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What is Public Addres ?

Horn loudspeakers are often used to broadcast sound to outdoor locations

A public address system (PA system) is an electronic sound amplification and distribution system with a microphone, amplifier and loudspeakers, used to allow a person to address a large public, for example for announcements of movements at large and noisy air and rail terminals.

The term is also used for systems which may additionally have a mixing console, and amplifiers and loudspeakers suitable for music as well as speech, used to reinforce a sound source, e.g., recorded music or a person giving a speech, and distributing the sound throughout a venue or building.

Simple PA systems are often used in small venues such as school auditoriums, churches, and small bars. PA systems with many speakers are widely used to make announcements in public, institutional and commercial buildings and locations. Intercom systems, installed in many buildings, have microphones in many rooms allowing the occupants to respond to announcements.

Sound reinforcement systems and PA systems may use some similar components, but with differing application, although the distinction between the two is not clear-cut. Sound reinforcement systems are for live music or performance, whereas PA systems are primarily for reproduction of speech. In Britain any PA system is sometimes colloquially referred to as a Tannoy, after the company of that name now owned by TC Electronic Group, which supplied a great many of the PA systems used in the past.

Small systems

Public address system in a school

The simplest PA systems consist of a microphone, an amplifier, and one or more loudspeakers. Simple and small 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. A sound source such as a Compact Disc player or radio may be connected to a PA system so that music can be played through the system.

Public address systems consist of input sources, amplifiers, control and monitoring equipment, and loudspeakers. The primary input sources are microphones for live announcements and a source of recorded sound. There may be a system which allows operators, or automated equipment, to select from a number of standard prerecorded messages. These input sources are fed into preamplifiers and signal routers that determine the zones to which the audio signal is fed.

The preamplified signals are then passed into the amplifiers. Depending on local practices these amplifiers will usually 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. This control equipment is also used for separating zones in a PA system. The loudspeaker is used to convert electrical signals into sound.

Large systems

Public Address System consisting of amplifiers, mixers and routers for a major international airport

Some PA systems have speakers that cover an entire campus of a college or industrial site, or an entire outdoor complex (e.g., an athletic stadium). A large PA system may also be used as an alert system during an emergency.

Telephone paging systems

Some analog or IP private branch exchange (PBX) telephone systems use a paging facility that acts as a liaison between the telephone and a PA amplifier. In other systems, paging equipment is not built into the telephone system. Instead the system includes a separate paging controller connected to a trunk port of the telephone system. The paging controller is accessed as either a designated directory number or central office line. In many modern systems, the paging function is integrated into the telephone system, and allows announcements to be played over the phone speakers.

Many retailers and offices choose to use the telephone system as the sole access point for the paging system, because the features are integrated. Many schools and other larger institutions are no longer using the large, bulky microphone PA systems and have switched to telephone system paging, as it can be accessed from many different points in the school.

PA over IP

PA over IP refers to PA paging and intercom systems that use an Ethernet or GSM-R network instead of a centralized amplifier to distribute the audio signal to all paging locations in a building or campus. Network-attached amplifiers and intercom units are used to provide the communication function. At the transmission end, a computer application transmits a digital audio stream via the local area network, using audio from the computer's sound card inputs or from stored audio recordings. At the receiving end, specialized intercom modules (sometimes known as IP speakers) receive these network transmissions and reproduce the analog audio signal. These are small specialized network appliances addressable by an IP address just like any other computer on the network.

Such systems are inter-connected by the networking infrastructure and thus allow loss less transmission to remote locations across the Internet or a local area or campus network. It is also possible to provide for multiple or relocatable transmission control stations on such a network.

Long line PA

London Underground Employee making a Long Line Public Address system   announcement using an RPA01 Radio Microphone at Bank Station

A Long-line public address (LLPA) system is any public address system with a distributed architecture, normally across a wide geographic area. Systems of this type are commonly found in the rail, light rail and metro industries and allow announcements to be triggered from one or several locations to the rest of the network over low bandwidth legacy copper, normally PSTN lines using DSL modems, or media such as optical fiber, or GSM-R, or IP-based networks.

Rail systems typically have an interface with a passenger information system (PIS) server, at each station linked to train describers which state the location of rolling stock on the network from sensors on trackside signaling equipment. The PIS system invokes a stored message to be played from a local or remote digital voice announcement system, or a series of message fragments to be assembled in the correct order, for example: / the / 13.29 / Virgin_Trains / sleeper_service / from / London_Paddington / to / Penzance / .... / will depart from platform / five / this train is formed of / 12_carriages /.

Messages are routed via an IP network and are played on local amplification equipment. Taken together, the PA, routing, DVA, passenger displays and PIS interface are referred to as the customer information system (CIS), a term which itself is often used interchangeably with passenger information system.

PA on Tour

Touring bands will source a large line-array PA system from an audio equipment hire company with reliable service to take from venue to venue along with various other equipment such as lighting and projection. PA hire companies typically will provide "sound solutions" for myriad purposes. Local companies may specialise in small systems tailored to clubs, pubs and small outdoor events whereas larger companies will cater for concert halls.

A line array speaker system and subwoofer cabinets at a live music concert

Large venue 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 system consists of microphones, a mixing board, sound processing equipment, amplifiers, and speakers.

  • 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 3000 to 5000 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 "foldback". The monitor system in a large club may provide 500 to 1000 watts of power to several foldback 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.

Acoustic feedback

All PA systems have a potential for audio feedback, which occurs when sound from the speakers returns to the microphone and is then re-amplified and sent through the speakers again. Sound engineers take several steps to maximize gain before feedback, including ensuring that directional microphones are not pointed towards speakers, keeping the onstage volume levels down, and lowering gain levels at frequencies where the feedback is occurring, using a graphic equalizer, a parametric equalizer, or a notch filter.


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