Public address
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.
In colloquial British English, a PA system is often referred to as a "Tannoy" after the company of that name.
See also
ja:公衆伝達