The first German magnetic-acoustic combination mine unit was recovered in Septem-ber 1941. This was the MA 1 and MA 1a type, which were nearly identical and laid in equal quantity. Later, the MA 2 and MA 3 units were developed. Both of these later units incorporate a rate-of-change acoustic system, and are nearly identical; but small vari-ations result in a substantial difference in the operational characteristics. The MA 1st is a further modification of the MA 1 type, but it was not used operationally.

The German MA 1, MA 1a, MA 1st, MA 2, and MA 3 units are magnetic-acoustic com-bination mine-firing devices. In each case, a special acoustic firing unit with vacuum-tube amplification is triggered by the standard magnetic-unit type M 1 or one of its modi-fictions. The principal modification used is M 1r. When MA 1a is used with M 1r, it is de-signated MA 1ar. Because of the use of the M 1 system, all these units are restricted to ground-mine use, because of the pendulum-type anti-countermining feature.