Non-Occluding (or "open") earmolds are designed for high frequency emphasis or CROS applications, and are either heavily vented or feature a small canal portion, just large enough in diameter to accommodate the tubing, that does not block the canal entrance. The latter allow amplified sound to pass naturally around the mold as well as go through the tubing from the hearing aid. Low frequency amplification is significantly reduced with open molds, of which there are several basic variants:
CROS molds are used for CROS and many IROS fittings, and are available in styles which address tubing length and position as well as degree of occlusion.
Free-field molds are more delicate than the CROS types, as they are reduced in the concha ring and bridge to offer even less occlusion at the canal opening.