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Types of Emergency Vehicle Sirens

Part 1: Emergency Vehicle Sirens - Audible Warning Devices

Mechanical Sirens

Mechanical sirens were the first to be used, replacing the traditional bells.  From the late 1700’s people began playing around with ways to make high pitched sounds using pneumatics, but it wasn’t until the late 19th century that the first functional mechanical siren was used in a lighthouse in Scotland. By the early 1900’s fire departments mounted the Decot or Sterling mechanical sirens on their station houses to call in firefighters.  At their hey-day, mechanical sirens compressed air to more than 130,000 inches/minute; that is 124 mph.  Comprised of a motor and a chopper, the air pump sucked in air through the front, and the sound came out the sides. The rotor has specifically placed holes which pulsed on and off to produce a ‘square wave’ form.   The spiraling sound was amplified and sent out through the throat of a speaker.

E-Sirens  Electronic Sirens

E-sirens make the familiar ‘woo-woo’ sound because the electromagnetic driver pulsates back and forth creating a ‘sine-wave’ form.  These new sirens made their debut in 1960 and many fire departments quickly changed to them because the new technology reduced the drain on fire truck and EMS vehicle batteries.  There was a huge difference between mechanical sirens which pulled 200-300 amps and these new devices which pulled between 12 and 17 amps, a staggering difference.  E-sirens were the perfect choice right up until the time when ordnary vehicles became overloaded with all sorts of entertainment equipment and engineering designs to dampen outside sounds. The outcome of this ever increasing sound insulation fitted to everyday cars was that motorists could no longer hear approaching electronic emergency vehicle sirens.  This ongoing problem gave the industry the chance to introduce the electro-mechanical siren.

Electro-Mechanical Sirens

Motorists ‘feel’ the sound created by electro-mechanical sirens.  The low frequency produced penetrates solid objects. Sirens with this unique advantage produce a low-frequency sound, so drivers sense that an ambulance or other first-responder vehicle is nearby, usually behind them, alerting them to start looking for flashing lights.

The technology takes the primary signal tone of a siren and creates a secondary signal tone which is reduced to 75%. Then it is routed through an amplifier, on to high-output speakers (woofers) that send the sound in all directions.  These sirens have an effective range of 200 feet.

There will always be preferences when selecting a siren, especially among first-responder agencies. Fire trucks, patrol cars, and EMS units are all state regulated agencies, each with specific tasks and guidelines for the use of sirens.  The main purpose of each, however, is the same: to catch the attention of people whether they are in a motorized vehicle or on the street, and to move them out of the path of first responders who are trying to do their jobs as quickly and safely as possible.

Fire Truck Sirens...Which is Better...Electronic Sirens or Electro-Mechanical Ones?

According to Chief Avsec, few topics can ‘get fire fighters going’ like a discussion about sirens and which type is better: electro-mechanical or the electronic siren.  The ‘mechanical’ group are the traditionalists who feverently believe that E-sirens are just ‘peanut whistles.’  While those who advocate E-Sirens believe they are the ‘attention-getters’ of the future!

Argument FOR Electro-Mechanical Sirens

Kevin O’Connell steps forward as a firm advocate of the electro-mechanical siren claiming that all the criticism levied against them are just myths. He rejects the complaint that they are too loud, pointing out that in all his years of service, his vehicle which had electro-mechanical sirens was never ‘hit’ and laid up because it wasn’t heard, unlike those that carried a pipsqueak E-siren.  He also points out that both types of sirens meet OSHA’s guideline for noise level, and that ‘real’ sirens are much better for the ‘long’ run.  

E-M sirens are not the power hogs that many claim them to be because they only draw on the battery during wind-up and full wail; while E-sirens draw continuously on the battery, though at a lower pull.  He explains that 6 to 8 E-sirens would be needed to do the work of one electro-mechanical powerhorse.  The reason that batteries drain, according to this emergency equipment specialist  is because systems are left fully lit while the engine is idling.  For every 50 Watt halogen lamp adds 4.2 amps to the load on the power source.   O’Connell also refutes the so-called fact that electro-mechanical sirens cost too much compared to E-sirens saying that the new ones last just a fraction of the older ones

Yes, opinions are strong when it comes to choosing the ‘right’ emergency vehicle siren.  

Mounting Sirens

Siren speakers and loudspeakers should be mounted on the front grill of apparatus rather than on the roof.  They should not be mounted behind the grille because by doing so there will be a 50% reduction of the projected sound.  The rule is always mount them as far in the front of a first responder vehicle as possible.

Sirens do not work alone, ETD sells Siren Sets with amplifiers and built-in PA systems; you purchase the siren speaker that meets your needs.  Mount any of ETDs products yourself, including sirens, by following the detailed instructions in the Installation Guide.  Of course, ETD always recommends that you get an EVT expert to install your purchases whenever you can.

Summary

Agencies have taken sirens for granted, believing that they will be heard and will get the job done, but it’s a fact that is NOT always the case.  As motorists and pedestrians become more distracted by their own entertainment systems, more and more “I never heard the siren” collisions are occurring. Some professionals say the reason is that E-sirens have replaced the ‘electro-mechanical’ ones and they simply are not heard as far or as well.  Others say that electro-mechanical sirens are ‘dinosaurs.’

Sirens are employed to grab the attention of anyone in hearing distance and to alert them to move out of the way...but a police officer often uses the siren for something more.   As retired Sergeant Rich Kinsey points out: “ Sirens are also used as a tool by police officers. Officers tend to go very loudly to “jobs” where they are trying to convey to the victim to “Hold on help is on the way!” There is no better sound when you are in trouble than multiple sirens in the distance...then tires squealing and  (close) sirens…then, thank God, car doors opening — The Cavalry Has Arrived!”

References:

Part 1: Emergency Vehicle Sirens - Audible Warning Devices

Further Reading