the measuring test were build by myself, for collapsing test I modified a circuit that was suggested by Mr. Sattler/Sennheiser USA for measuring capacitance with active polarization voltage. The set up has a resolution of 0.1 pico farad. With a fine stepped power supply the nonlinearity of polarization vs. capacitance can be viewed while slowly working to the point of collapse.
A 2nd test setup to collapse capsules is a 1 to 80 step up x-former, that injects a square wave into the capsule and a finely stepped power supply, any capsule can be slowly collapsed.
As I said, the capsule that collapse unevenly had the higher distortion.
Measuring distortion, the capsule is hooked up to a Reaktanz circuit, either a tube or a transistor is operated as an electronic blind-resistor, this circuit will amplify the output of the capsule without being active in the circuit. It is technical speaking a parallel amplifier that follows the movement/capacity changes of the capsule.
Technical an amplified copy of the original signal without effecting the capsule signal. THD of this circuit itself is not measurable.
The signal source were electronic stimulated tuning forks.
It is still not the best set up in my view to quantify distortion.
Got a bunch of good info over the years from Karl, but the capsule material data I got out of old Telefunken data that was thrown out when the audio archive was closed.
The distortion papers you mention, they read well from a technical point, but in my view they try to describe chemical composition of wine (I slightly modified Klaus comparison here)
with finer and finer measurments of what is in it, without knowing what needs to be measured to quantify why the wine taste the way it does.
Capsule distortion one thread? More for a technical minded forum in my view.