everyone's favourite

When you see a Bumblebolt you KNOW it's has a 6M blower. Also i tried to make this sound good with a square wave and 180 hz undertone, and tbh it sounds accurate.

Inspired by Hannah's Mill, GA Thunderbolt

RESUME

The Thunderbolt is a line of supercharged rotating sirens formerly manufactured by Federal Signal. At the time of its release, it sported nearly unrivaled sound output and continued to be a popular siren even as newer sirens surpassed it until its discontinuation 38 years later. The Thunderbolt is perhaps the most well-known of all North American civil defense sirens, even with those who are unfamiliar with sirens. It was sold alongside the naturally aspirated Model 500 rotational sirens.

Development of the Thunderbolt began as early as 1951. While originally envisioned with an angled down horn and a rounded rotator and blower housing, the design would be refined by 1952 with rectangular housings replacing both to simplify construction. The Series A1 Thunderbolt Type 1000 was released in 1952, sporting several differences from later models. Most units constructed between 1952 and 1954 had three grids of steel reinforcements in the horn as Federal was unsure of whether or not the horn was at risk of warping. As units began to be produced without these reinforcements, it was found the horn was strong enough without them. Mesh screens were added to the horn keep out debris as early as 1953. These early reinforced units are often called "jailbar" or "gridface" units by most enthusiasts. Later Series A1 units had slightly taller rotator enclosures.

HISTORY

Series A1

In 1954, the dual tone Thunderbolt 1000T was introduced. The 1000T originally used the 4/5-port dual tone rotor from a Model 2T; by 1959 5/6-port dual tone became the standard, and 4/5-port was discontinued shortly after. The Type 1000T became the standard model of Thunderbolt upon introduction, though the Type 1000 was still available as an option. Due to the Office of Civil Defense's requirements for air raid sirens to be dual tone, the Type 1000T was intended for civil defense purposes, while the Type 1000 was offered for other applications, though it could still be used as a civil defense siren if necessary.

Series A2

In 1959, the Series A2 was developed, which used a new Sutorbilt 5M blower, as well as a 3-signal model known as the Thunderbolt 1003. The 1003 was mainly meant for use at volunteer fire stations where the hi-lo signal would be used for fire calls, since using the wail signal could cause confusion and panic. The 5M blower was replaced by the 4M blower in 1965, which costed less but came at the expense of air volume and sound output. When Federal Sign & Signal became the Federal Signal Corporation, the Thunderbolt's nomenclature was changed. "Type" was changed to "Model" on all of the tags as well as in documentation and advertisements.

Series B & C

In 1980, Federal began development and production of a Thunderbolt "Series B" which included a complete rotator overhaul in an attempt to make the sirens more maintenance-free. The Series B was plagued with issues and did not last long in production, as they proved to be less reliable than the standard models. The primary issue was with its C-face motor, which was found to often fall out of alignment. This caused the chopper to scrape against the stator, which led to the early failure of the chopper or motor. The Series B was replaced by the Series C Thunderbolt models in 1982, which included new vertically-oriented Series C 4M blowers. While more reliable than the Series B, the Series C still proved to have quality control issues and worse reliability than the Series A models. Late Series B Thunderbolt sirens also made use of Series C blowers.

Discontinuation

In 1990, the Thunderbolt siren would finally be eliminated from Federal Signal's product line after a 38-year production run, as the newly developed DC-powered 2001-SRN outperformed it without the need of a blower and could run independently of grid power using battery backup. The last Thunderbolt sirens to be manufactured were a batch of Series C Model 1003 sirens sent to Michigan, though they have since been removed. Supposedly, there was a single year run in 1992 where Thunderbolt sirens were manufactured with a slightly shortened blower frame/cover; no known 1992 models are known to exist, and this information is unverifiable.

Despite the Thunderbolt line's discontinuation, elements of its design still remain in the 2001 and 508-128 sirens. The 2001-SRN made use of the Thunderbolt rotator, and even today the collector ring tube from the Thunderbolt is still used both of the former. Many Thunderbolt sirens are still in service today, though that number continues to slowly dwindle as these Cold War-era relics are replaced by newer, more efficient DC-powered sirens. Some see second lives after removal, being fully refurbished and put back into service as a cheaper alternative to a brand new siren.

