AK... active capsules are part of the variable miniature microphone system. Together with the KM 100 output stage AK... active capsules form a complete microphone. AK 30 and KM 100, for example, make up the KM 130.
AK 31 is a free-field equalized pressure transducer. The sensitivity in the free sound field is flat up to 20 kHz. In the diffuse sound field there is a roll-off above 5 kHz.