Then it would work better, although you could still get false positives from neighbors' appliances when your windows and their windows are open. That, however,

Then it would work better, although you could still get false positives from neighbors' appliances when your windows and their windows are open. That, however, is completely beside the point. The point is that the audio pitches aren't outside human detection and don't have distinct enough patterns right now. Changing pitch and patterns would require appliance manufacturers' support, at which point you might as well go with mature, well-defined and well-implemented solutions, such as WiFi connectivity, rather than defining and implementing your own protocol based on audio pitches and patterns.