How Do Sirens Work? (Part I-III) | Remastered

Added: 2026-05-11

[00:00:04][music] [music] [music] Electromechanical sirens are a big part of history. They were the main mode of sirens since the 1900s. Many countries adapted sirens during World War I and World War II as a way to warn citizens about incoming attacks and air raids.

[00:00:30]Many people were saved by the sirens since it gave them enough time to hide or evacuate. But once the wars were over in the 1940s, they stopped using the sirens as much. Some areas kept testing the sirens while others left them to weather or decommissioned them.

[00:00:44]Eventually, in the 1950s and60s, tensions started rising, making the sirens in demand again. This led to many iconic siren manufacturers like Federal Signal and Alerting Communicators of America to start making many models that were iconic in and of itself like the Federal Signal 3T22, the ACA Hurricane, the Federal Signal Thunderbolt, and the ACA Alert.

[00:01:08][music] Electromechanical sirens are not speakers and thus they cannot play any digital audio like voice, music, [music] nor electronically based tone generators like the sireone. Electromechanical sirens use choppers which are built out of two parts, a stator and a rotor. The stator is a fixed part which doesn't move or rotate unlike the rotor which is slightly smaller than the stator in order to fit which is driven by an electric motor or in purely mechanical sirens a gearbox connected to a hand crank. The rotor can be called the chopper or the impeller. Both the stator and the rotor have holes or ports. There is a housing for the siren where air can come in through the intake, then into the rotor and exit through the stator.

[00:01:51]The reason why the chopper has ports is so that it can make that familiar noise.

[00:02:01]When the rotor rotates, there is a time where all the air is blocked since the rotor is not aligned with the stator.

[00:02:07]then a time where the holes are open, letting the air exit through those holes. Since there is a big amount of air being moved at a certain frequency, you get an audible sound. The rotor often has holes drilled in for it to be balanced to avoid shaking, like in this ASCT128 or ASC 130, whatever.

[00:02:46]>> [music] >> There are many ways you can alter the siren's pitch and volume. There are two ways to control the pitch. [music] Having more holes in the chopper and making the rotor spin faster. Having more holes in the chopper makes it have a higher pitch, as shown here.

[00:03:11]and the faster the rotor spins, the higher the pitch, which is actually why the sirens go from low pitch to high pitch when starting up and winding down.

[00:03:20]Take a close look at the RPM meter, or rotations [music] per minute done by the rotor.

[00:03:32]But that's for single tone sirens. What about dual tone sirens? Dual tone sirens have, as the name suggests, two distinct tones. These sirens either have two choppers with different amounts of ports that may or may not be driven by the same motor, or one chopper with two rows [music] of ports with also different amounts of ports. As I said, having more ports makes the pitch of the siren higher relative to other sirens with different ports of the same RPM or rotor speed. Meaning that two choppers with different ports running at the same time and speed means you have two pitches of siren at the same time and speed. This is why when you may or may not have seen it before, see things like 8/10 port, [music] 1012th port, 912th port, 45thsport in a siren specification on a recording or wiki page. That is the amount of holes/ports seen in the choppers. This means that for example, chopper A of the Federal Signal 3T22 has 10 ports while chopper B of the same unit has 12 ports. This is called the port [music] ratio. You can also identify single tone sirens and dual tone sirens by the port ratio. If it has one number, then it's a single tone, two numbers, then it's a [music] dual tone.

[00:04:42]This is also why sirens of different models sound the same. Basically, they are running with the same port ratio as another model at almost the same speed.

[00:04:50]Like the Federal Signal 3T22 and the Federal Signal 500 SH-T sound identical. They both have 10 over 12 port, but a Federal Signal 2T using a 5 over6 port Thunderbolt chopper and single tone 5port stator sound identical, too.

[00:05:17]This is because the 5/6port siren is running at 2 times the speed of the 10/12port sirens. And since 5 * 2 = 10 and 6 * 2 = 12, it sounds basically the same. One thing to note is that some sirens of the same model can be produced with different choppers. Most notably, Federal Signals model series of sirens came in two types. the normal one with a single tone chopper and the one rotor dual tone one. The model 2 had a five port chopper with the model 2T having a 4/5port chopper. To give you a better understanding of port ratios, here's a demonstration of what each port ratio sounds like.

