Public address
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A public address system, abbreviated PA system, is an electronic amplification system used as a communication system in public areas. It is an amplification setup with amplifier and loudspeakers, used when playing live to an audience. The PA system will be driven from a mixer at which the engineer will put together the incoming microphones and instruments.
A simple PA system consists of a microphone, an amplifier and one or more loud speakers. Those three components have a potential for feedback. Feedback occurs when sound from the speakers finds its way back into the microphone and is then re-amplified and sent through the speakers again. This generally manifests itself as a sharp, sudden shrill.
"PA system" may also refer to an audio transmission system used in a building, most commonly seen in schools (used to read announcements and summon students) and hospitals (used to summon staff or declare states of emergency). Some such systems can address specific rooms and exclude others; others also have microphones in each room so that the occupants can reply to the central office. The latter is often called an intercom system.
In colloquial British English, a PA system is often referred to as a "Tannoy" after the company of that name.
A Public Address also may refer to any IP address that is not in RFC 1918 "Private Addressing" scheme and is routable on the internet.