Stebel Nautilus. One piece, dual tone, compact, integrated air horns
Sound Output: 139 dB
Operating Voltage: 12 Volts
Absorbed Current: less than 18A
Fundamental Frequency: 530 Hz & 680 Hz
Weight: 0.6 kg (1.3 lbs)
Dimensions: 5.9" (150mm) x 4.4" (112mm) x 3.7" (95 mm)
Reaction time: less than 90 milli-seconds
I placed them near the factory location and they respond better than the traditional air horn because there is no tube (well, it isn't visible, at least) and I can tap the horn and have them sound as opposed to having to wait for the air pressure to build.
And they are way loud.. 139db!
Go here to hear: http://www.bikerhiway.com/images/nautilushorn.mp3
I love em!