Locke, Antony James Steven; (1999) Acoustic power flow into the ear and the auditory microstructure. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An experimental technique to determine the acoustic power absorbed by the human ear at absolute threshold is described and applied to data recorded in adult subjects. A previously published method of electroacoustic probe calibration in terms of equivalent Thevenin source parameters is substantially ameliorated. Careful and detailed measurements of continuous tonal aural sound pressure (CTASP) are presented. Ear canal input impedance, reflectance and absolute power flow constituents are derived from CTASP data. Auditory microstructure, characterised by spectral periodicity, is observed and validated in CTASP, impedance, reflectance and power flow parameters at a 20 dB SPL stimulus level, but undetectable at 60 dB SPL. Periodicity in the ear canal acoustic parameters elicited at low stimulus levels is found to be commensurate with absolute threshold microstructure. An elementary analogue network model of the peripheral auditory system is formulated, enabling cochlear input impedance and reflectance to be inferred from ear canal acoustic parameters. At a 20 dB SPL stimulus level a non-zero cochlear reflectance is inferred, implying that energy propagates basally, as well as, apically. Microstructure amplitude in cochlear input impedance is shown to be 4 dB greater than that in ear canal input impedance, a consequence of decoupling of the probe from the tympanic membrane. A proportionality between transmittance and auditory sensitivity exists, implying that the ear couples more efficiently to the sound source, and consequently extracts proportionally more power, at peaks in sensitivity. However, the measured change in coupling is inadequate to wholly explain threshold microstructure. An explanation is offered by applying empirical data to a phenomenological model of power flow within the peripheral auditory system. It is argued that threshold microstructure arises predominately from a phasic interaction of the basalward and apical travelling waves effectively modifying the spatial distribution of energy within the cochlea.

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