Exactly what I was looking for, thanks David, Key quotes
It has to be mentioned that due to the limitations regarding the maximum displacement of an electrostatic speaker caused by the so called snap-in
effect, the maximum sound pressure is limited by the underlying actuator principle. Using a piezoelectric device would greatly enhance
the maximum stroke level and therefore the achievable sound pressureSo I think that confirms it - it's got to be piezo, so the original electrostatic patent isn't relevant. I'm interested what the "micro-pump" is as it seems like it's delivering better performance than I can find in any comparable devices
The authors would like to thank Alexander Bergmann, head of the
Institute of Electrical Measurement and Sensor Systems at TU Graz,
for providing the micro-pump used in the experimentsI wonder if this is a R&D sample or something that's manufactured at scale?
Their method of measuring acoustic response is a bit funky, normally you'd measure levels at 1m in an open area, instead we have a closed tube with a microphone in it, so I'll concede that this research at least demonstrates the audiopixel earphone speaker solution is possible, though with the caveats of: What was the micro-pump? They link to this
https://www.ipms.fraunhofer.de/en/research-development/mesoscopic-actuators-and-systems/microfluidics.html but the page doesn't work
(20min delay)