LACoFD squad trucks had something similar but with a (name escapes me) hi-lo that's powered by a small alternating compressor that they use for confined & populated areas. Those suckers is sorta loud and a doozy to adjust the horns to get maximum output.
1. LACoFD and LAFD use Fiamm / Stebel / Unus compressor horns, only Fiamm being still available from a US distributor located in Southern California.
2. A very loud disc horn (electromagnetic) is to use 2 sets of Hella Supertone horns (2 each, hi and lo) with an alternating flasher. These are not melodious,...they are intense and harsh, to be heard. At a slightly different frequency, the same horns are sometimes used in Germany on EVs, especially in the past on police cars (fire trucks mostly used/use Martin air horns). 4 horns are very loud (about 118-119 dbA in LAFD testing at 10'), but directional, straight ahead. LAFD used these in the mid 70's on ambulances, but found the Hi-Lo air horns to be better because they were less directional or some other reason, so they went back to them. The Hella horns cost about 70.00 per set, but they are about 3-4X louder than a regular horn.
The Hella horns are a combined 280 watts, 140 per side. A suitable relay must be used, and a flasher must be HD, preferably with an alternating rate of 70-80 per minute. When all 4 horns are sounded together, they are piercing and aggressive.
If you can't find an heavy duty alternating flasher. You could wire it so that the each output on the flasher turns on a relay. Then you could run any type of horn without worrying about the flasher being overloaded. I would like to see what a ULF44 could do to some horns.
in the past i have used two regular car horns with different tones hooked up to a alternating flasher for a do it yourself hi/lo two tone warning (attention getter). has anyone done anything similar?
I have done this several times over the last 20 years or so. I used a school bus light flasher for one set. For another set, I used the 537 flasher (the 537 flashers I used failed quickly), and on 2 others I used the Gall's wig wag headlight flasher. For two sets, I even went to the trouble of going to the junk yard and trying out the horns until I found two extra loud ones with just the 'right' high & low tones that I wanted to hear.
Good post topic. These are quite effective at a price that is hard to improve upon.