13. Sound Science class 8 in English Medium ncert book solutions 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.