Seriously, if it's for fire warning then the sound of a fire truck siren would make better sense. I believe they are electrically operated.
Fire trucks go racing toward fire, but warning needs to be lets say min of 5kms-10kms and up to 20kms away on windy days…
Truck sirens are quite directional.
In oz, helicopter is an ideal way to reach both remote homes on say 50 to 100 acres and homes on 2 or 4 acre blocks on the urban fringe.. would take hours to do it by road. Agricultural folk are more switched on to watching the horizon, it is those on the urban fringe in higher density that aren't quite as connected to nature as they think they are, cosseted inside with zircon on a hot day unaware that they are in the firing line.
The warning needs to be quite a different experience to hearing a passing fire truck to create what the experts call a trigger (to motivate people)
Stick a couple of ref whistles on the blade tips
Whistles on the main rotor would be fun to try, just need to be able to switch them off every now and then!
Metch, fantastic link, Robbie Coltrane chews gum like a true mechanic…
Mickjoebill