The popularity of hearing instruments deeply placed in the ear canal has been driven by their superior cosmetic characteristics. However, people fitted with these hearing instruments often complain about the sound quality of their voice, which is typically described as being hollow, echoing, or like talking in a barrel. This problem is caused by the occlusion effect. The effect is primarily due to vibrations of the walls in the soft part of the ear canal, which generate a sound
pressure that is trapped in the cavity between the tip of the occluding hearing aid and the tympanic membrane. If the hearing aid is fitted with a seal in the bony portion of the ear canal, preventing vibration in the soft part of the ear canal from reaching the tympanic membrane, then occlusion problems can be solved or at least reduced in most cases.