Bone conduction
This type of appliance is used to improve transmissive type hypoacusis (problems concerning the outer and/or middle ear).

In this case the appliance transforms the sounds into vibrations, transmitting them to the mastoid bone through a special vibrator placed at the end of the arm of the glasses. The vibrations reach the cochlea, exciting the ciliary cells, giving rise to nervous impulses which from that moment proceed along the normal hearing paths to the brain.