Describe impedance matching and the mechanism of sound transmission.

The Problem and the Solution

• Impedance matching is the function of the middle ear in transferring sound energy from air to the fluid of the inner ear.
• Without it, 99.9% of the energy would be reflected at the air-fluid interface, a loss of some 30 dB.
• Acoustic impedance is the resistance a medium offers to sound, and depends on its density and the velocity of sound within it. Fluid has a far higher impedance than air, so at the boundary most energy is reflected rather than transmitted — the same reason one hears little when submerged
• The middle ear IS A mechanical transformer that increases pressure at the expense of amplitude, which is exactly what is required to drive a stiff, dense medium

Applied Aspects

• Phase differential is the fourth requirement: fluid is incompressible, so it can only move if one window moves out while the other moves in. The intact drum and the air-containing middle ear shield the round window from sound, preserving the phase difference
• This explains the hearing loss in specific lesions:
• A small central perforation causes a modest loss, proportional to the area lost from the vibrating surface
• A large or subtotal perforation causes a disproportionately greater loss, because the round window is exposed and the phase difference abolished as well as the areal ratio being lost
• Ossicular discontinuity with an intact drum causes the maximal conductive loss, 55 to 60 dB.
• Ossicular discontinuity with a perforation causes a smaller loss (35 to 40 dB) than with an intact drum.
• Fluid in the middle ear loads the ossicular chain and abolishes the air cushion, giving a loss of 25 to 30 dB
• Otosclerosis fixes the stapes footplate.
• Surgical application — reconstructive middle ear surgery (tympanoplasty) aims to restore an intact vibrating membrane, a functioning ossicular linkage to the oval window, and an aerated middle ear that shields the round window. All three are needed.
• The various types of tympanoplasty are simply different ways of re-establishing that linkage when different ossicles are missing
• The transformer raises pressure at the cost of amplitude, which is exactly what is needed to drive a dense stiff medium
• Only about two-thirds of the drum vibrates effectively, which is why the working area is 55 rather than 90 square millimetres
• An air-bone gap cannot exceed 60 decibels, so a greater loss proves a sensorineural component whatever the drum looks like