The Gentle Bass and Treble Roll Off of Input or other Signal Transformers...

(Summary)


...can make flat on paper microphones to sound flat in frequency response from singer's vocal chords to listener's brain.


Otherwise the sound may be bass or treble heavy to the human brain perception.


Welcome to Psychoacoustics..




(A great scientist on the field of Psychoacoustics is S.S. Stevens. He was the man who made the Psychophysics lab at Harvard university. You mat have heard of the Steven's law. He has written some excellent books on the subject of sound and hearing.)


Back to transformers.


The gentle bass and treble roll off of a signal transformer (let's concentrate here more on the microphone input transformers) can make flat on paper microphones sound flat from singer's vocal chords to listener's brain.


Otherwise the sound is bass or treble heavy to our mind.


Since the impedance (reactance in ohms) of an inductor or coil is proportional to it's inductance and proportional to frequency...the game is played accordingly.


Parameters that influence are:


primary winding  inductance (input inductance)


turns ratio


mic drive impedance, or also called internal impedance or series impedance, output impedance etc


secondary terminating impedance


inductance index of the magnetic core


type of music, eg type of singing


distance of mic from source, eg proximity effect


acoustic level at reproduction


voice or singing effort


acoustics


etc


On the other hand transformers can help a mic at high frequencies when they are less loaded. Or bu creative use of high frequency resonance. Or by deliberate low cut or high pass by the use of lower input inductance, eg Pleiades filters, transformers etc. For example the following signal path:


Grampian DP4/L 25 ohm version - Pleiades V6 with 140mH, 1:10 input transformer - Sony Tc-D5 pro


25 ohms is transformed to 2500ohms and the 2500ohms (imagine the 2500 ohms in series) loaded by 100Kohm shunt. 100Kohms is the input impedance of the EF183 electron tubewhen the electron accelerating anode to grid resistor is connected. This makes the electron tube operate at only 4 volts at the anode or plate and in a very low noise way.


The frequency response of our ear mind combination depends on the acoustic intensity and is only reasonably flat at higher internsity levels. See Fletcher-Manson equal loudness curves. So softer acoustic sounds when recorded and reproduced louder will sound bass or treble heavy.


Welcome to Psychoacoustics.


References:


Flat frequency response from singer's vocal chords to listener's brain, Sound Picture Recording and Reprosucing Characteristics - Loye , Morgan - Journal of the Motion Pictures Sound Engineers


Elements of  Sound Recording - Frayne, Wolfe (Possibly the best book on the planet, they were brave enough to put the word elements in the title)


Radio engineer - F. Terman (excellent chapter on signal transformers)


Coil design and Construction Manual - Babani


Electromagnetism - Grant, Philips (great chapters on inductance, inductors, capacitors,resonance etc, a great university level physics textbook)


Applied Electronics - T.S.Gray - MIT (one of the best electronic engineering books on the planet, it os from MIT what do you expect/)


Electronics, a System's Approach Neil Storey (a great book to understand why the output impedance is in series, what does a load resistor (the resistor connected between anode, or drain or collector and power supply) do and why


Sound and Hearing - S.S.Stevens, Warshofsky


Hearing - Stevens, Davis