Federal Signal Modulators (also known as Modulator Speaker Arrays) are electronic warning devices produced by Federal Signal Corporation that are used to alert the public about tornadoes, severe weather, earthquakes, fires, lahars, tsunamis, or any other disaster. They are identified mostly by their distinctive stacked "flying saucer" design. The Modulator II is sold based on the more compact chassis of the siren compared to the original Modulators.

The modulator is composed of speaker cells (ranging from two to eight, with the exception of seven) that contain four speaker drivers per cell, although two models (model 6032 and 6048) had additional drivers available when they were still produced. Modulators have an inactive (dummy) speaker cell on the bottom of the stack that is used to help project sound in all directions. Due to the design of active cells, there would be unbalanced sound distribution without the inactive cell. Modulators that are being made now use the UltraVoice controller. When they were first made, they were used with Modulator Control Plus (MPC) and basic/standard Modulator Controls.

Modulator model numbers identify the number of cells, as well as the number of drivers. The first family of modulator arrays consisted of eight different models, as follows: 1004, 2008, 3012, 3024, 4016, 5020, 6024, 6032, and 6048.

In January 2013, Federal Signal released the Modulator II sirens, consisting of the 1004B, 2008B, 3012B, 4016B, 5020B, 6024B, and 8032B models. They provide the same alerting technology as the original modulator, with the exception of a smaller compact chassis and cylindrical modules instead of elliptical ones.^[1]

Similar to Federal Signal's previous Electronic Outdoor Warning Siren series, the Modulator can produce seven standard warning tones. The seven standard tones are Wail, Alternate Wail, Pulsed Wail, Steady, Alternate Steady, Pulsed Steady and Westminster Chimes.^[2] If properly equipped, the Modulator can also employ voice notification to give specific information or to give a more clear understanding of an emergency situation. They have also been known to play Star Spangled Banner during tests on or near the 4th of July, and also on military bases.

The sirens can be activated by radio using single tone, two-tone, DTMF, MSK or POCSAG over analog, digital and trunking systems, or by satellite, cellular, landline or IP. They are also capable of being activated automatically by the Emergency Alert System.^[2]

The Modulator has a similar setup to the Whelen WPS 2700, 2800, 2900, and OmniAlert omnidirectional speaker arrays. The Whelen arrays have single driver cells,^[3] while Modulators have multi-driver cells. The American Signal I-Force siren, which uses stacked, elliptical speaker cells that provide omnidirectional sound output, is also similar to the Modulator.

The Modulators in Chicago's siren system formerly used to run on the Alternate Wail tone during tornado warnings in order to vary from emergency vehicle sirens. However, in O'Hare International Airport, instead of using the main UV Wail or Attack, they use an Eclipse 8 (possibly) sound card doing attack. Before that is the Westminster Chimes and voice. The city of Chicago no longer does its Alternate Wail mode, and was replaced by Attack mode in the late 2010s.

The city of Moore, Oklahoma is well known for their Modulators being placed closer together in comparison to other cities. They are placed only a half-mile from each other in some parts of the city. Until 2017, some sirens ran on a major third dual-tone alert.

Baytown, Texas is one of only a few cities in the United States to have a whole system of Modulators. The city uses just over twenty 5020s and 5020Bs, primarily for notification of chemical releases. The warning tone is a custom Hi-Lo signal.

The Modulators in Olmsted County, Minnesota are known for their pre-test and post-test voice announcements, and also for only being located in parks.

The Modulators in Monroe County, Michigan are known for their pre-test voice announcements. There is a unique "rainbow" Modulator located in Monroe County as well.

Eden Prairie, Minnesota has a uniquely mounted Modulator 5020 on top of a parking garage which has a custom attack tone that cannot be found on other Modulators. This Modulator has never done a voice announcement, which is one of the primary reasons that some cities invest in them.

Additionally, in Cape Town, South Africa, several Modulator sirens are installed which use their standard wail tone to alert citizens in the Melkbosstrand, Duynefontein, Blaauwberg, Table View, Robben Island, Atlantis, Philadelphia and Parklands areas in case of any emergency which can occur at the Koeberg Nuclear Power Station.^[4] These Modulators are tested for about an hour annually on the first Wednesday of March during a so-called "Full Volume Siren Test" using a preset female voice (which reads "This is only a test; there is no need to take any action. This is only a test; there is no need to take any action. I repeat: this is only a test; there is no need to take any action. Listen to Good Hope FM or Kfm for further information") and live voice announcements (which are read by people and are often cite similar things said by the preset voice), other than the typical wail tone of the Modulator.^[5]

The Modulators (MOD-2008 & 1004) make up nearly all the warning sirens in the Netherlands, which are tested on the first Monday of the month at noon.

