U.S. patent 1,707,545 "Acoustic Device". Edward C. Wente, assigned to Bell Telephone Laboratories. Applied for on August 4, 1926. Patent awarded on April 2, 1929.
U.S. patent 1,707,544 "Electrodynamic Device". Albert L. Thuras, assigned to Bell Telephone Laboratories. Applied for on August 4, 1926. Patent awarded on April 2, 1929.