A clinical trial of the ReSound IC4 hearing device.

A manufacturer-sponsored clinical trial was conducted of ReSound Corporation's IC4 hearing device (HD), an in-the-ear application of their two-channel, fast-acting, wide-dynamic range compression sound processor. This study was a follow-up to an earlier clinical trial of ReSound's behind-the-ear version of the same sound processor, the BT2 Personal Hearing System (Walden, B. E., Surr, R. K., Cord, M. T., & Pavlovic, C. V. (1998). A clinical trial of the ReSound BT2 Personal Hearing System. American Journal of Audiology, 7, 85-100). Forty adult males with gradually sloping, moderate sensorineural hearing losses participated. All were experienced hearing aid users who wore linear Class D instruments with input compression limiting at the time of their enrollment in this study. The Connected Speech Test, presented at several presentation levels and under various conditions of signal degradation, and the scales and subscales of the Profile of Hearing Aid Benefit were used to evaluate hearing aid performance and benefit under four relatively independent prototype listening situations (Walden, B. E., Demorest, M. E., & Hepler, E. L. (1984). Self-report approach to assessing benefit derived from amplification. Journal of Speech and Hearing Research, 27, 49-56). Aided performance with the IC4 HD was compared with (a) unaided performance, (b) performance of persons with normal hearing, and (c) performance with linear amplification. Participants with hearing loss obtained significant benefit from the IC4 HD, although IC4-aided performance remained well below that of unaided performance of persons with normal hearing, especially on laboratory measures of speech recognition. Furthermore, small mean performance advantages were observed for the IC4 HD compared to linear hearing aids, although there was substantial variability across participants. Finally, when given a choice to either purchase the IC4 HD at a discount from the manufacturer or continue using their own government-issued linear hearing aids, the majority of the participants chose to purchase the IC4 HD.

Effects of temporal smearing on temporal resolution, frequency selectivity, and speech intelligibility.

Envelopes of speech were smeared in 23 parallel frequency channels. The smeared speech was presented to normal-hearing listeners, and the effects of different smearing magnitudes on speech intelligibility were measured by obtaining speech recognition scores. It was demonstrated theoretically and experimentally that the system consisting of the computer smearing and the auditory system had reduced temporal resolution but nearly normal frequency resolution. Speech intelligibility of the processed vowel-consonant nonsense syllables was tested for low- and high-pass filter conditions. The overall speech recognition scores as well as the recognition scores of the consonants grouped according to articulatory features were analyzed. The results indicated that smearing with a narrow temporal window did not degrade speech. The larger equivalent rectangular durations (ERDs) of the resultant temporal window (RTW) of the combined system (temporal smearing plus auditory system) produced a small but significant reduction in speech intelligibility for the low-pass filter condition. Scores for the RTWs > 16 ms were significantly different from the score for the 7.7-ms RTW for the high-pass filter condition, but this effect was small and did not differ across articulatory features.

Band importance functions for audiological applications.

Recently, band importance functions have been developed for a number of speech tests used in audiology. These functions, as well as the importance functions for average (everyday) speech, are also being considered for the inclusion in the revised Articulation Index standard (submitted for vote to the Acoustical Society of America). In this paper, the band importance functions for different speech materials (usually reported in literature for 1/3 octave bands) have been recalculated to correspond to frequency bands normally used in audiological applications. In addition, criteria for selecting appropriate importance functions and transfer functions are discussed.

Development of Phonetically Balanced Phrase Lists Typical of Communication in Noisy Work Environments.

The existing speech corpora are of limited value in assessing the combined effects of hearing loss, hearing protection, and noise on peech intelligibility in the work place. One reason for this is the differences between the acoustic characteristics typical of the elevated vocal effort in the work place and those typical of the normal vocal effort used in recording various existing speech corpora. The other reason for the limited value of the existing tests is related to the differences in nonacoustic factors (linguistic structure, predictability of the messages, etc.) between the available speech corpora and the actual messages used in the work place. In this study a speech test designed for use in industrial work places has been developed. The test material consists of eight 20-phrase lists. The lists are phonetically balanced and of approximately equal difficulty. The variability among lists is greatest at very low sensation levels and decreases progressively as the sensation level increases.

Addendum to "transfer functions and correction factors used in hearing aid evaluation and research".

In an earlier publication, various transformations used in hearing aid research and its application were summarized. As a result of continued interest and requests, additional transfer functions are provided in this addendum.

Reliability, sensitivity and validity of magnitude estimation, category scaling and paired-comparison judgements of speech intelligibility by older listeners.

