Public Address System

A P. A. System is an amplification system using an electronic amplifer and large loudspeakers to relay sound from a performance to a fill a larger space than the unamplified performance would by itself. 

They are used in many situations and venues - schools, churches, bars, theatres as fixed installations.

Bands carry P A equipment routinely and augment it with a mixing desk, microphones and pick-ups and fold-back or monitor speakers.