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I switched from a shelving filter to a low-pass filter, cutting below 75 Hz (Photo 3), roughly the frequency of my lowest note, the 6th string dropped to D. (To learn which musical notes correspond to which number in Hz, Google “pitch to frequency conversion”—or just use the “A = 440 cheat”: The A at your 1st string’s 5th fret is 440 Hz. Shifting by octaves doubles or halves the frequency, so A at your 3rd string’s 2nd fret is 220 Hz, and the open 5th string is 110 Hz. My lowest bass note—the dropped-D of my 6th string—is roughly half an octave below, midway between 110 and 55, or about 80 Hz.)
Adjusting the bandwidth on most high- and low-pass filters alters the resonance of the cutoff frequency, with high Q settings emphasizing a narrow swath of frequencies at the cutoff. In Photo 4, I’ve raised the Q, and the gentle bass cut of Photo 3 becomes a focused bass boost with relatively little low-mid buildup.
Also, since the strongest overtone is the octave above the fundamental, cutting there can restore clarity after a bass boost. For example, if you’re goosing 70-80 Hz, it may help to trim around 140-160 Hz (Photo 5 and Clip 5).