Voice warning systems for fires.
In particular, speech alarm systems are mandatory for almost all public buildings with an area of over 1000 m2 or with a number of floors higher than 3.
Unfortunately, the current document only touches upon the requirements for the system in general terms. Thus, zonality (sequentiality) of notification in multi-story buildings is required, as well as preliminary notification of the building personnel. There is a general recommendation to use pre-recorded messages, rather than a live voice (an insufficiently restrained announcer can, on the contrary, cause panic). There are absolutely no requirements for the quality of notification (intelligibility of speech), and in extreme conditions the external background noise level increases significantly. The designer is left with the discretion of the system self-monitoring issues.
Monitoring of operability
In many cases, periodic testing of the system is sufficient. Often this happens by itself — if the system is used on a daily basis to broadcast background music or work announcements. In this case, only specifically «fire» subsystems are subject to special regular testing. For example, a block of pre-recorded messages.
In special cases, when there is a large number of people (potentially increased number of victims), and when using the system to ensure the daily functioning of the enterprise (for example, announcements at the airport), automatic control of the main units and automatic connection of the hot standby should be used.
The main method of automated self-control is now the addition of a small test pilot signal of 20 kHz, constantly present in all channels. The sound is inaudible, especially since most loudspeakers are specially designed to reproduce this frequency poorly.
Monitoring the presence and amplitude of the pilot signal at the amplifier output allows constant monitoring of the amplifier operation. Additional detectors located at the farthest loudspeaker on each loop monitor the integrity of the loop. Detectors in the loudspeaker circuit also monitor the loudspeaker itself (current through the loudspeaker winding).
Dual-purpose systems
Having a wide network of loudspeakers and waiting for a fire to use them is a shame. Therefore, especially in public buildings, the warning system is initially designed with everyday use in mind. In this case, it should not be forgotten that the volume of «fire» announcements should significantly exceed the volume of advertising or everyday messages. In addition, amplifiers are often designed for relatively short-term (half an hour to an hour) operation at maximum power. To avoid overheating during long-term use, they should be loaded at 20-30% of maximum power. If the system is used to broadcast background music, volume controls (or program selectors) can be used in individual rooms. Indeed, background music is needed to please people, not to irritate them. In this case, one should not forget the forced full volume relay for the fire announcement.
Acoustic design
A difficult task for concert halls, sometimes leading to changes in the project and even the entire building structure. In ordinary rooms, as a rule, high-quality sound pressure distribution is quite easily achieved by any professional acoustician without particularly bloody measures, solely due to the competent selection and placement of loudspeakers. By the way, it is essential that announcements are made in a calm female voice, its spectrum is more uniform and matches well with the most important frequency range for speech perception. Relatively small rooms with low (up to 5 m) ceilings do not present a problem at all — the calculation of ceiling loudspeakers for these cases can be carried out using simplified formulas. In this case, the only problem may be the customer, who demands to reduce the cost and install loudspeakers «cheaper» and «sparsely». Unfortunately, in this case, increased unevenness of the acoustic pressure distribution is inevitable. And, as a consequence, speech intelligibility in some areas is significantly reduced, because even an increase in the overall power leads not only to an increase in the useful volume, but also to an increase in the level of reverberation background (the indistinct echo will still be louder than the useful signal). Reducing the relative background level is usually very expensive — it requires laying carpets on the floor and hanging tapestries on the walls. I would also like to remind you that rooms with high ceilings present another danger that is not taken into account by the widely used simplified formulas: closely located neighboring loudspeakers are a source of powerful interference, and if you move the speakers apart so that they do not interfere, you need carpets and tapestries, as described above. A typical solution is to lower the loudspeakers on cords (only sparrows listen to them under the ceiling anyway) — often meets objections from designers. Other solutions (narrowly directed loudspeakers) require very precise and complex calculations and more expensive equipment.
Multi-zone systems
The number of notification zones and the number of signal sources (switching matrix) are usually easily determined based on the tasks facing the system. There are two types of switching implementation: switching after the amplifier (powerful signals) or switching before the amplifier (which requires a separate amplifier for each zone). The choice is determined by the matrix size. If there are few sources and many notification zones (typical for a fire system), then it is better to get by with one or two amplifiers (although their total power must in any case be sufficient for the simultaneous operation of all notification zones). If there are many sources (as, for example, in an airport terminal), then it is more cost-effective to switch before amplification. In practice (90% of systems), a modified first option is used — one amplifier, several signal sources are switched to its input in accordance with their priorities, many notification zones are switched to the amplifier output. In this case, simultaneous transmission of different signals to different zones is impossible.
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