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CBSE Questions for Class 8 Physics Sound Quiz 2 - MCQExams.com
CBSE
Class 8 Physics
Sound
Quiz 2
Time taken to complete 6 successive compressions if the time between two successive compressions of a wave is 4 sec is :
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20 s
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50 s
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70 s
0%
15 s
Explanation
The time period of a longitudinal wave is defined as the time between two successive compressions or rarefactions.
As given the time between two successive compressions is 4s,
therefore time period of the wave will be $$T=4 \ s $$.
The time between 6 successive compressions will be equal to 5 time periods,
i.e. $$=5\times4=20s$$
An object attached to one end of a spring makes 20 vibrations in 10s. Its frequency is
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2Hz
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10s
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0.05Hz
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2s
Explanation
Answer is A.
The frequency (f) of a wave is the number of full wave forms generated per second. This is the same as the number of repetitions per second or the number of oscillations per second.
In this case, an object attached to one end of a spring makes 20 vibrations in 10 s. That is, 20 vibration in 10 seconds. S in one second it makes 2 vibrations.
Therefore, the frequency of the object is 2 Hertz.
Fill in the blank.
The time taken by ___ oscillation to complete is called time period.
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One
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Two
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Three
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Four
Explanation
Time period can be defined as time to complete one full oscillation. Hence A is the correct answer.
The number of cycles an oscillator completes in each second is called its _______________.
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Frequency
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Velocity
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Amplitude
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Angular velocity
Explanation
The number of cycles an oscillator completes in each second is called its frequency.
The frequency (f) of a wave is the number of full wave forms generated per second. This is the same as the number of repetitions per second or the number of oscillations per second.
A person is listening to a tone 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between the successive compression from the source ?
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5 milli second
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1 milli second
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2 milli second
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2 second
Explanation
Given - $$ n=500Hz$$ ,
the time interval between two successive compressions or rarefactions is called , time period of wave , which is given by ,
$$T=1/n=1/500=0.002s=2millisecond$$
What is the time period of a wave having frequency of $$50 \ Hz$$?
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$$0.02 \ s$$
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$$0.20 \ s$$
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$$2.0 \ s$$
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$$20 \ s$$
Explanation
$$\text {Time period} = \dfrac{1}{\text{frequency}}= \dfrac{1}{50}=\ 0.02 \ s$$
Vocal cords in men are about ____ long.
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20 mm
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20 cm
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20 m
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15 cm
Explanation
The vocal cords in men are of 20 mm of length.
Some animals get disturbed and start running here and there just before the earthquakes occur
because they can hear______________?
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infrasonic sounds
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ultrasonic sounds
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audible sounds
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none of these
Explanation
Answer is A.
An earthquake produces infra-sound before the main shock wave. Infra-sound is a low-frequency sound. It is sound that is lower in frequency than 20 Hz (hertz) or cycles per second, the normal limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infra-sound, the sound pressure must be sufficiently high.
Rhinos have extremely good ears, picking up infra-sound far deeper than the range of human hearing.
Rhinos can hear down to a frequency of four hertz, whereas even a human baby, with entirely undamaged ears, can normally only pick up sounds as low as 20 hertz. Giraffes and elephants can also hear in this infra-sound range.
Hence, we can see some animals get disturbed and start running here and there just before the earthquakes because they can hear infrasonic sound.
If a vibrator strikes the water 50 times in 5 second, then the frequency of wave is________.
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10 Hz
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0.5 Hz
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5 Hz
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0.1 Hz
Explanation
In $$5 seconds$$ it vibrates $$50 times$$
In one sec it will vibrate $$ \dfrac{50}{5} = 10 times$$
And the number of vibrations in one second is nothing but frequency defined as number of oscillations in one second .
$$freq = \dfrac{10 times}{1s} = 10 Hz.$$
Where $$1Hz$$ = one oscillation per second.
Which is true for a wave? (here n = frequency, T = time period).
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nT = 1
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$$\displaystyle\frac{n}{T}\,=\, 2$$
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n = T
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None of these
Explanation
Option A
Frequency is number of vibrations per (one) second, and time period is the time taken to complete one vibration.
Let a vibrating body completes n vibrations in 1 s,
therefore time taken to complete one vibration , $$T=(1/n)$$ second
therefore we have , $$T=1/n$$ ,
or $$nT=1$$
Trees must be removed from the road sides and around buildings to reduce the harmful effects of noise pollution.
