A public address or "PA" system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a given sound, e.g., a person making a speech, prerecorded music, or message, and distributing the sound to the general public around a building.
The Amplifier is used to increase the level of the electrical signal from the sound source so that it can be heard at sufficient volume from loudspeaker.
A Mixer is used to add the sound together so that a single signal can be sent to the amplifier.
