Ear asymmetries and asymmetric directional microphone hearing aid fittings.

PURPOSE: To determine whether an asymmetry between ears for speech understanding in noise was related to performance with, or preference for, 1 of 2 asymmetric microphone fittings in which omnidirectional processing was provided to 1 ear and directional processing to the other. METHOD: Twenty-eight adults with symmetric sensorineural hearing impairment were recruited from the clinic population. Sixteen individuals had symmetric hearing-in-noise ability between their right and left ears, and 12 participants had an asymmetry for speech understanding in noise between ears. A repeated measures design was used. Interactions between various microphone fittings and speech signal locations in noise were assessed in the laboratory. In addition, the listeners with asymmetry between ears for hearing in noise completed a field trial comparing the 2 fittings in everyday listening situations. RESULTS: Laboratory testing resulted in different patterns of performance for the 2 groups. Field trial results revealed that participants generally noticed little difference between the 2 fittings in everyday life and did not express a strong preference for 1 fitting over the other. CONCLUSION: An asymmetry between ears for speech understanding in noise did not result in preference for 1 asymmetric fitting over the other in everyday listening situations.

An objective measure for selecting microphone modes in OMNI/DIR hearing aid circuits.

OBJECTIVES: Studies have shown that listener preferences for omnidirectional (OMNI) or directional (DIR) processing in hearing aids depend largely on the characteristics of the listening environment, including the relative locations of the listener, signal sources, and noise sources; and whether reverberation is present. Many modern hearing aids incorporate algorithms to switch automatically between microphone modes based on an analysis of the acoustic environment. Little work has been done, however, to evaluate these devices with respect to user preferences, or to compare the outputs of different signal processing algorithms directly to make informed choices between the different microphone modes. This study describes a strategy for automatically switching between DIR and OMNI microphone modes based on a direct comparison between acoustic speech signals processed by DIR and OMNI algorithms in the same listening environment. In addition, data are shown regarding how a decision to choose one microphone mode over another might change as a function of speech to noise ratio (SNR) and spatial orientation of the listener. DESIGN: Speech and noise signals were presented at a variety of SNR's and in different spatial orientations relative to a listener's head. Monaural recordings, made in both OMNI and DIR microphone processing modes, were analyzed using a model of auditory processing that highlights the spectral and temporal dynamics of speech. Differences between OMNI and DIR processing were expressed in terms of a modified spectrotemporal modulation index (mSTMI) developed specifically for this hearing aid application. Differences in mSTMI values were compared with intelligibility measures and user preference judgments made under the same listening conditions. RESULTS: A comparison between the results of the mSTMI analyses and behavioral data (intelligibility and preference judgments) showed excellent agreement, especially in stationary noise backgrounds. In addition, the mSTMI was found to be sensitive to changes in SNR as well as spatial orientation of the listener relative to signal and noise sources. Subsequent mSTMI analyses on hearing aid recordings obtained from real-life environments with more than one talker and modulated noise backgrounds also showed promise for predicting the preferred microphone setting in varied and complex listening environments.

Evaluation of a "direct-comparison" approach to automatic switching in omnidirectional/directional hearing aids.

