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Sound distribution systems,Personnel call systems

Sound distribution systems

Sound-distribution systems consist essentially of loudspeakers permanently installed in suitable position in buildings or in open spaces associated with buildings. 

They are essentially part of the telecommunications systerm of buildings. The current which operate such systems are derived from a microphone , gramophone, radio receiver or other device, or from a wire broadcasting service. 

These currents are of a very small order and so requireto be amplified to values suitable for the operation of loudspeakers. Sound-distribution systems are found in schools, theatres and cinemas, churches, meeting halls, factories, offices and department stores, hotels and clubs, hospitals, railway stations and sports grounds. 

Though these systems generally operate from mains supplies, some systems, or parts thereof, operate from batteries or from mains-supplied rectified, producing low voltages

Personnel call systems

These systems are used in private dwelling, hotels, schools, factories and other premises where it is required to attract the attention of individuals to a situation or circumstance. The simplest system is where a person is called to a particular position by a called.In a private house, the householder is called to the door. A bell push or similar device is fitted at each such position and an indicator provide to show which push has been operated. A bell or buzzer is used to provide the sound which will attrack attention to the call. Bell pushes can be of the wall-mounted, table or pendant type,the contact point are of a metal which gives long service without becoming pitted or corroded.If the bellpush is to be installed outside,protection against the ingress of moisture must be provided.

Indicators are installed in a central position in the building. In large premises, such as hotels and factories,the indicator board is located in a room in which some person is always in attendance, e.g,kitchen or reception office. The use of lamps is necessary where the sound of bells might be either objectionable or useless e.g:in hospital at night or in noisy workshops.Hand-setting indicators should be mounted at a height convenient for access and visibility.

Multiple-call system are used in very large hotels where the call point are too many to be indicate conveniently on a single indicator board or pannel. Pushes are fitted at each call point but the circuits are grouped to serve a corridor or floor.Each group gives the indication in a central service room. In these systems, arrangement must be made to have attendants on duty in corridor or floor to deal wit the calls. Multiple-call systems use indicators which have to be reset by the attendant.

Time-bell systems are common in schools and factories to indicate the beginning or end of a time or period(e.g:break,class change etc), These systems usually have one or two pushes other switches connected in parallel and a number of bells throughout the building which are also connected in parallel.The bells can be controlled from a clock system, to eliminate the human element required with bell pushes. The burglar-alarm system is also a call system.The switches in this case are sets of contacts mounted at door and windows.There are two circuits type, open-circuits and close-circuit.The first type requires contacts to close to energise the bell circuit.In the closed-circuit type all contacts are closed.A series relay with normally-open contacts is energised by a circulating current.When a contact set is openned this current ceases to flow,de-energises the relay and closes the relay contacts to ring n alarm bell.Some alarm systems operate from photo-electric cells which work when an invisible light beam is broken.

The large plate-glass windows of jewellers shops often have a series length of very thin wire which if broken when the window is smashed in or ahole cut in it, will bring the relay into operation to ring a bell.In certain systems today no bell rings. But a buzzer and light indication circuit is wired from the protected building and terminated at a nearby police have the opportunity of catching the burglar red-handed.

The open-circuit system is seldom used because it can be inerfered with.For instance, a cut in a wire will render the complete system inoperative, where as such a break in the series circuit of a circulating-current (closed-circuit) system will immediately set an alarm-bell ringing.Supplies are sometimes from the mains, but in this instance a standby-battery supply is provided in the event of a power failure. Alarm bells are often installed in a placed inaccessible to un athorised person and outside the building.
Another type of call or alarm system is the watchman supervisory service. 

It is designed to provide a recorded indicator of the visits of watchmen or guards to difference parts of a building in the course of the duty round.The system uses a clock movement of the impulse synchronous-time controlled a.c or 8-day clockwork type installed at each contact station througth out the building.Each station has a box with a bellpush operated by the insertion of a special key.Operation of the contacts energises an electromagnetically-operated marker which records the time of the visit on a paper marked off in hours. In some systems, an alarm is given after a predetermined time if the watchmail fails to clock in at any contact station.


Luminous call system are used instead of bells.These system use colour edlight which  staff to fulfill a service duty.They are largely used in hospital and hotels.When the bellpush is presed in any position in the building,a small lamp lights in a duty room to indicate the general area from which the call has come.Alternatively a lamp outside the call room light and remains so untill an attendant extinguishes it by operating a reset push located just outside the room.Some systems incorporated in lift systems.


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