Without realizing it many of previous years Pleiades experiments were hearing aid experiments.
Perhaps generalizing a microphone amplifying studio setup serves a similar purpose.
In pop music a usually much softer, intimate voice is made to sound louder by increasing the acoustic intensity in dBs entering the listener's ear. A hearing aid?
It is not surprising that a top quality studio mic preamp setup such as for example one the Beatles used has electronic schematics so similar to a top quality electron tube hearing aid. Usually with 3 active devices we have so much gain as to accommodate the need of an almost deaf person, even 1 mW electrical output (0dBm) can be enough [Philips].
One typical Beatles EMI Abbey Road signal path, mic followed by a mic preamplifier set may be:
Neumann U47 (with its one VF14 electron tube head amplifier) - Telefunken V72s (a 2 electron tube preamplifier).
Electron tube hearing aids of the 50s used 3 electron tubes in very similar class A (electrons flowing all the time) schematics.
Even the beginning of the signal path was very similar as the mic was a crystal ie high impedance feeding the electron tube control grid. Diaphragm reasonance placed at high frequencies similarly to condenser capsule mic elements.
For an example, 2 possibly hearing aid schematics:
http://euroelectron.blogspot.com/2018/08/more-nice-hearing-aid-schematics.html
And while we might like to get a mic level to line level we are increasing the signal not few dBs.
When we say we need a preamplifier of 60dB gain we are very close to the hearing aid community decibel gain talk at least in the electric signal domain.
We do this for the possibility of having a softly strumming guitar sound as loud as an orchestra. Or for example My heart will go on - Celline Dion, Celline sound as loud as a full orchestra. Or generally making a voice loud while keeping the great quality of voice, eg Just Hold Me - Maria Mena. In general this means not flat frequency response form mic to loudspeaker or headphones.
Since the acoustic intensity of voice coming to the listener's ears is much higher than real life, our brain (equal loudness curves) perceives the sound more bass and treble heavy if the frequency response from mic to loudspeaker is flat.
The aim is flat frequency response form producer's brain to listener's brain:
References:
A transistor hearing aid - Blom, Boxman:
http://www.extra.research.philips.com/hera/people/aarts/_Philips%20Bound%20Archive/PTechReview/PTechReview-19-1957_58-130.pdf
Flat frequency response form producer's brain to listener's brain, Sound picture recording and reproducing characteristics - D. P. Lowe, K. F. Morgan - JSMPE
Schematics of electron tube hearing aids, of Neumann U47, Telefunken V72
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