The full-shell instrument fills the concha of the individual. It is the largest of the in-the-ear hearing aids (see Image 3). It typically can address more severe hearing losses with greater ease because of its ability to fill the canal and the concha of the external ear. It thus can reduce the chance of feedback from the hearing aid.

The half-shell is an instrument that fills only the concha cavum and the canal and is approximately half the size of a full-shell instrument. Because of its smaller size, it is cosmetically more appealing and could be appropriate for moderate-to-severe hearing losses.

The canal-sized in-the-ear aid primarily fits within the concha and in the outer half of the canal. The faceplate of this aid is accessible to the user to allow changing the volume control and turning the aid on and off. This aid provides some advantage in gain at higher frequencies because of its depth of insertion and the acoustic resonance in the unblocked concha.

The completely in-the-canal aid, or what may be termed a peritympanic hearing aid, is fitted deep into the ear canal and is the smallest of all hearing aids (see Image 4). It typically fits entirely within the ear canal, and the deepest portion of the aid is in close proximity to the tympanic membrane. The faceplate is usually not accessible to the user. The aid also needs a short cord or wire attached to the faceplate for the wearer to use while removing the aid.

These aids are regarded as the most cosmetically pleasing, and, because of the close proximity to the tympanic membrane, they can reduce or eliminate the occlusion effect. Additionally, patients with this type of aid can use the telephone as do individuals without hearing aids.

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