Glasses hearing-aids are classified according to the method used to transmit amplified sounds to the ear:

• Air conduction
  In air conduction appliances, the amplified sounds reach the eardrum, thanks to a made-to-measure earpiece which is inserted in the auditory canal.
• 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.