DESIGN

The Thunderbolt utilizes a 2-piece cylindrical pivotal point which encloses the motor and chopper and supports the siren's large steel exponential horn alongside 2 supports attached below the motor tube (bottom piece). At its core, the Thunderbolt is essentially a rotational supercharged Model 2. The siren's chopper and motor are reused from the Model 2, with the exception of the 5/6-port Thunderbolt 1000T which uses its own chopper design. Rather than using a typical stator with an equal number of ports to the chopper, the Thunderbolt makes use of a stator cap (upper piece) with a single port opening where the horn attaches to, focusing all of the siren's output through that port. The motor tube includes the siren's 2 hp series-wound brushed universal motor, as well as the collector rings at the bottom, where brushes underneath the motor frame transmit power to the chopper motor. The head includes a "drive-band" clamp that acts as a clutch, disconnecting the head from the rotator under high wind loads to prevent damage.

The Thunderbolt's horn and rotor assembly are rotated using a belt-driven rotator inside of the box underneath the chopper tube, with collector rings inside the rotator allowing the siren to rotate 360°. A motor drives a pulley attached to an Abart 2-1/4ATD, 60-1 ratio worm gear reducer, which then drives the main driven gear through a small drive gear. The pulley has three different sizes, which translates to 3 different rotation speeds: 2 rpm (1:2), 4 rpm (1:1), and 8 rpm (2:1). The speed can be changed by simply moving the belt to a different slot on the pulley. The vast majority of Thunderbolt units are set to 2 rpm or 4 rpm, while 8 rpm is uncommon.

The rotator motor on Thunderbolt Series B models faces the gear reduction drive and both are coupled directly. This eliminates the need for a belt, though the rotation speed was no longer adjustable and limited to 4 rpm. These changes required the rotator box to be enlarged and offset from the siren's horn and necessitated the use of external wiring to the chopper motor. The chopper motor was switched to a C-face motor, which proved to be problematic.

ROOTS BLOWER

While most sirens draw in air from the intake though natural aspiration, the Thunderbolt instead forces a high volume of compressed air into the chopper through the use of a belt-driven Roots-type positive displacement rotary lobe blower. By greatly increasing the pressure and volume of air through the chopper, the blower greatly amplified the siren's sound output. This "supercharged" concept would be later imitated by ACA in 1968, with the introduction of their competing Hurricane 130; contrary to popular belief, Federal never sued ACA for their similar design. A Thunderbolt without a blower is effectively a Model 2 with a horn attached and inferior airflow, and as such Thunderbolt sirens are quiet and high pitched without them.

A Roots blower has a pair of rotary lobes connected through gears inside of a thick rounded steel housing which compresses and supercharge the air flowing into it as they rotate against one another, which is then sent up into the siren itself through a standpipe, massively increasing the siren's volume and range. The Roots blowers used on the Thunderbolt were produced by Sutorbilt, and are dual-lobed blowers. The blowers draw in air from above through a filtered and silenced intake, and the lobes are driven by 3 belts and a 1 or 3 ph electric motor. The motors used on 3 phase models are 10 hp, while the motors used on single phase models are 7.5 hp. Both rotate at 1800 rpm. The main pipe includes a pressure release valve next to the blower; this helps prevent excessive air pressure from blowing holes in the standpipe, destroying the chopper, or potentially causing the blower to explode in the event of a blockage.

The Roots blowers on the Thunderbolt are only designed to spin in one direction (counterclockwise), and if wired backwards (known as reverse wiring, possible on 3 ph sirens only) the blower will run backwards, sucking air from the siren's horn and forcing it out of the blower's intake. This results in the Thunderbolt's chopper running faster than it should, causing a significant increase in pitch, and the blower itself becomes significantly louder to the point of being very clearly audible over the siren. This also causes the siren to have an almost nonexistent winddown, as the sudden change in air pressure causes the chopper to quickly come to a halt when the motor shuts off.

6M Blower

Type 6M (Series A1) blowers have a 6-inch diameter pipe and use a large pulley driven at a slower speed than other blowers, which results in a pulsating sound for both the blower itself and the siren. This is especially noticeable on single tone units. The 6M blower produces a pressure of 300 CFM. Due to their age, 6M blowers are less common than other blower models. Due to the lower sound pressure caused by the larger pipe, these tend to be slightly quieter than other blowers, though the pulsating effect helps mitigate this by being more attention grabbing. Single tone units are rated at roughly 129 dB on a 6M blower, while dual tone units are rated at 128 dB. Early 6M blower units used 4 belts instead of the standard 3.

