Here in this video i will present you a special "WW2 air-raid-siren" built in the 40ies. It is an electro-mechanical siren called "L138" (by Elektror). This rare kind of siren has two seperate engines with a power of 1000 W. (400 V AC /50 Hz)
The upper unit is the high-tone siren-unit. A rotor/stator system with 11 air-vents generates a frequency of about 500Hz at 2800 RPM:
It was in use to warn the population of air raids during the war-times.
The second lower visible unit is the low-tone unit: A rotor/stator system with 3 air-vents, it generates a frequency of 145 Hz at rotor-speed of 2800 RPM:
It has the purpose to call the fire fighter in case of fires during times of war.
Both different siren-tone frequencies are a very important acoustical indication to make an easy difference among the population between an air raid (high-tone) or a simple fire (low-tone) during the terrible WW2-times.
There are some other siren-sytems like this one shown here in the video. (One other system will presented soon by me).