Briefly stated the higher the intensity of a sound the more bass heavy it sounds to our mind.
Since we usually amplify voices or for example the sound of vibrating strings in an electric bass much more than reality the brain ends up perceiving a bathtub like frequency responce curve with exagerated low and usually high frequencies.
Is this why the Pultec production EQ was made for? With its nice ring core inductors an example is that it can artfully low cut or high pass so that the sound of an instrument is correct at the level we want it to be reproduced.
The Pleiades filter is an attmpt to do this at a gentle 6dB per octave low cut right at the output of the mic or for example the pickup of an electric bass.
It pays to get the sound close to being right, right at the beginning of the signal path.
Excessive low frequencies that will end up to be low cut later may intermodulate with higher frequencies inside active amplification devices, creating unmusical overtones that may compromise sound.
The Pleiades filter is just a wound coil or inductor connected directly in parallel with the mic voice coil or say an electric bass pickup coil.
It can be would on Magnetec Nanoperm nanocrystsline ring cores. These cores have an extremely high inductance index so given needed inductance can be achieved by relatively few turns.
One can decide on the bass cut (turnover frequency) needed by adjusting the number of turns wound on this toroidal core.
Such Pleiades filters ie inductors can be seen inside RCA ribbon microphones, Electro-Voice RE15, RE16 microphones etc at various needed turnover frequencies to compensate for the proximity bass effect.
They are very useful for compensating.
The proximity effect.
Fletcher Munson Equal loudness curves
Voice effort curves (the softer we sing the more bass and treble our voice produces, or conversely the louder we sing the more midrange we produce)
Pleiades schematics or concepts are open source.
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