During WW2, the Germans had at least five acoustic fuzes for anti-aircraft use under development, though none saw operational service.   The most developmentally advanced of the German acoustic fuze designs was the Rheinmetall-Borsig Kranich (German for Crane) which was a mechanical device utilizing a diaphragm tone filter sensitive to frequencies between 140 and 500Hz connected to a resonating vibratory reed switch used to fire an electrical igniter.  The Schmetterling, Enzian, Rheintochter and X4 guided missiles were all designed for use with the Kranich acoustic proximity fuze.