c) Nervousness
d) All the above
Ans: d) All the above
II. Fill in the blanks.
1. Sound is produced by __________.
Ans: vibrating bodies
2. The vibrations of a simple pendulum are also known as __________.
Ans: oscillation
3. Sound travels in the form of _________.
Ans: mechanical wave
4. High frequency sounds that cannot be heard by you are called _________.
Ans: ultrasonics
5. Pitch of a sound depends on the ________ vibration.
Ans: frequency of the
6. If the thickness of a vibrating string is increased, its pitch __________.
Ans: decreases
III. Match the following.
Ans:
IV. Consider the statements given below and choose the correct option.
1. Assertion: When lightning strikes, the sound is heard a little after the flash is seen.
Reason: The velocity of light is greater than that of the sound.
2. Assertion: Two persons on the surface of moon cannot talk to each other.
Reason: There is no atmosphere on moon.
A. Both assertion and reason are true and reason is the correct explanation of assertion.
B. Both assertion and reason are true but reason is not the correct explanation of assertion.
C. Assertion is true but reason is false.
D. Assertion is false but reason is true.
E. Both Assertion and reason are false.
Ans: A. Both assertion and reason are true and reason is the correct explanation of assertion.
V. Answer briefly.
1. What is vibration?
Vibration means a kind of rapid to and fro motion of a particle. This to and fro motion of the particle causes the substances around it to vibrate.
2. Give an example to show that light travels faster than sound?
Lightning. The reason we see a flash of lightning before hearing thunder is because light travels faster than sound. In the air, sound travels at around 332 metres per second. The light travels about 300000 kilometres per second.
3. To increase loudness of sound by four times, how much should the amplitude of vibration be changed?
To increase loudness of sound by four times, the amplitude of vibration to be increased by four times
4. What is an ultrasonic sound?
A sound with a frequency greater than 20000 Hz is called an ultrasonic sound.
5. Give two differences between music and noise.
6. What are the hazards of noise pollution?
• Noise may cause irritation, stress, nervousness and headache.
• Long term exposure to noise may change the sleeping pattern of a person
• Sudden exposure to louder noise may cause heart attack and unconsciousness.
7. Mention few measures to be taken to reduce the effect of noise pollution.
• People should be encouraged to refrain from excessive honking while driving.
• Industrial machines and home appliances should be properly maintained.
8. Define the following terms.
a. Amplitude b. Loudness
a. Amplitude
The measure of a sound wave is called amplitude. It is a distance between the resting position and the maximum displacement of the wave.
b. Loudness:
It is defined as the characteristic of a sound that enables us to distinguish a weak or feeble sound from a loud sound. The loudness of a sound depends on its amplitude.
9. How does planting trees help in reducing noise pollution?
Planting trees reduces noise pollution through a phenomenon called sound attenuation, which is the reduction of sound intensity.
Leaves, twigs, and branches of trees, shrubs absorb and deflect sound energy. Refraction of sound waves occurs when sound passes through vegetative barriers and bends around plants.
VI. Answer in detail.
1. Describe an experiment to show that sound cannot travel through vacuum.
Aim:
To determine that, the sound can not travel through vacuum.
Required materials:
Bell jar, Mobile phone and Vacuum pump.
Procedure:
Take a bell jar and a mobile phone.
Switch on the music in the mobile phone and place it in the jar.
Now, pump out the air from the bell jar using a vacuum pump.
As more and more air is removed from the jar, the sound from the mobile phone becomes feebler and finally, very faint.
Result:
It is clear from this experiment that sound cannot travel in vacuum and it needs a medium like air.
2. What are the properties of sound?
All sounds that we hear are not the same. There are some properties that differentiate one kind of sound from another.
Loudness:
•It is defined as the characteristic of a sound that enables us to distinguish a weak or feeble sound from a loud sound.
•The loudness of a sound depends on its amplitude. Higher the amplitude louder will be the sound and vice versa.
•When a drum is softly beaten, a weak sound is produced. However, when it is beaten strongly, a loud sound is produced.
•The unit of loudness of sound is decibel (dB).
Pitch:
•The pitch is the characteristic of sound that enables us to distinguish between a flat sound and a shrill sound.
•Higher the frequency of sound, higher will be the pitch. High pitch adds shrillness to a sound.
•The sound produced by a whistle, a bell, a flute and a violin are high pitch sounds.
Quality or Timbre:
•The quality or timbre is the characteristic of sound that enables us to distinguish between two sounds that have the same pitch and amplitude.
•For example in an orchestra, the sounds produced by some musical instruments may have the same pitch and loudness.
3. What steps should be taken to reduce the effect of noise pollution?
Noise pollution can be significantly reduced by adopting the following steps.
• Strict guidelines should be set for the use of loudspeakers on social, religious and political occasions.
• All automobiles should have effective silencers.
• People should be encouraged to refrain from excessive honking while driving.
• Industrial machines and home appliances should be properly maintained
• All communication systems must be operated in low volumes.
• Residential areas should be free from heavy vehicles.
• Green corridor belts should be set up around the industries as per the regulations of the pollution control board.
• People working in noisy factories should wear ear plugs.
• People should be encouraged to plant trees and use absorbing materials like curtains and cushions in their home.
4. Describe the structure and function of the human ear?
•Ear is the important organ for all animals to hear a sound.
•We are able to hear sound through our ears.
•Human ear picks up and interprets high frequency vibrations of air.
•Ears of aquatic animals are designed to pick up high frequency vibrations in water.
•The outer and visible part of the human ear is called pinna (curved in shape).
•It is specially designed to gather sound from the environment, which then reaches the ear drum (tympanic membrane) through the ear canal.
•When the sound wave strikes the drum, the ossicles move inward and outward to create the vibrations.
•These vibrations are then picked up by special types of cells in the inner ear.
•From the inner ear the vibrations are sent to the brain in the form of signals.
•The brain perceives these signals as sounds.
VII. Problems.
1. Ruthvik and Ruha hear a gunshot 2 second after it is fired. How far away from the gun they are standing? (Speed of sound in air is equal to 330ms-1)
Solution:
Given,
Speed of the sound in air v = 331ms-1
Time t = 2 seconds
Distance = ?
Distance = Time X Speed of sound
= 2s X 331ms-1
= 662 m
They are 662m away from gunshot.
2. A sound wave travels 2000 m in 8 s. What is the velocity of the sound?
Solution:
Given,
Time, t = 8s
Distance = 2000m
Distance
Speed of sound = --------
Time
2000m
= -------
8s
= 250ms-1
Speed of sound=250ms-1
3. A wave with a frequency of 500 Hz is traveling at a speed of 200 ms-1. What is the wavelength?
Solution:
Given,
Frequency n = 500 Hz,
Speed v = 200 ms−1
λ = ?
v = nλ
λ =v / n
λ = 200 ms−1 / 500 Hz
= 0.4m
Wavelength=0.4m
