The E6 EarSet should always be used with a protective cap in place to keep sweat, makeup and other foreign material out of the microphone. The three omni caps each have a different high frequency response characteristic that controls the amount of "crispness" or "sibilance" (response at 15 kHz).
The omni ships with the +4 dB protective cap fitted to the mic. We have found that this cap's frequency response meets the needs of the majority of users, providing a slightly increased response in the 15 kHz range. This will boost the perceived amount of "presence" in your sound, while leaving the lower frequencies unchanged. If you experience problems with high-frequency feedback, you should switch to the +0 dB cap.
Countryman Omnidirectional EarSets come with all three (+0dB, +4dB, and +8dB) protective caps in the matching color. Additional caps may be purchased separately; please specify color and frequency response characteristic.