Middle ear

The middle ear acts as an acoustic impedance transformer, reducing the reflection of sound energy. It couples the large, low-impedance tympanic membrane to the smaller, higher-impedance oval window. Acoustic impedance refers to the ratio of sound pressure to volume velocity in a transmitting medium.

e This transformer function is achieved by:

0 The smaller area of the stapes footplate and oval window, when compared with p y the tympanic membrane. The area ratio between the tympanic membrane and

1 the oval window is about 30 to 1. o y The lever action of the ossicles, the stiffness of which is controlled by the stapedius and tensor tympani muscles. Contraction of the stapedius muscle pulls the stapes in a direction perpendicular to its normal motion, reducing the amplitude of motion of the stapes. This protects the inner ear from acoustic damage.

The buckling motion of the tympanic membrane. Contraction of the tensor tympani muscle pulls the tympanic membrane inwards.

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