BACKGROUND: Hearing aids today often provide both directional (DIR) and omnidirectional (OMNI) processing options with the currently active mode selected automatically by the device. The most common approach to automatic switching involves "acoustic scene analysis" where estimates of various acoustic properties of the listening environment (e.g., signal-to-noise ratio [SNR], overall sound level) are used as a basis for switching decisions. PURPOSE: The current study was carried out to evaluate an alternative, "direct-comparison" approach to automatic switching that does not involve assumptions about how the listening environment may relate to microphone preferences. Predictions of microphone preference were based on whether DIR- or OMNI-processing of a given listening environment produced a closer match to a reference template representing the spectral and temporal modulations present in clean speech. RESEARCH DESIGN: A descriptive and correlational study. Predictions of OMNI/DIR preferences were determined based on degree of similarity between spectral and temporal modulations contained in a reference, clean-speech template, and in OMNI- and DIR-processed recordings of various listening environments. These predictions were compared with actual preference judgments (both real-world judgments and laboratory responses to the recordings). DATA COLLECTION AND ANALYSIS: Predictions of microphone preference were based on whether DIR- or OMNI-processing of a given listening environment produced a closer match to a reference template representing clean speech. The template is the output of an auditory processing model that characterizes the spectral and temporal modulations associated with a given input signal (clean speech in this case). A modified version of the spectro-temporal modulation index (mSTMI) was used to compare the template to both DIR- and OMNI-processed versions of a given listening environment, as processed through the same auditory model. These analyses were carried out on recordings (originally collected by Walden et al, 2007) of OMNI- and DIR-processed speech produced in a range of everyday listening situations. Walden et al reported OMNI/DIR preference judgments made by raters at the same time the field recordings were made and judgments based on laboratory presentations of these recordings to hearing-impaired and normal-hearing listeners. Preference predictions based on the mSTMI analyses were compared with both sets of preference judgments. RESULTS: The mSTMI analyses showed better than 92% accuracy in predicting the field preferences and 82-85% accuracy in predicting the laboratory preference judgments. OMNI processing tended to be favored over DIR processing in cases where the analysis indicated fairly similar mSTMI scores across the two processing modes. This is consistent with the common clinical assignment of OMNI mode as the default setting, most likely to be preferred in cases where neither mode produces a substantial improvement in SNR. Listeners experienced with switchable OMNI/DIR hearing aids were more likely than other listeners to favor the DIR mode in instances where mSTMI scores only slightly favored DIR processing. CONCLUSIONS: A direct-comparison approach to OMNI/DIR mode selection was generally successful in predicting user preferences in a range of listening environments. Future modifications to the approach to further improve predictive accuracy are discussed.

The robustness of hearing aid microphone preferences in everyday listening environments.

Automatic directionality algorithms currently implemented in hearing aids assume that hearing-impaired persons with similar hearing losses will prefer the same microphone processing mode in a specific everyday listening environment. The purpose of this study was to evaluate the robustness of microphone preferences in everyday listening. Two hearing-impaired persons made microphone preference judgments (omnidirectional preferred, directional preferred, no preference) in a variety of everyday listening situations. Simultaneously, these acoustic environments were recorded through the omnidirectional and directional microphone processing modes. The acoustic recordings were later presented in a laboratory setting for microphone preferences to the original two listeners and other listeners who differed in hearing ability and experience with directional microphone processing. The original two listeners were able to replicate their live microphone preferences in the laboratory with a high degree of accuracy. This suggests that the basis of the original live microphone preferences were largely represented in the acoustic recordings. Other hearing-impaired and normal-hearing participants who listened to the environmental recordings also accurately replicated the original live omnidirectional preferences; however, directional preferences were not as robust across the listeners. When the laboratory rating did not replicate the live directional microphone preference, listeners almost always expressed no preference for either microphone mode. Hence, a preference for omnidirectional processing was rarely expressed by any of the participants to recorded sites where directional processing had been preferred as a live judgment and vice versa. These results are interpreted to provide little basis for customizing automatic directionality algorithms for individual patients. The implications of these findings for hearing aid design are discussed.

Field evaluation of an asymmetric directional microphone fitting.

Laboratory evidence suggests that an asymmetric microphone fitting (omnidirectional processing in one ear and directional processing in the other) can provide a directional advantage in background noise that is as great, or nearly as great, as that provided by binaural directional processing (Bentler et al, 2004). The present study investigated whether the potential benefit of an asymmetric fitting observed in the laboratory extends to real-life listening. Specifically, ease of listening was compared across a variety of real-life listening situations for asymmetric microphone fittings and bilateral omnidirectional processing. These ratings were compared to determine whether the asymmetric fitting provided an advantage in listening situations in which directional microphone processing is generally preferred and/or a disadvantage in listening situations in which omnidirectional microphone processing is generally preferred. Results suggest that an asymmetric fitting may be a viable option for patients who cannot or do not switch microphone modes.

Effect of signal-to-noise ratio on directional microphone benefit and preference.

