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CBSE Questions for Class 8 Physics Sound Quiz 7 - MCQExams.com
CBSE
Class 8 Physics
Sound
Quiz 7
The heart of a man beats $$75$$ times a minute. What is its time period?
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$$0.75s$$
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$$1.33s$$
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$$1.25s$$
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$$0.8s$$
Explanation
To calculate the frequency in hertz, we have to measure the time in seconds.
That is, t= 1 minute = 60 seconds.
Number of time it beats = 75 times per minute.
Frequency is given as,
Number of times it beats/Total time taken in seconds = 75/60 = 1.25 Hz.
Hence, the frequency of a man's heartbeat is 1.25 times per second.
The time period is given as the inverse of frequency, that is, $$T=\dfrac { 1 }{ f } =\dfrac { 1 }{ 1.25 } = 0.8\; seconds$$.
Hence, the time period of a man's heart beat is 0.8 seconds.
Two people can hear the sound of each other on the moon's surface.
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True
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False
Explanation
Sound needs something to travel through to get from one place to another. On the Moon, since there is no air, sound cannot travel above the surface. So, there are no sounds on the surface of the Moon.
The Moon has no air because its gravity is too weak to hold an atmosphere. The gravity on the Moon is about six times less than the gravity on Earth, so any gases on the Moon (except for the very heaviest) would eventually float away into space.
Hence, the no sound can be heard on the moon's surface.
Complete the following sentence.
Sound can not travel through ..............................., but it requires a ................... .
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$$air, medium$$
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$$water, medium$$
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$$solids, medium$$
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$$vacuum, medium$$
Explanation
Sound waves need to travel through a medium such as a solid, liquid, or gas. The sound waves move through each of these mediums by vibrating the molecules in the matter. The molecules in solids are packed very tightly. Liquids are not packed as tightly as solids. And gases are very loosely packed. The spacing of the molecules enables sound to travel much faster through a solid than a gas. Sound travels about four times faster and farther in water than it does in air.
Sound waves are travelling vibrations of particles in media such as air, water or metal. So it stands to reason that they cannot travel through empty space, where there are no atoms or molecules to vibrate.
Hence,
Sound cannot travel through vacuum, but it requires a medium.
The sketches I to IV in Fig. show sound waves, all formed in the same time interval.
Which diagram shows a bass (low frequency) note?
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I
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II
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III
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IV
Explanation
Frequency is the speed of the vibration, and this determines the pitch of the sound. It is only useful or meaningful for musical sounds, where there is a strongly regular waveform. In other words, frequency is the number of cycles per second in a particular wave pattern.
In the question, figure II has the least number of cycles, and hence, it has a low frequency or a base note.
The heart of a man beats $$75$$ times a minute. What is its frequency?
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$$1.25 \ s^{-1}$$
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$$2.5 \ s^{-1}$$
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$$7.5 \ s^{-1}$$
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$$75 \ s^{-1}$$
Explanation
To calculate the frequency in hertz, we have to measure the time in seconds.
That is, t= 1 minute = 60 seconds.
Number of time it beats = 75 times per minute.
Frequency is given as,
Number of times it beats/Total time taken in seconds = 75/60 = 1.25 Hz.
Hence, the frequency of a man's heartbeat is 1.25 times per second.
Different musical instruments sound different even when they play a note with the same pitch.
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True
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False
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Ambiguous
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Data insufficient
Explanation
Quality is used to describe the quality of the waveform as it appears to the listener. Therefore the quality of a note depends upon the waveform. Two notes of the same pitch and loudness, played from different instruments do not sound the same because the waveforms are different and therefore differ in quality or tone.
The same note from different instruments has different qualities because the sounds from instruments are never pure notes, i.e. of one frequency,
the only exception being a tuning fork.
No sound is possible in a .......... .
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Vacuum
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Solid
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Gases
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Medium
Explanation
Sound is a mechanical wave which propagate in medium like solid, liquid and gas. In vacuum, there is no medium is present hence no sound is possible in a vacuum.
Sound waves requires a .......... to travel through.
