Impedance Matching

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.