The German assumed erroneously that the British, who had laid the first magnetic mine, would have developed sweeping techniques and established degaussing procedures to counter it. Consequently, in 1938, when the magnetic mine program was well advan-ced, they commenced research upon acoustic mine firing mechanisms.

Because of the unexpected initial succes of magnetic mines, acoustic research conti-nued at a low pace until the British inaugurated effective countermeasures in the spring of 1940. The German Navy in May assigned a sonic acoustic with highest priority to Dr. Hell Firma. By September Dr. Firma had developed the A 1 unit ready for operational use. Thereafter, he made steady improvements in it, and sought other types of acoustical units.

The A 3 acoustic unit for use with the EMF and SMA moored unfluence mines proved unsatisfactory. In one test, 100 of these units, fitted in EMF cases, were laid in the Kat-tegat, and almost all of them simulataneously prematured. Dr. Firma sought to remedy the defects of the A 3 in the A 7, which was in its final test stages when World War II ended.

Other research developed the AT subsonic units (also known as AA units) and the AE supersonic units. The AT units were used operationally from 1942 to the close of the war, but the AE units did not progress beyond the advanced development stage. The AE, which functions satisfactorily in greater depths than other acoustic types, was intended for use in moored mines to be laid in the relatively deep water off the American coast.

Other acoustic units embodying devices designed the A 4, AA 4 and Seismik. The first two differd from the A 1 primcipally in that the A 4 depended on the rate of the change of the sound level and the AA 4 depended on the directional characteristic of the sound. The Seismik (Sismograph Microphone) consisted of a simple electrical circuit utilizing a carbon-buttom microphone mounted within a modified D 1 pressure unit and intended to respond at the subsonic levels of 5-8 cps. The circuit was intended for use in combina-tion with other mechanisms such as the M 4, A 4, and D 2.