The selection and validation of output sound pressure level in multichannel hearing aids.

OBJECTIVE: To validate the Australian National Acoustic Laboratories' (NAL) procedure for prescribing output sound pressure level (OSPL) for multichannel hearing aids (Dillon & Storey, 1998) DESIGN: The NAL OSPL prescriptive procedure for multichannel hearing aids was used to calculate Predicted OSPL, Predicted Maximum Acceptable OSPL and Predicted Minimum Acceptable OSPL for 20 subjects with sensorineural hearing loss fitted with a 2-channel linear hearing aid. Subjects rated the speech clarity and quality of average (65 dBA) and loud (80 dBA) speech, in quiet and in noise, with the hearing aid set to a number of OSPL settings. These data were used to evaluate the validity of the Predicted OSPL. Frequency-specific loudness discomfort levels (LDLs) were measured to determine whether use of measured LDLs would improve the accuracy of the prediction. RESULTS: The Predicted Minimum Acceptable OSPL was in good agreement with the measured minimum acceptable OSPL for both the low- and high-frequency channels. The Predicted Maximum Acceptable OSPL was in good agreement with the measured maximum acceptable OSPL for the low-frequency channel, but was only a fair predictor for the high-frequency channel. The use of measured LDLs rather than predicted LDLs did little to improve the accuracy of the fitting. A direct comparison between the NAL single-channel and multichannel prescribed OSPL settings showed that most listeners rated speech clarity higher for the multichannel settings. CONCLUSIONS: In two channel hearing aids, the NAL Predicted Minimum Acceptable OSPL and Predicted Maximum Acceptable OSPL are reasonable predictors of minimum and maximum OSPL levels measured using sound clarity and quality ratings. The results of this study support the use of the NAL prescriptive formula for setting OSPL in multichannel hearing aids. Such settings should be verified by having the listener rate the loudness of an intense speech signal. If tolerance problems are evident, the OSPL in the high-frequency channel(s) should be reduced first.

An examination of the practicality of the simplex procedure.

OBJECTIVE: The practical importance of the simplex procedure, a subjective technique used to refine the frequency gain characteristic (FGC) of a hearing aid according to listener preference, was determined for individual listeners by measuring hearing aid benefit using both laboratory studies and field studies. DESIGN: A digital research hearing aid with two memories was used as the test hearing aid. The modified simplex procedure was used to select the FGC judged to yield the best speech clarity in the presence of low-level vent noise and again in higher-level cafeteria noise by 10 experienced hearing aid users. The FGCs assessed by the listeners varied systematically from The National Acoustic Laboratories Revised (NAL-R) response in the amount of low-frequency or high-frequency amplification. The benefit obtained with these two simplex-selected settings was compared with that obtained using the NAL-R FGC. Measures of benefit included speech recognition testing in the laboratory and ratings of speech intelligibility in the field. In the first field study, the two simplex settings were compared. In the second field study, the simplex-selected setting for higher level noise and the NAL-R setting were compared. RESULTS: In the laboratory, the majority of listeners selected an increase in the low-frequency channel gain compared with the NAL-R. Desired high-frequency channel gain was correlated with degree of hearing loss and type of background noise. The benefit as measured using nonsense syllables did not differ significantly among the three fittings, but differences in benefit were measurable with the rating procedure. Five of eight participants noticed a significant difference in their speech understanding in the real world for the FGCs selected in different background noises. Two of seven participants reported significantly better speech intelligibility with a simplex-selected FGC compared with the NAL-R FGC in the real world. The remaining subjects reported similar speech understanding capabilities with both hearing aid settings. CONCLUSIONS: The majority of subjects included in this study selected an FGC with real ear insertion gain different than the NAL-R prescription to improve subjective speech understanding in the laboratory. A small number of these listeners rated the selected FGC as providing improved speech intelligibility over the NAL-R FGC in the real world. This finding indicates that the simplex procedure should be used selectively to modify the NAL-R prescription. A screening technique would be useful in selecting those who might benefit from a modified fitting. The simplex procedure may also prove to be useful in selecting listeners who would benefit from multiple memory hearing aids.

Response time and word recognition using a Modified-Rhyme monitoring task: list equivalency and time-order effects.

