The best sounding car horns are a chord! Our old '64 Cadillac had a pair of horns. They were not terribly loud but we stood out because of the chord. Find two pitches that go well together or better: find a chord horn. The two together get you some additional volume but the two tone will get you the notice you are looking for.

A quick search found Bosch 400/500 Hz "fanfare" dual horns for $40. They are 110 dB which is probably close to normal horn but that's just a jumping off point

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)