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, a DJ playing prerecorded music, 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 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.
There is disagreement over when to call these audio systems Sound Reinforcement (SR) systems or PA systems. Some audio engineers distinguish between the two by technology and capability, while others distinguish by intended use, e.g., SR systems are for live music, whereas PA systems are usually for reproduction of speech and recorded music in buildings and institutions). This distinction is important in some regions or markets, while in other regions or markets the terms are interchangeable. In colloquial British English, a PA system installed for public address in a building is sometimes referred to as a "Tannoy" system after the company of that name. 
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. A sound source such as a CD player or radio may be connected to a PA system so that music can be played through the system.
Public address systems typically consist of input sources, preamplifiers 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 system. These input sources are fed into the 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 a country's regulations 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.
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).
Most modern telephone systems, such as PBX and VOIP, use a paging system that acts as a liaison between the telephone and a PA amplifier. In key telephone systems such as those by Nortel, Toshiba, Avaya or Alcatel-Lucent, paging equipment is usually built into the telephone system itself, and allows announcements to be paged over the phone speakers themselves, through external speakers or through both external and internal telephone speakers.
In PBX and larger VOIP telephone systems such as those by Nortel, Cisco, Avaya, Siemens or Alcatel-Lucent, used for larger enterprise applications, paging equipment is not built into the telephone system. Instead the system provider must provide a separate paging controller connected to a trunk port on the actual telephone system. The paging controller is accessed as either an unused directory number or unused central office line. Access to the paging system is provided through a "trunk access" code or a pre-programmed feature button on the telephone set itself.
Many retailers and offices choose to use the telephone system as the sole access point for the paging system, because the equipment is already "paging system"-ready. 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 of the school in an emergency. One disadvantage of telephone paging systems compared to microphone paging systems, is that the noise associated with hanging up the telephone can be heard over the speakers.
"PA Over IP" refers to PA paging and intercom systems that use an ethernet or GSM-R network instead of a centralized analog or DSP amplifier to distribute the paging to all of the locations in a building or company. Distributed network attached amplifiers and intercom units are used to provide the communication function. At the transmission end, a computer with specialized software broadcasts the audio data digitally over the local area network, using audio from the computer's sound card inputs or from stored audio files. At the receiving end, specialized intercom modules receive these network transmissions and reproduce the analog audio signal. These are small specialized network appliances with an IP address just like any other computer on the network.
Such systems are connected using standard networking infrastructure and thus allow more flexible configurations than those possible using traditional analog PA wiring. For example, a user can have multiple remote sites tied together through the local area network (LAN) and through the internet so that one location can be used to send audio signals to any or all of these locations. Use of the internet allows PA systems to span multiple buildings, and allows for long-range PA receivers in satellite locations. It is also possible to provide for multiple or relocatable transmission (control) stations on such a network.
Long Line Public Address (LLPA) describes any Public Address system in which the architecture is distributed, 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 IP based media such as optical fiber, or GSM-R. 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 signalling 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 the IP network and are played on local amplification equipment. Taken together, the PA, routing, DVA, passenger displays and PIS interface are commonly referred to as the Customer Information System or CIS, a term which itself is often used interchangeably with Passenger Information System (PIS)
As the number of album and singles sales falls each year, artist are relying more and more upon the income from live performances and tours. 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 a myriad of purposes. Local companies may specialise in small systems tailored to clubs, pubs and small outdoor events whereas larger companies will cater for concert halls.
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.
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.
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 prevent 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.
In recent years, a number of technological advances have been made to PA systems.
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 become a problem. Digital effects units may offer multiple pre-set and variable reverb, echo and related effects. Digital loudspeaker management systems offer sound engineers digital delay, limiting, crossover functions, EQ filters, compression an other functions in a single rack-mountable unit. In previous decades, sound engineers typically had to transport a substantial number of rack-mounted analog devices to accomplish these tasks.
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 powered subwoofer. Routing the low-frequency energy to a separate amplifier and subwoofer can substantially improve the bass-response of the system. Also, clarity may 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 switching power supplies and Class D amplifiers, which offer significant weight and space savings as well as increased efficiency. In the 1970s and 1980s, most PA amplifiers were heavy Class AB amplifiers. In the late 1990's these lightweight technologies spread into PA applications. Installations in railroad stations, stadia and airports, their high efficiency allow them to run with minimal additional cooling and with higher rack densities compared to older amplifiers.