New knowledge about the function of the human middle ear: development of an improved analog model

Conventional teaching regarding the acoustic function of the human middle ear is that it serves as an impedance matching system to offset the loss that occurs when sound passes from the low-impedance sound field to the high-impedance cochlear fluid. A transformer analogy is often used with the pressure transformation produced by the eardrum; footplate area ratio and the lever ratio considered to be approximately 27 dB. Recent data on middle ear function has shown this to be only partially correct. A transformer analogy is not appropriate since the pressure gain of the middle ear decreases above 1000 Hz and does not depend on the cochlear load at all frequencies. Experiments are described of umbo, malleus short process, and stapes displacement in human temporal bones using a laser Doppler measuring system (LDS). The measurements support previous studies that indicate that in addition to a roll-off in tympanic membrane function above 1000 Hz, there is slippage in the ossicular lever system that causes an increasing "lever ratio" above 1000 Hz, thought to be caused by translational movement of the ossicular rotation axis near the short process. An improved analog circuit model of the external and middle ear has been developed that produces results equivalent to those found in the temporal bones.