I searched and mostly found expensive solutions for a loud horn system until one day when visiting my local Harbor Freight Tools store found a very inexpensive horn system that is plenty loud. Sounds like a mid size to large car instead of a motorcycle. Runs off same wire oridiginal bike horn does without having to use air horns.
Oh, if you hook up both hi and lo freq horns then they sound like a loud car horn. One is pretty good but I like both together.
You can look them up at: http://www.harborfreight.com
Item numbers: 44759-0VGA and 44760-0VGA