168 Chapter 9
by many state licensure boards is hearing instrument specialist. However, most audiologists can
also dispense hearing aids. Currently, the terminal degree for audiologists is the clinical doctorate
(AuD). Audiologists are becoming increasingly involved in aural rehabilitation, the development
and integration of residual hearing to maximize auditory perception. Aural rehabilitation also
includes educational techniques to improve expression. Speech- language pathologists and audiolo-
gists overlap in the clinical management of persons with hearing loss and deafness. Hearing loss
is the reduced ability to sense sound, and deafness denotes the inability to perceive sound with
or without amplification. Sometimes, the term hard of hearing is used to refer to hearing loss and
deafness. According to the American Acad emy of Audiology (2015), approximately 35 million
Americans suffer from hearing loss and more than half are younger than 65 years old.
Hearing is sometimes called the second sense, after vision, in transmitting knowledge of the
world. However, when it comes to the acquisition of speech, hearing is primary and vital. For
example, a child born deaf will not naturally acquire the ability to speak, and a child born with
a hearing loss will have speech that ref lects the frequency and severity of the loss. Besides its
importance for speech communication, hearing conveys the sounds of the environment. It brings
persons in constant contact with real ity, alerting them to enjoyment and danger.
Anatomy and Physiology of the Hearing Mechanism
As Figure 9-1 demonstrates, the human ear can be separated into outer, middle, and inner sec-
tions. The external ear directs the acoustic energy generated by the sound source into the ear canal,
which is covered by a waxy secretion, cerumen, that protects it. The ear canal is a self- cleaning
structure; it is not necessary to remove cerumen in most persons. In the middle ear, acoustic
energy is transformed into mechanical energy. The tympanic membrane, or ear drum, vibrates in
response to the vibrations of the air particles in the ear canal. The smallest bones in the human
body, the ossicles, which are connected to the tympanic membrane, amplify the energy and direct
it to the cochlea. The ossicles are the malleus, incus, and stapes, also known as the hammer, anvil,
and stirrup, respectively. The middle ear is a small chamber ventilated by the eustachian tube,
which acts to equalize middle ear and atmospheric pressure. Mechanical energy from the middle
ear is transformed into hydraulic energy in the inner ear, consisting of the semicircular canals and
the cochlea. The semicircular canals are part of the vestibular mechanism and are impor tant for
balance. The hydraulic vibrations in the cochlea are transformed into electrochemical or nerve
energy at the vestibulo- cochlear cranial nerve (VIII), or auditory nerve, and sent to higher brain
centers, where auditory perception and association occur (Culbertson, Christensen, & Tanner,
2013).
Figure 9-1. Major landmarks of the
outer, middle, and inner ears.