Living Science 2019 2020 Solutions for Class 8 Science Chapter 13 - Sound

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Sound waves are longitudinal waves which require medium to travel from one place to another. As the moon does not have any kind of atmosphere, so without any medium the sound waves produced will not be able to travel anywhere. Hence, they will never reach the earth. 

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The minimum frequency required to hear a sound by humans is 20 Hz. Which means 20 vibrations per second. By waving his hand, a person can not produce 20 vibrations in a single second. Hence, no sound is produced when a person waves his/her hand.

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It is called Eardrum. When a sound with a certain frequency strikes our eardrum present inside our ear, it starts vibrating with the same frequency as of the sound wave. That's how we are able to listen to any sound. 

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Speed of the light is much higher than the speed of the sound. In normal conditions, the speed of the sound in air is just 332 m/s, but the speed of the light is 3$\times$10⁸ m/s. We can also observe this difference in our daily life during the phenomenon of lightning. When lightning occurs in the sky, the sound of the lightning comes after, but before we are able to see it in the sky. This proves that light waves travel faster and reach to us first and sound waves are slow, so we hear the sound of lightning later.

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When a sound wave travels in a medium, it creates a disturbance in the particles of the medium. Every particle passes this disturbance to the adjacent particle and that's how a sound wave moves through a medium. Speed of the sound is maximum in solids as the molecules in the solid medium are tightly packed together as compared to liquids or gases, allowing sound waves to travel faster. Hence, in the given case, the speed of the sound would be maximum in steel. 

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Vocal cords of women are shorter than the vocal cords of men, that's why the frequency of the sound produced by women is higher and have a high pitch.

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When a sound is produced by regular vibrations, it is more uniform and melody in nature. e.g. Music generated by a muscial instrument is produced due to regular vibrations. On the other hand, when a sound is produced by irregular vibrations, it is unpleasant and mostly noisy in nature. e.g. Sound produced due to racing of a motorbike is due to irregular vibrations. 

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(a) The musical instrument which produces sound by the vibration of a stretched string is Guitar.

(b) A tabla is a musical instrument in which sound is produced due to vibrations of a stretched skin.

(c) In a flute, Sound is produced by vibrations of an air column.

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Not just the noise but sometimes even loud music is disturbing and causes noise pollution. 

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(d) Frequency and time-period do not depend on amplitude of oscillation.

Frequency is inversely related to time-period and time-period does not depend on amplitude.

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(c) harmonics

Tuning fork and musical instruments will produce sounds of different harmonics and loudness.

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(d) all of these

Sound travels through the vibrations of molecules and molecules are present in all solids, liquids and gases.

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(c) It produces sound that we cannot hear.

Humans can hear sounds in the frequency range 20Hz - 20000Hz.

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(c) steel

The molecules of steel are much more closely packed compared to that of air, water and vacuum. So, molecular vibrations pass quite quickly in steel. Consequently, sound travels fastest in steel.

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(c) loudness

Loudness of sound depends on the amplitude of vibrations.

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(d) 512 Hz

Higher the frequency, higher is the pitch of sound.

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(c) 500 Hz

We can hear sound in the frequency range of 20 - 20000 Hz.

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Wool is the best absorber of sound among the given options.

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Sound is produced by vibrations.

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Tuning fork is used in the laboratory to produce sound of a fixed frequency.

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Loudness of sound depends on the amplitude of vibration. Higher the amplitude of vibration, louder is the sound produced.

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'Frequency' is the term for 'number of vibrations per second'.

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Frequency is measured in hertz (Hz).

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Yes, the pitch of sound depends on the time period of vibration.

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The sound of a singing bird has higher frequency than that of a singing man due to higher pitch of  the bird's song.

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It's true that most vibrating objects generate sound of several frequencies simultaneously.

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The quality of two sounds with the same fundamental frequency differs because of the number and relative loudness of the harmonics produced.

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False, a tuning fork produces harmonics that are very soft and so, barely audible.

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Flute is a wind instrument.

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It's true that the pitch of the sound produced by a guitar can be altered by reducing the vibrating length of the guitar string.

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The sound produced by irregular vibrations is called noise.

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The vibrations in air are picked up by the outer ear.

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Light travels faster than sound.

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The two kinds of sound that we cannot hear are infrasonic and ultrasonic.

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A soft board will absorb most of the sound falling on it.

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Loudness of sound is measured in decibel (dB).

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Prolonged exposure to noise louder than 70 dB can lead to permanent hearing damage.

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Hard surfaces reflect sound better than soft surfaces.

