My experience with horns is that volume alone is not the only factor to consider.
Two tours in Turkey where the average driver is just a terrible driver, and no enforcement to speak of anywhere, with bad city traffic (and everyone used the horn continuously instead of the brakes, as the horn apparently never wears out). So in a cacophony of horns, one louder one was ignored like all the others.
So I ordered the biggest JC Whitney Ah-Oo-Ga horn and put that on my El Camino. It was decently loud, but the Turks had never heard one before, and they found it highly insulting, and they turned around an looked like they had been goosed, and gave me insulting gestures back. So overall, it was much better than just a louder horn. (And a bunch of them wanted to know where
they could get one

)