Faculty Bibliography

As advanced signal processing algorithms have been proposed to enhance hearing protective device (HPD) performance, it is important to determine how directional microphones might affect the localization ability of users and whether they might cause safety hazards. The effect of in-the-ear microphone directivity was assessed by measuring sound source identification of speech in the horizontal plane. Recordings of speech in quiet and in noise were made with Knowles Electronic Manikin for Acoustic Research wearing bilateral in-the-ear hearing aids with microphones having adjustable directivity (omnidirectional, cardioid, hypercardioid, supercardioid). Signals were generated from 16 locations in a circular array. Sound direction identification performance of eight normal hearing listeners and eight hearing-impaired listeners revealed that directional microphones did not degrade localization performance and actually reduced the front-back and lateral localization errors made when listening through omnidirectional microphones. The summed rms speech level for the signals entering the two ears appear to serve as a cue for making front-back discriminations when using directional microphones in the experimental setting. The results of this study show that the use of matched directional microphones when worn bilaterally do not have a negative effect on the ability to localize speech in the horizontal plane and may thus be useful in HPD design

OBJECTIVE: The purpose of this study was to compare the accuracy of sound-direction identification in the horizontal plane by bilateral cochlear implant users when localization was measured with pink noise and with speech stimuli. DESIGN: Eight adults who were bilateral users of Nucleus 24 Contour devices participated in the study. All had received implants in both ears in a single surgery. Sound-direction identification was measured in a large classroom by using a nine-loudspeaker array. Localization was tested in three listening conditions (bilateral cochlear implants, left cochlear implant, and right cochlear implant), using two different stimuli (a speech stimulus and pink noise bursts) in a repeated-measures design. RESULTS: Sound-direction identification accuracy was significantly better when using two implants than when using a single implant. The mean root-mean-square error was 29 degrees for the bilateral condition, 54 degrees for the left cochlear implant, and 46.5 degrees for the right cochlear implant condition. Unilateral accuracy was similar for right cochlear implant and left cochlear implant performance. Sound-direction identification performance was similar for speech and pink noise stimuli. CONCLUSIONS: The data obtained in this study add to the growing body of evidence that sound-direction identification with bilateral cochlear implants is better than with a single implant. The similarity in localization performance obtained with the speech and pink noise supports the use of either stimulus for measuring sound-direction identification

Until recently, researchers used behavioral measures of identification and discrimination of speech and nonspeech stimuli to assess the effects of auditory deprivation, enhancement, and training. Recent advances in our ability to measure electrical activity in the auditory system in response to sound have made it possible for us to study how changes in auditory input (because of hearing loss, auditory input modification, or training) affect the function of the central auditory system. This article reviews the evidence of changes in the auditory cortex in mature animals and in humans with acquired sensorineural hearing loss as well as changes associated with auditory training in persons with normal hearing. The results of studies that measure psychoacoustic and speech-recognition performance of persons with hearing loss, with and without hearing aids, are interpreted within the framework of our new knowledge about plasticity of the auditory system. Applications of electrophysiologic techniques to hearing aid research and clinical practice are highlighted

The purpose of this study was to assess the accuracy of clinical and laboratory measures of directional microphone benefit. Three methods of simulating a noisy restaurant listening situation ([1] a multimicrophone/multiloudspeaker simulation, the R-SPACE, [2] a single noise source behind the listener, and [3] a single noise source above the listener) were evaluated and compared to the 'live' condition. Performance with three directional microphone systems differing in polar pattern (omnidirectional, supercardioid, and hypercardioid array) and directivity indices (0.34, 4.20, and 7.71) was assessed using a modified version of the Hearing in Noise Test (HINT). The evaluation revealed that the three microphones could be ordered with regard to the benefit obtained using any of the simulation techniques. However, the absolute performance obtained with each microphone type differed among simulations. Only the R-SPACE simulation yielded accurate estimates of the absolute performance of all three microphones in the live condition. Performance in the R-SPACE condition was not significantly different from performance in the 'live restaurant' condition. Neither of the single noise source simulations provided accurate predictions of real-world (live) performance for all three microphones

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

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

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

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