5M Blower

The Type 5M (early Series A2) is generally regarded as the loudest and punchiest of the blowers. These blowers made use of a much smaller pulley with a 3-inch diameter pipe, which allowed a pressure of 350 CFM. Because of this change, Thunderbolt sirens running on 5M blowers tend to be very punchy due to the additional air pressure, especially the Model 1000. These blowers lack the pulsating effect of the 6M, instead having a low-pitched hum with a bit of a hiss. The increase in air pressure allows the siren to boast a rating of 130 dB on single tone units, and 129 dB on dual tone units.

4M Blower

The Model 4M (Type 4M before 1975, late Series A2 and Series B) uses a 1.5-inch diameter pipe, alongside a slightly smaller blower, which results in an air pressure of 250 CFM. These blowers are the most common blowers found on Thunderbolt sirens and can be visually identified from other blowers as the Model 4M blower introduced a vent on the rear of the blower box. However, some early Type 4M blower boxes lack these vents. These blowers have a loud, high-pitched hum, and run at a faster speed than the other blowers due to the smaller pulley. These blowers are far less punchy than the Type 5M and Type 6M and instead have more of a "buzzy" sound to them. They're still very loud however, with single tone units being rated at 127 dB at 100 ft, while dual tone units are rated at 126 dB.

4M Blower (C)

The Series C Model 4M blower is essentially a Series A2 Model 4M blower flipped onto its side into a vertical position. On Series C Model 4M blowers, the pulley sizes are slightly larger on the blower compared to the motor. This causes the blower to run at a slower speed. Instead of the pipe being located below the blower, the pipe is mounted to the side of the blower, which causes the standpipe to be higher up on the blower box compared to earlier Thunderbolt models. The pressure release valve is also significantly enlarged.

MODELS

1000

The Thunderbolt Model 1000 (Type 1000 before 1975) is a single-tone, 2-signal supercharged electromechanical rotating siren introduced in 1952. Originally the only model available, the Thunderbolt 1000 was replaced by the 1000T in 1954 but remained in production as an optional alternative. Its primary use was for civil emergencies and volunteer fire departments, where its single tone sound could be differentiated from dual tone air raid sirens. The Thunderbolt 1000 reached 126 dB at 100 ft according to Federal Signal; however, the decibel rating of these sirens varies depending on the blower used. Typically, the higher air pressure created by the blower, the louder the siren will be. The Thunderbolt 1000 has a higher decibel rating and longer range than dual tone models, due to the lack of clashing tones causing sound cancellation.

1000T

The Thunderbolt Model 1000T (Type 1000T before 1975) is a dual-tone, 2-signal supercharged electromechanical rotating siren introduced in 1954. The Model 1000T utilizes the same overall design as the 1000, with the exception of the chopper and stator. Units produced between 1954 and 1959 were exclusively 4/5-port dual tone, reusing the chopper from the Model 2T. In 1959, a new 5/6-port chopper was designed specifically for the Thunderbolt 1000T, and became the standard for all 1000T units produced afterwards. The Thunderbolt 1000T came in 1 and 3 ph variants, the Model 1000BT and Model 1000AT respectively. The Model 1000T reaches 124 dB at 100 ft according to Federal Signal; however, the decibel rating of these sirens varies depending on the blower used.

1003

The Thunderbolt Model 1003 (Type 1003 before 1975) is a dual-tone, 3-signal supercharged electromechanical rotating siren introduced in 1959. The Thunderbolt 1003 utilizes a solenoid-operated mechanism placed in front of the stator cap which blocks one of the 2 ports in the horn—or both if desired—allowing for the siren to perform a "hi-lo" or "pulse" signal. By combining these signals with the wail function, it was also possible for the siren to perform "alternate wail" and "pulse wail" signals. The Thunderbolt 1003's horn is slightly moved forwards to make way for the solenoid mechanisms. The rotator box on the Thunderbolt 1003 is enlarged to make room for the additional collector rings that were needed to transmit power to the solenoids. Also included with the Thunderbolt 1003 was a specialized Model RCM3 controller, which allowed for individual control of all components as well as containing the flashers used to operate the solenoids. Due to their age, the return springs on the solenoids are prone to failure, with many units having suffered from one or both solenoids failing to operate due to the springs wearing out or the solenoids seizing.

