There are 4 warning range settings. Fighter, Bomber, Boat and Vehicle.
When either of those types enter the range needed to set the base flashing, you will get an audible warning.
This is not the Radar Ring that you can see visibly. Not sure what the MA settings are exactly, but typical default settings are something like 3 miles for Boats and Vehicles, 12 miles for Bombers and Fighters.
That's about half a sector out from the tower for the planes.