How Sirens Vary as Attention Getters
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Sirens help us get to fires faster and without accidents. They save us important seconds by warning other drivers and pedestrians that we are coming and that we want the right-of-way, even if that violates usual rules of the road.
To he useful, a siren needs to be loud enough to be heard, despite other urban noise and soundproofed cars, and must be recognized as a warning. At the same time, it would certainly help us if it did not cost more than the seconds saved are worth. Let us consider what goes into a good siren, and the best ways of using it.
Loudness is measured in decibels (db), with a whisper being about 20 db, a normal conversation about 60 db, and loud thunder about 120 db. On this scale, a 100-watt electronic siren can produce about 95 db at 100 feet straight ahead of the speaker. That is loud, in fact about as loud as is practical since at short distances this sound approaches levels that can damage hearing.
Decibel capabilities
Large coaster sirens produce slightly more than the electronics, while a mechanical siren of the size often paired with a siren light will produce about 85 db at the same distance. Small mechanical sirens, sold for perhaps less than $50 and often used by volunteers on their private vehicles, produce about 65 db at 100 feet.
Obviously, we should not waste money and deafen pedestrians with a siren that is louder than necessary, but how much sound is enough? Unfortunately, a modern car can attenuate outside sounds by 30 db when the windows are roller) up. That means a 90-db siren sounds like only 60 db inside a car. A car driving down the street with the radio on may have a 60-db noise (music?) level inside, so the siren will be no louder than the noise and very hard to hear.
We cannot increase the loudness of sirens very much without making them dangerous to nearby ears, so we are caught between maximum volumes and the “better idea” of soundproofing cars. I conclude that we want all the volume we can get with existing electronic sirens, and that small mechanicals are limited to helping volunteers get through low-speed traffic.
Differences in sound waves
Even in terms of loudness, though, decibels are not the entire story. The electronics emit a clean square-wave tone, while the mechanicals produce a wealth of different sounds jumbled together. Mechanical sirens are actually small centrifugal pumps. They take in air in their center and throw it outward, where it is chopped into wave fronts by spinning blades. These wave fronts make up the sound heard.
It’s a simple way to produce a lot of sound, but many frequencies are produced at the same time. The resulting tone is as complicated as a person’s screaming. A lot of sound produced then is lost, as it blends in with other urban and vehicle noises and does not contribute to motorist’s perception of the presence of a siren. Mechanical sirens vary in loudness as well as frequency as they change speed, while electronic sirens produce the same amount of energy at all frequencies.
Comparing mechanical and electronic sirens directly on the basis of decibels may be unfair. The mechanical siren is not as effective as the decibel figures suggest.
We tend to hear sounds that are changing and which are of importance to us. For example, if you stop at this moment and purposefully listen, you may hear the ventilating system of the building you are sitting in, or outside traffic noises, or a conversation going on in the next room. All these sounds were present all along, but some are constant, like the rumble of the ventilating system, or are unimportant to you, such as the conversation. You will suddenly hear the ventilating system if it changes by turning on or off. You will notice the conversation, on the other hand, if someone mentions your name in it. This quick attention to what is important is called the “cocktail party phenomenon,” since that is a good place to observe its occurrence.
These two phenomena are important to siren design. A changing siren tone, like a flashing light, is easily heard, while a constant sound is not. Up to about three or four patterns per second, the more frequent the change, the better. Faster than that, the slow response of the ear to the sound will result in the sound not being perceived as being as loud as it actually is.
Now note the frequency vs. time patterns for various siren patterns in figure 1. The wail and the yelp are the same pattern, with the yelp simply retreated at a higher rate. The yelp is repeated about three times per second, or as rapidly as is practical. The yelp, therefore, is far more effective in catching attention than is the wail.
The hi-lo substitutes simple discontinuities for progressive changes in frequency. The breaks are almost as effective, but the yelp should be the best. While these sounds are perceived as being quite different from each other, it is fair to say that motorists have come to recognize all of them as warning signals.
Crossing of speech frequencies
Unfortunately, our best hearing frequencies, and those used for electronic sirens, coincide with the frequencies we use for speech. That means that it can be very difficult to understand speech, say between the officer and the apparatus operator or over the radio, while a siren is operating. This is particularly true for the yelp pattern because in this case, the siren frequencies cross the speech frequencies several times a second so that the two sounds are confused by the listener. Add to this the 90-db noise level commonly found in the cab of an engine, and speech becomes impossible.
