This all sounds terrific. Can I make a couple of late suggestions? Could the mic gain be adjustable, even if, like is common for side tone rejection, it is via a trimmer. Last week we were at Minack where they had a mix of canford cans and ASL cans and the mic levels seemed to be quite different. Instead of a "siren", a vibrator as found in mobile phones might be a better alternative. However, as he design stands , it still sounds excellent, and you are to be congratulated and thanked for such a fine effort.