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Public Address

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.

Simple PA systems are often used in small venues such as school auditoriums, churches, and small bars. PA systems with a larger number of speakers are widely used in institutional and commercial buildings, to read announcements or declare states of emergency. Intercom systems, which are often used in schools, also have microphones in each room so that the occupants can reply to the central office.

Small PA systems

The simplest PA systems consist of a microphone, a modestly-powered mixer-amplifier (which incorporates a mixer and an amplifier in a single cabinet) and one or more loudspeakers. Simple PA systems of this type, often providing 50 to 200 watts of power, are often used in small venues such as school auditoriums, churches, and small bars.

Public Address systems typically consist of input sources, pre-amplifiers and/or signal routers, amplifiers, control and monitoring equipment, and loudspeakers. Input sources refer to the microphones and CD Players that provide a sound input for the PA system. These input sources are fed into the pre-amplifiers and signal routers that determine the zones that the 'audio signal' is fed to. The preamplified signals are then passed into the amplifiers. Depending on a countries' regulation these amplifiers will amplify the audio signals to 50V, 70V or 100V speaker line level. Control equipment monitors the amplifiers and speaker lines for faults before it reaches the loudspeakers.

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Large venue PA systems

For popular music concerts, a more powerful and more complicated PA system is used to provide live sound reproduction. In a concert setting, there are typically two complete PA systems: the "main" system and the "monitor" system. Each PA system consists of microphones, a mixing board, sound processing equipment, amplifiers, and speakers. There is disagreement over when to call these audio systems Sound Reinforcement (SR) systems or a Public Address (PA) systems. This distinction is important in some regions or markets, while in other regions or markets the terms are interchangeable.

" The "main" system (also known as "Front of House", commonly abbreviated FOH), which provides the amplified sound for the audience, will typically use a number of powerful amplifiers driving a range of large, heavy-duty loudspeakers including low-frequency speaker cabinets called subwoofers, full-range speaker cabinets, and high-range horns. A large club may use amplifiers to provide 1000 to 2000 watts of power to the "main" speakers; an outdoor concert may use 10,000 or more watts.

" The "monitor" system reproduces the sounds of the performance and directs them towards the onstage performers (typically using wedge-shaped monitor speaker cabinets), to help them to hear the instruments and vocals. In British English, the monitor system is referred to as the "fold back". The monitor system in a large club may use amplifiers to provide 500 to 1000 watts of power to the "monitor" speakers; at an outdoor concert, there may be several thousand watts of power going to the monitor system.

At a concert in which live sound reproduction is being used, sound engineers and technicians control the mixing boards for the "main" and "monitor" systems, adjusting the tone, levels, and overall volume of the performance.

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Digital signal processors

Small PA systems for venues such as bars and clubs are now available with features that were formerly only available on professional-level equipment, such as digital reverb effects, graphic equalizers, and, in some models, feedback prevention circuits (which electronically sense and prevent feedback "howls" before they occur). These digital signal processing multi-effect devices offer sound engineers a huge range of sound processing options (reverb, delay, echo, compression, etc.) in a single unit. In previous decades, sound engineers typically had to transport a number of heavy "rack-mounted" cases of analog effect devices.

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A number of PA companies are now making lightweight, portable speaker systems for small venues that route the low-frequency parts of the music (electric bass, bass drum, etc.) to a separately-powered subwoofer. Routing the low-frequency parts of the signal to a separate amplifier and low-frequency subwoofer can substantially improve the bass-response of the system. As well, the clarity of the overall sound reproduction can be enhanced, because low-frequency sounds take a great deal of power to amplify; with only a single amplifier for the entire sound spectrum, the power-hungry low-frequency sounds can take a disproportionate amount of the sound system's power.

Power amplifiers have also become lighter, smaller, more powerful and more efficient due to increasing use of Class D amplifiers, which offer significant weight and space savings as well as increased efficiency.