The MK 390 is a selected pair of 1/4" measuring microphone cartiges for sound intensity probes.
The constructive design of the cartidge is equal to that of the MK 301. The equalization of the static air pressure between the inside and outside of the cartridge is by means of a capillary tube rear-vented into the preamplifier.
The cartridge uses the international standard thread 60 UNS which ensures compatibility with a wide range of calibration equipment and measurement devices available from many manufacturers.