I have been using this 12v Fiamm Freeway Blaster horn on my bike for a few weeks now... I have since installed the same horn setup on my other bikes. ...
Having this device on my bike has boosted my confidence to unexpected levels... I can easily make myself known to cagers that don't see me, buzz me, ignore me, possibly right hook me... etc... and it works REALLY WELL !!!!
It is powered by a 12v SLA 5ah battery that also powers my lights.. I have it wired to a momentary flush mount horn button on the top of my handlebar.. Works perfectly !!!!!
I have tested it in the rain and it works fine =)