I use these as replacements for inside the cow bell horns:
http://www.amazon.com/FIAMM-72012-No...=5336075432-20


Here's one rated a few decibels more, but I don't remember seeing the spec for what distance the measurement is taken. The rating may be optimistic:
http://www.amazon.com/FIAMM-72112-Fr...=5336075432-20

 

I have a Stebel Nautilus Compact Air Horn.

139dB sounds like a little train. It mounts where the stock horn does if you have the bell style horn. You can't run it though the stock switches without using a relay though. Install took about an hour. You can get both the horn and a prefab wiring kit with the relay through Amazon for pretty cheap.

 

This is what I have. I mounted it to blow right. That seems to place it out of the fenders reach.

 

Stock horn is weak, so instead of the major air horn, I decided to install this new Harley-Davidson® Horn Kit -Loud for Sportster® - Chrome. Not like a train, but not bad either, noticeably louder and a bit more base.

 

What's the story on the brass looking head bolts? Is it powder? (Sorry for the derail)

 

My fiance bought this horn for my 2011 and it's not worth it. Too me it sounds very similar to the stock horn just with a chrome cap