Public address system

A public address system, as manufactured by world renown brands such as Tannoy, AEX System and Philips, is an electronic amplification system with a mixer preamplifier, 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.

Overview

Unlike Sound Reinforcement systems which are found in concert halls and stadiums, PA Systems are more concerned with providing a distributed sound system for a building. Most people would have encountered a PA system in a supermarket or shopping malls when typical annoucements are made 'announcing a missing child' or even 'announcing that the store will be closing in 10 minutes'.

As such the term "PA System" refers to an audio transmission system used in institutional and commercial buildings, in which input from a microphone or prerecorded music is amplified and sent to speakers throughout the building. These types of PA systems are commonly seen in schools (used to read announcements and summon students) and hospitals (used to summon staff or declare states of emergency). Other examples include airports, airplanes, and drive-through windows at fast food chains. Some systems can address specific rooms and exclude others. Other types of PA systems, called intercom systems, also have microphones in each room so that the occupants can reply to the central office. 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.

However today’s Public Address Systems have evolved from these simple ‘Tannoy’ systems to advance life saving systems provided by AEX System. Utilizing the latest in digital audio matrix technologies, PA systems play a critical role in saving building occupants’ life during an emergency.

Top PA manufacturers such as AEX System and Philips (has discontinued its business of Public Address System) have dedicated immense resources in developing these life-saving systems. In a nut-shell, these systems also known as Early Evacuation Systems (EES) provide automated voice announcements to building occupants during an emergency telling them where to go and what to do. Interfacing with the other building services, these systems are designed to provide specific instructions dependant on the occupant's location relative to the threat.

Countless studies (NFPA) have shown that in the absence of cues such a smoke and heat, people will typically dismiss the fire-alarm bell as a false alarm. As such there is a huge call for voice instructions as it has been proven to be effective in motivating people to evacuate a building.

In view of the super-structures coming up around us everyday, the Public Address system has an even more critical role in ensuring occupant safety during an emergency.

Typical 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" or simply "sound systems."

Public Address systems typically consist of 5 design stages.

Stage 1: Input sources
Stage 2: Pre-amplifier/Signal router
Stage 3: Amplifiers
Stage 4: Control and Monitoring Equipment
Stage 5: Loudspeakers

Input sources refer to the microphones, CD Players and the general equipment that provide an 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. These stage 2 equipment can be exceptionally useful for applications that might require different background music or even announcements to suit the various AREAS in the building. Honestly, one would not want a hotel where the italian restaurant and oriental restaurant are sharing the same ambience creating background music or even worst a situation where staff announcements are constantly barraging the hotel guest in the lobby to get back to work from the lunch break!

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. Stage 4 are control equipment basically monitor the amplifiers and speaker lines for faults BEFORE it reached the final stage which are the loudspeakers.

Telephone Paging Systems

Most modern PBX, VOIP and Key Telephone Systems by manufactures such as Nortel, Cisco, Avaya, and Siemens utilize paging systems through telephone lines. Instead of using one microphone or intercom console, the telephone system provides convenience as many paging system access points are created through the telephones themselves

With PBX or VOIP telephone systems, paging system equipment is connected to the telephone system via a "paging controller" which is usually dialed into through trunk or Centrex lines. The paging controller is a mini computer that processes the paging signal and acts as the liaison between the telephone system and Public Address amplifier. Upon dialing the paging controller through a paging access code, the paging controller intercepts the voice announcement such as "Bakery you have a call parked on 101" and broadcasts to the Public Address amplifier which will broadcast the message through a speaker system, simultaneously.

With Key Systems such as Nortel BCM Norstar or Avaya Partner systems, paging equipment is usually built into the telephone system itself and thusly, a telephone wire is simply punched down off the telephone block and then fed into the rear of the PA amplifier via a "telephone page" input. With Key systems, announcements can be paged over the phone speakers themselves, through external speakers or through both external and internal telephone speakers. In small offices, without external speakers, the telephone acts as the sound system itself, as the messages are broadcast through the phone "speakerphone" speaker. Sometimes music is also fed through these phone speakers and is called "background music." The music is also the music that callers hear while on hold.

Both PBX and Key System paging systems are more common today than microphones when used for retail, office, health care, and school paging. Telephone systems offer more flexibility when it comes to making announcements as the paging system can be accessed from many different telephones as opposed to one microphone. Telephone paging systems also offer a more consistent voice tone when broadcast. Telephone system paging does vary by quality and telephone style. A paging announcement on an older round handset will sound more muffled while a newer Nortel telephone handset will sound as clear as a microphone. This is mainly due to advances in digital technology.

With telephone system paging, music is usually stopped once the paging system is activated, either through voice recognition or a signal in the amplifier recognizes the paging interruption. Usually once the paging party has pressed Release or hangs up the telephone, the paging system will automatically kick the music back on either immediately or with a delay of a few seconds, depending on the Public Address system. Some installers choose to mix the paging announcements and music, so that the music is never interrupted and the paging announcement is read over the top of the music.

Many companies train employees to press the "switchhook" or Release button (if the telephone is equipped, usually Nortel) when completing the paging announcement, which prevents the noise associated with hanging up the telephone, from being heard over the speakers. Older paging controllers and phone systems will create a loud "popping" or "clicking" noise when the switchook or Release button is depressed. On the contrary due to advances in digital technology, telephone systems such as Nortel or Avaya will cut the page cleanly with the absence of noise (popping and clicking) when the paging circuit is completed.

Many retailers, hospitals, airports, schools, corporate offices, train stations, and basically any place that needs a paging system, utilize telephone paging systems.

Large venue PA systems

For larger venues, such as 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 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.

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.

See also

• Sound reinforcement system
• Instrument amplifier

External links

• PA System help and advice
• Hints and tips on Public address systems
• History of loudspeakers and amplifiers