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True
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False
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Ambiguous
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Data insufficient
Explanation
Trees absorb and block noise and reduce glare. A well-placed tree can reduce noise by as much as 40 percent.
Vegetation reduces noise pollution through a phenomenon called sound attenuation, which is the reduction of sound intensity. Leaves, twigs, and branches on trees, shrubs, and herbaceous growth absorb and deflect sound energy.
The characteristic of sound which helps to distinguish between a man's voice and a woman's voice is.
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pitch
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amplitude
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loudness
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frequency
Explanation
Answer is A.
Pitch of a sound depends upon the frequency of waves.
The greater the frequency, the higher is the pitch and vice versa. It is the pitch of sounds that enable us to distinguished between a sound of man and woman.
Sound of a woman is shrill due to high pitch. Sound of a man is grave due to low pitch and low frequency.
Time period of a sound wave having the wavelength 0.2 m and frequency 10 Hz will be
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2 s
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0.2 s
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0.1 s
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0.02 s
Explanation
Given - $$ n=10Hz$$ ,
we have , time period , $$T=1/n=1/10=0.1s$$
The difference between musical sound and noise is due to:
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Musical instruments
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wave lengths
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Pitch
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Intensity
Explanation
Pitch is that characteristic of a musical sound by which a shrill sound can be distinguished from a grave one, even though the two sounds may be of the same intensity. It is also defined as that characteristic of sound by which the ear assigns it a place on a musical scale. When a stretched string is plucked, a sound of a certain pitch sensation is produced. If the tension in the string is increased, the pitch (the shrillness) becomes higher. Increasing the tension also increases the frequency of vibration. Therefore, the pitch is intimately related to frequency. But frequency alone does not determine the pitch. Below $$1000\ Hz$$, the pitch is slightly higher than the frequency and above $$1000\ Hz$$ the position is reversed. The loudness of sound also affects the pitch up to $$1000\ Hz$$. An increase in loudness causes a decrease in pitch. From about $$1000$$ to $$3000\ Hz$$, the pitch is independent of loudness, while above $$3000\ Hz$$ an increase in loudness causes an increase in pitch.
Hence, the difference between musical sound and noise is due to pitch.
A sound wave completes $$24$$ cycles in $$0.8\;s$$. The frequency of the wave is :
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$$30\;Hz$$
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$$8\;Hz$$
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$$24\;Hz$$
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$$12\;Hz$$
Explanation
Frequency is the number of times the cycle repeats in a one-time unit.
Given that, the sound wave completes $$24$$ cycles in $$0.8\,s$$.
So, the number of cycles in $$0.1\,s =\dfrac{24}{0.8}=30\,Hz$$
Hence the frequency is $$30\, Hz$$
The sound of the human voice is produced due to
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movement of tongue
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vibration in the pharynx
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vibration in the vocal cord
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vibration below tongue
Explanation
Vibration means rapid to and fro movement of an object. Sound of the human voice is produced due to vibrations in the vocal cords.
A tuning fork vibrates with 2 vibrations in 0.4 seconds. Its frequency (in Hz) is
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5
0%
6
0%
8
0%
0.8
Explanation
Number of vibrations $$n = 2$$
Time taken $$t = 0.4$$ s
Thus frequency of sound $$\nu = \dfrac{n}{t}$$
$$\therefore$$ $$\nu = \dfrac{2}{0.4} = 5 \ Hz$$
A tuning fork vibrates with $$2$$ vibrations in $$0.4\ s$$. Its frequency is:
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$$5\ Hz$$
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$$6\ Hz$$
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$$8\ Hz$$
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$$0.8\ Hz$$
Explanation
Frequency is defined as the number of vibrations per second.
Given,
A tuning fork vibrates with $$2$$ vibrations in $$0.4\ s$$.
Number of vibrations $$=2$$
Time $$=0.4\ s$$
Frequency, $$f=\dfrac{number\ of\ vibrations}{time}$$
$$f=\dfrac{2}{0.4}$$
$$f=5\ Hz$$
A sound wave completes $$24$$ cycles in $$0.8\;s$$.
The time period of the wave would be:
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$$1/30\;s$$
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$$30\;s$$
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$$1/24\;s$$
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$$24\;s$$
Explanation
A time period is the time taken to complete one cycle.
