Phasor admittance measurements of the middle ear. I. Theoretical approach.

A parallel two-piston model of the middle ear is described. One piston represents vibrations of the ossicular chain and closely coupled vibrations of the tympanic membrane. The other piston represents uncoupled vibrations. To a first approximation, it is assumed that acoustic reflexes modify the former and tympanometry principally modifies the latter, although a comodulation model is also considered. The performance of the model is evaluated in terms of the trajectories of the input admittance phasor resulting from increases in stiffness assumed to occur during acoustic reflexes or tympanometry. This phasor trajectory approach places the various morphological categories of normal acoustic reflexes and tympanograms, including W-notched tympanograms, along a simple continuum. The model leads on to suggest a method by which the parameters of the two pistons can be evaluated using acoustic reflex and tympanogram data at two probe tone frequencies. The method is utilised in the subsequent paper.