Openings to the rear and side of a uni-directional microphone, allowing sound waves to reach the rear of the diaphragm.
"We are impressed with the quality of the CD. Two completely broken records, and only faint echoes of the breaks can be heard. That you were able to transfer anything from those two records is remarkable. The other old records (one from Germany circa 1930 and an Al Jolson from about the same period) sound like the artists are in the room with ..."