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Sound is produced by vibration. For example, when a guitar string is plucked, sound is produced due to vibrations produced in the string.

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The time taken by an object to complete one oscillation is called time period of the vibrating object.

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Amplitude is the distance covered by a vibrating object from its mean/zero position.

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Number of oscillations completed by a vibrating object per second is called frequency.
Shrillness of sound is directly dependent on frequency. Higher the frequency of vibration more is the shrillness of sound.

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Women have shriller voices than men because the sound produced by women is of higher frequency than that produced by men.

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Loudness of sound is proportional to the square of amplitude.
If the amplitude is tripled, the loudness increases by nine times.

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As light covers the distance instantly, the time gap is the time taken by sound to reach Ramesh from its point of origin.
Now, speed of sound in air = 340 m/s
Time gap = time taken to travel = 3 s
Distance between Ramesh and the clouds = (340 m/s) $\times$ (3 s)
                                                              = 1020 m.

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We cannot hear the echo because the time difference between the original sound and reflected sound will be less than 1/10^th of a second. So, we are not able to distinguish between the two sounds.

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Sound, in our throat is produced in the larynx by the vibration of two vocal cords when air passes through the opening between the vocal cords.

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The three types of musical instruments are:

1. Stringed instruments: They produce sounds using vibrations generated in metal wires fixed at both ends. For example: guitar.
2. Wind instruments: They produce sounds using vibrations in an air column trapped in a hollow structure. For example: flute.
3. Percussion instruments: They produce sounds using vibrations in a stretched membrane to produce sound. For example: drum.

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Any sound produced by irregular vibrations is called noise. Too much noise in our surroundings is harmful to us and is called noise pollution. Loud music, traffic on roads, machines in factories, mixers and grinders, etc. are major causes of noise pollution.

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Four ill effects of noise pollution are:

1. Exposure to sudden high noise level (e.g. explosion) can cause permanent hearing loss due to rupture of the ear drum.
2. Prolonged exposure to noise of volume higher than 70 dB can cause permanent hearing loss.
3. High noise can lead to nervous tension and increase in blood pressure.
4. Noise pollution can lead to sleep deprivation and headaches.

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Three methods of controlling noise pollution are:

1. Reducing noise emissions by developing low-noise products, such as better silencers for automobiles.
2. Controlling recreational noise, such as those of loudspeakers.
3. Making screens or enclosures around machinery to obstruct the path of noise.

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Quality of sound is the characteristic that allows us to distinguish sounds from two different sources even though the sound produced by them has same frequency and loudness. This happens because with every single frequency, called fundamental frequency, many other frequencies, known as harmonics, are also generated with differing loudness.

The quality of sound is determined by the frequencies present in it and their relative loudness.

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A vibrating object causes air molecules to vibrate. When these vibrations reach our ears, they are collected by the pinnae and funnelled into the ear tube. These vibrations strike the eardrum, which starts to vibrate at the same frequency. This causes the delicate bones of the middle ear to vibrate. This stimulates tiny hair in the hearing organ that sends a signal to the auditory nerve of our nervous system. The auditory nerve takes the signal to the brain and we can hear the sound.

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Hold a bell in one hand and dip it in water. Now bend till your ear touches the surface of water (caution: water should not enter the ear). On ringing the bell you will hear a clear sound. This shows that sound can travel in liquids.

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Take a container with a tightly held lid. Make a hole at the bottom of the container. Now, connect a vacuum pump to this hole with a rubber tube. Make sure that there is no gap between hole and tube. The vacuum pump is used to extract air from the container.
Put a cell phone inside the container and close it with the lid. Now call the number of the cell phone so that it rings. We can hear the sound clearly. Extract the air from the container by switching on the vacuum pump. Again, call the number of the cell phone. This time no sound is heard because there is no air (medium) to help the propagation of sound. This experiment shows that sound cannot travel through vacuum.

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(a) It will be seen instantly on earth, as light travels at a great speed of 300000000 m/s.

(b) It will not be heard on earth, as sound needs a medium to travel and there is no medium between the moon and earth.

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In a game of hide-and-seek, the blindfolded person guesses the player closest to him by following the sound generated by the movement and speech of the closest player.

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(1) Louder sound of the same pitch:



(2) Sound of the same loudness but greater pitch:



(3)  The sound that is louder and also has a greater pitch:

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This is totally not correct. Having party, having fun is okay but too loud music is annoying as well as dangerous for us. This does not only disturbs the neighbors but it harms our ears too. In my daily life, I always keep the sound at an optimum level, when I organize a party at my house. I never disturb my neighbors by too loud music.