Dear Sidewinder,
I'm sure you can Prove it with some theoretical aspects supporting with some illustrated diagrams as you felt the discussed topics here are way beyond to their (whose?) limits & with full of assumptions which are no where near to the reality.
I believe whatever you said are True !
Regards
IRONICAL
To understand a Loudspeaker magnetic circuit (motor structure) you will first got to know how this circuit is working.
(A) There are 2 types of magnetic field paying into action in a loudspeaker magnet....
1. DC or (static field).
2 AC field (generated)... when the coil is moving.
(B) The entire magnet assembly's main focusing point is the gap geometry. The advance engineering in a Loudspeaker is to get a linear or radial semmetrical feild between the T-yoke and Top plate.
And to focus the fringe feild up and below the gap more and more into the radial gap.
Several patented design had been implemented to achieve the highest and efficient gap strength with a semmetrical flux lines.
To know more and more about this one has to explore the unlimited datasheet and white papers on Loudspeaker magnet modeling and design.
The T-yoke or the return structure of a magnet assembly with the ring magnet in between and the top washer on the top is a cyclic path of magnetic field which flows from Noth to South or South to North .... between the gap width at the edge of the top washer and pole piece upper circumstance of the T-yoke.
The thickness of the top plate is the so called... "Gap-Height". This thickness is the main area of the entire magnetic assembly which has the strongest magnet energy.
Adding an extra magnet to the rear of the T-yoke will not do any better to the cyclic flow of the existing magnet circuit bringing any change to the actual gap geometry.
Apart from this many design had been implemented by adding an extra magnet on top of the pole piece for achieving linearity and suppressing flux modulations.
Generally rear attached magnets charged with an opposite polarity are used sometimes to sheild the entire magnet structure to prevent any leaking flux which can effect nearby electronics such as cathode ray tubes ... etc
The motor structure faces some real challenges in regards to eddy currents, inductance stabilization, flux stabilization, Fringe or leakage feild (around the gap), generation of 2 & 3rd harmonics, and many other issus related to temperature, offset DC, over excursion and bottoming.
Designer and engineers all around the globe are developing and bringing out new ideas and technology to overcome these serious issues and problems.
If only adding an extra magnet at the rear would increase the efficiency of the Loudspeaker then the designer would have simply used an oversized magnet intially. It is not that simple. Selection of a magnet size is a part of a particular loudspeaker design intended for a specific use with a specific parameters.
This discussion is endless ... it will further get complicated and can get offtrack.
I guess there a several books and information out there online to know more and more on this.
Thank you
Vivek