OBJECTIVE: The purpose of this study was to assess list equivalency and time-order effects of word recognition scores and response time measures obtained using a digital recording of the Modified Rhyme Test (MRT) with a response time monitoring task (Mackersie, Neuman, & Levitt, 1999). DESIGN: Response times and percent correct measures were obtained from listeners with normal hearing using the MRT materials presented at a signal to noise ratio of +3 dB. Listeners were tested using a word-monitoring task in which six alternatives were presented in series and listeners pushed a button when they heard the target word (as displayed on the computer monitor). Listeners were tested in two sessions. During each session each of the six MRT lists was administered once. Time-order effects were examined both between and within test sessions. RESULTS: All lists were equivalent for both speech recognition accuracy and response time except List 1, which showed slightly higher percent correct scores than the other lists. Varied patterns of systematic change over time were observed in 75% of the listeners for the response time measures and for 33% of the listeners for the percent correct measures. CONCLUSIONS: Lists 2 through 6 of this version of the MRT are equivalent, with List 1 producing slightly higher word recognition scores. Systematic changes over time in response time data for the majority of listeners suggest the need for careful implementation of the test to avoid time-order effects.

A comparison of response time and word recognition measures using a word-monitoring and closed-set identification task.

OBJECTIVES: The primary purpose of this study was to investigate the possibility of improving speech recognition testing sensitivity by incorporating response time measures as a metric. Two different techniques for obtaining response time were compared: a word-monitoring task and a closed-set identification task. DESIGN: Recordings of the Modified Rhyme Test were used to test 12 listeners with normal hearing. Data were collected using a word-monitoring and a closed-set identification task. Response times and percent correct scores were obtained for each task using signal to noise ratios (SNRs) of -3, 0, +3, +6, +9, and +12 dB. RESULTS: Both response time and percent correct measures were sensitive to changes in SNR, but greater sensitivity was found with the percent correct measures. Individual subject data showed that combining response time measures with percent correct scores improved test sensitivity for the monitoring task, but not for the closed-set identification task. CONCLUSIONS: The best test sensitivity was obtained by combining percent correct and response time measures for the monitoring task. Such an approach may hold promise for future clinical applications.

The effect of compression ratio and release time on the categorical rating of sound quality.

Two experiments were carried out to determine how manipulating the compression ratio and release time of a single-band wide dynamic range hearing aid affects sound quality. In experiment I, compression ratio was varied over the range from linear to 10:1 (low compression threshold, attack time = 5 ms, release time = 200 ms). In experiment II, compression ratios of 1.5, 2, and 3:1 were combined with release times of 60, 200, and 1000 ms (attack time = 5 ms). Twenty listeners with sensorineural hearing loss rated the clarity, pleasantness, background noise, loudness, and the overall impression of speech-in-noise (Ventilation, Apartment, Cafeteria) processed through a compression hearing aid. Results revealed that increasing compression ratio caused decreases in ratings on all scales. Increasing release time caused ratings of pleasantness to increase, and ratings of background noise and loudness to decrease. At the 3:1 compression ratio, increasing the release time caused increases in ratings of clarity, pleasantness, and overall impression, and a decrease in background noise. Significant correlations were found between scales. Regression analysis revealed that the contributions of the scales of clarity, pleasantness, background noise, and loudness to the prediction of overall impression differed as a function of the competing noise condition.

Consonant-vowel intensity ratios for maximizing consonant recognition by hearing-impaired listeners.

The effect of adjusting the consonant-vowel (C-V) intensity ratio on consonant recognition in 18 subjects with sensorineural hearing impairment was investigated. C-V intensity ratios in a set of 48 vowel-consonant nonsense syllables were adjusted in steps of 3-6 dB depending on the subject's dynamic range of hearing. An increase in consonant intensity is referred to here as consonant enhancement (CE). The value of CE producing the highest consonant recognition score (CRmax) is defined as CEmax. Both CEmax and CRmax were determined for each subject for each of the 48 nonsense syllables. Consonant type was found to have a highly significant effect on CRmax, the gain in consonant recognition, and CEmax. The effect of vowel environment was also significant, but of much smaller magnitude. Audiogram configuration was found to have a small effect and was only significant for CRmax. The results of the study also showed that individualized adjustment of the C-V intensity ratio for each subject and consonant-vowel combination can produce substantial improvements in consonant recognition. These data can be used to estimate upper bounds of performance that, in principle, can be obtained by appropriate adjustment of the C-V intensity ratio.

