Countryman's B3 lavalier 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).

Note: The image shown is for illustrative purposes. Actual product color and appearance may vary depending on the model. 

The B3 ships with the +0 dB (flat) protective cap fitted to the mic.  Switch to the +4 dB or +8 dB cap to 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.