The Modulators in Saudi Arabia are located in Air Force Bases in both Riyadh and Dammam.

The Modulators in Branson, Missouri sometimes use live voice with their Whelen counterparts.

The Concrete, Washington area has a system of eight 8032B Modulators to warn of possible breaches at the Baker River Dam. The system uses a distinctive "WHOOP" tone that was originally produced for Federal Signal's line of fire alarms. The system is tested on the second Monday of every month at 6:00pm.^[6]

Alaska has many Federal Signal Modulators and Eclipse 8s on their All Hazard Siren System. They sound for tsunamis as well. They use 3012s, 6024s and other potential models. During a tsunami warning, the sirens are activated in attack following a voice announcement.

Langley Air Base has Modulators as a part of the giant voice system at the local Air Force base. The sirens are known to play bugle calls and musical melodies. They play bugle calls such as The Star Spangled Banner, Wild Blue Yonder, and Taps.

Hawaii has a statewide system of green painted Federal Signal Modulators and Modulator II sirens throughout the state to warn of tsunamis, volcanic eruptions, bomb threats and any other dangerous emergencies or natural disasters. They are replacing a system of old mechanical Federal Signal sirens, such as Thunderbolts, SD-10s, among others, in which a very few still exist due to system replacements by the electronic sirens. Most of the Modulators are dual-tone. The system also consists of EOWS 1212s, EOWS 612s, DSAs, and American Signal I-Forces, mostly running on Siratone and UV controls. Some I-Forces also use the UV Modulator sound cards. The system is tested regularly and are tested both locally by Hawaii-EMA worker(s) and statewide. The Modulators are tested locally, normally started by a worker accessing the controller. They start with the PA feature on the UV by making a live announcement. Then they test every sound on the sound card.

The now decommissioned Pilgrim Nuclear Station in Plymouth, Massachusetts used Federal Signal Modulators and EOWS 612s. They still currently remain standing as of 2023, even though the power plant closed in 2019. They were placed within the towns of Plymouth, Massachusetts, Carver, Massachusetts, Duxbury, Massachusetts and Marshfield, Massachusetts. Their Modulators consisted of 5020s, 3012s, 6024s and 5020Bs. Their EOWS 612 sirens were part of their original SiraTone system and were later on upgraded to UltraVoice controls as the new system utilized them. All sirens sounded in a male voice message that says “ATTENTION! ATTENTION! THIS IS A TEST OF THE EMERGENCY SIREN SYSTEM. THIS IS ONLY A TEST!” The message repeated a few more times and followed by a steady 3-minute alert tone. Testings concluded with another male message (the same voice) that said “ATTENTION! ATTENTION! TESTING OF THE EMERGENCY SIREN SYSTEM IS COMPLETE. PLEASE REGARD ANY FURTHER ALARMS.” The Pilgrim Power Plant never had any major emergencies, but if there was one, the sirens would sound in a tone, followed by a voice announcement. As of 2023, the sirens are still there, but it is unclear whether they are still active. However, in a couple of the towns, they are active.

Manasquan, New Jersey has a system of MOD-3012s and 6024s. They are similar to the Chicago siren system because they use the rare signal, alternate wail, but after, it includes a voice message that says “THIS IS AN EARTHQUAKE. WARNING! TUNE YOUR AM RADIO TO 1620.”^[7] the message repeats at least a few times and then plays the signal.

Severn Estuary has a system of Modulator and DSA sirens. The system runs on both UltraVoice and MCP Controllers. They have very different tones compared to the American ones, potentially custom. The system is activated by radio signal at Avon and Somerset Police headquarters.