One partial answer to this is to switch to the wail, or better yet, the hi-lo pattern while talking. The wail crosses the speech frequencies less often and more slowly than does the yelp, while the hi-lo alternates constant frequencies which only interfere with the speech at the brief changes from one frequency to another. The complex frequency patterns of speech are easily distinguished from even rather loud sounds if the sound is constant in frequency. Of course, this is done at some expense to the effectiveness of the siren as a warning device.
Mechanical sirens cross the speech frequencies only as they accelerate and wind down, but the many complex frequencies produced mask speech effectively.
The wail and hi-lo patterns are less disruptive to other people as well. There are many situations in which we need an audible warning device, but do not want to disturb our neighbors more than absolutely necessary. Heavy traffic in front of a hospital or passing a church during services are examples. In really thick traffic, where the siren range necessary is minimal and each car will be near the siren speaker longer than we would like, the yelp pattern in the ear may do more to fray nerves and elicit angry letters to our city fathers than save us time. In this case, the hi-lo or wail would be a better choice. Some departments even use bells in these situations.
Air horns
A word about air horns is in order at this point. Air horns are cheap, often less than $50 for an engine that has air brakes, and even a modest horn can produce 90 db at 100 feet. The meaning of an air horn blast to drivers is not the same as the meaning of a siren. While a siren may mean “there is an emergency vehicle approaching,” an air horn means “there is something Very Big behind me that wants me to move.”
Air horns may not immobilize drivers as sirens sometimes do. After all, would you sit still with a 40-ton truck bearing down on you? Air horns can even be used to clear intersections by making repeated short blasts, as with a train’s horn.
While electronic sirens have distinct advantages, a mechanical with an air horn is a pair to be considered. The combination of an electronic siren and an air horn is the best of all worlds if you can afford it.
Deflection of sound waves
Sound waves are simple mechanical disturbances traveling through the air. They bounce off hard surfaces like tennis balls off a cement wall. We want to make sure that as they come out of our siren or siren speaker, they bounce toward other drivers, not off in useless directions. Primarily, that means straight ahead, but it also means to the sides to protect us at intersections.
Therefore, sirens should be high enough to clear the tops of cars immediately in front of the apparatus without being so high as to warn only low-flying aircraft. Also, sirens should be centered, not placed to one side.
In figure 2, similar electronic sirens are used to supply identical speakers. One is centered on a light bar on a police car, the other is mounted over the left fender of a commercial-chassis engine. Note the large drop-off on the right side of the engine. The hood bounced sound to the left that should have gone to the right.
The third line represents a mechanical siren also mounted on the left fender of an engine. The sound distribution is peculiar enough in mechanical sirens, since the sound leaves the siren around the sides of the impeller. In this case, the pattern is altered by the hood and the sound distributed to the right is reflected to the left.
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Cab-forward mounting
Mechanical sirens are often mounted in the flat front surface of a cab-forward engine. This would appear to be an excellent means of evenly distributing their sound.
Placing a siren or speaker under the hood of a vehicle invites trouble. Not only is the sound spread and blocked in strange ways, depending on what it hits, but the dirt, oil, and water present can do damage as well. I was embarrassed to find my small mechanical siren frozen solid on the way to one winter fire call. Under the hood and behind the radiator grill placements really should be left to unmarked police cars and volunteers’ private vehicles.
Finally, sirens and speakers should be separated from the driver and officer both by distance and sheet metal. Mounting directly on the cab top may cause the roof to vibrate, making the noise level intolerable inside. Placing a siren on the fender within an arm’s reach from the driver’s seat and then driving with the windows open is an experience few of us would like to repeat.
Types of speakers
Three basic types of speakers are available. They all have similar drivers, but their horn shapes differ. The most familiar is the reflected horn often seen on light bars. Instead of having a long bell like those on public address speakers at stadiums, the horn is folded into a small space. The driver is in the back and the sound is transmitted to the front via a thin tube. Here the path widens, with the sound reflecting to the back again inside the familiar cone in the center of the speaker. At the back, the sound reverses a second time and travels out the bell.
The idea is for a single clean wave front of sound starting at the driver to leave the speaker without distortion and directed forward. Obviously, some deterioration occurs as the sound is bounced back and forth, but a single non-folded horn would be too large to be practical.
