Bram writes to direct me towards this wonderful, strange and geeky clip of a Helmholtz Double Siren in action - it's a little wood and brass gadget powered by compressed air, which was used by Hermann von Helmholtz in the 1860s while he was researching is interesting-sounding book 'On the Sensations of Tone as a Physiological Basis for the Theory of Music' It comes across half way between Goodiepal and this guy. Incidentally, there's quite a thriving community of people who collect and restore vintage police/fire sirens - There's Siren Man (warning, some pages play sound files), who sells police sirens for up to $750, and here's a great feature about the scene by Michael Lamm: "I felt as though I could reach out and touch the sound, or lean on it or even sit on it, maybe even pick it up with a shovel or swim in it."
Finally, here's the genius of Bill Bailey on Foreign Police Sirens.
Posted by Tom Whitwell.
Great stuff! I'm a huge fan of Helmholtz - he discovered most of the principles of modern acoustics by using very basic equipment and before electricity (eg analyzing tones by sticking an assortment of tuned resonators in his ear while someone played the same violin note - live!). And he was actually a vet...