Assuming that it's not quite like what WiFiJeff said, then these are basically omnidirectional microphones arranged so that they keep their sonic focus even at rather great distances in a reverberant space.
Embedding a pressure transducer in the surface of a sphere and aiming the transducer forward toward the main sound source has two potentially useful effects for distant pickup: It creates a forward "preference" in the directional pattern above a few kHz, and it emphasizes the upper midrange (= presence) range of frequencies in the front hemisphere of the mike while doing more or less the opposite on the back side.
Attached is a polar diagram for one incarnation of the Neumann M 50 (which used a smaller internal sphere--only 40 mm, or about 1-1/2" in diameter) so that you get a general idea about what the sphere is doing. Among other uses, this is the prototypical "Decca Tree" type of microphone.
--best regards
Topic: ID these unusual spherical mics? DIY? Cool? Useful? (Read 3678 times)