[00:06:08]Volume can be controlled with five [music] things. A coating mechanism or a damper, the size of the chopper, the effectiveness of the housing, rotor design, [music] and the air supplied to it. Firstly, the coating mechanism. Most sirens don't have this, but the coding mechanism is used to be able to control the siren with an [music] electric solenoid by having the intake or the output blocked, making it so that minimal air goes through the siren, effectively muting it. The damper can be controlled in such a way to make a pulsing signal for single tone sirens and [music] dual tone sirens with one rotor and even a high low signal for dual tone sirens with two [music] rotors. Secondly, the size of the chopper. The model 2 siren has 7in 5port rotor at a sound output of 100 dB at 100 ft [music] while the ACAP50 has a 24 and 1/16th in with 135 dB at 100 [music] ft.

[00:07:01]Remember, the decibel scale is logarithmic, meaning that the P50 is about 11 times louder than the Model 2.

[00:07:08]Thirdly, the effectiveness of the housing. The Federal Signal ST10 has horns on the stator, but the intake is pointed to the ground. And since some of the sound actually comes from the intake, that sound is wasted and not spread as far. But take the Federal Signal 508-128.

[00:07:24]The intake is exposed with the sound coming out the chopper also coming from the sides of the projector horn. This means that most of the sound is directed where it should, [music] giving us minimal sound wastage. Next, the rotor design. The rotor or the spinning part is designed with veins that help lead the air out into the ports instead of just smashing into the wall and scattering everywhere. The veins not only just help with the volume, it also helps with the sound. This is a commonly seen problem in 3D printed mini siren design [music] and even vacuum cleaner designs. The Model 5's veins are short when compared to the STH10's fulllength veins, [music] which even if comparing them with the same 12-port rotor, the ST10 ultimately sounds smoother than the Model 5.

[00:08:18]Finally, the air supplied to it. Almost all sirens have their intakes just exposed, not connected to anything. But there are eight models of supercharged and pneumatic sirens that are special in a way that they use supercharged air directly supplied to the airtight chopper through a pipe. They are the Federal Signal Thunderbolt, ACA Hurricane, Henoku Turbo Siren, Gyri [music] Siren, and SoCal Edison 120 which was part of the songs siren system and the following which are pneumatic sirens. [music] the Chrysler siren, the Horman HLS series, and the Pinch Bama.

[00:08:55]And while you can call all pneumatic sirens as supercharged sirens, you can't call all supercharged sirens pneumatic.

[00:09:02]These sirens make use of a blower, or for [music] pneumatic sirens, an air compressor with an air tank, like the Thunderbolts type MX blowers to feed supercharged or highdensity air directly to the chopper. This adds the amount of air being moved through the siren, making it very loud. However, these sirens often break due to the amount of parts, especially the blower.

[00:09:25]Electromechanical sirens are built with different housings and configurations.

[00:09:29]Almost all of these are built to be omnidirectional, where you can hear the sound of the siren in all directions at the same time, except for the projector horn configuration, which is rotational.

[00:09:39]Rotational sirens make use of collector rings, which is a device that enables something wired to rotate without having to twist a wire. These sirens have an extra motor that runs at a very slow speed at around 2 to six revolutions per minute and makes the sound go in different directions at a time. Staying in one spot while a rotational siren goes off makes it sound like the volume is going up and down.

[00:10:16]Before we move on to the next part, and I had already said this in the last video, but to be more clear, the equation to get the frequency of a siren is hertz equals RPM [music] divided by 60* ports. This means that the number of ports affects frequency linearly. But since pitch perception is logarithmic, doubling the number of ports results in a [music] sound one octave higher at the same RPM.

[00:10:42]The tambber is the character or quality of the noise generated by the siren.

[00:10:46]From this point on, I will use a spectrogram, which is a visual representation of the spectrum of frequencies of a signal as it varies with time. [music] The tamber of a siren usually consists of a fundamental frequency, which is the loudest and main character of the sound, the harmonies, which add to the sound, and the undertone, which can occur due to uneven air flow or asymmetries in the rotor design. How the tambber works is different between single tone choppers, [music] dual tone single choppers, and sirens with a rotor and stator that don't match each other. Let's start with the single tone choppers. They consist of a fundamental frequency and may have an undertone. You can hear this when comparing the Sentry 10V with a T128.

[00:11:37]As you can hear and see, the 10V has a noticeable undertone, an octave lower than the fundamental frequency, and the T128 does not. To see why, I want you to look at their rotors and guess what makes them sound different.

