Almost everything that sounds like this on the web is an mp3 of a siren. This one is a siren.

Pull the breaker lever on the console and a two-tone wail winds up over the city. The tones are 515 Hz and 618 Hz — a 5:6 ratio, a minor third, the interval the UK Home Office settled on in 1940 because of what it does to people. They are not two oscillators tuned to those numbers. They are what falls out of the model: one rotor turning at 103 revolutions per second past two rings of ports in a stator, pitch being ports times rotor speed. Each tone is a pulse train shaped by the chopper duty cycle, mixed with a blower undertone and soft-clipped through a horn equaliser.

The reason to build it that way is audible in the parts nobody would bother to sample. The growl on spin-up and the long sagging coast-down are not separate sound effects cued at the right moment; they are the same model with the rotor speed ramping, which is why they sound continuous with the note instead of pasted onto it. The horn turns at 4 RPM — one sweep every fifteen seconds — and the angle drives directivity gain, an off-axis lowpass, a true Doppler delay and the stereo position all at once, so the sound genuinely goes past you rather than fading and returning. The distance knob runs from 150 m to 2.8 km and applies inverse-distance gain, air absorption as a lowpass, and a three-tap city echo. Turn it up and the siren stops being loud and starts being far.

The scene around it earns its keep. The skyline is painted once per window size to an offscreen canvas at up to twice screen resolution and reused every frame, so thirty frames a second costs almost nothing; buildings get floor lines, bays, perspective side faces, and damage — burned profiles, collapses, blast holes, boarded glass. One tower is still burning and its smoke bends with the wind. Weather walks slowly through calm, building, squall and passing. Lightning is mostly sheet flash inside the cloud, and the thunder arrives late, by distance.

Two honest notes. The rain and thunder are recordings — CC0 files from Pixabay — and only the siren and console are synthesized; the site says so itself rather than leaving you to assume. And on iOS the screen has to stay awake, because Web Audio stops when the phone locks. Otherwise: no account, no signup, no cookies, one HTML file, MIT licensed, and there is a shutter you can drop when you want the city to go quiet.