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Here is an explanation of how the Federal 3T22 and 1003 Thuderbolt sirens work. These 2 sirens are capable of doing 3 singals. One signal for "Alert", one signal for "Attack" and one signal for "Fire." Since the Federal 2T22 2-signal siren is a dual tone siren, which is essentially one siren on top of another siren with the motor in the middle, Federal added damper air valves to each air intake of the top and bottom parts of the 2T22 siren to make it possible to alternately shut off the air to each end of the siren. This siren is the 3T22. This produces a Hi-Lo alternating signal for the "Fire" signal for volunteer fire departments. Federal couldn't easily stop the incoming air to the 1003 Thunderbolt siren since the siren rotor (chopper) is located in a closed housing with the outlet at the base of the horn. The 1003 Thunderbolt uses 2 solenoid activated air valves to close off the air coming from each half of the chopper. Alternately closing each valve causes the 1003 Thunderbolt to do the Hi-Lo signal. The Fire signal of a 3T22 is almost musical sounding because the 2T22 has such a nice sound the Hi-Lo signal sounds really nice too. The Fire signal of the 1003 Thunderbolt sounds similar but more mean. Of course that just goes with the territory since the Thunderbolt is such a "mean" sounding siren. It's the blower that gives the Thunderbolt is distinctive "mean" sound.
Brett Jones Recording Of 3T22 Siren Fire Signal
This diagram of the 3T22 shows the air intakes and locations of the air dampers and solenoids that activate the dampers.
Here is a photo of the bottom air intake of a 3T22. You can barely see the damper open in the bottom intake tube. The dampers are normally open and the solenoids pull them closed.
Federal Thunderbolt 1003
The above photos show a comparison between the Thunderbolt 1000/1000T horn/rotator assy and the Thunderbolt 1003. You can see in the photo that the 1003 has a taller rotator box and and the additional feature of a solenoid housing between the horn and the chopper housing. The housing at the base of the horn on the 1003 contains the air valves and solenoids. The tubes coming off of each side of the housing contain the wiring to the solenoids. The wiring is connected to a set of brushes and a collector rings inside the rotator box. See detail of the solenoid housing at the bottom of this page. Only the 1003 Thunderbolt has this housing at the base of the horn. For more detail about how the Thunderbolt works see this section. Thunderbolt Restoration.
Here are two photos of the different types of Thunderbolt choppers. The chopper is the siren rotor of the Thunderbolt siren. This is what the chopper looks like with the top (stator) removed. The single tone chopper on the left has 5 large openings and the dual-tone chopper on the right has an upper row of 6 openings and a lower row of 5 openings. The two rows of openings on the same chopper cause the siren to have a dual-tone sound. The air valves in the 1003 solenoid/air valve housing act to close off the different rows of ports.
Here's a plan-view photo of the 1003 solenoid/air valve box showing the low tone solenoid (left) air valve (center) and the high tone solenoid (right). When the solenoids are at rest the air valves are open. When the solenoids are energized they close the air valves. The springs attached to the solenoid plunger hold the air valves open. For a video demonstration of the operation of the solenoids see my video on youtube. http://www.youtube.com/watch?v=V0lszXevCF0 Click photo to see larger version.