The performance of our hearing system degrades with age, causing a reduction in our ability to understand speech. This thesis describes the design of a personal audio system, aimed at enhancing the audio signal from a TV, for hard of hearing listeners. Such system generates a zone of high acoustic pressure in a specific area of a room, whilst minimising the sound radiated to other zones. In order to obtain a very narrow sound radiation, superdirective techniques are used. Such techniques improve the performance of an array of small dimensions at mid and low frequencies, but they can require the use of a large amount of power and can be very sensitive to errors in the source transfer functions. A practical balance between performance and array effort is achieved in a design that uses phase-shift sources. Two arrays have been built. The first array uses 8 phase-shift sources and provides a high directionality in the free field. The second array uses 4 phase-shift sources vertically plus 8 horizontally and is more directional in a 3D sense, hence reducing the reverberant field excitation. The design of superdirective filters has been considered in both time and frequency domains. The effect of the reverberant field in the performance of superdirective radiators has also been investigated, by first examining how the radiation pattern is modified, and then by observing how the reverberant field affects the robustness of the control. The performance has been assessed by means of off-line simulations with point source Green functions and measured transfer responses, and by real time measurements in free-field and reverberant environments. The later chapters of the thesis consider the subjective assessment of arrays as an aid for speech intelligibility. The assessment has first been performed using an objective metric, in particular the speech transmission index (STI). The performance has also been assessed in a subjective study, in which 30 participants have judged the increase in intelligibility. A cochlear model has also been utilised to investigate equalisation methods that minimise masking and provide a better speech intelligibility, particularly for the hearing impaired. Although this has not been tested with behavioural experiments, STI simulations have predicted that it can lead to an intelligibility improvement.

Skeletal muscle contractions emit a low frequency sound (<100Hz) that can be detected by vibromyography (VMG) or mechanomyography (MMG). The aim of this study was to determine the future application of the VMG signal as a clinical tool in reflecting the physiological state or function of muscle during rehabilifation and diagnosis. In order to achieve this aim, the following was investigated. A) The technical characteristics: Three different transducers (the capacitance and piezoelectric accelerometers and the contact sensor) and the transducer/ amplifier system. B) The repeatability of the VMG signal at different force levels. C) Physiological characteristics of the VMG signal comparing 'normal' and 'abnormal' muscle function. Past studies on VMG have revealed that these three areas have received little attention. The VMG signal was taken from the biceps brachii muscle at different contraction levels 10%, 50% and 80% maximum voluntary contraction (MVC) for within day (n=3) and between day (n=17) recordings. The signal from asymptomatic (n=29) and symptomatic (n=23) low back pain subjects was then compared. Recordings were taken from the erector spinae muscle at an incremental increase in voluntary contraction (10%-100%MVC). These recordings were repeated on the next day. Results from this study have revealed the following: A) Technical characteristics : the lower cut-off frequency is important and insufficient attention in past studies has been given to the transducer-amplifier system. Additionally, the capacitance accelerometer was the favoured transducer due to its greater sensitivity to low frequencies, simplicity of use and ease of stabilising and coupling techniques to the skin. B) Repeatability of the VMG signal: For successive recordings, compared with precision (%C.O.V and L.O.A) and reliability (I.C.C), correlation (r value) of the VMG signal appears a much stronger and a more consistent statistic in the time and frequency domains. C) Physiological Characteristics: Although having a high individual variation, the erector spinae demonstrated a strong curvilinear relationship with increasing submaximal voluntary contraction (%MVC) reflecting the recruitment and rate coding strategies for this muscle. The VMG RMS for symptomatic low back pain subjects was considerably less than asymptomatic subjects suggesting a deficiency in muscle function. In contrast, the PSD was less favourable in distinguishing between the two subject groups. Reasons for these findings are discussed. In future, the clinical application of the VMG RMS values may quantitatively assess paraspinal muscle force reflecting physiological changes in recruitment and rate coding strategies in 'normal' and 'abnormal' muscle function during rehabilitative care.

