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Originally Posted by desirableroasted

The outer is the visual range; the inner one with the triangle at the stern of the warship denotes its passive acoustic equipment.

The visual circle is a guideline for "normal conditions," so sea state, fog, day/night, affect when/if the escort will actually see you visually.

In the same way, in the acoustic circle, your silence, your depth and your angle in relation to the escort will determine your acoustical "visibility."

Whether you play stock or GWX, it is worth reading the GWX Manual Appendix B thoroughly to understand how things work within the acoustic circle, as they change over the course of the war.

...there you go, even more clarity. and a gentle nudge to RTFM!