Impedance Matching
Impedance matching is the process by which tympanic membrane and auditory ossicles convert the sound energy into mechanical vibrations in cochlear fluid with minimum loss of energy by matching the impedance offered by fluid.
Impedance means obstruction
or opposition to the passage of sound waves. When sound waves
reach inner ear, the fluid (perilymph) in cochlea
offers impedance, i.e. the fluid resists the
transmission of sound due to its own
inertia. Tympanic membrane and auditory ossicles effectively reduce the
sound impedance. Sound waves are conducted from external ear to inner ear, with an impedance of only 40%. Remaining 60% of sound energy developed in tympanic membrane is transmitted to cochlear fluid by the ossicles. Thus, along with the help of tympanic membrane, ossicles match the impedance offered by fluid to a great extent. It is because, the ossicles act like a lever
system so that stapes exerts a greater force
(pressure) against the cochlear fluid. This results in
generation of vibrations in the cochlear
fluid. The increased force is very essential to set up
the vibrations in cochlear fluid because of higher
inertia of the fluid. Force exerted by footplate of stapes on cochlear fluid is 17 to 22 times greater than the force exerted by sound
waves at the tympanic membrane. It is because of two structural features of
ossicles:
1. Head of malleus is
longer than long process of incus so that a higher force is generated in small
structure
2. Surface
area of tympanic membrane (55 sq mm) is larger compared to that of
footplate of (3.2 sq mm). So the pressure
increases when force is applied to small area. Thus, the tympanic
membrane and the auditory ossicles are capable of converting the sound
energy into mechanical vibrations in cochlear fluid with minimum
loss of energy.
Significance of impedance matching
Impedance matching is the
most important function of middle ear. Because of impedance matching the
sound
waves (stimuli) are transmitted to cochlea with minimum loss of intensity.
Without impedance matching conductive deafness occurs.
Types of Conduction
Conduction of sound from
external ear to internal ear through middle ear occurs by three routes:
1. Ossicular conduction
2. Air
conduction
3. Bone
conduction.
1. Ossicular conduction
Ossicular conduction is
the conduction of sound waves through middle ear by auditory ossicles. In
normal
conditions,
the sound waves are conducted through auditory ossicles.
2. Air conduction
Air conduction is the
conduction of sound waves through air in middle ear. If the ossicular chain is
broken,
conduction occurs in an alternate route of air conduction. Air conduction
is common in otosclerosis. Otosclerosis
is the disease associated with fixation of stapes to
oval window.
3. Bone conduction
Bone conduction is the
conduction of sound waves through middle ear by bones. It occurs when middle
ear is
affected. In this type of conduction, sound waves are transmitted to cochlear
fluid by the vibrations set up in skull bones. Bone conduction is tested
by placing vibrating tuning forks or other vibrating bodies directly on the skull. This route plays a role in transmission of extremely
loud sounds.
ROLE OF EUSTACHIAN
TUBE
Eustachian tube is not
concerned with hearing directly. However, it is responsible
for equalizing the
pressure on either
side of tympanic membrane.
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