The other two types use the first two sound paths of the full horn, but then change the final stage. The underthe-hood “scoop” substitutes a metal box for the final bell. It obviously sets up disturbances in the wave front that reduce the quality of the sound.
Light bar speakers
Worse yet are the speakers used in some light bars. In these, the final bell is left off entirely or is squared off, and the speaker is then enclosed in a slotted box between the two sets of lights. This makes for a very’ clean box-like light bar that may’ appeal to our desire for pretty equipment but it plays hob with the sound.
Mechanical sirens are just that, sirens. Electronic sirens have other uses, starting with multiple patterns and usually including public address and radio amplification. The public address feature may not be as useful as it appears. The speaker never seems to be pointed in the right direction on a fireground. Radio amplification is very useful, as any pumper engineer who has tried to monitor a radio over a pumping engine can attest.
One caution, however, siren amplifiers are really grossly overloaded, but the transistors are protected from overheating by the quick switching involved in making a square wave. Noise, which involves slow switching, will quickly burn out an electronic siren. Therefore, always be sure sure that a radio signal is squelched before you route it through your siren amplifier.
Touch controls suggested
The control knobs on most electronic sirens could use the services of a human factors engineer. With a large manual button and separate switches keyed by shape for each siren pattern, it would be possible to operate the siren by touch. At present, most require that wail and yelp patterns be used before reaching the hi-low, and they distract the officer or engineer from other responsibilities with small, easily fumbled knobs.
Engineers in my state are caught in the middle. By law, lights and siren must both be operating any time an emergency vehicle is violating the usual rules of the road. This allows very little discretion in the use of the siren. In general, discretion is the word because a siren is needed where drivers may not see flashing lights and not in other places. That means when coming out of the station, crossing intersecting traffic, and overtaking oblivious drivers.
Overuse not only irritates the citizens who hired us, but may cause traffic simply to stop rather than yield. Staying off the siren until it is really needed and using a beep on the horn where it will move but not scare a driver are marks of skilled emergency driving. As well, sirens interfere with even the most experienced apparatus operator. The sound of a siren triggers adrenalin flow in any fire fighter. In that excited state, a driver will go faster than he thinks he is and will often making driving errors that amaze him after the fact.
Selection considerations
Several factors may enter into the decision to buy a particular siren. A 100-watt electronic siren costs roughly $400, including the speaker, or twice the cost of a large mechanical siren. With air horns starting around $50, the combination of a mechanical siren and air horn is appealing for a department on a tight budget. However, an electronic siren draws about 8 amperes while a medium-sized mechanical one draws perhaps 80 amperes while starting and 60 while running. Current drain is not a trivial consideration with our desire for more lights and more labor-saving devices like electric rewind booster reels rapidly outstripping our alternators and batteries.
Lewis Baker of the Tennessee State Fire School has suggested to me that in his experience, drivers find it easier to locate the source of a mechanical siren than an electronic one. If true, that would mean that fewer drivers would be confused by the approach of a mechancial siren.
I cannot confirm that this is the case. The many different frequencies produced by a mechanical siren and the single square wave produced by an electronic might result in differences in the way sound is reflected off objects between the siren and the listener. This might in turn make it easier to locate the source of a complex tone, but only a laboratory analysis will tell us for sure. Until then, this is conjecture.
My major conclusions are these. Mechanical sirens are cheaper, may be easier to localize and in some cases are louder than electronic sirens. These factors are outweighed by the clarity, detectability, versatility, and lower current drain of the electronics. Air horns have appealing possibilities for the rapid clearing of traffic.
In placing a siren or speaker on an engine, a basic knowledge of what the siren will do can save us from making bad decisions, based more on esthetics than function. The various tonal patterns available on electronic sirens each has its advantages, and knowing what they are will reduce idle knob-twisting and frayed nerves in the cab.
Finally, discretion in the use of sirens is definitely to our advantage. Without the good graces of the city fathers, we will not be getting any new sirens of any sort.
Dr. Timothy Keith-Lucas is assistant professor of psychology at the University of the South at Seuianee, Tenn. He holds a H.A. in psychology from Swarthmore College and M.A. and Ph.D. degrees in experimental psychology from Duke University. A volunteer fire fighter, he is chief engineer of the Sewanee Community Fire Department and training officer of the university’s Student Fire Department.
