A study of tympanic membrane displacement
Presented in this thesis is a study relating to the measurement of tympanic membrane displacement. Contributions have been made under three broad headings: firstly, following a review of techniques used to measure various audiological parameters, particular emphasis is given to the design and development of a new method, the Tympanic Membrane Displacement (TMD) system; secondly, consideration is given to the techniques of signal processing, including the separation of the desired signal from the noise, which is both environmental and physiological in origin; thirdly, the physiology of the ear and the acoustic reflex are examined in detail. The TMD system was conceived to overcome various shortcomings of other measurement techniques. This device measures extremely small volumetric displacements, of the order of a few nanolitres, over a relatively wide bandwidth and under near free field conditions. Audiological aspects of the present work include comprehensive studies of the background noise level within the external ear canal, the displacement of the tympanic membrane in connection with the Eustachian tube function, and the steady state and dynamic characteristics of the acoustic reflex. An hypothesis is postulated which relates the latter reflex characteristics to the resting position and dynamics of the stapes footplate in the oval window, the interface between the middle and inner ear. It is thus shown that the TMD system enhances the physical understanding of the hearing mechanism, and so has a potential for identifying abnormalities pertaining to pathological conditions.