A piezoelectric actuator comprising: a first plate having a planar first plate surface and an oppositely disposed planar second plate surface separated by a first plate thickness; a first electrode disposed on one of the planar first plate surface and the planar second plate surface and having a plurality of first fingers (52, 60); a second electrode disposed on the same one of the planar first plate surface and the planar second plate surface and having a plurality of second fingers alternated with the plurality of first fingers; and a flexible diaphragm having a first diaphragm surface and an oppositely disposed second diaphragm surface, wherein one of the planar first plate surface and the planar second plate surface of the first plate faces and is rigidly secured to the first diaphragm surface, wherein the piezoelectric actuator is configured to apply a first electric field through the first electrode and the second electrode to cause in-plane tensile stresses to elongate the first plate in a first direction, to cause in-plane compressive stresses in a second direction, and to cause transverse compressive stresses in a third direction, and wherein the piezoelectric actuator is configured to apply a second electric field through the first electrode and the second electrode to cause compressive stresses to compress the first plate in the first direction, and to cause tensile stresses in the second direction and the third direction, and characterized by a second plate having a planar third plate surface and an oppositely disposed planar fourth plate surface separated by a second plate thickness, wherein one of the planar third plate surface and the planar fourth plate surface of the second plate faces and is rigidly secured to the second diaphragm surface; a third electrode disposed on one of the planar third plate surface and the planar fourth plate surface and having a plurality of third fingers; and a fourth electrode disposed on the same one of the planar third plate surface and the planar fourth plate surface and having a plurality of fourth fingers alternated with the plurality of third fingers, wherein the piezoelectric actuator is configured to apply a third electric field through the third electrode and the fourth electrode to cause in-plane tensile stresses to elongate the second plate in a fourth direction, to cause in-plane compressive stresses in a fifth direction, and to cause transverse compressive stresses in a sixth direction, and wherein the piezoelectric actuator is configured to apply a fourth electric field through the third electrode and the fourth electrode to cause compressive stresses to compress the second plate in the fourth direction, and to cause tensile stresses in the fifth direction and the sixth direction. a first plate having a planar first plate surface and an oppositely disposed planar second plate surface separated by a first plate thickness; a first electrode disposed on one of the planar first plate surface and the planar second plate surface and having a plurality of first fingers (52, 60); a second electrode disposed on the same one of the planar first plate surface and the planar second plate surface and having a plurality of second fingers alternated with the plurality of first fingers; and a flexible diaphragm having a first diaphragm surface and an oppositely disposed second diaphragm surface, wherein one of the planar first plate surface and the planar second plate surface of the first plate faces and is rigidly secured to the first diaphragm surface, wherein the piezoelectric actuator is configured to apply a first electric field through the first electrode and the second electrode to cause in-plane tensile stresses to elongate the first plate in a first direction, to cause in-plane compressive stresses in a second direction, and to cause transverse compressive stresses in a third direction, and wherein the piezoelectric actuator is configured to apply a second electric field through the first electrode and the second electrode to cause compressive stresses to compress the first plate in the first direction, and to cause tensile stresses in the second direction and the third direction, and characterized by a second plate having a planar third plate surface and an oppositely disposed planar fourth plate surface separated by a second plate thickness, wherein one of the planar third plate surface and the planar fourth plate surface of the second plate faces and is rigidly secured to the second diaphragm surface; a third electrode disposed on one of the planar third plate surface and the planar fourth plate surface and having a plurality of third fingers; and a fourth electrode disposed on the same one of the planar third plate surface and the planar fourth plate surface and having a plurality of fourth fingers alternated with the plurality of third fingers, wherein the piezoelectric actuator is configured to apply a third electric field through the third electrode and the fourth electrode to cause in-plane tensile stresses to elongate the second plate in a fourth direction, to cause in-plane compressive stresses in a fifth direction, and to cause transverse compressive stresses in a sixth direction, and wherein the piezoelectric actuator is configured to apply a fourth electric field through the third electrode and the fourth electrode to cause compressive stresses to compress the second plate in the fourth direction, and to cause tensile stresses in the fifth direction and the sixth direction.