We put a Stebel on my husbands bike.... and you definitely need the relay for the signal button and to run a hot wire back to a solid amp source... we took it all the way back to the battery main lead....and the ground wire needs to be capable of 20a as well.
It is definitely loud enough to alert people...but 139 db it aint unless they measured right in the throat of the thing. It does a much better job than the wimpy stock horns tho fer sure.
Biggest problem we had was making a custom bracket... it does NOT usually go where the old horn was.... much bigger / interferes with the wheel motions, etc. Ended up using a Kuryakin clamp on the engine guard and a small L bracket. My pic is posted here somewhere in one of the many threads... ( cant find it now my computer got a virus and I am now back to only basic programs until I load the back up stuff...)