Late-onset auditory deprivation: a review of past research and an assessment of future research needs.

This paper includes an overview of the research on late-onset auditory deprivation, an evaluation of the evidence from retrospective and prospective studies, recommendations for future research, and a consideration of the theoretical and clinical implications of the research. The studies reviewed offer convincing evidence of the late-onset auditory deprivation effect both in groups of listeners and in substantial numbers of individual listeners included in the group studies. The effect appears to be reversible in some cases with the use of amplification in the previously unaided ear. This preliminary evidence supports the recommendation of binaural amplification for persons with bilateral symmetric sensorineural hearing loss. There is much that is still unknown about the deprivation effect and recovery from deprivation. Longitudinal prospective studies are needed to obtain a better understanding of the role of subject-related variables and amplification-related variables on the magnitude and time course of the deprivation effect. Behavioral and electrophysiologic measures of monaural and binaural performance using speech and nonspeech stimuli would also further our understanding of the mechanisms underlying late-onset auditory deprivation.

Effect of release time in compression hearing aids: paired-comparison judgments of quality.

Paired-comparison judgments of quality were obtained from 20 hearing-impaired listeners for speech processed through simulated compression hearing aids varying in release time (60, 200, 1000 ms) at three different compression ratios (1.5, 2, 3:1) and for three different background noises (ventilation, apartment, cafeteria). Analysis revealed that the main effect of release time did not have a significant effect on perceived quality. The interaction between release time and noise type was found to be significant. While no significant difference in preference for release times was evident for the ventilation noise, the longer release times (200 and 1000 ms) were preferred for the higher level noises (apartment noise, cafeteria noise). Post hoc testing revealed that the mean preference scores for the 200- and 1000-ms release time were significantly greater than that of the 60-ms release time with the competing cafeteria noise (p < 0.05). Analysis of individual subject data revealed statistically significant preferences that differed from the group mean, suggesting that individualized fitting of this parameter of a compression hearing aid might be warranted.

Preferred listening levels for linear and slow-acting compression hearing aids.

OBJECTIVE: The purpose of the present experiment was to determine the relationship between most comfortable listening level and preferred listening levels for linear and slow-acting compression hearing aids as a function of variations in speech and noise level. DESIGN: A digital hearing aid test system was used to simulate six hearing aids having compression ratios of 1, 1.5, 2, 3, 5, and 10:1. Speech was presented in three different noises (vent, apartment, and cafeteria), with speech input level being varied (55, 70, 85 dB SPL). Subjects were 20 listeners with sensorineural hearing loss (half with a dynamic range < or = 30 dB and half with a dynamic range >30 dB). The boundaries of the most comfortable listening range were measured to estimate most comfortable listening level. Preferred listening level was measured by having subjects adjust the output of the hearing aid for satisfactory listening. RESULTS: On average, the deviation of preferred listening level from most comfortable loudness (MCL) was less than 5 dB. Dynamic range, noise type, and input level were all found to have small, but significant, effects on the deviation of preferred listening level from MCL. On average, subjects with a small dynamic range listened slightly below MCL, and subjects with a larger dynamic range listened slightly above MCL. For favorable signal-to-noise ratios, preferred listening levels were highest for high input levels and for conditions that resulted in high output levels before level adjustment. Although the pattern of average performance differed slightly at poorer signal-to-noise ratios, all preferred listening levels were close to MCL. CONCLUSIONS: The gain of a slow-acting compression hearing aid should place the output within 5 dB of MCL. The output for low and medium inputs should approximate MCL and the output for high input levels should be slightly above MCL. This pattern of gain may be obtained with mild compression ratios and a gain rule that places a speech input of 70 dB at MCL.

Effect of compression ratio in a slow-acting compression hearing aid: paired-comparison judgments of quality.

