The Thunderbolt siren is a discontinued electro-mechanical warning siren that was once manufactured in the United States by Federal Signal Corporation. The Thunderbolt siren consists of three major components: the blower, rotator and siren chopper. The blower, which is made up of a motor-driven Roots-type supercharger, supplies the large volume of air for the siren head. It is housed inside a large rectangular box typically found at the base of the siren head. The blower is connected to an air standpipe which pumps 250 cubic feet per minute of air at 6-7 psi up to the siren head. The rotator is a motor-driven gear unit which rotates the directional horn/projector to direct the sound in all directions. The chopper is located in the cylindrical housing that the horn/projector attaches to at the top of the siren. The siren chopper motor in the Thunderbolt siren is actually the same siren motor used in the Federal's Model 2 siren. The siren chopper in the Thunderbolt operates in the same manner as any conventional siren. As the chopper rotor spins air pulses are generated by the rapid opening and closing of the chopper openings. This generates the sound the siren makes. The primary difference in the Thunderbolt siren is that the air supplied by the blower is pumped through the siren resulting in a greatly increased sound volume and distinct sound generated by the Thunderbolt siren.
The Thunderbolt siren is operated by a control panel called an RCM Panel. This panel consists of the three relays that switch the three components of the siren on and off. The RCM panel also has a time delay relay and circuit. This time delay section keeps the blower and rotator operating in the attack signal while the siren chopper motor is switched on and off which produces the up-and-down pitch of the attack signal. With the blower and rotator oprerating as the siren motor is switched on and off the Thunderbolt maintains a high level of sound output. An additional important feature of the RCM panel is a transformer in the siren/chopper electrical circuit. This transformer has seven different voltage taps from 120 volts up to 240 volts. This allows the top pitch of the siren to be changed as desired when the siren is installed. A siren with the chopper connected to the 120volt transformer tap will sound at a lower top pitch than a siren connected at the 240volt tranformer tap.
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The Federal Thunderbolt was the siren of choice by many Civil Defense authorities and emergency management agencies from the early 1950s through the late 1970s. The main marketing point of the Thunderbolt by Federal Signal was the fact that the siren, since it uses a blower to move air through the siren rotor, maintains a high sound output at all siren pitches when the siren is in operation. Normally aspirated sirens lose sound output because of the reduced air flow through their rotors as the siren runs down during the coast down during the wail or attack signal. Since the Thunderbolt was introduced in 1952 during the beginning of Cold War tensions as primarily an attack warning siren the high volume during all phases of the attack signal was stressed in advertising the siren by Federal. The attack signal was a series of wailing tones to alert of immediate danger. One of the most distinguishing features of the Thunderbolt siren's sound is the dramatic volume in the down-pitch phase of the signal. It's usually this part of the signal that carries the farthest and is the most "attention-getting." [1]
With the discontinuation of federal matching funds for civil defense equipment in the late 1970s-early 1980s the sales of the Thunderbolt siren declined. The Thunderbolt was a very expensive siren to purchase new. The January 1, 1979 Federal Signal Price List lists the price of a new 1000 Thunderbolt siren from $6684.25 up to $7716.75 depending on the model. In addition to that the necessary control panels to operate the siren were $837.00 for the main motor control panel and $236.00 for the additional control panel needed to operate the 1003 model of Thunderbolt.
Federal's development of new types of sirens also led to the end of the Thunderbolt. In Late 1988, Federal Signal introduced a new single-tone siren called the 2001 Series [2]. This siren can utilize a DC (battery) backup power feature, enabling the siren to be operated during a power failure, operates on only 2 motors, and requires far less maintenance. As the Thunderbolt only operates on AC power, the introduction of the 2001 Series spelled the end of the Thunderbolt siren, and Federal Signal phased out the Thunderbolt series, as well as many other of its sirens, by 1990.
The Thunderbolt was first produced by Federal Enterprises (now Federal Signal Corporation) in 1952 and its use continued through the Cold War.
There were 3 models manufactured, most of them producing approximately 125dB sound pressure level at 100 feet (30 m):
The "System 7000" line [3] was a generator back up version of the Thunderbolt. These were available as 3 separate models:
![[1]](http://michigan3.ipower.com/sitebuildercontent/sitebuilderpictures/fedsigadaug1952.jpg){kind=link}
![[2]](http://www.jmarcoz.com/sirens/baraboo2_wi.jpg){kind=link}