Good qualities we identified in a discussion:

- directional

- many short duration rather than continuous

- sonorous (melodic or consonant)

- amplitude appropriate to time of day and danger

A big mistake we make is confusing "attention", "arresting" and "annoying" qualities. Dissonant sounds seem like they grab your "attention" quicker, but that's not necessarily true, it's really about the spectrum morphology in the first 100ms - those sounds (like bells) have better localisation qualities - they tend to elicit an orientation response and have better front-rear ambiguity so we can position them better. Knowing where an emergency vehicle is, is as important as just knowing it's there. From video evidence of interchanges and roundabouts what we see is that the sound of sustained sweep sirens often creates chaos, with drivers panicking, pulling on to pavements or making other rash and dangerous sudden manoeuvres.

Here in Germany, I'm seeing more and more emergency vehicles having their siren off by default in low to medium traffic situations (even in a large city like Berlin) and only activating it when they enter dangerous situations (like large crossings or passing red lights) or need space.

I don't know whether that's current regulation already, but I think it's a rather nice gesture and definitely reduces the noise background ever so slightly.

In Northern Ireland where I am ambulances only seem to turn their sirens on when approaching junctions where they will have to go through a red light, otherwise they just have their lights on

They'll also blip them when traffic isn't getting out of the way, but that also seems incredibly reasonable. I honestly can't remember the last time I saw a "needless" siren on an emergency vehicle. But, like you, I am in the UK and the rules are different here

In Munich, they seem to regard every crossing as dangerous. :(

It really is quite obnoxious.

In Germany police and emergency vehicles are only allowed to use "Sonderrechte" (special privileges) when using both the lights and the sirens. So if they want to pass a red light or go the wrong way in a one way lane they have to use the sirens. Note that this is not technically a legal requirement as the law grants them a general exemption but the administrative regulations make this mandatory.

That said, I have seen police cars go the wrong way using hazard lights instead of the blue lights (and no sirens) so I'm not sure what the actual consequences for not following these regulations are.

I also noticed just how loud sirens are in Berlin. They're far louder than what I've heard in other European cities. Does anyone know why that is?

There is the "European" siren (Martinshorn in German) which is just two tones (typically a' followed by d'') and sounds like [1]. There is also a slightly higher pitched version used by vehicles stationed inside cities.

It's still loud, but subjectively less stress inducing and objectively easier to locate. The most obvious feature is that you can very easily tell if the emergency vehicle is moving towards you or away from you as well as the approximate relative speed because the doppler shift is really easy to pick up by ear.

1: https://freesound.org/people/DominikBraun/sounds/459880/

It would sound more like broad-band static. That kind of sound is much easier to localize, so it doesn't have to be as loud. They would need to mix in a bit of siren sound so you can distinguish it from a backup alert.

What's odd is that 15-20 years ago they were trying out adding white noise to sirens, they still had the recognisable sound but also a blast of static noise. It would be interesting to know why they didn't follow through.

Also it doesn't focus all its energy on a narrow range of cilia in the inner ear. Like the difference between a small, intense light in your eyes vs. a larger, less intense light.

Whoooooeeeeew, whoooooeeeeew, whoew, whoew, whoew, whoew.

I know it goes against the spirit of HN etiquette, but damn this comment was funny ^_^

It already exists in the form of silent flashing blue lights. Anyone who hasn't already seen these lights approaching will usually panic when they suddenly hear a loud siren and become more of an obstacle than if they just carried on moving forward as normal.

I see this scenario played out every weekend when I visit a friend who lives near a busy intersection of two major routes. Emergency vehicles impede their own progress nearly every time by deploying sirens.