Just purchased a 132db horn for $16.00 delivered. Replaces the HD factory horn . Yes it is loud and doesn't require a relay. Very easy install. Search the recent postings in the forum for the horn. Purchased it through Amazon.
I usually question posts with just a link and absolutely nothing else. BUT, I heard someone mention Banshee Horn not to too long ago, this made me look it up and found their website through google (the one I posted). Does look like a well thought out product and the price isn't too bad considering the price point on a lot of other things that have recently come out.
It's a Stebel/Wolo/Mini Beast horn & relay. Add a module to flash the hi beam & activate the air horn. But that 1/2 second delay to activate the air horn may be too late!