No description available.

This thesis describes the development of a novel game-like method, the AnimalSeek method, which can be used, along with motion tracking technology, to measure localisation ability of a child under five years of age. For the game-like task to be successful, a high number of responses (in particular correct head turn responses) was required. Previous studies, although not all looking at localisation ability, have used many different techniques to obtain the maximum number of responses from a child. The children were engaged inside a custom-built environment inside an anechoic chamber. Three large video screens onto which backgrounds and animated characters were projected and manipulated and used to engage the child in the game-like task. Behind the video screens were loudspeakers from which the auditory stimulus where presented. A correct response to the auditory stimulus i.e. a head, hand or eye movement towards the target speaker was rewarded with a animated character presented on the screens (incorrect responses were presented with a static character). The location of the reward in relation to the auditory stimulus was a point of interest and was investigated to see how it affected the number of responses. The method shows it was possible to engage the child with the visual environment and obtain responses, however, the results showed generally fewer head turn responses than expected, especially in the younger age groups. Motion tracking technology was used to measure the localisation ability of the children, as well as measuring the responses, the motion tracking data was used and programs developed which could automatically classify the responses the children made to the sounds. The thesis has shown that it is possible to devise a new method which can be used to engage the child in the task and extract and classify their responses to auditory stimuli in order to measure their localisation ability.

Experimental meningitis was induced in pigmented guinea pigs by subarachnoid inoculation of \(1 \times 10^9\) Escherichia coli K-12 or \(3 \times 10^7\) CFU Streptococcus pneumoniae serotype 2 D39 (NCTC 7466) or PLN-A, \(\Delta\)NA1 or \(\Delta\)HY1, defined isogenic derivatives of D39 deficient in pneumolysin, neuraminidase or hyaluronidase respectively. All animals developed a meningeal inflammatory response and a labyrinthitis. Hearing loss in pneumococcal meningitis was measured by recording the evoked auditory nerve compound action potential from the round window membrane. Animals infected with PLN-A sustained significantly less hearing loss than those infected with wild-type D39 (12 dB vs. 50 dB 12 h post inoculation; P<0.0001), Neuraminidase deficiency did not alter the course of the meningeal inflammatory response nor affect hearing loss. The \(\Delta\)HY1 mutant survived poorly in the cerebrospinal fluid and blood but still caused hearing loss. Both pneumococcal and E. coli meningitis induced specific ultrastructural lesions in the organ of Corti as judged by high-resolution scanning and transmission electron microscopy, and these lesions were most severe with pneumolysin-sufficient pneumococcal infection. Microperfusion of \(5\times10^6\) CFU S.pneumoniae D39 directly into the scala tympani of guinea pigs also resulted in electrophysiological and ultrastructural damage to the organ of Corti that could be diminished by pretreatment with antibiotics. The data confirm the cochlea as the site of meningogenic deafness. They suggest that pneumolysin expression is chiefly responsible for meningogenic deafness and that if pneumococci invade the inner ear during bacterial meningitis, cochlear deafhess will rapidly ensue.

The vestibular system provides vital information about head position and motion; which is used for the control of balance through vestibulospinal reflexes. Chapter 2 explores the process of transforming head position to body coordinates, with and without vision. The results show when vision is available, the evoked response is less precise. Chapter 3 explores the transformation process before and after 60 days of bedrest. After this period of inactivity, participants swayed more, and their EVS-evoked sway response was less precise. This decrement in precision appears to begin recovery 6 days postbedrest. Chapter 4 focuses on vestibulo-ocular reflexes rather than postural reflexes. Electrical vestibular stimulation is used to evoke measurable torsional eye-movements. The magnitude of the response is modulated by stimulus frequency. Results suggest that CNS interprets electrical vestibular stimulation as a velocity signal rather than a position or acceleration signal. This technique is an ideal measure of pure vestibular function, Chapter 5 utilised the technique in a clinical environment. Vestibular schwannoma patients, with known unilateral vestibular deficit, were tested to identify if the proposed technique can detect this deficit. Results showed that asymmetries could be detected, and, the test may be more sensitive than previously used measures of vestibular asymmetries.

