A PA (Public Address) system is a collection of items that includes a mixer or mixer amp, one or more power amplifiers, various types of speaker housed in cabinets and one or more vocal and instrument microphones. The objective of the PA system is to relay audible information to a public gathering or provide amplification of music, speech or other sound sources for entertainment or educational purposes.
PA systems above a certain size that have been constructed specifically for live music or theatre performances are more commonly called 'Sound Reinforcement Systems'. Over and above their standard equipment the SRS may also contain foldback capabilities, which is a means whereby the sound that comes out of the main front of house speakers is routed back through a separate mixer, amplifiers and speakers. When audible to the performer(s), the foldback monitors allow them to hear the instrument they play and their own lead or backing vocal for the purposes of accuracy of pitch and musical orientation.