Sirens: Let's make some noise
There is more than one choice in sirens, and understand how and why each works will help you make a better decision
By Robert Avsec
Sirens. Firefighters love them and more than a few in the general public detest them — as evidenced by letters to the editor in many daily newspapers. And lawyers have grown increasingly fond of sirens as a new revenue stream.
Today, more than ever, it is important for fire and EMS departments to find auditory signaling devices that are safe and effective.
Researchers consider the hearing as one of our primary warning senses. Studies have shown that reaction time to a visual warning lights from an approaching emergency vehicle improved when an audible warning signal was added. This should also be the rationale behind never using warning lights without the siren during an emergency response, which most states prohibit anyway.
Improved sound-reduction engineering in automobiles along with the prevalence of air conditioning has created a "sound insular cocoon" for today's driver. The system does a great job of keeping unwanted noise out of the vehicle and desired sound, from state-of-the-art entertainment systems, in. It also does fairly well at shielding the driver from an approaching emergency vehicle sounding its siren.
In one report, the U.S. Department of Transportation evaluated a siren's effective frequency range for signal attenuation — the amount of loss of the signal's intensity as it moves through a medium. In this case the medium was a closed-windowed automobile body combined with typical masking noise.
The results showed that at an urban intersection the sound from the test siren did not penetrate the vehicle until the siren was within 26 to 40 feet of the test vehicle.
Additional studies by both public and private sector organizations found only modest improvement in the situation for suburban intersections and straight-ahead highway conditions. Based upon the various findings, emergency vehicle operators should approach any intersection — where it is imperative that they capture the attention of other drivers in the intersection—at no more than 10 mph.
Electro-mechanical sirens take air, compress it and then accelerate it to more than 130,000 inches per minute (or 124 mph). This compressed and accelerated air then passes through a rotor, which pulses it on and off to create a square wave form.
The resulting column of air spirals at 9,000 revolutions per minute as it expands from the siren's 2¾-inch diameter guiding throat. The electro-mechanical siren's spiraling wave is like an ocean's wave curl projected from a short guiding throat, on off on off, producing the variable sound. The operator is able to control the volume and the pattern using a foot switch.
Electronic sirens (e-sirens) take a transistor-generated signal and send it to an electro-magnetic driver, which pulses a ¾-inch diameter diaphragm back and forth. This action causes the air to move in a sine-wave form, creating the familiar "woo-woo" sound that gets projected from the speaker horn.
Today's electronic sirens are designed to gain the attention of a vehicle's driver or pedestrian when they are within 50 to 100 feet. Most produce their highest volume near and in front of the siren speaker.
Beyond 100 feet or so, all electronic sirens begin to lose their effectiveness and volume. Stand in front and then to the side of your truck with the siren on and see what a difference you can hear.
Controversy: mechanical and electronic sirens
It seems that few topics in the fire service can do more to spur the passions of firefighters than a discussion about which is better: electro-mechanical or electronic siren? Electro-mechanical sirens and their predecessors, the mechanical siren, have served as audible warning devices since the early days of the fire service. So where did electronic sirens come from, and how did they become part of the debate?
Electronic sirens made their first appearances in the emergency vehicle warning device business in the 1960s. Many fire service leaders quickly embraced this new technology as a means of coping with an increasing demand for electric power aboard fire and EMS apparatus.
This electrical demand was fueled by more lights (emergency warning lights, spot and flood), mobile radio sets, on-board computers, and new EMS equipment (bio-medical equipment, IV fluid warmers, etc.). The electro-mechanical sirens of the day drew anywhere from 200 to 300 amps compared with only 12 to 17 amps for the new electronic sirens.
These sirens seemed to fit the bill as a comparable alternative to power-hungry electro-mechanical sirens because automobile air conditioning was not common in most cars and sound-reduction engineering was pretty much non-existent. Also, the variety of sounds they could produce were quick to catch the attention of people; electronic tones from alarms, buzzers, games, and computers were not yet commonplace.
So what changed? Start by going back to the aforementioned sound proofing of today's automobile. Then add in all the ambient noise that's found in today's urban environment.
And finally, for good measure, consider all the other things that people are doing besides driving — and even walking for that matter — ear phone/plugs in with the volume cranked up, talking on the phone, texting and more.
And there you have the scenario that has fueled the comeback of the electro-mechanical siren with its more penetrating wave form. But don’t think for a moment that the electronic siren manufacturers have become shrinking violets in this market.
Low-frequency electronic sirens
Enter the low-frequency system (LFS). This 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.
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.
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 grab the attention of distracted drivers and pedestrians. This also enhances our ability to alert those pedestrians who have physical hearing impairments.
There are a few simple measures that firefighters and EMS personnel can take to reduce their risk from siren noise.
- Wear hearing protection — which in many cases now can be your radio/intercom headset — at all times while operating the siren during responses.
- Keep the windows to the cab closed.
- Ensure that you employ the siren in accordance with state laws and regulations and the regulations of your department. If you don't need it, and it's not required, don't create unneeded exposure.
The department's apparatus and vehicle policies and procedures also can have a positive impact on siren noise exposure.
- Ensure that personnel know and understand when they must use the siren, when it's at their discretion, and when it should not be used.
- Provide appropriate hearing protection and ensure that personnel use it whenever the vehicle is in motion, not just when responding to alarms.
- Ensure that siren speakers are mounted on the front grill of apparatus rather than the roof.
- Do not let the manufacturer mount your siren speakers behind the apparatus bumper (the little grille). Doing so will result in a 50 percent reduction in the siren's sound projection because the sound bounces off the bumper backwards.