Over the years I've noticed a variety of siren sounds for emergency vehicles. Is it true siren sounds have to be changed periodically, particularly in urban areas, to prevent drivers from getting used to a particular sound and not paying attention? —Dylan, Chicago

Varietywise I don't know that siren sounds are in the same league as ice cream flavors, or, for that matter, the olive department at Whole Foods, but there are more than there used to be. Two are reliably found in just about all U.S. emergency vehicles: 1. the wail, the traditional Dragnet-type siren, whose pitch in olden days rose and fell with the vehicle's speed; and 2. the yelp, whose pitch rapidly alternates, reflecting the frantic pace of modern life. Other common sounds include 3. the European-style high-low or two-tone siren, which nowadays is often interspersed with whoops and other noises; 4. what's sometimes called the "phaser" siren, which does sound a bit like something you'd use to take out the Klingons; and 5. the braying "air horn" (actually an electronic reproduction of an air horn), admittedly not a siren in the strictest sense, for when you can't get the attention of space cases at intersections any other way.

Siren use policy varies. Some jurisdictions, such as Hawaii and California, limit the permissible siren sounds to the wail and yelp; Hawaii further specifies that the wail is to be used by police for routine emergencies with the yelp reserved for traffic offenders oblivious to everything else. More commonly, though, the idea is to mix up the sounds. Partly, as you suggest, that's to get drivers' attention, which is more of a challenge than it used to be what with car stereos, ubiquitous air conditioning (and rolled-up windows), yammering GPS units, iPods, cellphones, and so on. Emergency drivers will often cycle through the siren types if civilians don't move out of the way immediately.

The main reason for multiple sounds, though, is a little different and frankly a lot more urgent: If all emergency vehicles used the same type, their drivers wouldn't hear each other's sirens while speeding to the same intersection — this is known as the wash-out effect — and would be more likely to crash into each other.

That raises a key question. Do sirens do what they're supposed to do, namely get non-emergency traffic out of the way without getting people hurt or killed in the process? A lot of experts think they don't.

Some claim lights and sirens actually cause more accidents. Statistically, most ambulance crashes happen at intersections while the vehicle is running "hot — i.e., using lights and siren. Of course, vehicles running cold are also less likely to speed, drive on the wrong side of the road, or run red lights, so you'd expect more accidents in emergency mode than in the McDonald's parking lot.

Still, you have to wonder whether sirens in many instances are worth the trouble. Studies show ambulances typically make it to their destinations less than a minute faster with the aid of the siren, which is rarely enough to do anything medically useful. Another problem is what emergency-response expert Jeff Clawson calls the wake effect — drivers collide with each other or stationary objects while trying to get out of an emergency vehicle's way. Sirens often confuse people. They don't penetrate background noise very well; at busy urban intersections, they're only audible within 25 to 40 feet, and frequently it's hard to tell what direction the sound is coming from. A Department of Transportation study concluded that "sirens will never become an effective warning device — and that was in 1977, when drivers had fewer distractions. On top of that, several studies have concluded that sirens deafen emergency personnel and cause driver stress and disorientation, doubtless leading to more accidents. Nonetheless, till somebody perfects the ratchet-the-car-up-on-stilts technology familiar from cartoons, priority vehicles will need to clear a path through traffic, and sirens are pretty much the only way of doing it.

So which of these marginally effective siren sounds is best? Good luck getting a straight answer. The International Encyclopedia of Ergonomics and Human Factors says the most audible and localizable sounds are very high or very low frequencies, with low preferred, because high can be irritating. (Although I have to think a gut-buzzing low vibration could be plenty disconcerting.) Moreover, a complex sound is easier to hear over background noise than a single tone. Beyond that, the experts are pretty useless. A 1991 review in Annals of Emergency Medicine found some studies claiming the European high-low siren was the best, others claiming it was the worst, two professing to find no difference between sounds, and one commentator recommending different sounds for different conditions. The latest in siren technology is the Rumbler, which uses ultralow frequencies as advocated above, perhaps on the theory that if you can't reach drivers through their ears, maybe you'll get their attention if they puke.

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