Public Address

Small systems				Information System or CIS, a term which itself is
The simplest PA systems consist of a				often used interchangeably with Passenger
microphone, a modestly-powered mixer-amplifier				Information System (PIS)
(which incorporates a mixer and an amplifier in a				PA on Tour
single cabinet) and one or more loudspeakers.				As the number of album and singles sales falls
Simple PA systems of this type, often providing				each year, artist are relying more and more upon
50 to 200 watts of power, are often used in				the income from live performances and tours.
small venues such as school auditoriums, churches,				Touring bands will source a large line-array PA
and small bars. A sound source such as a CD				system from an audio equipment hire company
player or radio may be connected to a PA				with reliable service to take from venue to venue
system so that music can be played through the				along with various other equipment such as lighting
system.				and projection. PA hire companies typically will
Public address systems typically consist of input				provide "sound solutions" for a myriad of
sources, preamplifiers and/or signal routers,				purposes. Local companies may specialise in small
amplifiers, control and monitoring equipment, and				systems tailored to clubs, pubs and small outdoor
loudspeakers. Input sources refer to the				events whereas larger companies will cater for
microphones and CD Players that provide a sound				concert halls.
input for the system. These input sources are fed				Large venue systems
into the preamplifiers and signal routers that				For popular music concerts, a more powerful and
determine the zones to which the audio signal is				more complicated PA System is used to provide
fed. The preamplified signals are then passed into				live sound reproduction. In a concert setting, there
the amplifiers. Depending on a country's				are typically two complete PA systems: the
regulations these amplifiers will amplify the audio				"main" system and the "monitor" system. Each
signals to 50V, 70V or 100V speaker line level.				system consists of microphones, a mixing board,
Control equipment monitors the amplifiers and				sound processing equipment, amplifiers, and
speaker lines for faults before it reaches the				speakers.
loudspeakers.				A line array speaker system and subwoofer
Larger systems				cabinets at a live music concert
Some PA systems have speakers that cover an				The "main" system (also known as "Front of
entire campus of a college or industrial site, or an				House", commonly abbreviated FOH), which
entire outdoor complex (e.g., an athletic stadium).				provides the amplified sound for the audience, will
Horn loudspeakers are often used to broadcast				typically use a number of powerful amplifiers
sound to outdoor locations				driving a range of large, heavy-duty loudspeakers
Telephone paging systems				including low-frequency speaker cabinets called
Most modern telephone systems, such as PBX				subwoofers, full-range speaker cabinets, and
and VOIP, use a paging system that acts as a				high-range horns. A large club may use amplifiers
liaison between the telephone and a PA amplifier.				to provide 3000 to 5000 watts of power to the
In key telephone systems such as those by				"main" speakers; an outdoor concert may use
Nortel, Toshiba, Avaya or Alcatel-Lucent, paging				10,000 or more watts.
equipment is usually built into the telephone				The "monitor" system reproduces the sounds of
system itself, and allows announcements to be				the performance and directs them towards the
paged over the phone speakers themselves,				onstage performers (typically using wedge-shaped
through external speakers or through both				monitor speaker cabinets), to help them to hear
external and internal telephone speakers.				the instruments and vocals. In British English, the
In PBX and larger VOIP telephone systems such				monitor system is referred to as the "foldback".
as those by Nortel, Cisco, Avaya, Siemens or				The monitor system in a large club may provide
Alcatel-Lucent, used for larger enterprise				500 to 1000 watts of power to several foldback
applications, paging equipment is not built into the				speakers; at an outdoor concert, there may be
telephone system. Instead the system provider				several thousand watts of power going to the
must provide a separate paging controller				monitor system.
connected to a trunk port on the actual telephone				At a concert in which live sound reproduction is
system. The paging controller is accessed as				being used, sound engineers and technicians
either an unused directory number or unused				control the mixing boards for the "main" and
central office line. Access to the paging system is				"monitor" systems, adjusting the tone, levels, and
provided through a "trunk access" code or a				overall volume of the performance.
pre-programmed feature button on the telephone				Acoustic feedback
set itself.				All PA systems have a potential for audio
Many retailers and offices choose to use the				feedback, which occurs when sound from the
telephone system as the sole access point for				speakers returns to the microphone and is then
the paging system, because the equipment is				re-amplified and sent through the speakers again.
already "paging system"-ready. Many schools and				Sound engineers take several steps to prevent
other larger institutions are no longer using the				feedback, including ensuring that directional
large, bulky microphone PA systems and have				microphones are not pointed towards speakers,
switched to telephone system paging, as it can be				keeping the onstage volume levels down, and
