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2. Piezoelectric crystals in flexural vibration;Keller;Wireless Eng.,1951
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4. L.A. Petermann and L.C. Oakley, Bender tuned array, U.S. Patent No. 2 842 685 (July 8, 1958).
5. J.T. McNaney, Electrostrictive apparatus for changing displays, U.S. Patent No. 3 146 367 (Aug. 25, 1964).