Let us explore the role played by the ossicles in matching the impedance between the air in the outer ear and the fluid in the inner ear. More specifically, let us model the hearing mechanism under the condition that the middle ear did not exist . The diagram below illustrates the situation we are attempting to model. The diagram shows a pressure wave in air striking the oval window of the inner ear. Behind the oval window is the cochlea, which is filled with a fluid .
Water has a higher density and elasticity than air.
The ratio of the impedance of water to the impedance of air is about 4000. The fraction of the energy transmitted across the boundary can be increased by inserting an intermediate device to match the impedances across the two media . To understand how to match the impedances across the two media, we need to know how the impedance is related to the dynamical variables that determine the energy in a wave. The impedance of a medium is proportional to the pressure . Thus, to increase the impedance in the middle ear side of the ear in order to avoid a large impedance mismatch, the pressure would have to be increased in the middle ear side of the oval window.