I'm ready to put a horn on my bike. I am sold on the Airzound being really loud, but I ride year round and from what I understand, the Airzound doesn't really work in cold temps (below freezing for months at a time, sometimes below zero).
Then there's the Galeforce Blaster, which I'm sure would work in the cold, but I don't know how effective it is. People say it's pretty loud, but not as loud as the Airzound. The stuff I've seen on youtube sounds to me like it's not nearly as loud.
It says it'll work on a 6 volt source so I may be able to wire it to my Magicshine 7.4v pack. It says it's louder on a 12v battery than a 9v, but I don't know if that's due to voltage or current; a 12v SLA can generate way more current than a wimpy little 9v battery.