This study examined speech intelligibility and preferences for omnidirectional and directional microphone hearing aid processing across a range of signal-to-noise ratios (SNRs). A primary motivation for the study was to determine whether SNR might be used to represent distance between talker and listener in automatic directionality algorithms based on scene analysis. Participants were current hearing aid users who either had experience with omnidirectional microphone hearing aids only or with manually switchable omnidirectional/directional hearing aids. Using IEEE/Harvard sentences from a front loudspeaker and speech-shaped noise from three loudspeakers located behind and to the sides of the listener, the directional advantage (DA) was obtained at 11 SNRs ranging from -15 dB to +15 dB in 3 dB steps. Preferences for the two microphone modes at each of the 11 SNRs were also obtained using concatenated IEEE sentences presented in the speech-shaped noise. Results revealed that a DA was observed across a broad range of SNRs, although directional processing provided the greatest benefit within a narrower range of SNRs. Mean data suggested that microphone preferences were determined largely by the DA, such that the greater the benefit to speech intelligibility provided by the directional microphones, the more likely the listeners were to prefer that processing mode. However, inspection of the individual data revealed that highly predictive relationships did not exist for most individual participants. Few preferences for omnidirectional processing were observed. Overall, the results did not support the use of SNR to estimate the effects of distance between talker and listener in automatic directionality algorithms.

Relationship between laboratory measures of directional advantage and everyday success with directional microphone hearing aids.

The improvement in speech recognition in noise obtained with directional microphones compared to omnidirectional microphones is referred to as the directional advantage. Laboratory studies have revealed substantial differences in the magnitude of the directional advantage across hearing-impaired listeners. This investigation examined whether persons who were successful users of directional microphone hearing aids in everyday living tended to obtain a larger directional advantage in the test booth than persons who were unsuccessful users. Results revealed that the mean directional advantage did not differ significantly between patients who used the directional mode regularly and those who reported little or no benefit from directional microphones in daily living and, therefore, tended to leave their hearing aids set in the default omnidirectional mode. Success with directional microphone hearing aids in everyday living, therefore, cannot be reliably predicted by the magnitude of the directional advantage obtained in the clinic.

Predicting hearing aid microphone preference in everyday listening.

Seventeen hearing-impaired adults were fit with omnidirectional/directional hearing aids, which they wore during a four-week trial. For each listening situation encountered in daily living during a total of seven days, participants selected the preferred microphone mode and described the listening situation in terms of five environmental variables, using a paper and pencil form. Results indicated that hearing-impaired adults typically spend the majority of their active listening time in situations with background noise present and surrounding the listener, and the signal source located in front and relatively near. Microphone preferences were fairly evenly distributed across listening situations but differed depending on the characteristics of the listening environment. The omnidirectional mode tended to be preferred in relatively quiet listening situations or, in the presence of background noise, when the signal source was relatively far away. The directional mode tended to be preferred when background noise was present and the signal source was located in front of and relatively near the listener. Results suggest that knowing only signal location and distance and whether background noise is present or absent, omnidirectional/directional hearing aids can be set in the preferred mode in most everyday listening situations. These findings have relevance for counseling patients when to set manually switchable omnidirectional/directional hearing aids in each microphone mode, as well as for the development of automatic algorithms for selecting omnidirectional versus directional microphone processing.

Identifying dead regions in the cochlea: psychophysical tuning curves and tone detection in threshold-equalizing noise.

OBJECTIVE: Recent studies indicate that high-frequency amplification may provide little benefit for listeners with moderate-to-severe high-frequency hearing loss, and may even reduce speech recognition. Moore and colleagues have proposed a direct link between this lack of benefit and the presence of regions of nonfunctioning inner hair cells (dead regions) in the basal cochlea and have suggested that psychophysical tuning curves (PTCs) and tone detection thresholds in threshold-equalizing noise (TEN) are psychoacoustic measures that allow detection of dead regions ([Moore, Huss, Vickers, Glasberg, & Alcántara, 2000]; [Vickers, Moore, & Baer, 2001]). The experiments reported here examine the consistency of TEN and PTC tasks in identifying dead regions in listeners with high-frequency hearing loss. DESIGN: Seventeen listeners (18 ears) with steeply sloping moderate-to-severe high-frequency hearing loss were tested in PTC and TEN tasks intended to identify ears with high-frequency dead regions. In the PTC task, pure-tone signals of fixed level were masked by narrowband noise that slowly increased in center frequency. For a range of signal frequencies, noise levels at masked threshold were determined as a function of masker frequency. In the TEN task, masked thresholds for pure-tone signals were determined for a fixed-level, 70 dB/ERB TEN masker (for some listeners, 85 or 90 dB/ERB TEN was also tested at selected probe frequencies). RESULTS: TEN and PTC results agreed on the presence or absence of dead regions at all tested frequencies in 10 of 18 cases (approximately 56% agreement rate). Six ears showed results consistent with either mid- or high-frequency dead regions in both tasks, and four ears did not show evidence of dead regions in either task. In eight ears, the TEN and PTC tasks produced conflicting results at one or more frequencies. In instances where the TEN and PTC results disagreed, the TEN results suggested the presence of dead regions whereas the PTC results did not. CONCLUSIONS: The 56% agreement rate between the TEN and PTC tasks indicates that at least one of these tasks was only partially reliable as a diagnostic tool. Factors unrelated to the presence of dead regions may contribute to excess masking in TEN without producing tip shifts in PTCs. Thus it may be appropriate to view tuning curve results as more reliable in cases where TEN and PTC results disagree. The current results do not provide support for the TEN task as a reliable diagnostic tool for identification of dead regions.

