Information about 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.

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.^[1]

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.

In North America, PA systems are also sometimes referred to as "sound reinforcement systems" ^[2] or simply "sound systems." ^[3]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. ^[4]

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 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.

Telephone paging systems

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 or Avaya, 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 or Siemens, 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 preprogrammed 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. That is, the business does not have to buy a separate intercom or microphone. An additional advantage, is that each telephone can access the paging system, which makes initiating a page much more convenient than having just one microphone. 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 unless the user takes the initiative to press the "switchhook" on the telephone or if the phone is equipped, pressing the Release (RLS) button, which is most commonly found on Nortel telephone systems.

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 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.^[5]

• 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.

Acoustic feedback

All PA systems have a potential for feedback, which occurs when sound from the speakers returns to the microphone and is then re-amplified and sent through the speakers again. This generally manifests itself as a sharp, sudden high-volume piercing sound which can damage the loudspeakers' high-frequency horns or tweeters - and audience members' hearing.

Sound engineers take several steps to prevent feedback, including ensuring that microphones are not pointed towards speakers, keeping the onstage volume levels down, and lowering frequency levels where the feedback is occurring, using a graphic equalizer, parametric equalizer a combination of both devices, or a notch filter.

Recent developments

In recent years, a number of technological advances have been made to PA systems.

PA speakers

High-end PA speakers have been made lighter by using neodymium speaker magnets, and horns are often wired using protective circuitry such as light bulbs (which illuminate and absorb excess wattage) or polyswitches that protect the horn from damage in the event of feedback or a dropped microphone. These new approaches to speaker protection are more convenient than the formerly used approach of fuses, because the sound system needs to be turned off to change fuses.

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.

Amplifiers

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. Many power amplifiers now use digital switching transformers, for significant weight and space savings as well as increased efficiency.

Other meanings

The term "Public Address" also may refer to any IP address that is not in RFC 1918 "Private networking" scheme and is routable on the Internet.

Footnotes

See also

• Sound reinforcement system
• Instrument amplifier

External links

• PA System Advice
  History of loudspeakers and amplifiers