neural-electrical signal that is translated by the brain as speech, music, noise, etc. The external ear, middle ear, inner ear, brainstem, and brain each have a specific role in this transformation process (see Figure 2-1).
The external ear includes the pinna, which helps capture sound in the environment. The external ear canal channels sound to the tympanic membrane (eardrum), which separates the external and middle ear. The tympanic membrane and the three middle ear bones, or ossicles (malleus, incus, and stapes), assist in the transfer of sound pressure in air into the fluid- and tissue-filled inner ear. When pressure is transferred from air to a denser medium, such as the inner ear environment, most of the pressure is reflected away. Thus, the inner ear offers impedance to conducting sound pressure to the fluid and tissue of the inner ear. The transfer of pressure in this case is referred to as admittance, while impedance is the restriction of the transfer of pressure. The term “acoustic immittance” is used to describe the transfer process within the middle ear: the word “immittance” combines the words impedance and admittance (im + mittance). As a result of this impedance, there is as much as a 35 dB loss in the transmission of sound pressure to the inner ear. The outer ear, tympanic membrane, and ossicles interact when a sound is present to