Pair of electric Fiamms. On the T100, I've mounted one off each of the fuel tank front mounting studs (extended) with the relay on the old horn frame bracket under the tank. On the T160's, I use the spare frame lug above the oil cooler mounting for a stud that takes one horn on each side.
PM me if you want more details, stud dimensions, etc., etc.
An open face helmet has advantages in that you can inform car drivers cerbally exactly what you think of their lack of driving skills but really the answer is a set of those twin Maserati air horns that were fitted to the BSA police bikes flogged to Saudia Arabia and quickly thrown by the Saudis into big piles in the desert....the button allow the twin dar -dur of our olde police horns or a single note when the fuzz are around. A quick very loud dar-dur up some sleeping car drivers arse does wonders for their anal retention!!! The idea of horns was to warn people..the British motorcycle and car industry thought it was to wake the operator up. The Italians thought they wrere musical instruments...LOUD ones............The Italians got it correct but those ex BSA ones need at least one HP to carry the compressor around!!
A buddy showed me a little air horn at Harbor Freight. He had one on his car. The reason I bring this up is it is a high quality air horn but it has been minaturized with a small compressor and the horns wrapped around the compressor so it is a compact package. My first thought was, "Finally an air horn that will fit a bike."
:p I did set up a small pair of air horns on my Norton a very long time ago. The dual Hi-Low horns are mounted to my left dowtube and the mini compressor is mounted to the front of my battery tray. Yes, it does turn heads, gets noticed even in loud traffic and looks good too.
guys, look at the specs!.. the harbor freight bad boy is $45(chrome accents), the Stebel is $40(black). the bad boy 119db, stebel 139(!)db. 119 is loud. 139 is L O U D as in hearing damage range, remember the db scale is log, not linear. Not to mention that the stebel claims to be rated for motorcycle vibration. Though if you look, other than trim details and output, they appear to be the same unit. Twistedthrottle has a good writeup
Stebel horn just came in. going to take a bit of metalwork to fabricate a bracket, but i touched it to the battery and the whole house ran out to the garage to see what dad blew up. I had tried it off a 10amp battery charger and barely got a squeak, so it wants some amperage, but give it power and it is impressive. Can't wait to pull up behind some harley rider poking along at 45 in a 50 zone.....
all done. looks pretty inconspicuous tucked up close behind the down tubes and close under the tank. How loud is it? in my 2-car garage, a blast ALMOST causes ear pain. It can be heard loudly throughout the house. Now to find some blond in a convertable talking on her cell phone who hasn't noticed the light has turned green.... The fun has just begun