I offer a collection of 107 patents of RCA microphone technique, published between 1930 and 1982 in USA or Great Britain (all in english). They are busy in most cases with various microphones, like carbon or condenser or uniaxial, reflector type microphone, electroacoustical / sound pick-up apparatus...
All patents are complete, that means contain the full text and all drawings (see example). You will get a professional overview about the technical developments of this company which you find nowhere in the world in this good quality / completeness. You can learn a lot about the technical details of microphones