From DPA, the APE L6 are a set of pressure equalization caps for the 4003 and 4006 microphones. The caps use acoustic pressure to modify tonal and frequency response characteristics.
DPA MicrophonesAPEL6 Stereo Acoustic Modification Set for the 4003 and 4006 Microphones - Includes: Pair Acoustic Pressure Equalizers 30mm, 40mm and 50mm
40mm Acoustic Pressure Equalizer
50mm Acoustic Pressure Equalizer
30mm Acoustic Pressure Equalizer
1 Year Warranty