Help! on variable modulus polymer films
David Josephson
intensity. Most of our devices are "condenser microphones" (archaic
terminology retained in deference to common usage)... a thin diaphragm
of metallized plastic is spaced a short distance away from a rigid
conductive backplate... sound causes the spacing to vary, thus the
capacitance, which is converted to an analog voltage for recording or
amplification.
The entire behavior of the transducer is dependent on several
resonant circuit equivalents that sci.physics.acoustics can expound
on (and where we are a lot more at home). The two most critical
characteristics of the stretched diaphragm material are its stiffness
and resonant frequency (the stiffness greatly affects the Q of the
resultant resonant device). The diaphragm behaves essentially like a
drum head -- it's a hinged membrane clamped at its circumference. As in
a drum, the air behind the diaphragm also has a significant influence
on the loading of the diaphragm, and thus its resonant frequency and Q.
Years ago, the most common material for microphone diaphragms was
nickel... typically 0.8 to 2 micrometers thick. Thin films of other
metals have also been used, but have several drawbacks, the most
important being that there can often be arcing between the
diaphragm and backplate when applied voltage is too high. Now, it's
usual to find metallized plastic used as the diaphragm (with the
metallization on the outside, so even if the diaphragm collapses
against the backplate, there is still a layer of plastic to insulate
them from each other). Polyethylene terephthalate (Mylar) is the
most common, with PTFE (Teflon) used in electret types and polycarbonate
or polystyrene sometimes used. Typically, plastic films have about the
same range in mass as the old metal films, which results in thicknesses
of 3 to 15 micrometers. The metallization is of negligible mass and
has no effect on resonant behavior.
Problem is this: when aiming for a particular tension and stiffness,
we stretch the material. A certain elongation is expected, of course,
but the modulus of elasticity changes dramatically according to how
much the material has previously been stretched. There is also a
variable amount of creep, where the material relaxes tension over a
period of days (also dependent on how much the material has been
stressed before relieving some of the tension). Heat also affects the
results; sometimes heat causes the tension of stretched material to
relax, sometimes to shrink further (increasing the tension).
DuPont has been unable to help... they aren't concerned with the
behavior of these films at 70-90% of breaking strength. My guess is
that some re-orientation of the polymer molecules is being promoted
or impeded by the tension they've been subject to, and heat (there
is also the idea of using a solvent such as cyclohexane to control
these phenomena) may accelerate the process.
I'd like to be able to predict, by other than empirical means,
what the stiffness and final tension will be from a particular
tension/heat/solvent/moon phase regimen. Any comments posted or
emailed will be greatly appreciated.
--
David Josephson <dav...@rahul.net>