Law Officer Volume 4 Issue 8 As kids, we all remember the thrill of watching police or fire vehicles whiz by, with their lights and siren blazing, on the way to some unknown incident. The piercing noise, the visual overload and the rumbling of exhaust made our pulses run wild with wonderment. Where are they going, and what could possibly be at the end of their hurried journey?
As peace officers, we know that often the most frightening aspect of responding to an urgent radio call isn't the call itself, but the trip getting there. We have greater risk of catastrophic injury and even death as we make our way through traffic, intersections and freeways than we do after arriving on scene. To offset this danger, our cars are equipped with overhead lights, powerful sirens and unique markings all designed to announce our presence in traffic. But just how effective are they, and what else can we do to increase response safety? Recently, we detailed the importance of proper overhead lighting configurations (see "Seeing the Light," March 2008, Law Officer). Now, we'll look at the realistic effectiveness of sirens and other emergency response considerations when heading to your next call.
Siren Audibility
The first time you flip the siren on in your patrol car, it seems as though the entire world should be able to hear you coming. After all, the standard 100W siren box for patrol cars puts out approximately 120 dBs of noise at approximately 30 feet, which meets Class-A standards set by the National Institute of Justice. This standard is the minimum government requirement for siren audibility. So how loud is 120 dBs? Pretty darn loud. Consider this: A chain saw hums along at 100 dBs, a typical rock concert riffs at 110 dBs, and the human threshold for hearing pain matches our siren box at an even 120 dBs. So why don't people hear us coming?
It goes back to basic physics and car manufacturing. To be effective, a siren has to compete with soundproofing, a vehicle's audio and climate control systems, and physical distance. In fact, a U.S. Department of Transportation study concluded that siren audibility at intersections was only 25 40 feet, which translates to a safe entry speed of approximately 10 mph. The bottom line: As loud as patrol car sirens seem, unless you happen to be a pedestrian most sirens don't really project far enough for other drivers to hear them.
The issue gets more complicated when considering the effectiveness of multiple-tone sirens. In my last patrol vehicle, I could manually control three different siren noises and the air horn. In all honesty, the different siren tones didn't seem to affect driver behavior. The air horn, with its low-frequency tones, did. My informal finding seems to match that of researchers.
In a landmark study by Dr. John Patterson at the University of Glascow, Scotland, auditory analysis of high- and low-frequency sounds determined that humans have difficulty locating high-pitch noises. Because most sirens depend on high-pitch wails, it would make sense that my low-pitch air horn elicited the best responses from drivers.
Apparently, Federal Signal Corp. feels the same way. The company has introduced a new siren augmentation device called the Rumbler. The system uses an amplifier and a pair of woofers to create low-pitch vibrations a la Funkmaster Fly in his Escalade to alert motorists of oncoming emergency vehicles.
Sound Reduction
So, what about the many siren-pitch options we have? Informal studies have shown that siren pitch has little affect on overall audibility, again because of physics.
In a study by Michael Miller and Robert Beaton, several factors were determined to have a direct effect on siren audibility. First, the degree of effectiveness is relative to the distance between the siren and the listener. This is simply the inverse square law of physics. Second, vehicle designs tend to mask outside noises. The typical sound reduction of most modern production vehicles is 20 30dBA, which typically lowers a 120-dB siren to 90 or so dBA's the dB level of the typical vacuum cleaner. This reduction is significant and the typical experience for most individuals inside a car with closed windows. This is because the decibel scale is exponential in nature. Example: A sound that makes 10 dBAs is 10 times the intensity of the bottom threshold of human hearing (0 dBA), but a sound making 20 dBAs is 100 times more intense than the 10-dBA sound. This exponential equation continues to move up the chart to a point where a 120-dBA siren is 10,000 times more intense than an 80-dBA vacuum. So sound reduction decreases siren effectiveness as the sound enters an enclosed vehicle space. According to Miller and Beaton, this absorption by the vehicle body is called "insertion loss."
Third, noise inside a car makes it difficult for drivers to detect sirens and horns. Interior car noise stems from many sources, including the radio, engine and drive train components, air conditioner and heater fans, the road noise of tires and open windows, and talking passengers. Miller and Beaton maintain that, for almost any auditory signal to be detectable, its sound-pressure level must exceed the background noise by about 8 12 dBAs, which is essentially 10 times louder. This difference in sound-pressure levels is known as the auditory signal-to-noise threshold. Because the interior noise of a car traveling on a modern highway is often measured at approximately 70 dBAs with the stereo off and windows up, one can expect that siren levels of 78 82 dBAs at the driver's ear are needed to ensure it is heard.
It gets more complicated when considering closing distances as dBA levels fluctuate. Suppose that a motorist is traveling at approximately 57 mph, and an emergency vehicle is traveling at 74 mph. Without getting into too much math, the closing distance is approximately 25 feet per second. If the motorist needs four seconds to detect the emergency vehicle and respond appropriately, the two vehicles would have to be 100 feet apart. At that distance, according to Miller and Beaton, the intensity of the siren is 77 dBAs, which means the sound doesn't exceed the signal-to-noise threshold of the normal in-vehicle sound level. Simply put, the siren can't be heard until the emergency vehicle is much closer to the car, which means the reaction time of the civilian driver is considerably shortened.
Of course, one way to solve this problem would be to make sirens louder, but the level would need to be much higher than the human threshold for pain, which leads to other issues.
The Bottom Line
Data shows that varying siren tones and the decibel level for motorists in vehicles with closed windows decrease overall siren effectiveness for alerting motorists of your intentions. Yes, there's some value to a siren for pedestrians, as well as individuals in stopped traffic with windows rolled down, but in terms of clearing traffic, the only real value seems to be for alerting vehicles in very close proximity to your patrol vehicle. Keep this in mind the next time you enter an intersection while running code. Other drivers might see you because of your lights or your paint scheme, but data shows it most likely won't be because of your siren.
In our next segment on "Factors to Consider When Running Code-3," we'll discuss the importance of patrol vehicle color on visibility and safety.