STH-10

Federal Signal STH-10
Company	Federal Signal
Produced	1955-2005 (STH-10A) 1955-1995 (STH-10B)
Type	Omnidirectional Electromechanical
Sound output	115 dB @ 100 ft
Frequency	50/60 Hz
Horsepower	7.5 hp 10 hp
Voltage	240 V AC 1 ph (STH-10B) 208-240/480 (STH-10A) V AC 3 ph
Preceded by	Federal Electric Fedelcode Federal Signal STL-10
Succeeded by	Federal Signal Model ECLIPSE
Documentation	Manual Product sheet

The STH-10 (Single Tone High, 10 Horsepower) is an omnidirectional electromechanical siren that was produced by Federal Signal over a span of 50 years. It proved to be one of the most popular of Federal Signal's omnidirectional sirens and gained a reputation as a reliable, effective multipurpose siren. It is an extremely common siren today despite being discontinued, a testament to its reliability and performance.

History and design

The STH-10 was introduced in 1955 as a higher-pitched option to supplement Federal's existing 10 hp STL-10 siren, using a 20.5-inch, 12-port aluminum rotor instead of the 24-inch 7-port rotor the STL-10 uses. The STH-10 can also be seen as a single tone variant of a 2T22 or SD-10. The siren is very simple in design, being little more than the chopper, stator, motor, intake and horns. The siren uses a motor-over-rotor design, like the STL-10. There are 12 horns on the siren, attached to each stator port. The sirens were furnished 7.5 and 10 Horsepower electric motors located on top of the siren, and the siren was available as both single and three phase variants, known as the STH-10B and STH-10A respectively.

The siren has a cylindrical, tubular intake located below the siren, in between the siren's three included mounting legs. These legs could be bolted to a pole, or attached to a rooftop, making it versatile. The siren's very simple design meant that the siren was very reliable and easy to keep maintained, and the horns allowed it to reach 115 dB at 100 feet, 1 dB more than its lower-pitched counterpart. The motor is typically bare, although several identical units have been found with the tops of a C3 1/2 Model 5 housing covering and protecting the motor, which may have been a custom order factory option.

A three-signal variant of the STH-10 was also produced, using a damper to produce a pulse signal. These units are designated as the STH-10C by enthusiasts. Due to its efficient rotor design and good motor bearings, the STH-10 is known for having long wind-downs, with the rotor often coasting for over five minutes after the siren shuts off.

The STH-10 was designed as a multipurpose siren mainly for use at fire stations, as it had better range and volume than Federal's Model 5. While it was mainly used as a fire siren, it easily doubled as a standard civil defense and weather siren when necessary. The STH-10 became a popular siren, outselling the STL-10 significantly. The STH-10 would also be produced in the form of the RSH-10 Thunderbeam, and the early 500-SH, which both used the rotor and stator of the STH-10. The STH-10's rugged design made it suitable for a wide variety of applications, often finding itself in areas where other sirens wouldn't be suitable, such as industrial plants. One STH-10 has the distinction of being the northernmost siren in North America, located in one of the coldest inhabited settlements in the world, Grise Fiord, Nunavut, and many other STH-10s can be found in the remote northern territories of Canada.

In 1995, the STH-10 was discontinued along with the STL-10 as part of restructuring, though the STH-10A would be reintroduced not long after due to popular demand. The STH-10B was not reintroduced and was discontinued, leaving the three-phase STH-10A as the only model produced. Unfortunately, STH-10s that were produced after 1995 were made much more cheaply, and as a result tend to be more unreliable, often suffering motor problems. The STH-10 would finally be discontinued in 2005, being replaced by the ECLIPSE-8, which offered similar performance while also using DC power. Today, the STH-10 remains a common sight throughout the US and even Canada, often in use at volunteer fire stations as a fire siren, or as a tornado siren for smaller towns. As a result of its popularity vs the STL-10, the STH-10 can be found much more commonly than the STL-10.

Darley 10-Horsepower (Darley STH-10)

W.S. Darley & Co. also produced their own variant of the STH-10, which was essentially an STH-10 that was flipped upside down to match the rotor-over-motor Model 5 configuration, with the intake on top and the motor on the bottom. Darley used their own custom cylindrical housing, which uses 8 short round horns and a flat, rounded rainshield with an eyebolt on top. The horns do not line up with the stator ports, but the housing overall provides much better protection from the elements than a standard STH-10. These would be produced until Darley's partnership with Federal Signal ended in the 1960s. While enthusiasts often refer to this siren as the "Darley STH-10" for obvious reasons, Darley appears to have internally referred to it as the "Darley 10-Horsepower Siren".