[_Microft]

These speakers seem to work like this:

There is a rubber layer, coated with conducting material to serve as electrodes. The signal is applied in form of a high voltage which makes the electrodes attract each other and contract the rubber in between perpendicular to the surface (i.e. the rubber layer gets thinner). Since the rubber material is relatively incompressible though (volume of the material doesn't change), the surface area of the membrane has to increase in return. To generate sound from that, the membrane is stretched over a cavity that is under higher than ambient pressure which helps expand the 'balloon' when its surface area increases. This displaces surrounding air which means the contraption is emitting soundwaves.

(I could only find a thumbnail of the first page of a paper from that professor and extracted this from it)

https://www.aes.org/e-lib/browse.cfm?elib=21108

[yummypaint]

This sounds like an "electroactive polymer." there are some squishy tapes made by 3m that incidentally have this property out of the box. Make a spot of conductive carbon paint on each side, apply a few kV, and watch the spots double in area as the electrostatic forces squish the material.

[atoav]

The article mentiones "conductive grease" that is added to the membrane.

[nrp]

It sounds a little like Bayer’s ViviTouch, but for higher frequency bands. ViviTouch was only used for haptics and sunwoofers as far as I know. https://www.innovationworldcup.com/finalist/vivitouch-eap-he...

[TylerE]

Other than the vacuum bit, it sounds a lot like planar headphones.