Given that, the sound wave completes $$24$$ cycles in $$0.8\,s$$
So, the time taken to complete one cycle $$=\dfrac{0.8}{24}=\dfrac{1}{30}\,s$$
Hence the time period is $$\dfrac{1}{30}\,s$$
Select the loudness level that is above the normal hearing range
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150 decibels
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40 decibels
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70 decibels
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90 decibels
Explanation
In order for us to be able to hear a sound at all, it has to be above a certain level. This level is called the auditory threshold or hearing
threshold. Humans have a hearing threshold of around $$0$$ decibels
. Above this threshold, sounds with higher sound pressure levels are heard as louder noises. Sounds above $$90\, dB$$ can lead to chronic
hearing damage if people are exposed to them every day or all the time. Hearing becomes uncomfortable if the sound pressure level is above $$110$$ decibels (threshold of discomfort), and it becomes painful above $$130$$ decibels (threshold of pain).
Select the odd one out (soft horns, ear plugs, silencers,
loudspeakers)
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soft horns
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ear plugs
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silencers
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loudspeakers
Explanation
Except loud speaker all are preventive measures of noise pollution while loud speaker creates noise pollution
Select the odd one out (loudspeakers, aeroplanes, vacuum cleaners, ear plugs)
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Ear plugs
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Aeroplanes
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Vacuum cleaners
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Loudspeakers
Explanation
Loudspeakers, aeroplanes and vacuum cleaners are sources of sound, while ear plug is not.
Hence, option A is correct.
Sounds having frequency more than 20,000 Hz are called -
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sonic sound
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ultrasonic sound
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infrasonic sound
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none of these
Explanation
Sounds having frequency more than 20,000 Hz are called Ultrasonic sound. As this is above the normal hearing range for humans, we cannot hear ultrasonic sound.
We, humans, produce sound by using :
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Oesophagus
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Larynx
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Thyroid Gland
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Mouth
Explanation
Larynx is known as the voice box. It is located at the upper end of the windpipe. The vocal cords stretch the larynx in such a way that a passage is created between them. When air passes through this passage the vocal cords vibrate and produce sound.
W
hat is the name of the strings which vibrate in our voice box when we talk?
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Oesophagus
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Pharyngeal slits
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Vocal cords
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All
Explanation
The name of the strings which vibrate in our voice box when we talk is vocal cords. These are two bands of elastic tissue located side by side in the voice box (larynx) just above the trachea.
Why Trees are planted along the roads?
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To make the roads beautiful
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To reduce noise pollution
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To provide shade
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All of the above
Explanation
Trees absorb the sound vibrations produced by various sources and hence leading in prevention of noise pollution. It also provides shade and also makes the road appear beautiful.
So, the correct answer is '
All of the above'
Mark the correct statement.
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Time taken by two consecutive compressions or rarefaction is called the time period of the wave.
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Sound of a violin and a flue are different due to the difference in the medium through which they travel.
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How the brain interprets the frequency of an emitted sound is called pitch.
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All true
Explanation
Time taken by two consecutive compressions or rarefaction is called the time period of the wave.
The sound of a violin and a flute is different due to their different pitches. The human brain interprets the sound frequency is called pith of sound.
Hence Option A and C are correct.
The frequency of sound is 500 Hz. How many times it vibrate in a minute :
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30,000
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300
0%
30
0%
8.33
Explanation
Frequency of vibration is defined as number of vibrations produced per second .
Given , frequency of sound 500Hz ,
it means that in $$1$$ second , number of vibrations produced $$=500$$
therefore , in 60 second (1 minute) , number of vibrations produced $$500\times60/1=30,000$$
Muscles attached to the vocal cords can make them
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Tight only
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Loose only
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Both loose and tight as per requirement
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Neither loose nor tight
Explanation
The muscles attached to the vocal cords can expand and contract, thus making them tight and loose. The voice quality changes depending on whether the vocal cords are tight or loose.
A source produces 60 rarefactions and 60 compressions in 0.6 s. Distance between a compression and the next (consecutive) rarefaction is 150 cm. Find the frequency of the wave :
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500 Hz.
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0.02
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0.01 Hz
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100 Hz
Explanation
Frequency of wave is defined as the number of waves produced in 1 second. A longitudinal wave is formed by one compression and one rarefaction, we have 60 compressions and 60 rarefactions in $$0.6s$$ i.e. 60 waves produced in 0.6s,
In $$0.6s $$ the number of wave produced $$=60$$
In $$1s$$ the number of wave produced $$=\dfrac{60}{0.6}=100Hz$$
Therefore , frequency $$f=100Hz$$
Here, the distance given
between a compression and the next (consecutive) rarefaction is of no use. It is wavelength. We need only number of waves and time to calculate frequency.
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