This study investigated the reliability, sensitivity, and validity of speech intelligibility judgments for hearing aid evaluation. Subjects aged 60-87 years judged the intelligibility of sentences using either magnitude estimation, category scaling or paired comparisons. The 60+ age group was chosen as representative of the majority of hearing aid wearers. Speech recognition scores for Central Institute for the Deaf (CID) sentences and Northwestern University Auditory Test No. 6 (NU-6) words were also obtained. The speech was bandpass filtered using filter settings that produce a monotonic increase in predicted intelligibility based on articulation index theory. Speech recognition scores and intelligibility judgments were obtained for each of eight filter conditions. Test-retest reliability was poorest for paired comparisons and CID sentence scores. There were no differences in sensitivity among the three psychophysical procedures. Intelligibility judgments and NU-6 scores were more sensitive than CID sentence scores to differences among conditions. The results indicated that intelligibility judgments are valid measures of speech recognition.

Speech intelligibility considerations in specifying characteristics of a programmable hearing aid.

With the use of sophisticated speech processing techniques in auditory prosthetic devices, it will be possible to convert the speech signal to maximal satisfaction for the user. However, the user's satisfaction is a multidimensional percept that may include speech naturalness, intelligibility, pleasantness, etc. This paper discusses the place of speech intelligibility considerations in the ensemble of criteria that could be associated with these perceptual attributes. It also discusses some of the problems that may be responsible for the relative inadequacy of the available speech intelligibility prediction procedures for use in digital hearing aids.

A signal processing scheme for output limitation.

For both normal listeners and mild to moderately sensori-neurally impaired (probable etiology, presbycusis) listeners a summation of discomfort (S) occurs for complex stimuli, proportional to the logarithm of the number of components. Regression equations of S = 2.05 + 11.51 log n for normals and S = 3.95 + 12.88 log n for hearing impaired (where n equals the number of components or bandwidth in the complex) were found to be significantly different. This change in percept with increased bandwidth (or components) has not been satisfactorily explained in terms of power or amplitude peak density of the stimuli. The regression equations could be applied to future digital hearing aids so that maximum output levels (MPO) would account for this summation.

Use of the magnitude estimation technique for assessing the performance of text-to-speech synthesis systems.

As text-to-speech systems develop, it becomes necessary to compare various solutions and to evaluate whether a change in the synthesis procedure has an effect on the listener's attitude to the system. The possibility of directly scaling intelligibility, naturalness, and user's satisfaction (i.e., acceptability) with the magnitude estimation technique is investigated. A magnitude estimation protocol suitable for this purpose is described. In general, within the limits of the methodological constraints discussed in this paper, the procedure appears to be reliable and valid for quantifying the perceived attributes of synthesized speech.

Comparison of discomfort levels obtained with pure tones and multitone complexes.

The relationship between threshold of discomfort (TD) estimates and the number of components in a complex signal has been investigated. The thresholds of discomfort were first obtained for 16 pure tones located at the center frequency of critical bands from 250 to 4000 Hz. Subsequently, thresholds of discomfort were obtained for 2, 4, 8, and 16 tone complexes. The pure-tone components of the complexes were systematically selected from the same 16 pure tones. For each subject, the relative intensities of the components in the four complexes were determined in such a way so as to parallel the pure tone TD contour obtained for that subject. Data were obtained from 15 normal and 15 hearing impaired adults. The individuals in the latter group all had mild to moderate sensorineural hearing loss. Summation of discomfort (S) was defined as the difference between the threshold of discomfort for a pure tone presented in isolation and within the complex. The two groups demonstrated different summation values. For both groups, however, the summation was shown to be a linear function of the logarithm of the number of components in the complex: S = a + b log (n) where n is the number of components (2, 4, 8, 16). For the normal hearing group, a and b are 2.05 and 11.51, respectively, while for the hearing impaired group, they are 3.95 and 12.88, respectively. While the future digital hearing aids can easily regulate their limiting levels so as to accurately account for this summation, present day hearing aids may underestimate this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfer functions and correction factors used in hearing aid evaluation and research.

In hearing aid research and application it is often necessary to transform the sound pressure level or acoustic gain measured at one physical point (e.g., 2 cc coupler) to an equivalent value at another physical point (e.g., tympanic membrane). Results of currently available studies on various transformations are summarized in this report. When no data were available, the desired transfer function was calculated as a linear combination of known contributing transfer functions. Values are given for the center frequencies of 1/3 octaves and critical bands.

Speech spectrum considerations and speech intelligibility predictions in hearing aid evaluations.

This report deals with the issue of the amplified and unamplified speech spectra as they relate to the residual auditory range. The methods for predicting intelligibility of both amplified and unamplified speech are discussed. Based on Pavlovic (1987) an articulation index-based procedure applicable to clinical data is derived (A1). A simplified version (A0) is also proposed.

Frequency importance functions for a feature recognition test material.

The relative importance of different parts of the auditory spectrum to recognition of the Diagnostic Rhyme Test (DRT) and its six speech feature subtests was determined. Three normal hearing subjects were tested twice in each of 70 experimental conditions. The analytical procedures of French and Steinberg [J. Acoust. Soc. Am. 19, 90-119 (1947)] were applied to the data to derive frequency importance functions for each of the DRT subtests and the test as a whole over the frequency range 178-8912 Hz. For the DRT as a whole, the low frequencies were found to be more important than is the case for nonsense syllables. Importance functions for the feature subtests also differed from those for nonsense syllables and from each other as well. These results suggest that test materials loaded with different proportions of particular phonemes have different frequency importance functions. Comparison of the results with those from other studies suggests that importance functions depend to a degree on the available response options as well.

