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Biomedical subjects

H Levitt

Publications and source records attributed to H Levitt.

At least 55 records · Page 3Linked to original sources

Towards a general measure of distortion.

An approach to the measurement of distortion in hearing aids is presented. Distortion is first defined in general terms. The technique is then applied to the specific problem of predicting the detectability of non-linear distortion. Distortion indices have been obtained for two common, but very different forms of distortion: peak clipping (common to most conventional hearing aids); and, quantization (a form of distortion common to digital systems including digital hearing aids). Distortion indices were derived for the two forms of distortion in steady-state vowels. The results showed good agreement between the distortion indices at the just-detectable level of distortion.

Acoustics↗

Experiments with a programmable master hearing aid.

A series of experiments is described tracing the development and application of adaptive paired-comparison testing to the prescriptive fitting of hearing aids. The equipment needed to implement the test procedures became progressively more complex with each new experiment, leading to the development of a digital master hearing aid.

Adult↗

Effect of two-microphone noise reduction on speech recognition by normal-hearing listeners.

An idealized 2-channel noise reducing adaptive filter of the type developed by Widrow requires that one channel contain noise only and that the microphones be fixed in position relative to the signal and noise sources. These conditions are unlikely to be met in a wearable hearing aid. In a typical situation, the microphones will be mounted in close proximity on a moving head in a room that is moderately reverberant. Experimental data have been obtained showing that, despite these deviations from the ideal conditions, significant improvements in speech intelligibility can be obtained using 2-channel adaptive filtering.

Acoustics↗

Log-linear modeling of consonant confusion data.

Log-linear models, in conjunction with the G2 statistic, were developed and applied to several existing sets of consonant confusion data. Significant interactions of consonant error patterns were found with signal-to-noise ratio (S/N), presentation level, vowel context, and low-pass and high-pass filtering. These variables also showed significant interactions with error patterns when categorized on the basis of feature classifications. Patterns of errors were significantly altered by S/N for place of articulation (front, middle, back), voicing, frication, and nasality. Low-pass filtering significantly affected error patterns when categorized by place of articulation, duration, or nasality; whereas, high-pass filtering only affected voicing and frication error patterns. This paper also demonstrates the utility of log-linear modeling techniques in applications to confusion matrix analysis: specific effects can be tested; variant cells in a matrix can be isolated with respect to a particular model of interest; diagonal cells can be eliminated from the analysis; and the matrix can be collapsed across levels of variables, with no violation of independence. Finally, log-linear techniques are suggested for development of parsimonious and predictive models of speech perception.

Humans↗

Evaluation of a cochlear prosthesis using connected discourse tracking.

A multichannel cochlear prosthesis was evaluated using the method of Connected Discourse Tracking. Data were obtained from five subjects over a 10-week period. Significant learning effects were obtained both with and without the prosthesis. The method of orthogonal polynomials was used to obtain a statistically precise fit for each learning curve. The curves differed both in terms of shape and average rate of learning. The two best subjects showed substantial improvements, reaching tracking rates in excess of 90 words-per-minute. A method for representing prosthesis-based improvements, which takes learning effects into account, is developed and discussed.

Analysis of Variance↗

A digital master hearing aid.

The use of computer simulation in evaluating conventional and experimental hearing aids is described. Two illustrative examples are provided. The first involves the simulation of a conventional master hearing aid and its application in evaluating different adaptive strategies in the prescriptive fitting of hearing aids. The second example involves the simulation of an experimental hearing aid embodying modern digital signal-processing techniques for the reduction of background noise. A high-speed array processor is used in order to accomplish these simulations in real time.

Computers↗

Predicting consonant confusions from acoustic analysis.

