The Type 144 ASDIC Set (1943)

The ASDIC is a sonar system for submarine detection developed by British, French and American scientists during WWI.
The name is derived from that of the Anti-Submarine Detection Investigation Committee.

The ASDIC emits a sound signal at regular time intervals. The sound waves travel through water and, when they hit a solid body, bounce back as an echo, which is intercepted, amplified and then heard by the operator.
The return sound wave also sets in motion a stylus that records the echo on a chart.
Position is estimated based on the direction of the echo, and distance based on the delay between emission and interception.
The operator will immediately notify the bridge of any suspicious reading.

The ASDIC system has its limitations: it is affected by the turbulence created by propellers or by the motion of ships; it becomes inefficient, therefore, once the submarine has succeeded in slipping inside the convoy. In addition, when there are layers of water of contrasting temperatures, the signal is deflected and readings are unreliable, as Canadian operators realized when tracking German submarines in the Gulf of St. Lawrence.