Hearing loss afflicts as many as 10\% of our population.Fortunately, technologies designed to alleviate the effects ofhearing loss are improving rapidly, including cochlear implantsand the increasing computing power of digital hearing aids. Thisthesis focuses on theoretically sound methods for improvinghearing aid technology. The main contributions are documented inthree research articles, which treat two separate topics:modelling of human speech recognition (Papers A and B) andoptimization of diagnostic methods for hearing loss (Paper C).Papers A and B present a hidden Markov model-based framework forsimulating speech recognition in noisy conditions using auditorymodels and signal detection theory. In Paper A, a model of normaland impaired hearing is employed, in which a subject's pure-tonehearing thresholds are used to adapt the model to the individual.In Paper B, the framework is modified to simulate hearing with acochlear implant (CI). Two models of hearing with CI arepresented: a simple, functional model and a biologically inspiredmodel. The models are adapted to the individual CI user bysimulating a spectral discrimination test. The framework canestimate speech recognition ability for a given hearing impairmentor cochlear implant user. This estimate could potentially be usedto optimize hearing aid settings.Paper C presents a novel method for sequentially choosing thesound level and frequency for pure-tone audiometry. A Gaussianmixture model (GMM) is used to represent the probabilitydistribution of hearing thresholds at 8 frequencies. The GMM isfitted to over 100,000 hearing thresholds from a clinicaldatabase. After each response, the GMM is updated using Bayesianinference. The sound level and frequency are chosen so as tomaximize a predefined objective function, such as the entropy ofthe probability distribution. It is found through simulation thatan average of 48 tone presentations are needed to achieve the sameaccuracy as the standard method, which requires an average of 135presentations.

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The aim of this study was to develop, digitally record, evaluate, and psychometrically equate a set of Taiwan Mandarin bisyllabic word lists to be used for word recognition testing. Frequently used bisyllabic words were selected and digitally recorded by male and female talkers of Taiwan Mandarin. Twenty normally hearing subjects were presented each word to find the percentage of words which they could correctly recognize. Each word was measured at 10 intensity levels (-5 to 40 dB HL) in increments of 5 dB. Logistic regression was used to include 200 words with the steepest logistic regression slopes in four psychometrically equivalent word lists of 50 words each with eight half-lists of 25 words each. Digital recordings of the psychometrically equivalent bisyllabic word recognition lists are available on compact disc.

The purpose of this study was to develop digitally recorded speech audiometry materials in the Japanese language to evaluate Speech Reception Threshold (SRT) and speech discrimination. Trisyllabic words were used to evaluate the SRT and bisyllabic words were used for speech discrimination. Words were recorded by one native female talker and one native male talker who were judged as having standard Japanese dialects. Twenty native Japanese speakers between the ages of 20 and 32 were used as subjects to evaluate 69 trisyllabic words across 13 different intensity levels. The 25 trisyllabic words with the steepest psychometric function (%/dB) were selected for inclusion in the final CD. The final trisyllabic words were digitally adjusted so that the threshold of each word was equal to the mean PTA (3.42 dB HL) of all the subjects. The mean psychometric function (%/dB) at 50% for the trisyllabic words was 9.6 %/dB for the male talker and 7.7 %/dB for the female talker. The same 20 subjects were also used to evaluate 240 bisyllabic words across 10 different intensity levels. A logistic regression was used to obtain regression slopes for each of the 240 words. The 200 bisyllabic words with the steepest slope were selected for inclusion in the final CD. Four lists of 50 words each and eight half-lists of 25 words each were created from the selected bisyllabic words. A chi-square statistic revealed no significant differences among the lists or half-lists. The mean psychometric function at 50% for the bisyllabic lists and half-lists was 5.9 %/dB for the male talker and 5.2 %/dB for the female talker.

The development of digitally recorded speech audiometry materials in Mandarin Chinese has been limited to date. High quality materials proliferate in the English language and have been developed for other languages such as Brazilian Portuguese, Spanish, Japanese, Korean, French, Italian, Polish, and Russian. The aims of this study were to develop, digitally record, evaluate, and equate words and word lists in Mandarin Chinese to use for speech reception threshold testing and for speech discrimination testing. The words that were evaluated were chosen from a Chinese frequency usage dictionary. One native male and one native female talker recorded the words onto a compact disc. These words were then evaluated on 20 normally-hearing subjects, all of whom were natives of mainland China. Following the tests, the words and word lists were analyzed using logistic regression. For speech reception threshold testing, 24 Mandarin Chinese trisyllabic words with steep psychometric function slopes were selected. Their intensities were adjusted to match the mean subject pure-tone average of 3.0 dB HL. The mean slopes for these selected male and female trisyllabic words were 11.3 %/dB and 12.1 %/dB, respectively. For the speech discrimination testing, the 200 bisyllabic words with the steepest logistic regression slopes were divided among four psychometrically equivalent lists of 50 words each and eight half-lists of 25 words each. The intensity of each word list was digitally adjusted so that the threshold of each list was equal to the midpoint between the mean thresholds of the male and female half-lists. All lists were homogeneous with respect to audibility and psychometric function slope. Digital recordings of the psychometrically equivalent word lists are included on a compact disc.