Influence of environmental factors on hearing aid microphone preference.

The purpose of this study was to identify characteristics of everyday listening situations that influence user preferences for omnidirectional versus directional hearing aid microphones. Eleven experienced hearing aid users were fitted with digital hearing aids featuring switchable omnidirectional (OMNI) and adaptive-directional (DIR) modes (programs). For 6 weeks, their task was to identify and describe at least one listening situation each day in which one program performed better than the other using a checklist daily journal format. All participants reported difficulty identifying situations in which they could perceive a difference between the two microphone modes. Although an equal number had been requested, descriptions favoring the DIR outnumbered those for the OMNI. Chi-square tests were used to compare the distributions of 60 descriptions favoring the OMNI and 155 favoring the DIR across variables associated with the primary talker to whom the hearing aid user was listening, background noise, and other environmental characteristics. The results indicated that location of the primary talker, presence or absence and type of background noise, and type of space in which the communication occurred influenced microphone choice.

Performance of directional microphone hearing aids in everyday life.

This study explored the use patterns and benefits of directional microphone technology in real-world situations experienced by patients who had been fitted with switchable omnidirectional/directional hearing aids. Telephone interviews and paper-and-pencil questionnaires were used to assess perceived performance with each microphone type in a variety of listening situations. Patients who used their hearing aids regularly and switched between the two microphone configurations reported using the directional mode, on average, about one-quarter of the time. From brief descriptions, patients could identify listening situations in which each microphone mode should provide superior performance. Further, they reported encountering listening situations in which an omnidirectional microphone should provide better performance more frequently than listening situations in which the directional microphones should be superior. Despite using the omnidirectional mode more often and encountering situations in which an omnidirectional microphone should provide superior performance more frequently, participants reported the same level of satisfaction with each microphone type.

A Clinical Trial of the ReSound BT2 Personal Hearing System.

The Food and Drug Administration requires that hearing aid manufacturers substantiate benefit claims in advertising with clinical research. Recently, Walden (1997) described a model protocol that might be used to assess hearing aid benefit in manufacturer-sponsored clinical trials. The Walter Reed protocol includes laboratory measures of speech recognition ability using the Continuous Speech Test (CST, Cox, Alexander, & Gilmore, 1987; Cox, Alexander, Gilmore, & Pusakulick, 1988) and the scales and subscales of the Profile of Hearing Aid Benefit (PHAB, Cox & Gilmore, 1990) to assess user benefit in four prototype listening environments.A clinical trial of the ReSound BT2 Personal Hearing System (BT2 PHS) using the Walter Reed protocol is reported here. The results for 40 adult participants with moderate-to-severe acquired sensorineural hearing losses revealed significant benefit from the BT2 PHS as compared to unaided performance on most of the CST and PHAB measures.Compared to performance (unaided) of persons with normal hearing, the individuals with hearing impairments obtained substantially poorer performance on the CST and reported slightly poorer BT2-aided performance on the PHAB. Finally, on average, participants reported significantly more success on the PHAB with the BT2 PHS as compared to their own linear hearing aids, and 70% of the participants preferred the BT2 PHS enough to be willing to purchase it rather than to continue to use their own government-issued linear hearing aids.