Question

what is sound ??,..........
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Answer 1

Heya!!

Sound is a form of energy that is produced when an object or a membrane vibrates to and fro about a mean position.

Hope it helps :)

Answer 2

sound is the form of energy which travels in the from of waves through various kinds of medium and it also produces the sensation of hearing in our ears 
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sound travels the fastest in solid and slowest in the gas/air , but it cannot travel in vacuum or space

sound above 20khz is called ultrasonic sound
sound below 20khz is called infrasonic sound

sound travels in the form of waves 
only when there are vibrations created we can hear sound