The workings of a Model 1003, courtesy of the Civil Defense Museum.
The Thunderbolt 1003 came in 1 and 3 ph variants, the Model 1003B and Model 1003A respectively. The Model 1003 reaches 124 dB at 100 ft according to Federal Signal; however, the decibel rating of these sirens varies depending on the blower used. Typically, the higher air pressure created by the blower, the louder the siren will be.

2000

The Thunderbolt Type 2000 is a single-tone, 2 signal supercharged gas-powered rotating siren. It was released in 1952 as a variant of the Thunderbolt 1000 that could generate its own power. Rather than using an electric motor, the siren's blower is powered by an Onan twin-cylinder gasoline engine. This engine also serves as a generator for the siren, which allows it to operate independently of external power sources, which was important for areas where grid power was either unreliable, or would be in danger of being lost in an attack. The head of the siren is the same as a typical Thunderbolt 1000. Due to the size of the engine, the blower enclosure is much larger than enclosures used on the other models.

While the Thunderbolt 2000 (and engine-driven sirens in general) was a good idea in concept, it never sold many units due to the increased maintenance the engine required over a basic electric motor. Grid power was widely and reliably available by the time the siren released. The majority of Thunderbolt 2000 units were installed around California and Arizona, which were at high risk of Soviet attack. Due to the increased maintenance required as well as the impracticality of operating gasoline powered sirens today, only a few units remain, none operating on their engines. This is a fate that is shared across most other engine-powered sirens.

Two surviving units have been converted to use an electric motor for the blower, one in Phoenix, Arizona at Fire Station 22, and the other in Beebe, Arkansas which replaced a Whelen Engineering VORTEXR4 in 2019 that was struck and fried by lightning. There is only a single known Type 2000 with its original engine intact, which is located in Windsor Hills, CA. It was converted to propane at some point and is inactive and dilapidated like most of California's former civil defense siren systems.

System 7000

The Thunderbolt System 7000 is a variant of the Thunderbolt which is entirely self-contained and able to run independently of grid power. It can be seen as a spiritual successor to the Thunderbolt 2000. The blower, optional RadioTrol and control devices are housed inside of a large (2375mm x 2438mm x 2223mm or 93.5'' x 96'' x 87.5'') aluminum and steel enclosure which also houses a gasoline-driven AC generator which powers the siren when grid power is lost. The fuel tank has a capacity of 72.8 litres, or 20 gallons. The total weight of a System 7000 unit is 2725 kg, or approximately 3 short U.S. tons and was explicitly not recommended for installation on the roof of a building. Both high and low mounted System 7000 configurations were offered.

The System 7000 was introduced sometime in the 1980s for use in areas where reliable power was unavailable, such as war zones and remote locations. The siren is driven by standard mains power when available, using an automatic transfer switch to run the siren with the generator when power is lost. However, the generator will not start during a power failure unless a siren warning signal is actually needed during the power outage. The transfer switch will operate even if power is lost while the siren is operating; this results in minimal interruption to the siren. Mains power is also used to maintain the generator battery's charge.

The enclosure is weather, corrosion and heat resistant, with thermostatically controlled cooling fans allowing it to operate even in hot, hostile environments. It is also tamper-resistant to prevent vandalism or unauthorized activation and optionally includes a security system that activates an audible alarm and sends an alert in the event of unauthorized access; a key has to be turned to disarm it. A powerplant exerciser could also be installed, which would periodically (at preselected intervals of 0 to 168 hours for 15 minutes at a time, on chosen days of the week) operate the engine in order to maintain the voltage of the generator's battery while reducing corrosion by keeping the engine's internals lubricated.

The AF*22Z timer used on System 7026 models.
The System 7000 was available with Model 1000, Model 1000T and Model 1003 heads, known as the System 7012, System 7022, and System 7026 respectively. The System 7012 is rated at 127 dB at 100 ft, while the System 7022 and System 7026 are rated at 126 dB at 100 ft. Most known documented units were System 7026 units sent overseas to Kuwait during the Gulf War, all of which have since been replaced by Federal Signal Modulator electronic sirens, some of which are mounted on top of the old (likely reused) enclosures. 1 intact System 7026 unit is confirmed to still exist, on display at the Kuwait National Office of Civil Defense. No System 7012 or 7022 units are known to survive, if any were produced at all.

The System 7026 came with an exclusive 6-signal variant of the AF timer named the Model AF*22Z. This timer allowed for steady, wail, alternating steady, alternating wail, pulsed steady, and pulsed wail signals at the push of a button.

Requires Tone Generator mod