It's the ping rate and volume I hear change when they've got you bracketed with the pings getting closer together and louder as they get closer in the game. I believe WWII active sonar was clearly audible. It takes a LOT of energy (lower frequencies) to travel distances and return with enough energy to get clear detection. Especially back then. We can hear wales and dolphins when they echo range and the environmentalists sure are concerned our pings are killing wales. It's doesn't take frequencies we can't hear to be "supersonic" in water since transmission is faster than in air.
-Pv-
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