What are the application of speaker (device)?

Question

What are the application of speaker (device)?

Answer 1

If you mean a loudspeaker ... it's "a device that converts analog audio signals into the equivalent air vibrations in order to make audible sound" (ZDnet).  It must be combined with some type of amplifier to translate the low level sound from a microphone, radio receiver or recorded signal into audible sound.

It is used in radios, TVs, hi-fi systems, movie theatres, etc, etc ... wherever sound needs to be reproduced.