They are passive (resistor, inductor) gentle slope high pass filters.
They are connected directly to the 200Ω, 600Ω (different R, L values needed) microphone line.
What they do is attenuating frequency content that causes bass heaviness, mid bass, mid heaviness to listener's brain. So singing voice has a chance to sound close to the quality of for example When I Fall in Love - Nat King Cole, The Power of Love - Frankie Goes to Hollywood etc.
How are they made?
By simply connecting a ring wound coil (toroid) in series with a resistor and the resultant in parallel with mic's output, ie to pins 2.3 of an XLR female to male adaptor. The Pleiadss (R,L) EQ can be so small as to fit inside a Neutrik module female to male XLR adaptor. A painted color stripe can be used to differentiate between EQs with differncy frequency response slope, turnover frequency, ie different values of R or L.
Where is the bass, mid heaviness coming from anyway?
It has to do with how the human body, ear, brain works. Phychoacoustics (branch of physics) research early in the 20th century revealed for example that: [Lowe, Morgan].
The softer a singer sings the more bass or treble frequencies are produced compared to mid frequencies. In other words the more we shout the more mid heavy is the frequency spectrum of our voice. Curves showing this are called voice effort curves. [Lowe, Morgan]
At reproduction, the higher the acoustic intensity the more our brain becomes sensitive to low and high frequencies. Curves showing this are called equal loudness curves or Fletcher-Munson curves. [Lowe, Morgan]
A possible reason nature doing this is survival. For example a baby crying in the desert (great mid content) can be heard miles away (low SPL due to distance) because a human listener is most sensitive to mid range at low SPL.
The above effects create 2 bathtubs like frequency response curves (as perceived by our brain). Bass heaviness is prominently perceived.
Added to this is the proximity effect of directional microphones and no wander our voice becomes unrecognisable. It can be unrecognisable even when recording with non directional mics with flat frequency response from mic to loudspeaker.
Early pioneer sound engineers at Hollywood discovered that the goal is flat frequency response from singer's, actor's vocal chords to listener's brain [Lowe, Morgan).
Pleiades R,L filters or passive inline EQs are an attempt to create flat frequency response from producer's brain to listener's brain in one go, being directly connected to the microphone's electrical signal output.
So depending on application an expensive to a world class microphone can be effectively used with a minimal, low component count, low distortion signal path.
Reference:
Sound Picture Recording and Reproducing Characteristcs - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers
Further reading:
The published work of director of Harvard University's Phychophysics laboratory, S. S. Stevens, for example the Sound and Hearing encyclopedia, Life Science Series - Stevens, Warshofsky
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