On May 30, 10:11 am, RdM wrote:

Yet it occurs to me thinking of it that it must make a difference.

In your single side coated case, one side sees its distance to stator with air
dielectric, and the other side sees that distance + the membrane thckness,
with a mixed dielectric of air and the membrane. The mylar will increase the
capacitance, and the distance increase decrease it; by less, I imagine ...

Then there's the slightly unequal by mylar thickness electrostatic forces.
Theorise, discuss, etc?

Then again, it occurs to me - in a sense, with two coatings, effective, one
each side, not only do we get symmetry, but maybe double the charge capacity?

Plus the very sandwich itself has two equal sign charges either side of a
dieletric, the whole central. I intuit that this is far better than one-sided.

All of the above (in my opinion) pales against the most basic reason I
can see for coating both sides of the mylar. Allow me to generalize
from some specific cases:

a) All good quality plywood is an odd number of layers _usually_ in
symmetrical pairs about a slightly thicker central layer. By
symmetrical, I mean in grain direction, thickness and general
composition.

b) High-quality melamine-laminated furniture - Formica - (almost a
contradiction-in-terms) has a "paper backing" on the blind-side to
prevent warping of the substrate.

c) Bi-metalic coils warp based on the different coefficients-of-
expansion of each metal...

It would seem from these general examples of materials-behavior that
unless the coating and the mylar are absolutely and exactly equally as
responsive to heat, humidity, static, skin-effects and other
environmental phenomena that the combined material would have some
unpredictable response as a two-layer material that could be obviated
by making it a three-layer material. Even if the conductive admixture
were not included in the 'outside' coating.

I am speculating that the increased effect of the charge on two vs.
one layer would make the system more responsive if the additional
attraction did not cause contact.

It would seem that the conditions in Australia range from severely dry
to near rain-forest, and in NZ from temperate to near-rain-forest. I
can see these factors being at issue as well. Would a double coating
allow the mylar diaphram to be positioned further from the stator to
get equal/better results? Would this be useful under more-than-normal
humidity?

Interesting point. Thanks for bringing it up. My brother keeps a pair
of Maggies (MG1) that I snagged for him some years ago... they were
far too large for my use then and now, but he keeps them under near-
ideal conditions by way of humidity and temperature. They are lovely
speakers which he treasures.

Maybe some day.

Peter Wieck
Wyncote, PA