High frequency sonar is an active kind of sonar, but because the frequency is very high the sound doesn't travel far. In reallife this may be detected (at short range), but not in the game. Also, it works a bit like a special camera view. Not like the regular active pinging sonar 2d plan-view representation. In the 688i it is a black rectangular screen on the helm display. On the Seawolf it is part of the active sonar suite, but only when set to high frequency. Akula I don't remember. Clicking on something that shows up on that screen puts a mark on the map.
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