Acoustic measurements of nonsense syllables in quiet and in noise were used to predict the pattern of consonant confusions made under those conditions. Eleven sets of nonsense syllables were presented to normal-hearing subjects in quiet and at a +5 dB speech-to-noise ratio, at five speech levels. A set of acoustic characteristics of the speech stimuli were chosen for analysis and measured using digital processing techniques. Results of the recognition task revealed significant effects of consonant voicing, position and vowel context on syllable recognition. The performance-intensity function of the quiet condition rises more steeply than the function obtained in noise. The effect of noise on consonant recognition is dependent upon the manner in which the consonant is produced, and the location of maximum constriction. Differences in the absolute values of the acoustic parameters of syllable pairs were used to predict their percentage of confusion. A set of acoustic variables was isolated which was found to be the best predictor of confusion percentages. Although the sets of acoustic variables were different for various syllable types and test conditions, three variables (consonant energy, consonant spectral peaks, consonant-to-noise ratio) were used in a majority of the predictions.

Adult↗

Evaluation of a protocol for fitting a wearable master hearing aid.

An experimental protocol for the prescriptive fitting of a wearable master hearing aid was evaluated. The protocol consisted of four stages: (i) acquisition of baseline audiometric data, (ii) fixed test battery using speech-in-noise as the test material, (iii) adaptive adjustments, and (iv) evaluative comparative data. The results showed that systematic improvements in hearing-aid performance were obtained as a result of using the experimental protocol and that the largest improvements were obtained during the fixed-test-battery stage. The protocol in its present form is not clinically practical for most situations.

Audiometry↗

The structure of segmental errors in the speech of deaf children.

A quantitative description of the segmental errors occurring in the speech of deaf children is developed. The method of description separates out those errors or characteristics that are common to all deaf speakers from those that are common to different subpopulations of the deaf (e.g., severely versus profoundly hearing impaired) from those errors or characteristics that are unique to individual speakers. Using this technique, the pattern of segmental errors is specified for several groups of children including deaf children at a school for the deaf as well as hearing-impaired children who have been mainstreamed into the regular school system.

Adolescent↗

Computer applications in audiology and rehabilitation of the hearing impaired.

Computers are playing an ever-increasing role in audiology. The potential applications of computers in audiology, speech pathology, and aural rehabilitation are described. These include the use of computers in adaptive testing, speech analysis and synthesis, automatic extraction of speech features, supplemental speechreading aids, analysis and synthesis of speechreading cues, language analysis, and computer simulation of the effect of specific speech training strategies.

Audiology↗

The effect of timing errors on the intelligibility of deaf children's speech.

The purpose of this study was to quantify the effect of timing errors on the intelligibility of deaf children's speech. Deviant timing patterns were corrected in the recorded speech samples of six deaf children using digital speech processing techniques. The speech waveform was modified to correct timing errors only, leaving all other aspects of the speech unchanged. The following six-stage approximation procedure was used to correct the deviant timing patterns: (1) original, unaltered utterances, (2) correction of pauses only, (3) correction of relative timing, (4) correction of absolute syllable duration, (5) correction of relative timing and pauses, and (6) correction of absolute syllable duration and pauses. Measures of speech intelligibility were obtained for the original and the computer-modified utterances. On the average, the highest intelligibility score was obtained when relative timing errors only were corrected. The correction of this type of error improved the intelligibility of both stressed and unstressed words within a phrase. Improvements in word intelligibility, which occurred when relative timing was corrected, appeared to be closely related to the number of phonemic errors present within a word. The second highest intelligibility score was obtained for the original, unaltered sentences. On the average, the intelligibility scores obtained for the other four forms of timing modification were poorer than those obtained for the original sentences. Thus, the data show that intelligibility improved, on the average, when only one type of error, relative timing, was corrected.

Adolescent↗

A note on simultaneous and interleaved masking.

Speech and noise were switched periodically from ear to ear at a rate high enough to create a binaurally fused image. Two masking conditions were investigated. In one the speech and noise were switched in unison from ear to ear such that both speech and noise were presented simultaneoully to the same ear. In the other case, the speech and noise were interleaved such that there was no simultaneous overlap of the speech and noise at any one ear. The difference in masking levels between these two conditions was found to be on the order of 20 dB. The technique offers some interesting possibilities for investigating the relative effects of central and peripheral masking in both normal and hearing-impaired listeners.

Acoustic Stimulation↗