The low pass and high pass dials are filters to tune out certain frequency ranges. The low pass lets low frequencies pass, while attenuating/weaken the higher frequencies (squeaky/hissing) above the dial setting (in kHz, thousands Hertz) From 0 to the dial setting the amplitude of the sound is constant. Above that they get increasingly weaker. Similarly the high pass dial allows you to filter out low frequencies (rumbles) below the dial setting. Above the dial setting the frequency amplitude is constant, below it is increasingly weaker.
There is no real specific setting that needs to be set. If you have a lot of low frequency rumble and you want to pay attention to high pitch noises to identify sounds you turn up the high-pass a bit above 0. If you want to count the prop/shaft beat you lower the low-pass until you can follow it clearly.
Counting prop/shaft beats in skirmish doesn't work well to get actual speeds. As the devs somehow included a random element in it. Though you can tell if it speeds up or down. In workshop missions or tutorial world this prop/shaft beat relation is constant. But at all times, make sure you relate it to actual movement seen with the scope or other kind of measurements. Don't count on a formula for speed and later find out that the formula changed at some point.
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