Low Frequency Siren Systems

Clearing traffic is getting to be more and more of a challenge.  Modern cars are far more soundproof than older vehicles, stereos are louder, and many drivers (and pedestrians) wear noise-cancelling earbuds.

Too often, regular sirens aren't as effective as we need them to be.  While we could add another siren speaker to the car's existing system, in many circumstances the effectiveness can be limited.

Rather than simply adding another speaker on the pushbar or in the grille, consider adding a different kind of siren system.

Low Frequency Siren systems have been in use since about 2005 and are available from various manufacturers.  For the sake of this article, we'll focus on the Federal Signal "Rumbler" system.

The theory behind LF (low frequency) siren systems is kind of a "work smarter, not harder" idea.  Lower frequency sounds tend to penetrate barriers better.  Whereas higher frequencies are easily deflected by glass or other solid surfaces, LF tones will penetrate them.  It takes a lot of mass to stop low frequency sound waves.   

If you've ever been a pedestrian and notice a car driving by, with a very loud stereo playing in the car, all you can hear from your perspective is the thump of the deep base (subwoofer).   While the car's occupants are hearing full-spectrum sound (low to high frequencies), the only tone that escapes the car's cabin are the lower frequencies.

The LF siren system works along the same theory, but in reverse.   While standard siren tones can 'bounce' off the vehicle's glass, the lower tones penetrate the car's hard surfaces.

The LF siren system generally emits about 1/4 of the frequency that your main siren does.  Since the LF system's amplifier is either linked to, or shared by, your standard speaker's amp, the LF system knows what tone your regular siren is generating, divides that frequency by 4, and outputs the resulting frequency.  For example, if your main siren system's "Wail" tone runs between 600hz - 1000hz during its cycle, the LF siren system attached to that tone would emit 150hz - 250hz at the same time.

If LF sirens are so great, why do we still use standard siren tones?   There are a few reasons;

• Low Frequency siren tones do not fall within the SAE siren specifications
• LF tones do not project as far in front of the emergency vehicle as standard tones
• LF tones penetrate other vehicles better, but also penetrate the emergency vehicle.  We limit the amount of time the LF tones are on, in order to protect the emergency vehicle's occupants' hearing.

Due to the factors noted above, LF sirens do not replace standard siren speakers.  Rather, the LF tones work in conjunction with standard siren tones.

While a standard siren will sound the entire time you're on your response call, the LF tones are set up so that they are only used when approaching an intersection, or approaching a car that isn't pulling over.   LF tones are not sounding continuously .   All LF sirens on the market have a timer so, when you activate the LF tones, they'll sound for about 8-15 seconds then automatically shut off, waiting for the next time you need to activate them. 

It's important to limit the LF sound exposure to the emergency vehicle's occupants as the SPL (sound pressure level) is higher with a LF siren than without.  These numbers are from a test in a 2013 Ford Explorer...

This means you should limit the Rumbler tone to < 1 hour of exposure per day (under US NIOSH standards).  However, if the Rumbler is installed correctly so that it only sounds when needed and the timer limits the tone to 8 seconds per button-push, that's 450 activations per day.   It's unlikely that any first responder will come anywhere near that level of activity. 

So, how effective are the LF systems on 'target' vehicles that you're trying to clear out of your way?  Here are the results from a test done in an SUV at various distances from the police car.  You can see a very dramatic increase in SPL in the 'target' vehicle, especially at closer distances.

At 50 feet, for example, the increase is nearly 9db, and closer to the police car the increase is up to 19dB.  Since, from our previous blog, you know that 10dB increase is 10x the volume, the addition of the Rumbler system generates a very dramatic increase in SPL inside the 'target' car!

While a standard siren emits about 120dB (measured 10 feet from the siren speaker), the Rumbler emits 110dB.  So, while the Rumbler emits less volume (SPL) than a standard siren system, more of its sound gets to the occupants of the 'target' vehicle than that of a standard siren tone.   This kind of a 'work smarter, not harder' theory.  If your department gets noise complaints from your residents, the LF siren system might be a good option to consider (rather than adding a second standard siren speaker) because a) it emits lower sound pressure levels than a standard speaker and b) it's only active when needed, and then for only for a short duration.  

This video shows some great examples of the Rumbler in action.  While these videos and your computer speakers do not replicate the low-frequency tones well, this gives you an idea of the use.

Physical Setup

When your car is being upfitted, the regular siren system will be installed.  The system will include some kind of controller/amplifier that resides inside the veihcle, and a standard siren speaker (1 or 2) mounted behind the vehicle's grille or on its push bar.

If a LF system is also being installed, the upfitter will install a separate speaker system that is specific to the low-frequency tones.  You cannot run Low Frequency tones through a standard siren speaker.  LF tones require dedicated speakers, just like a home stereo system requires a separate speaker for low frequency tones.

Normally, LF speakers are outfitted in pairs.  It's quite common for an emergency vehicle to have one standard speaker and a pair of LF speakers.  In terms of power draw and the demand on the amp, a pair of LF speakers draws about the same as one standard speaker (If you're a numbers geek, LF speakers actually output about 68 watts each, so a pair is closer to 136 watts).

Whether you need another control/amplifier module for the LF siren system will depend on the brand of controller/amp you are using, and may also depend on how many standard siren speakers you're using.  For example, most controller/amps can power 200 watts of sound.  This means they'll power 2 x standard speakers (which are 100 watts each) or 1 x standard speaker and 1 x low frequency speaker system.

If you have a siren system that's capable of 200 watts and you already have 2 x standard speakers on the car, you may need an additional module to power the LF speakers.   Or, you can get a controller such as the Federal Signal Pathfinder 400 which has 4 x 100 watt outputs.  The PF400 will power 2 x standard speakers and a pair of Rumbler (low frequency) speakers without any additional modules, and still have another 100 watt channel to spare!

Most manufacturers have a large LF speaker that are roughly 8" diameter and 10" long.  While there are vehicle-specific brackets available for some vehicles, the size of these speakers can be prohibitive when trying to mount them in many vehicles.   In late 2023, Federal Signal released their second generation of Rumbler system that consists of two different versions of speaker system;  the Solo and the Compact.

The Rumbler Solo is a housing of similar size as others, but contains TWO drivers in the housing (other manufacturers' systems contain only 1, which means you need two full speakers/housings in your car).  This means you only need to mount one speaker unit in your vehicle.  

The Rumbler Compact is a pair of speakers but they are far smaller than the standard LF speakers.  The Compacts can be used in a wide variety of vehicles where the standard LF couldn't possibly fit.

The performance / effectiveness of the Compact is about on par with the previous generation Rumbler.  Because the Solo has two speakers in a tuned housing, their effectiveness is a bit better than the Compact and a bit better than the previous generation Rumbler speakers.  

We generally start by considering the vehicle that you want to mount the Rumbler in.  That may determine whether we can only mount the Compact speakers, or whether we have the option of the Solo.   If the Solo will fit, then the next consideration is your budget, since the Solo - while more effective - is priced a bit higher than the Compact system.

Advantages and Drawbacks

Here is a quick summary of pros and cons of standard speakers and low-frequency systems

Summary

We hope this helps give you a basic idea of low-frequency siren systems.  If you would like more information about these unique siren systems, please feel free to contact us and we'll answer any questions you might have.  While we are happy to help Canadian customers, if you are viewing this from the US or other location outside Canada, we would encourage you to contact your local Federal Signal representative.