Sure, some in-ear monitors have vents that allow some air to pass through. Their purpose is to reduce the sensation of sound pressure in exchange for a more natural sound and deeper bass. Still, some models don’t employ the vents, and they are sold as sound isolating in-ear headphones. In addition, the seal offered by closed-back in-ear headphones is generally better than that offered by its on-ear counterpart, mainly because the seal is formed by the ear canal and not the ear cups.