I have an AirZound, but I'm working on a 12v car horn as well. The AirZound is fine for most of the year but they become useless at about 30*F (they're not real loud below about 40*F) so for at least 4 or 5 months a year I have no horn.
Besides, a real car horn is still louder than the AirZound, and it commands more respect as well.
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Work: the 8 hours that separates bike rides.