The purpose of this study was to create psychometrically equivalent word lists in the language of Cantonese for word recognition testing. Frequently used bisyllabic Cantonese words were recorded by a native female and male talker. The word lists were evaluated by administering the word recognition lists to 20 native speakers of Cantonese with normal hearing. Each list was presented at 10 different intensity levels ranging from -5 to 40 dB HL in 5 dB increments. Logistic regression was used to determine the words with the steepest logistic regression slopes. The 200 words with the steepest slopes were then formulated into four lists of 50 words and eight half-lists of 25 words. The mean psychometric slope value at the 50% location for the male talker was 7.5%/dB while the mean slope for the female talker was slightly steeper at 7.6%/dB. The word lists were digitally recorded on compact discs for worldwide use.

The speech recognition threshold (SRT) is an important measure, as it validates the pure-tone average (PTA), assists in the diagnosis and prognosis of hearing impairments, and aids in the identification of non-organic hearing impairments. Research has shown that in order for SRT testing to yield valid and reliable measures, testing needs to be performed in the patient's native language. There are currently no published materials for SRT testing in the Samoan language. As a result, audiologists are testing patients with English materials or other materials not of the patient's native language. Results produced from this manner of testing are confounded by the patient's vocabulary knowledge and may reflect a language deficit rather than a hearing loss. The present study is aimed at developing SRT materials for native speakers of Samoan to enable valid and reliable measures of SRT for the Samoan speaking population. This study selected 28 trisyllabic Samoan words that were found to be relatively homogeneous in regard to audibility and psychometric function slope. Data were gathered on 20 normal hearing native speakers of Samoan and the intensity of each selected word was adjusted to make the 50% performance threshold of each word equal to the mean PTA of the 20 research participants (5.33 dB HL). The final edited words were digitally recorded onto compact disc to allow for distribution and use for SRT testing in Samoan.

In addition to the use of pure-tones for testing hearing, speech signals are highly valuable diagnostic tools for identifying and evaluating hearing impairment. Speech audiometry involves the implementation of such signals in the measurement of hearing acuity. One aspect of speech audiometry involves assessment of the speech recognition threshold (SRT) which evaluates an individual's ability to hear and understand speech. While live speech has been used in the past to assess SRT, recorded materials are preferred and have been shown to be advantageous over live speech. High-quality digitally recorded speech audiometry materials have been available in English for some time, but assessment of individuals using speech materials from a language that they do not speak natively has been shown to be both inadequate and inaccurate. Speech audiometry materials have recently become available in many languages. Currently, however, there are no known published recordings for assessment of SRT in the Tagalog language. The goal of this study was to develop psychometrically equivalent speech audiometry materials for measuring speech recognition threshold in Tagalog. During this study Tagalog words were initially recorded by a native speaker selected for accent and vocal quality. The words were reduced down to 90 words to be evaluated in the study. Each of the 90 trisyllabic words were evaluated at 2 dB increments from -10 to 16 dB HL by 20 native Tagalog speakers, all having normal hearing. Based on the results, 34 trisyllabic Tagalog words were selected based on their familiarity to native listeners, relative homogeneity with regards to audibility and psychometric function slope. Each word was then adjusted to make the 50% performance threshold equal to the mean PTA of the 20 research participants (4.3 dB HL). The final edited words were then digitally recorded onto compact disc for distribution and for use in assessing SRT in native Tagalog speakers worldwide.

The purpose of this study was to evaluate an updated list of digitally recorded Speech Recognition Threshold (SRT) materials for test-retest reliability. Chipman (2003) identified 33 psychometrically equated spondaic words that are frequently occurring in English today. These digitally recorded words were used to determine the SRT of 40 participants using the American Speech-Language Hearing Association guidelines. The participants were between the ages of 19 and 83 years and presented with hearing impairment ranging from normal to severe. The individual's pure-tone averages classified 16 participants with normal hearing to slight loss, 12 participants with mild loss, and 12 participants with moderate to severe hearing loss. The speech materials were presented to participants in one randomly selected ear. The SRT was measured for the same ear in both the test and retest conditions. The average SRT for the test condition was 22.7 dB HL and 22.8 dB HL in the retest condition with an improvement of 0.1 dB for retest but no significant difference was identified. Using a modified variance equation to determine test-retest reliability resulted in a 0.98, indicating almost perfect reliability. Therefore the test-retest reliability was determined to be exceptional for the new SRT words.

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