Short Notes - Sound, Science, Class 8 Class 8 Notes | EduRev

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Class 8 : Short Notes - Sound, Science, Class 8 Class 8 Notes | EduRev

The document Short Notes - Sound, Science, Class 8 Class 8 Notes | EduRev is a part of the Class 8 Course Science Class 8.
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Facts that Matter     

  • Sound is produced by the vibration of materials.   
  • Vibration or oscillation: If a body moves to and fro repeatedly about a mean position, its motion is said to be oscillatory motion.

One complete back and forth motion of oscillating body about its mean (central) position is called a vibration or an oscillation.

Short Notes - Sound, Science, Class 8 Class 8 Notes | EduRev

For example, when we vibrate a tightly stretched metal string (Fig. 13.1), it makes a to and fro motion about its central position. During to and fro motions, string goes from upward extreme position to downward extreme position. When the string goes from upward extreme position to downward extreme position and then back to upward extreme position, we say that string has completed one vibration or one oscillation.   

  • The whole musical instrument is forced to vibrate, by plucking string, striking membrane etc., and it is the sound of the vibration of the instrument that we hear.     
  • In humans, the sound is produced by vocal cords present in voice box (larynx), when the lungs force air through the slit present between cord. Air passing through the slit causes vibration in stretched vocal cords producing sound.     
  • Sound travels through a medium. It may be gas, liquid or solid. It cannot travel in a vacuum.

Short Notes - Sound, Science, Class 8 Class 8 Notes | EduRev

  • We hear sound through our ear. The human ear has three main parts:
    — Outer ear: It has a canal at the end of which is a stretched membrane, called eardrum. The sound waves pass through ear canal and eardrum to vibrate.
    Middle ear: It has a set of three bones which are linked together and attached to the eardrum. These bones receive vibrations from eardrum pass on to the inner ear.
    — Inner ear: The inner ear receives vibrations from middle ear and changes them into nerve impulses. These nerve impulses are carried by nerve (auditory nerve) to the brain. The brain interprets impulses. That is how we hear.

Short Notes - Sound, Science, Class 8 Class 8 Notes | EduRev

  • Musical Sounds: The sounds which produce pleasant sensation to the ear are termed as musical sounds.     
  • Musical instruments: Musical instruments are generally of the following types:
    String instruments: The vibration of string produces musical sound as in sitar.
    Wind instruments or reed instruments: The column of air is vibrated by blowing air directly through tubes is known as reeds as in Shehnai or Flute.
    Membrane instruments: The musical sound is produced due to the vibration of skin or membrane mounted on the instruments such as Tabla.
    — Ghana vadya: The instruments which are simply beaten or struck as in the Manjira (cymbals) and Jal Tarang.     
  • Noise: The sounds which produce unpleasant sensation to the ears are termed as noise.     
  • Time Period, Frequency, and Amplitude:
    — Time Period: The time taken by a vibrating body to complete one oscillation is called the time period.
    — Frequency: The number of oscillations completed by a vibrating body in one second is called the frequency.

Frequency is equal to number of oscillations divided by time taken.
The unit of frequency is hertz. It is denoted by Hz. If a vibrating body makes 20 oscillations in a second, we say that its frequency is 20 Hz.
The human voice can produce sounds with a frequency between 60 Hz and 13,000 Hz. It is interesting to note that a normal human ear can hear sound of frequency between 20 Hz to 20,000 Hz. The sounds of frequency higher than 20,000 Hz and less than 20 Hz cannot be heard by a human ear. The ears of some animals like bat, dog respond to sounds of frequency higher than 20,000 Hz. The sound of frequency greater than 20,000 Hz is called ultrasonic.

— Amplitude: During to and fro motion, a vibrating string goes a certain distance in the upward or downward direction from its central position, i.e. position of rest. The maximum distance to which the string goes upward or downward from its central position is called the amplitude.
The maximum displacement of an oscillating body from its central position is called amplitude.     

  • Loudness: Loudness of sound depends on the amplitude of vibration. It is proportional to the square of the amplitude of the vibration producing the sound. It is, if the amplitude becomes twice, the loudness increases by a factor of 4.

The loudness is expressed in a unit called decibel (dB).     

  • Shrillness or Pitch: Frequency determines the pitch of a sound. If the frequency of vibration is higher, higher is the pitch, and shriller is the sound.   
  • Causes of Noise and Harms of Noise Pollution:
    — Causes of noise: Sounds of vehicles, machines, crackers, explosions, loudspeakers including television and transistor radio at high volumes, aeroplanes etc. contribute to noise pollution.

    — Harm of noise pollution:
    Presence of excessive noise in the surroundings may cause:
    – Lack of sleep
    –Hypertension (high blood pressure)
    – Anxiety and other health disorders
    –Temporary or even permanent impairment of hearing. 

    Measures to limit noise pollution:
    – Noise producing industries should be set up away from residential areas.
    – Airports should be made away from residential areas. Silencing devices must be installed in aircraft engines.
    –Use of automobile horns should be minimised.
    – TV, music systems, loudspeakers should be run at low volumes.
    – Trees must be planted along the roads and around building.
    – Use of high noise creating crackers should be avoided.
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