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Medium
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Vacuum
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Both A and B
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None of these
Explanation
Sound needs a material medium for their propagation like solid, liquid or gas to travel because the molecules of solid, liquid and gases carry sound waves from one point to another. Sound cannot progress through the vacuum because the vacuum has no molecules which can vibrate and carry the sound waves.
A sound of wave produces $$40$$ crests and $$40$$ troughs in $$0.4s$$. What is the frequency of the wave?
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$$10\ Hz$$
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$$50\ Hz$$
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$$100\ Hz$$
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$$150\ Hz$$
Explanation
One complete cycle consists of one crest and one trough.
The given source produces 40 crests and 40 troughs, hence it produces 40 wave cycles in the given interval 0.4 seconds.
Therefore, number of wave cycles produced in 1 second $$=\quad \dfrac { 40\times 1 }{ 0.4 } \quad =\quad 100$$
We know that the number of wave cycles produced in one second is equal to the frequency.
Hence, the frequency of the wave is 100 Hz.
A sound wave has a frequency of 192 Hz and travels the length of a football field, 91.4 m, in 0.27 s. What is the period of the wave?
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$$1.3\times 10^{-3}s$$
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$$2.6\times 10^{-3}s$$
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$$5.2\times 10^{-3}s$$
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$$7.8\times 10^{-3}s$$
Explanation
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.
Time Period (T) is the number of seconds per waveform, or the number of seconds per oscillation. It is clear that frequency and period are reciprocals.
That is, $$T=\dfrac { 1 }{ f } =\dfrac { 1 }{ 192 } =0.0052\quad seconds$$.
Hence, the time period of the wave is $$5.2\times { 10 }^{ -3 }\quad seconds$$.
The waves that transfer vibrations and disturbance are:
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Sound waves
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Ultrasonic waves
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Infrared waves
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All of the above
Explanation
A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. Sound is produced due to the vibration of different objects. Sound travels as a longitudinal wave through a material medium.
Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”.
The relation between frequency(f) and time period(T) is given by
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f = T
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f = $$\frac{1}{T}$$
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T = f$$^2$$
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T = f$$^3$$
Explanation
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.
Time Period
(T) is the number of seconds per waveform, or the number of seconds per oscillation. It is clear that
frequency and time period are reciprocals.
That is,
T
=1/
f
.
Water waves in a shallow dish are $$6 cm$$ long. At one point, the water moves up and down at a rate of $$4.8$$ oscillations/s. What is the time period of the water waves?
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$$\displaystyle 0.280\space s$$
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$$\displaystyle 0.408\space s$$
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$$\displaystyle 0.208\space s$$
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$$\displaystyle 0.308\space s$$
Explanation
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.
Time Period (T) is the number of seconds per waveform, or the number of seconds per oscillation. It is clear that frequency and period are reciprocals.
That is, $$T=\dfrac{1}{f}$$
$$\dfrac{1}{f}=\dfrac{1}{4.8}=0.208\;seconds$$
Hence, the time period of the water wave is 0.208 seconds.
In a ripple tank, 10 full ripples/s are produced. The distance between peaks of consecutive trough and crest is 15 cm. Calculate the frequency.
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$$5 Hz$$
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$$10 Hz$$
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$$15 Hz$$
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$$20 Hz$$
Explanation
Frequency is the number of complete vibrational cycles of a medium per a given amount of time. The frequency would have units of cycles per second, waves per second or vibrations per second. The unit for frequency is the Hertz (abbreviated Hz) where 1 Hz is equivalent to 1 cycle per second.
In the question it is mentioned that
10 full ripples per second are produced.
Hence, the frequency is 10 Hz.
What is necessary for the production and transmission of sound?
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Medium
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Acceleration
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Reciever
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None of the above
Explanation
The vibrations that create sound must travel through a medium, such as air or water, or anything made of molecules.
Which figure indicates vibrations of sound produced by n
oise
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(i) only
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(ii) only
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(i) and (ii)
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None
Explanation
Noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In communication systems, noise is an error or undesired random disturbance of a useful information signal.
How are noise and music interrelated?
