During World War II, Shure manufactured a "Battle Announce" microphone. What was unique about it?
The Battle Announce microphone was resistant to an explosive blast and the shock wave from the blast. The mic had to be physically rugged to withstand the vibration of the blast, but also had to operate properly after the blast.
The mic element was controlled reluctance, also called controlled magnetic. The mic diaphragm connects to a solid metal pin. The pin moves a ferrous armature that disturbs a magnetic field. The fluxuating magnetic field produces a signal voltage in a stationary coil of wire.
To make such a mic element blast-resistant, a small spring is inserted between the solid metal pin and the armature. During normal operation, the spring is stiff enough to maintain constant contact between the pin and the armature. When a blast occurs, the spring is compressed and absorbs the shock wave, thus avoiding damage to the armature.