accessed from many different points of the				lowering gain levels at frequencies where the
school in an emergency. One disadvantage of				feedback is occurring, using a graphic equalizer, a
telephone paging systems compared to				parametric equalizer, or a notch filter.
microphone paging systems, is that the noise				Recent developments
associated with hanging up the telephone can be				In recent years, a number of technological
heard over the speakers.				advances have been made to PA systems.
PA Over IP				Digital signal processors
"PA Over IP" refers to PA paging and intercom				Small PA systems for venues such as bars and
systems that use an ethernet or GSM-R network				clubs are now available with features that were
instead of a centralized analog or DSP amplifier to				formerly only available on professional-level
distribute the paging to all of the locations in a				equipment, such as digital reverb effects, graphic
building or company. Distributed network attached				equalizers, and, in some models, feedback
amplifiers and intercom units are used to provide				prevention circuits which electronically sense and
the communication function. At the transmission				prevent feedback "howls" before they become a
end, a computer with specialized software				problem. Digital effects units may offer multiple
broadcasts the audio data digitally over the local				pre-set and variable reverb, echo and related
area network, using audio from the computer's				effects. Digital loudspeaker management systems
sound card inputs or from stored audio files. At				offer sound engineers digital delay, limiting,
the receiving end, specialized intercom modules				crossover functions, EQ filters, compression an
receive these network transmissions and				other functions in a single rack-mountable unit. In
reproduce the analog audio signal. These are small				previous decades, sound engineers typically had to
specialized network appliances with an IP address				transport a substantial number of rack-mounted
just like any other computer on the network.				analog devices to acomplish these tasks.
Such systems are connected using standard				Amplifiers
networking infrastructure and thus allow more				A number of PA companies are now making
flexible configurations than those possible using				lightweight, portable speaker systems for small
traditional analog PA wiring. For example, a user				venues that route the low-frequency parts of the
can have multiple remote sites tied together				music (electric bass, bass drum, etc.) to a
through the local area network (LAN) and through				powered subwoofer. Routing the low-frequency
the internet so that one location can be used to				energy to a separate amplifier and subwoofer can
send audio signals to any or all of these locations.				substantially improve the bass-response of the
Use of the internet allows PA systems to span				system. Also, clarity may be enhanced, because
multiple buildings, and allows for long-range PA				low-frequency sounds take a great deal of power
receivers in satellite locations. It is also possible to				to amplify; with only a single amplifier for the
provide for multiple or relocatable transmission				entire sound spectrum, the power-hungry
(control) stations on such a network.				low-frequency sounds can take a disproportionate
Long Line PA				amount of the sound system's power.
Long Line Public Address (LLPA) describes any				Power amplifiers have also become lighter,
Public Address system in which the architecture is				smaller, more powerful and more efficient due to
distributed, normally across a wide geographic				increasing use of switching power supplies and
area. Systems of this type are commonly found				Class D amplifiers, which offer significant weight
in the rail, light rail and metro industries and allow				and space savings as well as increased efficiency.
announcements to be triggered from one or				In the 1970s and 1980s, most PA amplifiers were
several locations to the rest of the network over				heavy Class AB amplifiers. In the late 1990's
low bandwidth legacy copper (normally PSTN lines				these lightweight technologies spread into PA
using DSL modems), or IP based media such as				applications. Installations in railroad stations, stadia
optical fiber, or GSM-R. Rail systems typically have				and airports, their high efficiency allow them to
an interface with a Passenger information system				run with minimal additional cooling and with higher
(PIS) server, at each station linked to train				rack densities compared to older amplifiers.
describers which state the location of rolling stock				Other meanings
on the network from sensors on trackside				The term "Public Address" also may refer to any
signalling equipment . The PIS system invokes a				IP address that is not in RFC 1918 "Private
stored message to be played from a local or				networking" scheme and is routable on the
remote Digital Voice Announcement system, or a				Internet.
series of message fragments to be assembled in				See also
the correct order. for example //the//13.29/				Sound reinforcement system
virgin_trains//sleeper_service//from/				Instrument amplifier
London_Paddington//to//Penzance//....//will depart				Footnotes
from platform//five//this train is formed of /				^ Borgerson, Bruce. "Is it P.A. or SR?." Sound
12_carriages//. Messages are routed via the IP				& Video Contractor. 1 Nov. 2003. Prism
network and are played on local amplification				Business Media. 18 Feb. 2007 .
equipment. Taken together, the PA, routing, DVA,				^ Compact Oxford English Dictionary "Tannoy":
passenger displays and PIS interface are				^ Mesnik, Bob.
commonly referred to as the Customer

Wire It

Public Address