Helo's, aircraft and ultralights used for cattle mustering are often fitted with a siren or multi-tone air horns to encourage those recalcitrant beasts get a move on.
Shark spotting aircraft also come with a siren to warn swimmers that something nearby is looking for lunch.
In days gone by it was usual during gliding competitions to carry either a starting pistol or a bicycle bulb type horn to alert other glider pilots that you were in the same thermal, electronics have taken over that role these days.
CC