A planar diaphragm type magnetic transducer with an acoustically transparent magnetic backing, and a diaphragm overlying and spaced from the magnetic backing, the magnetic backing having magnetized strips lying parallel to each other and adjacent magnetized strips having opposite polarities at their...http://www.google.com/patents/US4471173?utm_source=gb-gplus-sharePatent US4471173 - Piston-diaphragm speaker
