Let's dispense with the obvious.
Your hydrophones will pick up noises when you are operating the hydrophone station, and you are submerged. The big needle points to where you are listening.
When you are running all stop and submerged, you won't hear much. Spin your props a little, 1/3 speed in enough. Turn the needle to 180, and you should hear your own screws. That's a bit more advanced.
Now, here's the not-so-obvious: click and hold on the top of the volume knob, and drag the mouse in a little circle, like as if you were twisting the knob. This will change the ambient volume for you. If you "twist" too far, the volume will step down to minimum again. It's a little tricky, but not difficult. Some people move the mouse up and down on the knob, and this will work too, but I find the control is "looser" for this technique.
Once you have adjusted your volume, stop the engines, and do a hydrophone sweep.
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