WITHvoice warning systems for fires The effectiveness of civil defense training, as well as the activities of those responsible for the premises appointed before the fire, is low. Especially in public buildings, where, in addition to permanently working staff, there is a significant number of people from the street. Most normal people, when they hear the word “fire,” lose the ability to think sensibly. The only thing that can help is guidance from professionals who have maintained their composure in an emergency situation. The technical basis for connecting a few professionals with a frightened public is voice warning systems.
In particular, voice warning systems are mandatory for almost all public buildings with an area of over 1000 m2 or more than 3 storeys.
Unfortunately, the current document only in general outlines the actual requirements for the system. Thus, zoning (sequence) of notification in multi-story buildings is required, as well as preliminary notification of building personnel. There is a general recommendation to use pre-recorded messages rather than a live voice (an insufficiently self-controlled speaker can, on the contrary, incite panic). There are absolutely no requirements for the quality of notification (speech intelligibility), but in extreme conditions the external background noise level increases significantly. Issues of system self-control are left to the discretion of the designer.
Performance monitoring
In many cases, periodic testing of the system is sufficient. Often this happens by itself — if the system is routinely used to broadcast background music or work announcements. In this case, only specifically “fire” subsystems are subject to special regular inspection. For example, a block of pre-recorded messages.
In special cases, when there is a large number of public (potentially increased number of victims), and when the system is used to ensure the daily functioning of the enterprise (for example, an announcement at the airport), automatic control of the main nodes and automatic connection of the hot reserve should be used.
The main method of automated self-testing now is the addition of a small 20 kHz test pilot signal, 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 you to constantly monitor the operation of the amplifier. 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-use systems
Having a wide network of loudspeakers and waiting for a fire to use them is a shame. Therefore, especially in public buildings, the public address system is initially designed with everyday use in mind. In this case, we should not forget that the volume of “fire” announcements should be significantly higher than 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 prolonged 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) may be used in individual rooms. Indeed, background music is needed to make people feel good, not to irritate them. In this case, you should not forget the relay for forced activation of full volume for the fire announcement.
Acoustic design
This is not an easy task for concert halls, sometimes leading to changes in the design and even the overall design of the building. 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 pose a problem at all — the calculation of ceiling loudspeakers for these cases can be carried out using simplified formulas. The problem in this case can only be posed by the customer, who demands to reduce the cost and supply loudspeakers “cheaper” and “less often”. Unfortunately, this inevitably results in increased unevenness of the acoustic pressure distribution. And, as a result, speech intelligibility in some areas deteriorates significantly, because even increasing the overall power leads not only to an increase in the useful volume, but also to an increase in the level of the reverberation background (unintelligible 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 pose another danger that is not taken into account by widely used simplified formulas: close adjacent speakers 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. The typical solution — lowering the speakers on cords (after all, only sparrows listen to them under the ceiling) — often encounters objections from designers. Other solutions (narrow directional loudspeakers) require very precise and complex calculations and more expensive equipment.
Multi-zone systems
The number of warning zones and the number of signal sources (switching matrix) are usually easily determined based on the tasks facing the system. The implementation of the switching itself is of two types: 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 size of the matrix. If there are few sources and there are many warning zones (typical for a fire system), then it is better to get by with one or two amplifiers (although their total power in any case should be sufficient for the simultaneous operation of all warning zones). If there are many sources (such as 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 warning zones are switched to the amplifier output. In this case, simultaneous broadcasting of different signals to different zones is impossible.
“SQUID TD” — when calmness becomes a habit!