The KA 40 is a sphere accessory (40 mm diameter) and is meant for use with omnidirectional microphones. It is plugged on the microphone.

The KA 40 is an acoustic filter that off-axis provides a larger shadowing at medium (1-4 kHz) frequencies. As long ago there were no good pressure gradient microphones, this was the only way to produce directivity. For example, the Neumann M 50 is a famous vintage microphone, which also had such an acoustic filter.

The KA 40 creates the illusion of "closer placement".

The higher directivity is also advantageous in the Decca tree, because it creates some level differences in this otherwise purely time-of-arrival-based stereophonic microphone techniqu