CBSE – 8th Standard Science

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Sound

  • Sound plays an important role in our life. It helps us to communicate with one another.

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  • To and fro or back and forth motion of an object is termed as vibration.
  • Sound is produced by vibrations.
  • The sound cannot travel through vacuum. . sound needs a  medium to travel
  • Sound can travel through any solid liquid or gas.
  • In humans, the sound is produced y the voice box or the larynx.  When the lungs force air through the lit, the vocal cords vibrate, producing sound.
  • When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.
  • The vocal cords in men are about 20mm long. In women these are about 5mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.
  • Sound can travel through wood or metal and even through solids.
  • When sound enters in it, it travels down a canal at the end of which a thin membrane is stretched tightly. It is called the eardrum.

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  • The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear.
  • The number of oscillations per second is called the frequency of oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second.
  •  Amplitude and frequency are two important properties of any sound.
  • To and fro motion of an object is known as vibration. This motion is also called oscillatory motion.
  • The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.
  • The frequency determines the shrillness or pitch of a sound.
  •  If the frequency of vibration is sound is shrill and has a higher pitch. If the frequency of vibration is lower, sound has a lower pitch.
  • Unpleasant sounds are called noise.
  • The fact is that sounds of frequencies less than about 20 vibrations per second (20 Hz) cannot be detected by the human ear. Such sounds are called inaudible.
  • Sound is produced by vibrating objects.
  • In human beings, the vibration of the vocal cords produces sound.

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  • Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.

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  • The eardrum senses the vibrations of sound; it sends the signals to the brain. This process is called hearing.
  • The number of oscillations or vibrations per second is called the frequency of oscillation.
  • The frequency is expressed in hertz (Hz).
  • A frequency of 1 Hz is one oscillation per second.
  • Larger the amplitude of vibration louder is the sound.

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  • Amplitude and frequency are two important properties of any sound.
  • Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.
  • The loudness is expressed in a unit called decibel (dB).
  • Unpleasant sounds are called noise.
  • Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.
  • A person who is exposed to a loud sound continuously may get temporary or even permanent impairment of hearing.
  • Attempts should be made to minimize noise pollution.
  • Plantation on the roadside and elsewhere can reduce noise pollution.
  • Is it possible that the decreasing amount of air in the tumbler had something to do with decreasing loudness of the ring?
  • Indeed, if you had been able to suck all the air in the tumbler, the sound would stop completely.
  • Actually, sound needs a medium to travel.

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  • When air has been removed completely from a vessel, it is said that there is vacuum in the vessel.
  • The sound cannot travel through vacuum .Does sound travel in liquids.
  • The shape of the outer part of the ear is like a funnel.
  •  When sound enters in it, it travels down a canal at the end of which a thin membrane is stretched tightly.
  • It is called the eardrum.
  • It performs an important function. To understand what the eardrum does, let us build a tin can model of the eardrum.
  • We find that sound can travel through wood or metal. In fact, sound can travel through any solid.
  • The loudness is expressed in a unit called decibel (dB).
  • A whistle has a high frequency and therefore, produces a sound of higher pitch.
  •  A bird makes a high-pitched sound whereas a lion makes a low-pitched roar.
  • However, the roar of a lion is very loud while the sound of the bird is quite feeble.
  • The frequency of the voice of a child is higher than that of an adult
  • Usually the voice of a woman has a higher frequency and is shriller than that of a man.
  • Some animals can hear sounds of frequencies higher than 20,000 Hz. Dogs have this ability. The police use high frequency whistles which dogs can hear but humans cannot. The ultrasound equipment, familiar to us for investigating and tracking many medical problems, works at frequencies higher than 20,000 Hz.
  • You already know about air pollution. Presence of unwanted gases and particles in air is called air pollution.
  • Similarly, presence of excessive or unwanted sounds in the environment is called noise pollution.
  • Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers etc.

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  • Television and transistor radio at high volumes, some kitchen appliances, desert coolers, air conditioners, all contribute to noise pollution.
  • The total hearing impairment, which is rare, is usually from birth itself.
  • Partial disability is generally the result of a disease, injury or age. Children with impaired hearing need special care.
  • By learning sign language, such children can communicate effectively. Because speech develops as the direct result of hearing, a child with a hearing loss may have defective speech also.
  • Technological devices for the hearing-impared have made it possible for such persons to improve their quality of life.
  • Society can do much to improve the living environment for the hearing-impaired and help them live normal lives.

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