Study Materials

NCERT Solutions for Class 8th Science

 

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Chapter 13. Sound

Chapter Review

 

 

 

Chapter Review:

  • When an object vibrates, it produces sound.

  • When any sound reflects by striking on any medium, that sound heard us again and again. This is called eco sound.

  • The audible range of human ears is 20 Hz to 20,000 Hz. 

  • The S.I unit of frequency is Hz (Hertz).

  • The S.I unit of wave length is meter.

  • The to and fro or back and forth motion of an object is termed as vibration.

  • In humans, the sound is produced by the voice box or the larynx.

  • The vocal cords in men are about 20mm long. In women these are about 5mm shorter.

  • When air has been removed completely from a vessel, it is said that there is vacuum in the vessel.

  • 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.

  • The to and fro motion of an object is known as vibration. This motion is also called oscillatory motion.

  • The number of oscillations per second is called the frequency of oscillation.

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

  • Unpleasant sounds are called noise.

  • Some animals can hear sounds of frequencies higher than 20,000 Hz.
    Dogs have this ability.

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

 

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