Paired-comparison judgments of quality were obtained from 20 hearing-impaired listeners (half with a small dynamic range and half with a large dynamic range) for speech-in-noise (vent, apartment, and cafeteria) processed through a slow-acting compression hearing aid. Compression ratio was varied (1, 1.5, 2, 3, 5, and 10:1). Compression threshold, attack time, and release time were fixed. Sound quality judgments were significantly affected by compression ratio, noise, and dynamic range. Preference decreased with increasing compression ratio. The selection of compression ratio. The selection of compression ratios < or = 2:1 was significantly higher than of compression ratios > 3:1. Less compression (no compression or 1.5:1) was preferred with the highest level noise (cafeteria noise) than with the lower level noises (vent or apartment). In particular, the small dynamic range group preferred compression with the vent and apartment noises (noise below the compression threshold), but preferred a linear hearing aid with the cafeteria noise (above the compression threshold). The large dynamic range group showed a slightly greater preference for the linear hearing aid for all three noises.

Evaluation of orthogonal polynomial compression.

In orthogonal polynomial compression, the short-term speech spectrum is first approximated by a family of orthogonal polynomials. The coefficients of each polynomial, which vary over time, are then adjusted in terms of their average value and range of variation. These adjustments can be used to compress (or expand) temporal variations in the average level, slope, and various forms of curvature of the short-term speech spectrum. The analysis and reconstruction of the short-term speech spectrum using orthogonal polynomials was implemented using a digital master hearing aid. This method of compression was evaluated on eight sensorineurally hearing-impaired listeners. Speech recognition scores were obtained for a range of compression conditions and input levels with and without frequency shaping. The results showed significant advantages over conventional linear amplification when temporal variations in the average level of the short-term spectrum were compressed, a result comparable to that obtained with conventional amplitude compression. A subset of the subjects showed further improvement when temporal variations in spectrum slope were compressed, but these subjects also showed similar improvements when frequency shaping was combined with level-only compression. None of the subjects showed improved speech recognition scores when variations in quadratic curvature were compressed in addition to level and slope compression.

Evaluation of a dereverberation technique.

A two-microphone dereverberation technique was evaluated by obtaining speech recognition measures and preference judgments from normal-hearing and hearing-impaired listeners. Monaural speech recognition performance was measured for two reverberation conditions (0.4 second and 1.2 seconds) with and without processing. Binaural speech recognition performance was also measured for the unprocessed conditions. In addition, paired-comparison judgments of preference were obtained for all combinations of the processed and unprocessed monaural stimuli. For both groups of subjects, scores at the shorter reverberation time were significantly higher than scores for the longer reverberation time. For the normal-hearing subjects, processing to dereverberate had no significant effect on speech recognition performance. Binaural presentation of the unprocessed signal yielded significantly higher scores. For the hearing-impaired subjects, performance was significantly better in the unprocessed condition than the processed condition, but was not significantly different from the binaural condition. Paired-comparison judgments revealed differences in patterns of preference between normal-hearing and hearing-impaired listeners.

An evaluation of three adaptive hearing aid selection strategies.

Paired-comparison judgments of intelligibility of speech in noise were obtained from eight hearing-impaired subjects on a large number of hearing aids simulated by a digital master hearing aid. The hearing aids which comprised a 5 X 5 matrix differed systematically in the amount of low-frequency and high-frequency gain provided. A comparison of three adaptive strategies for determining optimum hearing aid frequency-gain characteristics (an iterative round robin, a double elimination tournament, and a modified simplex procedure) revealed convergence on the same or similar hearing aids for most subjects. Analysis revealed that subjects for whom all three procedures converged on the same hearing aid showed a single pronounced peak in the response surface, while a broader peak was evident for the subjects for whom the three procedures identified similar hearing aids. The modified simplex procedure was found to be most efficient and the iterative round robin least efficient.

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.

Children's perception of speech in reverberation.

Recordings of nonsense syllables (VCV construction) were presented to groups of children aged 5, 7, 9, 11, and 13 years and young adults under monaural (reverberation time = 0.6s) and binaural (reverberation times = 0, 0.4, and 0.6 s) conditions of reverberation. Phoneme identification performance was affected by age, reverberation, and mode of presentation (monaural versus binaural). The major findings were (1) phoneme identification scores in reverberant conditions improved with increasing age and decreased with increased reverberation time; (2) children's performance in reverberant conditions did not reach asymptote until age 13; (3) binaural performance was consistently better than monaural performance for all age groups, with 5-year-olds showing the largest binaural advantage.