STL-10

Federal Signal STL-10
A Federal Signal STL-10 in Bloserville, PA.
Company	Federal Signal Corporation
Produced	1953-1995
Type	Omnidirectional Electromechanical
Sound output	114 dB @ 100 feet
Frequency	50/60 Hz
Horsepower	7.5-10 hp
Voltage	208-240/480 V AC 1 or 3 ph
Preceded by	Federal Electric Fedelcode Federal Signal Model 5
Succeeded by	Federal Signal Model ECLIPSE
Documentation	Manual

The STL-10 (Single Tone Low, 10 Horsepower) is an omnidirectional vertical siren that was produced by Federal Signal during the mid to late 20th century. It was produced between 1953 to 1995 as an alternative to Federal's popular STH-10, SD-10, and XT22 sirens and found popularity as both a fire siren and a civil defense siren.

History and design

The STL-10 at its core sports a 20.5-inch 7-port rotor and stator, a 10 hp AC induction motor on top, and 7 rounded metal horns attached to each stator port to project the sound directly from the chopper. Its low tone is very distinct from other sirens and can be heard for miles due to its pitch, and the siren boasts a decibel rating of 114 dB at 100 ft. The STL-10 was available with either a single or three-phase electric motor, being designated as the STL-10B or STL-10A, respectively. The rotor and stator of the STL-10 were also used in the prototypical phase of the RSH-10, as the RSL-10. However, Federal ultimately chose to use the STH-10's rotor and stator instead.

The STL-10 is a very basic siren in design, using a motor-over-rotor configuration. The bare motor is atop the siren with an eyebolt attached for mounting and has no shielding from the elements. there are 7 large round horns attached to each stator port, which assist in projecting the sound from the rotor, and a cylindrical intake is located below the siren in between the siren's three included mounting legs. Mesh screens are included to protect the rotor from debris. Early STL-10s used square horns, but these were quickly switched to round horns as they proved easier to manufacture and were more durable. A few remain today, but they are very hard to find.

The low pitch made it more effective in areas where range was more important than pitch, or where a higher-pitch sound may be lost in other background noise. The siren was especially suitable in urban areas, where higher pitched sirens may lose range from traffic noise and the many obstructions such as buildings. The STL-10, along with its smaller brother the STH-10, were mainly sold as fire sirens. While they were also used for civil defense purposes, their main purpose was to alert volunteer firefighters to report to the fire station to go fight a fire. The STL-10 was nowhere as popular as the STH-10, and as a result, they are harder to find than STH-10s. The STL-10 saw most of its success in urban areas such as New York and San Francisco, where its unmistakable piercing low sound was distinctly heard over the noise of traffic at great distances.

Due to waning sales compared to the STH-10 as the use of fire sirens diminished, the STL-10 was discontinued in 1995, while the STH-10 continued to be sold until 2005. Both were effectively replaced by the ECLIPSE-8 in Federal Signal's lineup, as it had battery backup capabilities and improved performance while operating at a similar pitch.