The APE50RS Acoustic Pressure Equalizer, 50mm Ball for 4006A and 4006C Microphone from DPA Microphones is for modifying a microphone's on-axis and off-axis frequency response. It is used to give your recordings greater presence, nearness and clarity on-axis under reverberant conditions.
The acoustic pressure equalizers are passive acoustic processors that function as both spatial and spectral modifiers. They use diffractions on the surface to modify the sound field near the microphone diaphragm. This technique works only with omnidirectional pressure microphones.
With the APES50, two changes occur. An upper-midrange/high frequency boost (without changing the low end) due to the pressure build-up at certain frequencies depending on the element's size and more directionality at higher frequencies (focus).