Public address
From Free net encyclopedia
A public address system, abbreviated PA system, is an electronic amplification system used as a communication system in public areas. It is an amplification setup 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. The PA system is controlled from a mixer which blends the incoming signals from microphones and musical instruments, modifies the tone, and controls the overall level of the signal that is sent to the amplifier.
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 are often used in small venues such as school auditoriums, churches, and small bars. In colloquial British English, a PA system is often referred to as a "Tannoy" after the company of that name. In North America, PA systems are also sometimes referred to as "sound reinforcement systems" or simply "sound systems."
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.
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Large Venue PA Systems
For larger venues, such as popular music concerts, a larger, 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, 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. 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.
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. 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 device called a graphic equalizer.
Recent Developments
Since the mid-2000s, a number of technological advances have been made to PA systems. Modestly-powered PA systems for small venues 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). As well, 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. This innovation can substantially improve the bass-response of the PA system and enhance the clarity of the overall sound reproduction.
Professional-level live sound reproduction equipment has also been substantially improved. PA speakers have been made lighter by using neodynium speaker magnets, and horns are often wired using fuses or other circuitry that protect the horn from damage if feedback occurs. As well, compact digital multi-effect devices have been developed that offer sound engineers a huge range of tonal 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. Another innovation is the development of "in-ear" monitors, which allow the musicians to hear the musical performance without using loud onstage monitor speakers.
Other Meanings
The term "Public Address" also may refer to any IP address that is not in RFC 1918 "Private Addressing" scheme and is routable on the internet.