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STL-10 Chopper

The STL-10's chopper, with 7 vanes.

The STL-10 (Single Tone Low, 10 Horsepower) is a discontinued civil defense siren that was produced by Federal Signal. It is an omnidirectional electromechanical siren with a 10HP motor driving the chopper. It was produced between 1953 to the 1990s as an alternative to Federal's popular STH-10.

The STL-10 is simple and rugged in its design, sporting a 7 port rotor and stator, and 7 large metal horns to project the sound directly from the chopper. Its low tone is very distinctive and can be heard for miles due to its pitch, boasting 114db at 100ft. This made it more effective in areas where range was more important.

The STL-10, along with its 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.

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, but they are very hard to find.

Due to its lack of popularity and likely to save costs, the STL-10 was discontinued somewhere in the 1990s, while the STH-10 continued to be sold until 2005. Both were effectively replaced by the Eclipse-8 in Federal Signal's lineup.

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