Flat frequency response from acoustic input to microphone to acoustic output of loudspeaker may lead especially in pop music production to a bathtub frequency response curve from producer's brain to listener's brain. Low frequencies and high frequencies are perceived too emphasized.
The higher the acoustic stimulus the more the listener's brain exgerates bass and treble frequencies. The lower the acoustic output from a singer's voice production apparatus the less the content of midrange. [Lowe, Morgan]. These 2 effects can be visualized from the known Fletcher - Munson or voice effort curves respectively.
There is also the proximity effect on directional microphones. While attempting to compensate for the directional microphone proximity bass exaggeration objective effect many inductor filters were build (Pleiades filter) each with a different inductance. They are 6dB per octave high pass or low cut wave filters made up of the microphone output impedance and the shunt inductance forming a 1st order low pass filter. It was found that although they can compensate for the proximity effect the above 2 effects need more subtle and gentle low cut and high cut slopes.
This became apparent while experimenting with world class flat frequency response microphones such as the Sennheiser MD211.
After experimenting with a non flat microphone which sounded flat from singer's to listener's brain, the Grampian DP4/L (25Ω amplified by the Pleiades V6 head amplifier) it was found that a Pleiades (R,L) shunt filter is needed to get the same effect from the flat frequency response omnidirectional MD211.
The R,L filter includes a resistance in series with the inductor and the total connected across the microphone's voice coil output. The included resistance crates the needed subtler and less than 6dB per octave slope.
However on a recent experiment (yesterday's post on Pleiades V1) it was found that the equally excellent and flat Sennheiser MD441 could be connected to a low inductance (140mH) military Freed input transformer feeding the CV2269 electrometer electron tube with nothing needed to be subtracted. No need for an R,L filter. The transformer used on the Pleiades V1 electron tube AAA battery head amplifier is Freed NO.33116 TFIRXIOYY SC-B-105074 150Ω:320KΩ. The transformer must have been donated from an electron tube modem, Western Electric?
Assuming the proximity effect is dealt by the low 140mH primary inductance, a similar inductance transformer wound on the Magnetec 070 core was used as an input transformer on the Pleiades V6 head amplifier. The sound was mid peaky and not comperable to the perceived naturalness of the military transformer. The HF response was less on the Freed transformer possibly compensating for the 2 above effects at HF too but why the mid exaggeration on the Pleiades transformer.
Older measurements revealed that the inductance itself of the military transformer is not constant but varies with frequency. So the reactance in Ohms departs from the theoretical 6dB per octave increase with increasing frequency.
Change of inductance with frequency is a property of some magnetic materials such as Mumetal. [Babani]
Specifically the inductance of the military Freed transformer is 140mH at 97Hz but becomes 40mH at 1028Hz. So when it is driven by the semi constant current of the relative high microphone 200Ω output impedance...
This should account for a much gentler slope of low cut, less than 6dB per octave. It was such that this transformer plays a major possibly role towards the ideal of flat frequency response form producer's brain to listener's brain.
Later addition. How would ordinary power transformer core sound? Or winding simultaniously on 2 rings of different material? Or using E, I laminations of different materials sandwiched? Eg. Silicon steel, Mumetal, iron etc. It sounds more and more like cooking. The art of electronics.
Acknowledgment to Lefteris Logaridis for suggesting earlier than 2000 the use of military signal transformers to pro audio amplifiers.
References:
Towards the end of the following post:
http://euroelectron.blogspot.com/2018/07/shure-unidyne-iii-reconnected-with.html
Flat frequency response from brain to brain, Sound Picture Recording and Reproducing Characteristics - D. P. Lowe, K. F. Morgan - JSMPE
Coil Design and Construction Manual - B. B. Babani BP160
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