Mechanical Sirens
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Mechanical sirens are the oldest of the three common types of sirens. They generate the sound via mechanics, and in essence, these are single-phase or three-phase electromotors, which spin fundamental parts in order to create a loud sound. In the past, mechanical sirens were a reliable way of warning the population against any threats, as they are nowadays. However, by today's standards in the Czech Republic, they are no longer acceptable in warning systems, as their technology is rather old and does not fit most requirements. Mechanical sirens are therefore being replaced by modern electronic sirens. As of today, there are about 4 000 mechanical sirens in Czechia.
Mechanical sirens are made of three fundamental parts:
Motor - a three-phase/single-phase electromotor responsible for spinning of the rotor
Stator - stationary part of the siren, which does not move
Rotor - the main, rotary part responsible for generating the sound
Other parts:
Protective cover - The cover protects the insides of the siren from rain, snow and debris and also helps to guide the sound towards the ground, so It is well spread
Principle of function
The process of generating the sound starts with the electric motor, which spins the rotor in very high speeds. The rotor is an open "fan", with multiple air ports (holes), through which air flows. The rotor is placed inside a stator, a steady part mounted to the motor. The stator contains equal number of ports as the rotor does, and also includes multiple air intakes either in the bottom or the top. As the rotor spins, air is sucked through the air intakes in the stator into the rotor. The rotor then "chops" the air between the rotor's and stator's air ports, which results in strong, impulsive air bursts which then create a mechanical sound. Steady signals are made simply by switching on the power to the motor and then switching it off. Fluctational siren signals are made by repatedly cutting power to the motor, and then re-enabling it. The oldest mechnical sirens were controlled completely manually. Newer models with control boxes controlled the power (and thus the signal) semi-automatically via a tonewheel or a geared code wheel. Modern replacement control boxes control the signals electronically.
Pitch of the sound
The parts responsible for the frequency and pitch of the sound are the air ports in the stator and rotor, and the rotational speed of the motor. The number of the ports, their shape and the speed of the rotor is what gives the siren its unique sound. The wavelenght of the sound is directly influenced by these factors.
The frequency of the siren's sound can be calculated with the following formula
Example:
List of mechanical sirens in the Czech Republic
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