When recording voice (speech or singing) one often assumes that a low cut turnover of 100Hz is enough.
In practice this usually sounds still bass heavy even on good quality omnidirectional microphones.
The aim is flat frequency responce from the vocal chords of actor-singer to listener's brain rather from microphone to loudspeaker. In practice a gentle 6dB per octave filter as a Pleiades filter of 200Hz-300Hz -3dB point restores flat frequency responce to brain perception.
The reason seems to be the Fletcher- Munson curves and the voice effort curves.
These frequency domain (as a function of acoustic intensity) mind perception effects take place when the acoustical intensity of the reproduced sound is different to the acoustical intensity emitted by the signer as for example on pop music. He-she may sing softly while recording but the acoustical intensity of the singer reaching the listener at reproduction may be much higher.
Similar reasoning may explain why soft instruments like guitar (which as reproduced louder too compared to when recording it) need this equalisation for correct brain perception. The opposite is perhaps when an orchestra may need the reverse i.e. some reduction to the mid frequency content or increasing the lower and higher frequencies. It may be that at real conditions we listen to an orchestra at less intensity that when the actual orchestra is being recorded
The above perhaps explain why inexpensive tape recorder microphones such as the Philips N8207 sound so natural at voice and pop singing which may need even less bandwidth for flat frequency responce to brain perception.
For using for example an excellent microphone such as the MD21 for soft speach or speach level singing a Pleiades filter or a Pleiades step up transformer may be used.
A Pleiades filter which is just an inductor in parallel with the output of the microphone can be for example greater than 82mH or less than 140mH.
If we make a transformer we may have the advantage of increasing the signal too. The microphone interface transformer can have a ratio of 1:2 thereby stepping up a impedance of 200Ω to 800Ω.
This increase in voltage increases the signal to noise ratio at the preamplifier output.
If a primary inductance of 120mH is found suitable this can be easily made. By using a Magnetec Nanoperm Nanocrystaline toroidal core of extremely high inductance index it becomes a joy to wind. Typicaly less than 100 insulated wire turns are needed.
And at the same time this lower chosen input inductance gently low cuts at a higher starting frequency and below giving a flat frequency responce from the vocal cords of singer to listener's mind.
For example if a Magnetec 070 Nanoperm magnetic tape wound core is used, only 35 turns are needed for a primary inductance of 140mH which may already be high. The secondary will still be less than 70 turns.
For amazing signal to noise ratio a much higher turn secondary may be wound so that the impedance of the micro one is stepped to 50KΩ for example. Then an ultra low noise Pleiades series V preamplifier can be made using just one tube, one or two resistors or capacitors, and a battery of 3.7Volts on a schematic similar to the Neumann U47 internal amplifier.
Example of passive setup:
Sennheisr MD21 - Pleiades compensating transformer - Sony TCD5 ProII - Sennheiser HD580 precision
References:
Flat Frequency Responce from Vocal Chords to Brain, Sound Picture Recording and Repreoducing Characteristics - Loye, Morgan - Journal of the Society of Motion Pictures Engineers - Presented at the 1939 Spring Meeting at Hollywood, California, received April 17, 1939
Pleiades open source posts on Euroelectron Blogspot
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