XT22

Federal Signal 2T22 Federal Signal 3T22
Federal Signal 3T22
Company	Federal Sign & Signal Company Federal Signal Corporation
Produced	1955-1992
Type	Omnidirectional Electromechanical
Sound output	113 dB @ 100 feet
Frequency	50/60 Hz
Horsepower	7.5-10 hp
Voltage	208-240/480 V 1 or 3 ph
Preceded by	Federal Electric Fedelcode Federal Electric Type Mach. B Federal Signal Model 5
Succeeded by	Federal Signal Model ECLIPSE
Documentation	Manual

The XT22 (2 or 3 Tone, 22 horns/ports) sirens are dual rotor omnidirectional sirens that were produced by Federal Signal over a span of nearly 40 years, and included two sirens, the 2T22 and 3T22. These are some of the most well-known and iconic sirens with a distinctive sound, and are still commonly found today.

Note that XT22 is not the name of the siren, rather a way of describing both models at once for brevity. It is also a commonly used name when one is unsure whether a unit is a 2T or 3T.

History and design

Overview

The XT22 is a simple vertical dual rotor siren, with 15-inch rotors/stators on each side of a dual shaft AC electric motor, with the top rotor being 12-port and the bottom rotor being 10-port, resulting in the XT22 being a 10/12-port dual tone siren. Each rotor has a long intake tube with mesh screens to prevent debris from getting inside, with the upper intake including a rain shield to keep moisture and debris from getting into the top rotor. Each port on the stator has a rounded horn of its own, similar to the STH-10 and STL-10, with 12 on top and 10 on the bottom. Due to being made of thin sheet metal, these horns are prone to warping and damage, and it is not uncommon to find XT22s with damaged or even missing horns.

Three-phase units (designated 2T22A and 3T22A on Federal Signal-era units) are powered by a bare dual sided 7.5 hp or 10 hp AC motor, which was originally manufactured by GE for early FS&S models, and by Marathon for later models. Units with GE motors are slightly shorter than units with Marathon motors. The XT22 also was available in single-phase (designated 2T22B and 3T22B on Federal Signal era units), using a 7.5 hp Northwestern Electric brushed motor. These are often lower pitched due to peaking at a lower RPM, but are capable of reaching a higher pitch than three phase units if there is low enough air resistance due to being non-synchronous (typically due to a blocked intake). These motors are physically taller than the other motors, making the XT22B easy to differentiate from a XT22A.

The XT22 is able to reach 113 dB at 100 ft, which is respectable for a dual tone omnidirectional siren and outperforming most of Federal's omnidirectional offerings at the time of its production.

While the XT22 is a simple siren that requires little maintenance, it is not without its issues. The cover on the top intake proved insufficient for keeping debris out, which often lead to these sirens having a blocked top intake, rendering the 12-port side near useless. This is especially the case for the 3T22, because the damper makes it even easier for the intakes to clog. In addition, the solenoids which operate the dampers on 3T22 units can freeze up and stop working if left unused for an extended period of time, or the return spring can fail and leave the damper stuck shut.

Because of the increasing age of the design, as well as the lack of a need for dual tone sirens after the Cold War ended, the XT22 was discontinued in 1992. However, a healthy, normally functioning 2T22 or 3T22 is a great siren, which is why they are still a common find across the United States, being used as fire or severe weather sirens. While some have since been replaced by newer DC-operated sirens such as the ECLIPSE-8 or ASC's Tempest-121, or rotational sirens, many cities have chosen to keep these aging beasts as with proper maintenance, these continue to be great sirens.

Models

3T22

The 3T22 was first produced in 1955, at the dawn of the Cold War. Federal lacked a siren with three-signal capabilities that also met Federal Civil Defense requirements for a dual tone siren. Federal's existing dual tone sirens used a single rotor, which were not capable of creating a hi-lo signal. Federal decided to take inspiration from the dual rotor sirens that were used by Britain during World War 2, but with enhanced performance through the use of horns and a more powerful 60 hz motor. From the beginning, the 3T22 was designed as a three-signal siren, which could be used for civil defense and domestic use, particularly for fire departments and industrial zones.

The 3T22 is equipped with two solenoid-driven dampers, with one on each rotor's intake. When the solenoids are energized, the dampers slam shut, restricting airflow to the chopper and greatly reducing the noise. When the solenoid is de-energized, a return spring forces the damper back into the open position, allowing air to flow again. When the two dampers alternate between open and closed while the siren is running, this creates a "hi-lo" signal. The dampers are also capable of operating in unison, creating a "pulse" signal. When the siren shuts off, both dampers automatically de-energize and open. Unfortunately, if the dampers are not maintained properly, the return spring can fail, causing the damper to be sucked shut when the siren activates, which prevents the rotor from reaching its full volume and leading to one rotor massively overpowering the other. The solenoid mechanisms themselves can also become stuck if they aren't maintained or used regularly. Because of this, it can be very difficult to find a 3T22 that has both dampers working, as they are not used frequently and most have opted to cease using a third signal.

2T22

Later on at an unknown date, the two-signal 2T22 would be released as an alternative to the 3T22. The 2T22 lacks the dampers and solenoid mechanisms that the 3T22 has, which makes the siren lighter and cheaper to produce, but also renders it incapable of performing a third signal. This can be a benefit, however, as the lack of dampers means the siren is less prone to intake clogs and requires far less maintenance. The 2T22 was fairly popular, though many places opted for the 3T22, even though most did not utilize the three signal capabilities. The 2T22 is entirely identical to the 3T22 with the exception of the dampers, and retains the extended intakes of the 3T22, likely as a cost saving measure. The 2T22 would be produced alongside the 3T22 until their discontinuation in 1992, with Federal opting to continue producing the STH-10 instead.