Federal Signal XT22

* Federal Electric Type Mach. B

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 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, or if it is unknown whether a unit is a 2T22 or 3T22.

## History and Design

### Overview

The XT22 is a simple siren beneath all of its horns. The siren is a dual rotor siren, with the rotors on each side of a dual sided motor, with the top rotor being 12-port and the bottom rotor being 10-port, resulting in the 3T22 being a 10/12-port dual tone siren. The 14-inch rotors are smaller than Federal's other omnidirectional sirens at the time, being similar in size to those used on the C1 1/2 Model 3 and 2001 series. Each rotor has a large intake tube with mesh screens to prevent debris from getting inside, with the top rotor having 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. These horns are prone to warping and damage, and it is not uncommon to find 2T22s with damaged or even missing horns. The three-phase 3T22A (and 2T22A) is powered by a bare dual sided 7.5HP or 10HP AC motor, which was originally manufactured by GE for the early FS&S models, and by Marathon for the rest of the models. The units with GE motors are slightly shorter than the other models. The 3T22 also was available in single-phase, using a 7.5HP Northwestern Electric AC motor. These are known as the 3T22B (and 2T22B), and are often lower pitched due to peaking at a lower RPM. These motors are taller than the other motors, making the XT22B easy to differentiate from a XT22A. The XT22 is able to reach 113 dB at 100ft, which is respectable for a dual tone omnidirectional siren.

### 3T22

The 3T22 was first produced in 1952, at the dawn of the Cold War. Federal lacked a dual tone omnidirectional siren for civil defense purposes, with their other brand new siren, the Thunderbolt, being rotational and single tone. 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 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

In 1955, the two-signal 2T22 would be released. The 2T22 lacks the dampers and solenoid mechanisms that the 3T22 has, which makes the siren lighter and cheaper, 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 cannot be messed up from solenoid failure. 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.

While the XT22 is a great siren for its purpose, it was not without its own issues. The cover on the top intake is insufficient for keeping debris out, which leads to these sirens often 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. 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.