Woody:
Get a pair of Fiamm dual tone horns.  These have two slightly differing tones
which combine to produce a beat.  I first had these on a fully faired BMW RS.
 I guess the engineers fell victim to the stylists because they were mounted
behind the fairing and the first time I hit the horn button, I had to stop to
clean out my BVD's.  They are LOUD in an authoritative way.  If you go this
route, use a relay and make sure to get the made in Italy Fiamm's and not the
atrocious Taiwanese knockoffs.  A couple of years ago, I picked up a pair at
a rural chain store called Farm and Fleet for $8 each.  After realizing this
was less than half the going rate for these horns, I returned to get a
lifetime supply only to find they had switched their stock to the junk
look-a-likes.  They are a BMW motorcycle stock part/accessory.  I once read
in the BMWMC newsletter that these horns are also available from Ford as an
accessory part.
Rich