Electronic Sirens

Electronic sirens are modern devices intended for rapid warning and information of the population in case of an emergency. Unlike their mechanical counterparts, they do not contain any mechanical parts, and generate sound fully electronically. In the Czech Republic, there are about 2 000 modern electronic sirens. They are gradually increasing in count, as they're replacing old mechanical sirens. All electronic sirens must fit strict regulations and requirements in order to be included in the "unified warning and notification system".  Some of these regulations state that electronic sirens must be capable of sounding not just warning signals, but also, in general, any sound files, mainly verbal information and announcements. They must be also capable of operation in an event of a power outtage and should allow live voice input.

Principle of Function

The siren works completely electronically. Electronic sirens are basically "speakers" hooked up to a small computer and a memory storage. All sound files are kept stored in the siren's memory card, or EPROM inside the control box. Once activated, the siren "plays" selected sounds from the memory card (or live input/broadcast), which are processed in the CPU (main processor), de-coded, and electronically sent to amplifiers. Amplifiers amplify the the output voltage and send it to drivers hooked up to horns. These change the electronic signal into high pressure sound. The sound is then radiated from the aluminium horns into the space. Thanks to the specific narrow design of the horns, the sound is not radiated only directly straight, but can bend and curve around the speakers, which allows 180 degree radiation. This design takes adventage of the diffraction of sound, which is a phenomenon during which sound waves are capable of moving around an object if it's smaller than (or about equal) to the wavelenght of the sound. Electronic sirens are capable of operating during power outtages thanks to back-up accumulators. Thanks to the presence of various communication modules inside control boxes, electronic sirens can be controlled remotely.

Means of activation

Electronic sirens can be activated by local triggers or remotely. Local means include activation directly via the control box, or by external call points and other external activators. Remote activation is either carried out by VHF-FM radio activation or other means, which are included in the SSRN JSVV system (JSVV - Jednotný systém varování a vyrozumění - Unified system of warning and notification). The protocols used within the system are mostly POCSAG, DMR and RDS. Thanks to the VHF-FM receiver, the siren can be also co-activated along with the BMIS (PA warning) system. The speciality is the option to activate the siren by cell phone thanks to GSM modules.

List of electronic sirens in the Czech Republic