Clemson University has a system of MOD-5020s, MOD-4016s and 2001s. They test at the same schedule as the city of Clemson, South Carolina’s siren system for the nearby power plant. When testing, the modulators normally play a voice that says “THIS IS A TEST OF THE CAMPUS EMERGENCY ALERT SYSTEM. THIS IS ONLY A TEST.” which is followed by attack mode. ^[8] When a thunderstorm is detected, the modulators sound in hi-lo followed by a voice that says “LIGHTNING WARNING! CLOUD TO GROUND LIGHTNING IS EXPECTED IN THE AREA. PLEASE SEEK SHELTER IMMEDIATELY!”^[9] When a tornado warning occurs, the Modulators sound in attack, followed by a voice warning that says “A TORNADO WARNING HAS BEEN ISSUED FOR CLEMSON UNIVERSITY. PLEASE SEEK SHELTER IMMEDIATELY!” ^[10] The system is not only used for tornadoes, and severe storms, therefore, it is also used in case of an emergency at the power plant nearby the facility. All of the Modulators are ran on UltraVoice controllers.

Little Silver, New Jersey has a system of two Modulator 2008s and one 3012. These Modulators are unique because they play off a custom air horn recording rather than the standard UV/MCP files even though Little Silver never had an actual horn system prior to the Modulators. These sirens replaced a Federal Signal SD-10 and a Model 2. These sirens are used for calls, funerals, Santa escorts, flooding, curfews, blackouts, and natural disasters such as hurricanes and tornadoes.

Point Pleasant Beach, New Jersey has a Modulator 3012 that plays off a Gamewell Diaphone recording in addition to a custom attack tone used primarily for calls even though Point Pleasant Beach never had a Gamewell Diaphone. However, the 3012 got replaced with a 3012b in January 2024 so it is unclear if it still has that tone.

Several controllers were made to run the Modulator sirens. These include the MC (Modulator Controller), MCP (Modulator Power Plus Controller), and UV (UltraVoice). The older SiraTone controller can also run on Modulator sirens but the SiraTone was mainly used on the predecessor EOWS Series sirens. All of the controllers built for the Modulators can be compatible with the Predecessor EOWS sirens, the Successor Modulator II sirens, and the DSA sirens. The Modulator II series started production in 2013 and the older Modulators started production in 1991. The reason why the EOWS sirens can be compatible with these controllers is because the EOWS 408, 812, 115, and 1212 lasted production until 2001. The EOWS 612 was kept in production only on special order until August 3, 2007. All 3 controllers were being made or were made at the time so Federal Signal decided to make the EOWS sirens compatible with these controllers.

The MC Controller came out around 1990 when the Modulator series were being produced. The controller was a microprocessor operated platform which used digital, solid state systems to produce the 7 signals provided by Federal Signal. Unlike the SiraTone, the MC had tones pre-loaded onto an audio ROM chip so there was no need for an analogue tone generator and timer circuit. Audio from the ROM chip was programmed by Federal Signal, with the auxiliary tone being Westminster Chimes by default unless a different tone was requested for by the customer. This could be overwritten by an engineer using software on site, should signaling needs change. The system was activated by two-tone or by DTMF by the controller's internal radio, locally by keying in a two digit code on the controller's keypad, or by operating the controller via landline. When using two-tone or DTMF, the siren has to be "armed" in order to operate (unless programmed not to). To do this a DTMF arm string would first be broadcast to the controller, then a signal DTMF string would be broadcast. Arm period would last for 5 minutes until automatic disarm, unless a "disarm" DTMF string was broadcast before the time expired. This is different, and more secure, to the SiraTone which required a jumper to be added in order to permanently arm the controller. After a signal was activated the digital audio was sent through a digital/analogue converter to each amplifier and onto the speaker array in the form of a square wave. The MC was discontinued in 1996 when it was upgraded to a newer version, the MCP.

The MCP or Modulator Power Plus Controller controller was pretty much the same thing as a MC except there were a few additions. More signals could be generated through the tone generator, and it could also be activated by Federal Signal's FSK (Frequency Shifting Keys) activation tones. Nothing else changed between the MC and the MCP. The MCP was discontinued around 2002 for the newest version of the series, the UV controller.^[11]

The UV or UltraVoice Controller is the newest electronic siren controller made by Federal Signal. It now is compatible with the newer type Modulators, the Modulator II series which started production in 2013. The UltraVoice comes in 2 variants: UV and UVIC. The UV is for outdoor use with large speaker arrays like the Modulator, where the UVIC is for indoor use with small speakers and/or intercom systems. UV is capable of holding up to 8 UV400 400 Watt amplifiers, for a total of 3200W. The UV comes standard with 7 signals: Steady, Wail, Alternate Steady, Alternate Wail, Pulsed Steady, Pulsed Wail, and Westminster Chimes. It is offered from the factory in Single or Dual tone, however it is easily changeable after purchase.