Public address
From Wikipedia, the free encyclopedia
A public address system, abbreviated PA system, is an electronic amplification system with an amplifier and loudspeakers, used to reinforce a given sound (e.g.,a person making a speech, prerecorded music, or a live musical performance) so that the audience can hear it clearly.
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[edit] Overview
The PA system is controlled from a mixer which blends the incoming signals from microphones, musical instruments and other devices, modifies the tone, and controls the overall level of the signal that is sent to the amplifier. In certain venues, there is a permanently-installed sound system which is called the "house system". When live performances are put on in other venues such as arenas, stadiums and fields which are mainly used for live sports, portable sound systems are set up by sound companies.
The term "PA system" also 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. [1]
[edit] Simple 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]
[edit] 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.
[edit] 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.
[edit] Recent developments
In recent years, a number of technological advances have been made to PA systems.
[edit] 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.
[edit] 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.
[edit] 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. Another innovation is the development of "in-ear" monitors, which allow the musicians to hear the musical performance without using loud onstage monitor speakers.
[edit] 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.
[edit] Footnotes
- ^ Compact Oxford English Dictionary "Tannoy": http://www.askoxford.com/concise_oed/tannoy?view=uk
- ^ An Indiana, US company offers a seminar "...for those who install, configure and calibrate sound reinforcement systems." Website is at: http://www.synaudcon.com/technicians.htm
- ^ For example, Rat Sound Systems Inc. calls itself a "sound system" supplier. The company supplies touring sound systems and sound systems for regional and corporate events. Website is at: http://ratsound.com/vdosc.htm
