AT 1. The AT 1 unit was the first attempt to produce a subsonic firing device. In the AT 1, a simple sonic-acoustic triggering system started a clock which kept the subsonic system armed for a period of about two minutes. It was found that this unit has too easily swept and was especcially subject to firing due to explosions. This may be seen in figure ???. The diagram showing the sound level ???????? as caused by remote detonations shows ????? transmission through the ground reaches ??????? approximately 2-1/2 seconds (in the case shown) before transmission through the water. It would be possible, in the AT 1, for the ground wave to trigger the unit by actuation of the sonic system and the water-transmitted wave to fire the mine. This fault was eliminated by the use of a different triggering system in AT 2. Therefore all existing AT 1 units, includ-ing those in the field, were converted to AT 2's, the modifications necessary for conver-sion lying primarily in the improvements in the acoustic triggering circuit.