Abstract
M.Ing. (Electrical and Electronic Engineering)
This dissertation presents a comprehensive electrical model of the human auditory periphery. The model focuses on the generation and transmission of otoacoustic emissions
(OAEs) under biometric conditions. The auditory system model was divided and studied in three sections, namely the outer, middle and inner ear sections. Existing models
were used and improved for the study. The outer ear model was derived using electroacoustic analogies. The middle ear model was derived empirically. The inner ear model
was derived by relating the mechanical properties of the inner ear to electrical principles.
The outer ear model includes an analog diffraction circuit and a linear transmission line
representation of the auditory canal and the concha. The variation of the radius of the
auditory canal along its length was incorporated when computing the model of the outer
ear. A pair of second order polynomials were used to create a new radius-length function
which approximates the relationship between the radius of the auditory canal and its
length. The frequency response of the outer ear model obtained using the radius-length
function gave a wide frequency range representation of the outer ear characteristics.
The middle ear is modelled using an analog network. Only the linear operation region of
the middle ear was considered, thus excluding its reflex nonlinear mechanisms, namely;
the stapedius muscle action and the stapes clipping displacement. The influence of the
middle ear on the transmission of OAEs was evaluated by considering both the forward
and reverse transmission characteristics/path of the middle ear. The middle ear response
demonstrated great sensitivity to changes in the terminal loads connected to the middle
ear as well as the transformer ratio.
The inner ear behavior is represented by means of a nonlinear transmission line model.
The nonlinear mechanism of the outer hair cells, which are taken as the primary sources
of OAEs, are modelled using nonlinear voltage sources. The inner ear model was evaluated for conditions of both the active and inactive outer hair cells voltage sources. Due to
limitations in the simulation software, a reduced active inner ear model was computed.The influence of the number of segments of the inner ear was explored. A reduced inner
ear model having 40 segments was found to be sufficient in representing the frequency
characteristics of the inner ear, whilst preserving the frequency-latency relationship of
OAEs.
The study not only improved the model of the auditory periphery, but also suggested
several factors that can be incorporated in future research in order to better design
signal acquisition and processing methods for OAE biometric applications.