Derivation of primary parameters and procedures for use in speech intelligibility predictions.

The literature on various parameters that appear in the articulation index-type calculations of speech intelligibility is reexamined. Based on the reported data, the best estimates of these parameters and the most appropriate procedures for their use are suggested. These included: (1) the analysis and specification of the importance of various frequency bands to speech intelligibility; (2) the procedures used for measuring threshold and the calculation of threshold-based parameters used for predicting intelligibility of low-level speech; and (3) the calculation and measurement of relevant speech parameters. All results are given so that the calculations can be performed either in critical bands, 1/3 octaves, or octaves.

A frequency importance function for continuous discourse.

Normal hearing subjects estimated the intelligibility of continuous discourse (CD) passages spoken by three talkers (two male and one female) under 135 conditions of filtering and signal-to-noise ratio. The relationship between the intelligibility of CD and the articulation index (the transfer function) was different from any found in ANSI S3.5-1969. Also, the lower frequencies were found to be relatively more important for the intelligibility of CD than for identification of nonsense syllables and other types of speech for which data are available except for synthetic sentences [Speaks, J. Speech Hear. Res. 10, 289-298 (1967)]. The frequency which divides the auditory spectrum into two equally important halves (the crossover frequency) was found to be about 0.5 oct lower for the CD used in this study than the crossover frequency for male talkers of nonsense syllables found in ANSI S3.5-1969 and about 0.7 oct lower than the one for combined male and female talkers of nonsense syllables reported by French and Steinberg [J. Acoust. Soc. Am. 19, 90-119 (1947)].

An articulation index based procedure for predicting the speech recognition performance of hearing-impaired individuals.

An articulation index calculation procedure developed for use with individual normal-hearing listeners [C. Pavlovic and G. Studebaker, J. Acoust. Soc. Am. 75, 1606-1612 (1984)] was modified to account for the deterioration in suprathreshold speech processing produced by sensorineural hearing impairment. Data from four normal-hearing and four hearing-impaired subjects were used to relate the loss in hearing sensitivity to the deterioration in speech processing in quiet and in noise. The new procedure only requires hearing threshold measurements and consists of the following two modifications of the original AI procedure of Pavlovic and Studebaker (1984): The speech and noise spectrum densities are integrated over bandwidths which are, when expressed in decibels, larger than the critical bandwidths by 10% of the hearing loss. This is in contrast to the unmodified procedure where integration is performed over critical bandwidths. The contribution of each frequency to the AI is the product of its contribution in the unmodified AI procedure and a "speech desensitization factor." The desensitization factor is specified as a function of the hearing loss. The predictive accuracies of both the unmodified and the modified calculation procedures were assessed by comparing the expected and observed speech recognition scores of four hearing-impaired subjects under various conditions of speech filtering and noise masking. The modified procedure appears accurate for general applications. In contrast, the unmodified procedure appears accurate only for applications where results obtained under various conditions on a single listener are compared to each other.

An evaluation of some assumptions underlying the articulation index.

The purposes of this investigation were to derive measurement and calculation procedures for the articulation index (AI) that were as precise and accurate as possible, and to evaluate the significance of the differences in assumptions made by early investigators. A nonsense syllable test specifically designed for this application was recorded. The implications of various assumptions utilized in different approaches to predicting speech sound identification were tested using 13 different conditions which differed with respect to filtering, level of interfering noise, or level of the speech signal. The listeners were three normal hearing individuals. A set of preferred assumptions was chosen based on a post hoc evaluation of the data. Predictions of speech sound identification were good, both for the average listener and for individual listeners.

Use of the articulation index for assessing residual auditory function in listeners with sensorineural hearing impairment.

Two experiments were performed to explore the feasibility of deriving an articulation index (AI) based scheme for quantifying residual auditory function in listeners with sensorineural hearing impairment. In the first experiment the AI procedure was applied to audiograms of normal and sensorineural hearing-impaired individuals in order to predict speech discrimination under various conditions. Good predictions were found for the normal and the less impaired subjects, but not for those with greater impairments. These subjects exhibited a disproportionate loss in speech discrimination compared to that predicted on the basis of the AI procedure. In the second experiment it was investigated whether the AI predictions for presbycusic individuals could be improved by making use of the proficiency factor that is not a function of frequency. The results indicate that the deficit in suprathreshold speech processing is frequency-specific and that, therefore, the proficiency factor is not independent of frequency. It assumes the value of 1 at those frequencies where hearing sensitivity is normal and is substantially reduced for frequencies where hearing threshold is elevated.