Biomedical Engineering Reference
In-Depth Information
Cochlea
Length
35 mm (cochlear channels)
Tympanic canal
1 mm high
2 mm
2
Round window
To give some perspective to the incredible sensitivity of the ear, consider that the
radius of a hydrogen atom is 5
10
−
11
m (Bohr radius), which is five times larger than
the required displacement of the tympanic membrane at the threshold of hearing.
.
3
×
6.4
HEARING LOSS
Hearing loss is defined as a deterioration in hearing ability, which, when it becomes
profound, is referred to as deafness. Hearing loss, which becomes much more common
among older people, has many causes, some of which are discussed in the following
section. Between 30 and 40% of people aged 65 and older are significantly affected.
Some children are born deaf or develop hearing loss in their first few years, and as this
is detrimental to language and social development it is important to diagnose any hearing
issues as early as possible.
6.4.1 Causes
The most common cause of hearing loss is the long-term exposure to loud noise. How-
ever, even the brief exposure to very loud noise can permanently harm hearing. It is
interesting to note that as industrial exposure to loud noise has been reduced thanks to
increased awareness and institutional use of personal protective equipment (PPE), hearing
loss caused by listening to loud music has become more common.
Degradation in hearing capability may be caused by a mechanical problem in the ear
canal or the middle ear that disrupts the conduction of sound. This is known as conductive
hearing loss. Blockage in the external ear canal can be caused by a buildup of wax,
the insertion of a foreign object, or the growth of a tumor. The most common cause
of conductive hearing loss in the middle ear is an accumulation of fluid. This is most
often a childhood problem brought about by an infection that blocks the Eustachian tube.
This tube drains fluid from the middle ear and helps equalize the pressure across the
eardrum.
Hearing loss may also be caused by damage to the eardrum itself or to the bones
that conduct sound through the middle ear into the cochlea. Damage to the hair cells
of the inner ear, the auditory nerve, or auditory nerve pathways to the brain—known as
sensorineural hearing loss—lead to hearing loss and can be caused by drugs, infection,
tumors, or injuries to the skull.
Age-related hearing loss is called presbycusis and occurs either as the structures of
the ear become less elastic and undergo changes that make them less able to respond to
sound waves or to the die-off of sensory hair cells. In many people, this process is hastened
by exposure to loud noise over many years. In most people, age-related hearing loss starts
at about 20 but doesn't become noticeable for a further 30 years.
As a person ages, hearing loss first affects high-frequency sensitivity and slowly works
down the spectrum. Loss of ability to hear high frequencies often makes understanding
speech difficult because it reduces the intensity of some of the consonants, particularly C,
D, K, P, S, and T, as illustrated in Figure 6-2. Sounds become muffled, and to people with