[00:11:51]If you said that it's because of the port amount or size, then you are wrong.

[00:11:55]It's actually because of the veins. As you can see, the T128 has eight full veins, and the 10V has four full and four half veins. The T128 having eight full veins gives it a pure eight port sound. But the 10V having four full veins out of eight total veins leads more air going to the full veins, giving it a four- port undertone over the eight port fundamental frequency. This also applies to dual tone sirens with separate choppers, only that the two frequencies may form a resultant tone or a slight dissonance. Now, let's head on to dual tone single choppers. They consist of two fundamental frequencies and always have an undertone. The undertone will have the pitch that is the greatest common factor of the two fundamental frequencies. For example, a 912 port chopper will give out a 3 port undertone. A 1012 and 810 port chopper will give out a two port undertone. And an 812 port chopper will give a 4-port undertone. For example, here's a 912 recording of an SD10.

[00:12:57]As you can see, there are two fundamental frequencies for the 9 and 12 ports.

[00:13:04]But there is another line below that is for the three port undertone.

[00:13:10]Here is what the full thing sounds like again.

[00:13:16]Finally, sirens with non-matching rotors and sters. Sometimes people installing a disassembled siren mistakenly swap the rotors. These sirens give heavy undertones and produce a chorus effect.

[00:13:39]or an insane rasp.

[00:14:01]It is also possible to reverse wire a siren by swapping any two of the three supply lines. Either by reverse wiring the chopper motor with directional veins or reverse wiring the blower motor for supercharged sirens like in the Federal Signal Thunderbolt, causing it to suck in air instead of sending supercharged air, which in both instances would lead to the sound output being decreased greatly.

[00:14:40]Each siren model is different, which is why some sirens have some niche features that [music] not a lot of other models have. We'll start off with the rotator.

[00:14:48]Sirens with rotators can either have a separate motor for the rotator or use the same motor for the chopper and the rotator. The opposite end of the motor armature has a belt, chain, or other gear down mechanism linking to the turntable base. Because of this, rotation speed is dependent on the current speed of the chopper and will slow down/ speed up accordingly.

[00:15:22]Some sirens don't use collector rings, which as I said is the device required for it to spin without twisting a wire, where the siren changes its direction of rotation every full spin. Another thing, the Chrysler siren, which is widely considered the loudest siren ever built, uses a cylindrical rotor with holes instead of a common veined rotor. Also, I will restate this because apparently many people don't know this, but when you supply power to the siren, the motor doesn't start spinning at its maximum speed [music] instantly. The speed slowly goes higher and higher, in turn making the pitch higher. [music] And when you stop supplying power, the motor also doesn't stop spinning instantly.

[00:16:00]The motor keeps spinning while slowly losing speed until it completely stops as there isn't a constant magnetic force spinning it. The time it takes for this windown period to finish depends on the quality of the ball bearings and the weight of the rotor with some sirens having another weight just to make the wind down longer.

[00:16:19]Now let's talk about electricity. Sirens can either use one-phase or three-phase AC electricity or DC electricity which is commonly used for battery powered units. One-phase sirens are usually small or have a small motor. They have less starting torque and may require a start capacitor. Three-phase sirens are used in bigger units and can provide smoother and continuous power delivery.

[00:16:43]DC sirens are often used for their battery backup capabilities. The mains frequency also changes the speed of the motor. Sirens running at 60 Hz run about 20% faster than those running at 50 Hz.

[00:16:55]You may be wondering what their controls look like, and the way they are controlled is different for each siren manufacturer and era. These are the Federal Signal PGA timer, AR/ AF timer, Cyrrol, Federal Signal FC, and many other controllers. I will go in depth about these controls in the next video.

[00:17:14][music] For now, I will talk about the AF timer and the FC controller. Older federal signal sirens used the Cyrol, which was a siren activation radio that enables towns to activate their sirens from a location with a radio using two-tone activation, which sounds like this.

[00:17:36]or DTMF, which sounds like this.

[00:17:40][music] The Cyrrol connects to the AR/ AF timer with an ARCH/ AFCH panel. The timers used clock timer motors, which operate a system of rotary cam operated switches to activate the siren signals for a 3minut cycle. Later timers used transformers so that they can be connected with 240 or 120 volts.

