ELI5: Why are the sirens of emergency vehicles in the USA so vastly different than those in Europe?

So you'll probably be surprised to know why mechanical sirens aren't typically found on car-type vehicles -- generally, you find them on fire engines only. The reason being that a mechanical siren (typically manufactured by Federal Signal Corporation, and known as a "Federal") swallows up about 200 amps at 12 volts during operation. This is about the entire electrical load of a passenger car. Fire engines have monstrous energy requirements, so adding on 200 more amps to the electrics isn't a big deal.

Compare that to an electronic siren/speaker unit, which takes far less than that -- maybe 1/10 or 1/20th the number of amps. (Mine, which is rated for 100w of "get the hell out of my way", just needs an 8 amp fuse.) You can stick one in a car, in a van, on a motorcycle; pretty much anything bigger than a bicycle, and you've got enough juice to run the thing.

Air horns obviously need a source of air: either they get it off the brake system, or they mount a small compressor somewhere. If it's the former, you've got a vehicle that needs brakes big enough that they can also continuously drive a pair of air horns for the time you're using the siren. If it's the latter, you've got an electrical system that's robust enough to drive the compressor for all the time you're using the siren. If there's a lot of traffic to blast through, that could be several minutes of continuous operation. You're not talking about small amounts of energy.

A Federal is huge -- they're the size of a small toaster oven, because they need a big electric motor to run them. Modern electronic sirens are small. Mine is the size of a couple paperback books -- for GoT fans, maybe one and a half books. Speakers are similarly sized. It's not hard to find space for them in the driver's area and up front behind the bumper. When either poops out, they're cheap and easy to replace. A decent weather-proof speaker costs about $100 and a good dual-tone siren (one that can do two tones simultaneously, which anecdotally seems to wake people up pretty well) runs about $250-$400, depending on options. You run a wire from the battery to the siren controller, and from the controller to wherever you stashed the speaker, and you're done. Installation takes about an hour.

Of equal importance, in smaller vehicles, the cumulative impact of driving near a siren can be enough to cause hearing damage. A big honkin' Federal, while it does a superlative job at "parting the Red Sea" of traffic, feels like being behind a jet engine at full throttle. They're insanely loud. In the US, OSHA actually stepped in and banned certain siren placements, because emergency personnel were losing their hearing. (Putting the siren on the roof is a no-no these days.) Similarly, air horns are freakishly loud (my agency has a pair on the front bumper, which are awesome) and if they were on all the time, I can't imagine the damage they'd cause.

I can't speak for why the UK has vehicles that feature one style rather than the other. One thing I'll point out is that electronic sirens are cheap, and there are at least three in the US that have long and robust track records of performance. So it might just be that agencies in the UK are sourcing them from the US. In the US, two-tone sirens have never caught on, so the UK might be de facto changing over just because of a difference in who they're sourcing sirens from.

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