Good vibrations: Jury still out on low-frequency sirens

They are much-needed attention grabbers in a world of insulated and distracted motorists, but they have drawbacks

In a growing number of cities and small towns motorists, pedestrians and people sitting in their homes are experiencing sound vibrations that have them asking that question: "Was that an earthquake?" So what is the vibration if it's not an earthquake?

The next best guess would be an emergency vehicle equipped with a low-frequency amplifier attached to its standard siren package. More law enforcement, fire, and EMS organizations are getting on board with this latest technology that's designed to alert motor vehicle operators and pedestrians alike that an emergency vehicle is approaching. 

Those agencies are looking to this new technology, as they have with other advances such as intersection management systems, as a means to arrive at the emergency scene more safely, effectively and efficiently.

That journey to a scene becomes more precarious every day as the distracted driver is clearly in the majority on our roadways today. Those drivers are operating wireless phones and tablet computers, listening to music on "supersonic" vehicle sound systems, applying makeup, reading a newspaper, and countless other activities all in a vehicle cabin that reduces outside noise to a minimum. 

To those drivers, and the sound engineers who design their mobile cocoons, police cars and fire trucks and ambulances are just examples of that outside noise to be minimized.

And most pedestrians today don't get a free pass on this either. When's the last time you saw someone on the sidewalk not connected to a wireless phone or ear buds? In the Washington D.C. Metro area, I constantly see such distracted pedestrians walk into an intersection without ever looking anywhere but straight ahead.

Low-frequency system
Enter the low-frequency system (LFS). It is an adjunct that interacts with the vehicle's regular 100/200-watt siren by taking the primary signal tone and creating a secondary signal tone, which it reduces by about 75 percent. 

Next, the LFS takes the reduced secondary signal tone and routes it through an amplifier and then on to a couple of high-output speakers (woofers) designed to disperse the sound in all directions from the vehicle. 

The resultant low-frequency sound waves have the distinct advantage of penetrating solid materials allowing vehicle operators and nearby pedestrians to feel the sound waves. The sound waves have an effective range of approximately 200 feet and they don't discriminate between vehicles, people outdoors or people indoors.

Such a system presents some clear advantages in our pursuit to warn others of an approaching emergency vehicle so that they can steer clear. The sound waves penetrate a solid material, which can gain the attention of those distracted drivers and pedestrians. This also enhances our ability to alert those pedestrians who have physical hearing impairments.

The drawbacks
While we in public safety may view LFS as an effective device, noise-control groups have voiced their opposition to low-frequency siren technology calling the siren "disorienting."

According to a report on, "The intense sound caused by the [LFS] siren easily triggers an involuntary stress response commonly known as 'fight or flight.' This results in the secretion of adrenaline, with ensuing spikes in cardio-respiratory rates, muscle tension, and elevated blood pressure.

"Vibroacoustic disease is a cumulative and chronic disease caused by exposure to infrasound. Infrasound is low frequency sound energy that affects the nervous system and prolonged exposure can lead to progressive medical conditions."

LFS also presents an increased risk of noise exposure to the vehicle operator; the sound waves travel in all directions and that includes the passenger compartment of the vehicle with the system.

LFS manufacturers include a shut-off timer with their devices — the timers can be set to disengage the LFS anywhere between 8 and 60 seconds — and the manufacturers recommendations are that the timer be set in the 8- to 10-second range as a standard operating guideline. Further, they recommend that vehicle occupants wear approved hearing protection while operating the device. 

Currently, the primary marketers of LFS technology are Federal Signal, manufacturer of the Rumbler, and Whelen Engineering, the manufacturer of the Howler. Both models sell for $450 to $500.

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