In order to better understand signal propagation in the ear, a time-domain model of the tympanic membrane (TM) and of the ossicular chain (OC) is derived for the cat. Ossicles are represented by a two-port network and the TM is discretized into a series of transmission lines, each one characterized by its own delay and reflection coefficient. Volume velocity samples are distributed along the ear canal, the eardrum, and the middle ear, and are updated periodically to simulate wave propagation. The interest of the study resides in its time-domain implementation--while most previous related works remain in the frequency domain--which provides not only a direct observation of the propagating wave at each location, but also insight about how the wave behaves at the ear canal/TM interface. The model is designed to match a typical impedance behavior and is compared to previously published measurements of the middle ear (the canal, the TM, the ossicles and the annular ligament). The model matches the experimental data up to 15 kHz.
Hearing thresholds expressed in terms of the forward-going sound pressure wave are repeatable, and with in-situ calibration, may be superior to the current coupler-based method.
Tympanic membrane (TM) and ossicular (OSC) models have been studied for many years. Most are implemented in the frequency-domain via Kirchhoff’s equations. However, the frequency domain is less intuitive than the time-domain models and does not apply given non-linear phenomena. This research presents a human time-domain TM+OSC implementation, extending our cat model [J. Acoust. Soc. Am. 122, 918–931 (2007)] as a cascade of slightly missmatched transmission lines. The canal plane-wave volume velocity is uniformly distributed across the canal cross-section, and updated at ≈1 MHz (to accurately simulate sound propagation). The time-domain model includes both forward and backward waves, enabling for the first time, the computation of one-way delays, thus explaining why multi-modal propagation observed on the TM are not critical. Results are validated with data from normal and pathological ears. Additionally, comparisons between the acoustic impedance in living and dead (temporal bones) ears shows significant temporal-bone artifacts. Because of ear-dependent impedance missmatch, canal standing waves are observed. It is frequently assumed that the umbo pressure is the best reference pressure. In fact the “forward-pressure” P+ is a better estimate of the ideal reference, the cochlear pressure [J. Acoust. Soc. Am. 125, 1605–1611 (2009)].
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