[00:18:05]These timers had a test, alert, attack, and cancel button. [music] The test button supplies power to the siren for as long as the button is pressed. This is helpful to make a growl signal, which is used for testing since the siren starts winding down even if it hasn't finished winding up completely, making a quieter noise than if you were to run it fully.

[00:18:37]The alert button supplies power to the siren continuously. This has historically been and in some places is still used as an allclear signal.

[00:18:59]The attack button supplies power to the siren intermittently, producing a wailing sound. This is often used for warnings and alerts.

[00:19:20]And finally, the cancel button stops the current signal. In AF timers, there is an extra button called the fire button.

[00:19:26]This button controls the solenoids of the damper of a siren instead of the motor running the chopper. This button alternates each solenoid, producing a high low signal.

[00:19:46]>> [music] >> The modern alternative to this setup is the FC, which is basically an AF timer with more and custom signals combined with the Cyrrol into one. It's also digital and can connect not just via two-tone and DTMF, but also via EAS.

[00:20:09]Poxag and AFSK.

[00:20:18][music] Omniirectional sirens are sirens that can be heard in all directions all at once. This is the more common type of siren, especially back in the day.

[00:20:30]Cities tend to install omniirectional sirens because rotational sirens used to be more fragile. These sirens have many configurations including exposed sirens, horned sirens, [music] shrouded sirens, and box sirens.

[00:20:45]Birectional sirens are sirens with two heads facing away from each other. They give a good sound coverage [music] as the sounds from the intakes cover the front part while the sounds from the outlets or ports cover [music] the sides. However, this often leads to dead space in the middle where the sounds are not brought out that far. Rotational sirens are sirens that use projectors, either by using a projector horn or [music] unique designs. Projector horn sirens rotate along with the chopper, which is inside or attached to it. They use slip rings to maintain an electrical connection without twisting the wire.

[00:21:23]Most rotational sirens have the entire siren and projector horn rotating. But some unique designs actually have only the projector rotating while the siren stays stationary. From one point, the siren will be heard for some time, then disappear, then heard again. This is because the sound is being directed at another location. [music] However, the sound is brought farther than omniirectional sirens because much of the siren's sound output is directed at a single direction. Also, more about veins. The amount of veins in a rotor will give a resultant tone which sounds as if another siren with the same amount of ports as the veins is sounding. I explained this earlier with the Sentry 10V comparison, but it is also seen in the Federal Signal 3T22.

[00:22:12]If you listen to it, you will hear [music] a four five port undertone. This is because the 12port rotor has four veins while the 10port rotor has five.

[00:22:28][music] [music] It's about time we talk about electronic sirens. Now, they are still sirens.

[00:22:46]However, the way they work is completely different from electromechanical sirens.

[00:22:50]These sirens are basically just loudspeakers. You can play digital tones, voices, anthems, and even songs.

[00:23:02][music] Oh, and I forgot to tell you that Yamaha literally made an electromechanical siren, or I guess we should say sirens that vary in pitch and have dampers that when played in an order makes music. But electronic sirens are more flexible for that. Electronic sirens have way different controllers, which can either be analog or digital. Analog controllers produce tones using electronic circuitry while digital controllers produce tones using microchips and CPUs.

[00:23:42]They also use audio amplifiers which turn the weak signal from the tone controller into a powerful sound which will be [music] set to the speaker drivers which are usually connected to horns [music] or Mentos modules.

[00:23:59]This type of siren is way more common.

[00:24:01]And some siren manufacturers don't even make electromechanical sirens, only electronic ones. Some examples of electronic sirens are the rotating federal signal EOS siren with a terrifying chime signal.

[00:24:21][music] >> [music] >> the Federal Signal Modulator, ASC Force, Whan, Vortex, and Louden ES5000-DC.

[00:24:45]Here ends the how do sirens work series.

[00:24:48]After this, I will be regularly posting videos about every model series by all manufacturers, including international ones. And I need your help. Please comment some international sirens that are not commonly known by enthusiasts outside your country. It will be a great help for my research [music] and that's how I make sure that every siren is covered. Also, I want to share with you guys a program that I have been working on which is a siren synthesizer capable of simulating real electromechanical sirens. This is actually the program I used for that 3T22 demonstration earlier.

[00:25:24]>> [music] >> Thanks for 1,000 subs and have a great day.

[00:25:32][music] [music]