Public address system

This article is about audio systems. For public IP addresses, see IP address.

"Public announcement" redirects here. For the musical group, see Public Announcement (group).

Horn loudspeakers are often used to broadcast sound in outdoor locations

US Navy Rear Admiral Michelle J. Howard uses the 1MC shipboard public address system to address the crew of USS Wasp (LHD 1)

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 or at a sports stadium. 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, such as recorded music or a person giving a speech or 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 previously in Britain.

Early systems

Early public-address system from around 1920 using a Magnavox speaker. The microphone (then called a "transmitter") had a metal reflector which concentrated the sound waves, allowing the speaker to stand back so it wouldn't obscure his face. The early vacuum tubes couldn't produce much gain, and even with six tubes the amplifier had an output power of only about 10 watts. To produce enough volume, the system used a horn loudspeaker. The cylindrical driver unit under the horn contained the diaphragm which was vibrated by the voice coil, and the sound waves produced were conducted to the open air through the flaring horn. The function of the horn was to couple the diaphragm more efficiently with the air, so horn speakers produced far more sound power from a given amplifier than a cone speaker. Horns were used in virtually all early PA systems, and in most systems today.

Edwin Jensen and Peter Pridham of Magnavox began experimenting with sound reproduction in the 1910s; working from a laboratory in Napa, California, they filed the first patent for a moving coil loudspeaker in 1911. Four years later, in 1915, they built a dynamic loudspeaker with a 1-inch (2.5 cm) voice coil, a 3-inch (7.6 cm) corrugated diaphragm and a horn measuring 34 inches (86 cm) with a 22-inch (56 cm) aperture. The electromagnet created a flux field of approximately 11,000 G.

Their first experiment used a carbon microphone. When the 12 V battery was connected to the system, they experienced one of the first examples of acoustic feedback. They then placed the loudspeaker on the laboratory's roof, and claims say that the amplified human voice could be heard 1 mile (1.6 km) away. Jensen and Pridham refined the system and connected a phonograph to the loudspeaker to be able to broadcast recorded music. They did this on a number of occasions, including once at the Napa laboratory, at the Panama-Pacific Exposition, and on December 24, 1915 at San Francisco City Hall alongside Mayor James Rolph. This demonstration was official presentation of the working system, and approximately 100,000 people gathered to hear Christmas music and speeches "with absolute distinctness".

The first outside broadcast was made one week later, again supervised by Jensen and Pridham. On December 30, when Governor of California Hiram Johnson was too ill to give a speech in person, loudspeakers were installed at the Civic Auditorium in San Francisco, connected to Johnson's house some miles away by cable and a microphone, from where he delivered his speech. Jensen oversaw the governor using the microphone while Pridham operated the loudspeaker.

The following year, Jensen and Pridham applied for a patent for what they called their "Sound Magnifying Phonograph". Over the next two years they developed their first valve amplifier. In 1919 this was standardized as a 3-stage 25 watt amplifier.

This system was used by former US president William Howard Taft at a speech in Grant Park, Chicago, and first used by a current president when Woodrow Wilson addressed 50,000 people in San Diego, California. Wilson's speech was part of his nationwide tour to promote the establishment of the League of Nations. It was held on September 9, 1919 at City Stadium. As with the San Francisco installation, Jensen supervised the microphone and Pridham the loudspeakers. Wilson spoke into two large horns mounted on his platform which channelled his voice into the microphone. Similar systems were used in the following years by Warren G. Harding and Franklin D. Roosevelt.

By the early 1920s, Marconi had established a department dedicated to public address and began producing loudspeakers and amplifiers to match a growing demand. In 1925, George V used such a system at the British Empire Exhibition, addressing 90,000 via six long-range loudspeakers. This public use of loudspeakers brought attention to the possibilities of such technology. The 1925 Royal Air Force Pageant at Hendon Aerodrome used a Marconi system to allow the announcer to address the crowds, as well as amplify the band. In 1929, the Schneider Trophy race at Calshot Spit used a public address system that had 200 horns, weighing a total of 20 tons.

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 IP network instead of a centralized amplifier to distribute the audio signal to paging locations across a building or campus, or anywhere else in the reach of the IP network (including the Internet). 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, either 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 / 23.30 / First_Great_Western /Night_Riviera_sleeper_service / from / London_Paddington / to / Penzance / .... / will depart from platform / one / 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 the term passenger information system.

Small venue systems

Small clubs and bars use a fairly simple set-up, with large Front of House speakers and subwoofers aimed at the audience, and smaller monitor speakers aimed back at the performers so that they can hear their vocals and instruments. In many cases, the Front of House speakers are elevated, either by mounting them on poles or by "flying" them from anchors in the ceiling. The subwoofers do not need to be elevated. In the smallest coffeehouses and bars, the audio mixer may be onstage so that the performers can mix their own sound levels. In larger bars, the audio mixer may be located in or behind the audience seating area, so that an audio engineer can listen to the mix and adjust the sound levels. The adjustments to the monitor speaker mix may be made by a single audio engineer using the main mixing board, or they may be made by a second audio engineer who uses a separate mixing board.

This small venue's stage shows a typical PA system.

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 a mixing board, sound processing equipment, amplifiers, and speakers. The microphones that are used to pick up vocals and amplifier sounds are routed through both the main and monitor systems. Audio engineers can set different sound levels for each microphone on the main and monitor systems. For example, a backup vocalist whose voice has a low sound level in the main mix may ask for a much louder sound level through her monitor speaker, so she can hear her singing.

• 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.

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

Touring productions will travel with relocatable large line-array PA systems, sometimes rented from an audio equipment hire company. The sound equipment moves from venue to venue along with various other equipment such as lighting and projection.

Acoustic feedback

All PA systems have a potential for audio feedback, which occurs when sound from the speakers is picked up by the microphone and is then re-amplified and sent through the speakers again. It often sounds like a loud high-pitched squeal or screech, and can occur when the volume of the system is turned up too high. Feedback only occurs when the loop gain of the feedback loop is greater than one, so it can always be stopped by reducing the volume sufficiently. Sound engineers take several steps to maximize gain before feedback, including keeping microphones at a distance from speakers, 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.

See also

• Announcer
• Instrument amplifier
• Sound reinforcement system
• Sound system

External links

• History of Public Address