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The x2 option turns the low pass filter from a 12dB/Octave filter into a 24dB/Octave filter.
How to explain what that means...
As you probably know, a low pass filter stops high frequency sound getting through. So the lower you set the cutoff, the more 'underwater' it sounds.
The 24dB/Octave filter has a steeper slope, so as you lower the cutoff, it rapidly and aggressively kills sound frequency above the cutoff.
The 12dB/Octave filter has a shallow slope so as you lower the cutoff, you can still hear much higher frequency sound above the cutoff point, but its greatly diminished.
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