Distortion product otoacoustic emissions (DPOAEs) arise within the cochlea in response to two stimulus tones (f1 and f2) at frequencies such as 2f1 − f2 and 2f2 − f1. Each DPOAE derives from two contributing mechanisms within the cochlea: a distributed distortion source and a reflection source. They are used for hearing screening, but a better understanding of their cochlear origin and transmission could potentially extend their clinical application to facilitate objective hearing loss assessment, differential diagnosis of sensorineural hearing losses and improved auditory rehabilitation using hearing aids. In this thesis a numerical model of the human cochlea is developed to study the generation of DPOAEs. It is based on a pre-existing active nonlinear model, the micromechanics of which are carefully re-tuned to simulate the response of the human cochlea to single- and two- tone stimulation. Particular attention is paid to the form and position of the nonlinearity within the model to best match experimental results. The model is also reformulated to verify its stability and ensure computational convergence of the iterative frequency domain solution method. Its predictions are validated against estimated time domain simulations and documented experimental DPOAE measurements. Additionally a novel method is developed for decomposing each frequency component of the cochlear response into forward and backward travelling waves, which is applied to investigate the multiple sources of both the 2f1 − f2 and 2f2 − f1 DPOAEs. The model is used to explain and predict a variety of phenomena observed in experimental DPOAE studies. It also confirms for the 2f1 − f2 emission, that the two source mechanisms are spatially separated and that the only significant reflection contribution is associated with the 2f1 − f2 travelling wave. In contrast, it predicts that the two source mechanisms will overlap in the case of the 2f2 − f1 DPOAE, which can be influenced by reflection of both the primary and 2f2 − f1 travelling waves.

No description available.

Normal-hearing adults have no difficulty in recognising consonants accurately, even in moderately adverse listening conditions. By contrast, users of multichannel cochlear implants have difficulty with the accurate perception of consonants, even in good listening conditions. Cochlear implant users are known to show systematic deficits in recognition of consonant features, with perception of the place feature, which relies on spectral information, being worst. These deficits may be attributed both to signal distortions introduced by the processing of the implants and to other factors, in particular the spectrotemporal distortions which occur at the interface between electrode array and auditory nervous system, including cross-channel interaction. The objective of the work reported here was to attempt to partial out the relative contribution of these different factors to consonant recognition. This was achieved by comparing cochlear implant users’ perceptual errors, analysed in terms of information transmission, with errors made by normal-hearing subjects listening to acoustic models of implant processing, in various conditions. Two initial experiments were undertaken to develop and refine an acoustic model of the Nucleus 24 cochlear implant. Findings from these two experiments informed the design of the main acoustic model experiment, which was undertaken in parallel with a further experiment involving users of the Nucleus 24 device. In both experiments, subjects listened to nonsense syllables with and without the addition of stationary background noise, in three different configurations of implant processing parameters. Additionally, in the acoustic model experiment, a simulation of cross-channel spread of excitation, or “channel interaction”, was varied. Results showed that acoustic model experiments were predictive of the pattern of consonant feature transmission in cochlear implant users with better baseline consonant recognition scores. Deficits in consonant recognition in this subgroup could be explained by the loss of phonemically relevant acoustic information in speech due to the nature of cochlear implant processing, while channel interaction appeared to play a smaller role in accounting for problems in consonant recognition. The work also evaluated the effect of changes in channel number and stimulation rate and failed to find any changes in consonant recognition as these parameters were varied. The lack of a stimulation rate effect was consistent with acoustic measurements of the temporal modulation transfer function of the processor, which showed almost no change across stimulation rates.

No description available.