Warning systems

Purpose of public address systems

In daily life, we often meet the public address systems of facilities and open objects. This may be an ad on the subway, supermarket advertising, emergency search of an employee at the plant, station announcements, music at the cafeterias, etc. Any of industrial, health care or culture object can not be imagined without sound alert system – a so called public address system. It should also be noted that the main function of these systems - warning of all people in case of fire or some other emergency and the management of evacuation from dangerous areas. The major piece of legislation to equip object’s public address systems in Ukraine is ДБН B.1.1-7-2002 (“The fire safety of the building”). According to this document, public address systems should ensure the timely dissemination of information about a fire or other emergency and to facilitate rapid evacuation of people from the dangerous zone. In daily life, public address system is usually used for voice, informative or advertising messaging all over the area of an building.

Development and installation of advanced public address system is performed while taking into account not only the area, but the geometric shape of rooms, the ceiling height, wall material, the noise level of premises and so on.

To determine the level of noise at the facility you can measure it or define it using the tables with approximate noise levels for different types of premises. Based on that data and considering the architecture, the elongation of the room, the ceiling height, the geometric shape of the object, different types of speakers and acoustic solutions must be used.

The right choice of speaker type and its location in the room affects the uniformity of the sound pressure and prevents the imposition of sound waves and the formation of "sound holes", reduces excessive reverb and general boominess of premises. This contributes to the intelligibility and even distribution of sound in indoor and outdoor areas (platform stations, parade grounds, school yards).

Speakers used in public address systems vary by conditions of location and usage. These speakers main difference from common speakers is the adaptation to work with the sound source operating at a voltage of 70-100 V. This in turn provides a high quality signal transmission to a large number of speakers of different capacities and long transmission distances without use of expensive speaker cable. In public address systems speakers are mounted in parallel connection to one or more lines (in case of multi-zone alert system) by two conductor copper wire in a grounded metal hose. In multi-zone systems speakers are grouped into zones, determining the number of cables going from the amplifier.

There are several types of speakers:

- Ceiling type speakers are used in relatively low premises with large horizontal area. They provide an even distribution of sound pressure regardless of the furniture and the occupancy of the room. The sound of these speakers is directed downward. Speakers are mounted directly to the ceiling – surface or recessed.

- Wall type speakers are used in premises where there is no technical possibility of implementing ceiling speakers. These speakers have better quality and intelligibility of sound, since they have a full SB-plastic case. Also wall speakers are made in a wooden case. But while using a wall-mounted speakers great attention must be paid to the place, direction and mount height of the speakers. For example, the location of wall speakers in corners of the room is likely to cause the effect of a "standing wave" and will lead to the imposition/overlay, and sometimes increase the damping of sound out of low frequencies and have a negative impact on the quality and clarity of sound programs.

- Weatherproof type horn loudspeakers are used in open and closed facilities (indoor) with sufficiently low temperatures or high humidity. Loudspeakers are usually not suffer from direct moisture and work reliably at low temperatures, constant exposure to moisture and direct sunlight. These speakers are manufactured in metal or plastic case. These speakers are used for messages in manufacture, mines, a large warehouse premises, terminals, where there is a possibility of condensation at frequent temperature changes. This type of loudspeakers has a narrow frequency working range, but quite a large sound pressure and accurate angle direction of acoustic signal. So these loudspeakers are great for using at large, open spaces. It should be noted that the use of loudspeakers in areas with limited space will lead to a sharp decrease in the intelligibility of sound signals.

Power amplifiers are used to ensure a stronger broadcast signal. These amplifiers have a high power output, and advanced protection system on case of line failures.

Amplifiers used in complex public address systems can diagnose the state of the loudspeaker lines, making it possible to quickly identify faulty or improperly connected one. Small public address systems with a low number of independent zones are built using amplifiers having switching up to 5 alarm zones. This greatly simplifies the system itself making all additional delimiters useless. Sometimes amplifiers contain digital playback sound-modules and gong-generators making complete sound solution.

The remote microphone panels are also used to supply voice messages to selected zones or all zones simultaneously. To mix a large number of input signals a so-called mixers must be used. Use of graphic equalizers and other devices to improve and correct audio signal characteristics are widely used in modern public address systems.

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