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Noise is unpleasant whereas music is pleasant
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Noise pollution causes health hazards whereas music has soothing effect
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When musical instruments produce loud sound it becomes noise
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All of the above
Explanation
The unpleasant sounds are called noise. Pleasant sound is called music. Noise pollution may cause many health hazards like lack of sleep, hypertension, etc. Music has a soothing effect. Music can also become noise when the musical instruments produce loud sounds.
Sound waves travel in all directions from the source through a medium.
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True
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False
Explanation
Sound is produced due to the vibration of different objects. Sound waves travel in all directions from the source through a medium.
Unwanted sound is called .......... .
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Echo
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Noise
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Decibel
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Vibration
Explanation
Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to the ear are known as music. (g) True. Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sounds continuously for a long time, then it may cause temporary hearing impairment.
The audible range of sound for human is:
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$$20Hz\ to\ 20000Hz$$
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$$20Hz\ to\ 2000Hz$$
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$$20Hz\ to\ 200000Hz$$
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$$5Hz\ to\ 10000Hz$$
Explanation
Humans can detect sounds in a frequency range from about $$20\ Hz \ \ to \ \ 20\ kHz$$. (Human infants can actually hear frequencies slightly higher than $$20\ kHz$$, but lose some high-frequency sensitivity as they mature; the upper limit in average adults is often closer to $$15–17\ kHz$$)
Which graph represents music ?
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$$(i)$$
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$$(ii)$$
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Both
$$(i)$$ and
$$(ii)$$
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None of the above
Explanation
Wave pattern is regular in music which feels pleasant to listening while it is quite irregular in noise which creates irritation.
Graph (i) represents wave pattern of music
Graph (ii) represents wave pattern of noise
Why can supernova explosions in space be seen but not heard on earth?
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Because sound cannot travel in vacuum but light can.
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Because light cannot travel in vaccum but sound can
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Because sound produced is beyond audible range of human ears.
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None of these.
Explanation
Light does not require a medium to travel through but sound requires a medium to travel, so it cannot be transmitted through space. That is why we are able to see the supernova explosions but cannot hear them.
Pattern of wave in noise is____.
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Regular
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Irregular
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Sinusoidal
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None of the above
Explanation
Wave pattern is regular in music which feels pleasant to listening while it is quite irregular in noise which creates irritation.
The highness or lowness of sound is -
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Frequency
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Pitch
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Vibrations
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None
I am the cause of production of all sounds.
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Vibrations
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Amplitude
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Frequency
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Echo
Explanation
Sound is produced due to the vibration of different objects.
The magnitude of the maximum
disturbance in the medium on either side of
the mean value is called the amplitude of the
wave.
The number of complete oscillations per unit time is called the frequency.
Echo is the repetition of sound caused by the reflection of sound.
(7) Which is
an undesirable sound and harmful for health.
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Noise
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Echo
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Music
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None of the above
Explanation
Noise is described as any disturbing or unwanted sound that interferes with or harms human health or wildlife. Sound levels between 120 dB and 140 dB can cause pain, and sound levels over 137 dB can cause an acoustic shock (sudden hearing loss).
(6) I am the kind of sound produced by earthquakes.
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Infrasonic sound
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Ultrasonic sound
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Audible sound
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None of these
Explanation
Sounds of frequencies
below $$20\ Hz$$ are called infrasonic sound or
infrasound
Earthquakes produce low-frequency
infrasound before the main shock waves
begin.
The pitch is determined by the _________________ of the vibrating body.
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Frequency
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Amplitude
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Speed
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Loudness
Explanation
Pitch of sound depends upon frequency of vibrating body. A high frequency produce a high pitched sound and a low frequency produces a low pitched sound.Hence pitch depends on frequency of vibrating body.
Sound of frequency below 20 Hz is called
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Audio sounds
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Infrasonics
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Ultrasonics
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Supersonics
The frequency of a given sound is $$1.5 kHz$$. How many vibrations does it complete in one second?
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$$1.5$$
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$$750$$
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$$1500$$
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$$6.6\times 10^{-4}$$
Explanation
The frequency of a vibration is defined as number of vibrations happening in one second.Since frequency is given as $$1.5kHz=1500Hz=1500Vibrations/sec$$
So the number of vibrations in one second is $$1500$$
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