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A siren with two rows of ports is known as a dual tone siren. By placing a second stator over the main stator and attaching a solenoid to it, one can repeatedly close and open all of the stator ports thus creating a tone called a pulse. If this is done while the siren is wailing (rather than sounding a steady tone) then it is called a pulse wail.
The sirens are mainly electronic ones (German made type ECN units or other high powered speaker systems on top of public buildings). The signals may include - attack (a 1-minute wailing tone used to indicate an incoming danger) and alert (sometimes referred to as the "all clear" signal which is a continuous single tone).
The sirens were able to produce these tones: Alert - A steady siren tone [3]; Attack - A wailing siren tone [4]; Alternating Steady - An siren tone alternating between a high and low frequency, at half-second intervals, akin to a typical European emergency vehicle siren.
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The Federal Signal 3T22 was originally designed as the 2T22 in 1952 or 1954. The 2T22 has the same number of ports and cones. It can produce two main signals (it can produce more but the other signals are rarely used), hence the name "2T22" (the 2 at front representing the 2 choppers, and the 22 representing the amount of horns).
A Federal Signal Modulator siren in Bay Head, New Jersey.. Federal Signal Modulators (also known as Modulator Speaker Arrays) are electronic warning devices produced by Federal Signal Corporation that are used to alert the public about tornadoes, severe weather, earthquakes, fires, lahars, tsunamis, or any other disaster.
Duke typically tests sirens around Shearon Harris between 10 a.m. and 11 a.m., with the alarms sounding between five and 30 seconds. Emergency alert that went out to many Wake County residents ...
Footage posted on social media showed warning sirens wailing in downtown Chicago, as well as heavy rain and lightning striking trees in the suburb of Mokena to the south.