The German had attempted since 1941 to produce a mine embodying subsonic firing, with only partial success. The result was the AT 1 group of units, which includes the AT 1, AT 2, and AT 3 units. These units were formerly designated AA 1, AA 2, and AA 3; but AT is a more recent official German designation (T-Tieftone). It is claimed that units of this type had been laid since 1942, but, until the war in Europe was over, only small parts of the units had been captured and none had ever been recovered.

The German AT units are subsonic mine-firing units for ground mines. The subsonic firing feature has some vertaical directional characteristics, but, since the mines in which it is used are cylindrical, the Germans assumed that any position in which the mine might lie would not inhibit its proper operation. Each AT unit is fitted with a sonic-acoustic triggering system designed to save the amplifier batteries of the subsonic component. The subsonic component is designed to operate in the range between 20 and 25 cps. The different units are likely to have slightly different resonances, with quite sharp re-sonance peaks, but this is considered a good operational property by the Germans.

Figure 219 – AT 2 Unit

Figure 220 – AT 3 Unit