Efficacy of directional microphone hearing aids: a meta-analytic perspective

The literature suggests that directional microphone hearing aids (DMHAs) are a viable means for improving the signal-to-noise ratio (SNR) for hearing-impaired listeners. The amount of directional advantage they provide, however, remains relatively unclear because of variability observed among individual studies. The present investigation was undertaken in an attempt to establish the degree of advantage provided by DMHAs. Data were synthesized from 72 and 74 experiments, respectively, on omnidirectional hearing aids and DMHAs representing both favorable and unfavorable outcomes. Using a meta-analytic approach, 138 weighted averages were derived for a variety of comparable independent and dependent variables. Comparisons were made for hearing-impaired and normal-hearing listeners. Findings are discussed with regard to their clinical and research implications.     

Making sense of directional microphone hearing aids

We have witnessed a large increase in the availability of directional microphone hearing aids over the past few years. Directional microphone technology is now available in analog, digitally controlled analog, and digital hearing aids, and has been implemented into both behind-the-ear and in-the-ear styles. This Short Course reviews basic design differences across directional microphone hearing aids. A number of different laboratory and clinical evaluation methods used for assessment of both electroacoustic and behavioral directivity are then reviewed. In addition, the potential impact of test conditions such as room reverberation and type and position of competing noise(s), on listener performance when fit with directional hearing aids are considered. Recommendations and suggestions relating to the clinical and laboratory assessment of directional hearing aids are provided.     

Evaluation of an adaptive directional system in a DSP hearing aid

The effectiveness of an adaptive directional microphone design, as implemented in the Phonak Claro behind-the-ear hearing aid, is evaluated. Participants were fit bilaterally and tested in 2 environments, an anechoic chamber and a moderately reverberant classroom, with the microphones in the fixed (cardioid) setting and the adaptive setting. Five speakers were placed between 110 degrees and 250 degrees azimuth around the listener. Speech-weighted noise was presented from those speakers at an overall level (OAL) of 65 dB (A). Noise was increased by 8 dB from 1 speaker at a time, using 2-s modulation and random assignment, while the output from the other speakers was reduced to maintain the constant OAL. Results of 2 speech perception tasks used as outcome measures indicated that the adaptive system was not able to follow the dominant noise source in the presence of lower level noise sources. Self-report measures obtained after blinded home trials were consistent with laboratory findings that the participants did not perceive this adaptive microphone design to be more effective than the default fixed-microphone option.     

Directivity of different hearing aid microphone locations

In this study, we present a new method to derive a single-number measure of the directivity of hearing aids. The method is an extension of the conventional directivity index (DI), and is called overall directivity index (ODI). The directivities of five different hearing aid styles were compared with that of the open ear. The behind-the-ear (BTE) style showed the lowest directionality compared to the other hearing aid styles and the open ear. There were only minor differences in directivity between any of the four different hearing aid styles placed in the ear and of the open ear canal. The conventional measure of DI is less suitable for hearing aids, because it ignores sound coming from other than the frontal direction. To quantify directionality of a hearing aid in a real-life situation, we suggest the single-number ODI, weighted with the articulation index and related to a listening segment of 0-30 degrees. An application of the data is shown for a BTE with a directional microphone.     

The development of a biologically-inspired directional microphone for hearing aids

The development of novel micro-fabrication techniques for producing a directional microphone for hearing aids is here described. The mechanisms underlying both the structure and function of these unusual microphones were originally inspired by the ears of an inconspicuous insect, the parasitoid fly Ormia ochracea. The structure of Ormia's ears inspired new approaches to design directional microphones that are more sensitive and have lower thermal noise than that typical of those using traditional approaches. The mechanisms for directional hearing in this animal are discussed along with the engineering design concepts that they have inspired, because they illustrate how basic research can inspire technology development-translational research. However, to realize the potential of bio-exploitation this microphone diaphragm concept would have been very difficult to realize without the availability of new silicon micro-fabrication technologies. Thus, this report can be viewed as an example of what may be possible with the application of new fabrication methods to microphones. Challenges and opportunities provided by the use of silicon micro-fabrication technology for microphones are discussed.     

Impact of compression and hearing aid style on directional hearing aid benefit and performance.

OBJECTIVE: To evaluate the impact of low-threshold compression and hearing aid style (in-the-ear [ITE] versus behind-the-ear [BTE]) on the directional benefit and performance of commercially available directional hearing aids. DESIGN: Forty-seven adult listeners with mild-to-moderate sensorineural hearing loss were fit bilaterally with one BTE and four different ITE hearing aids. Speech recognition performance was measured through the Connected Speech Test (CST) and Hearing in Noise Test (HINT) for a simulated noisy restaurant environment. RESULTS: For both the HINT and CST, speech recognition performance was significantly greater for subjects fit with directional in comparison with omnidirectional microphone hearing aids. Performance was significantly poorer for the BTE instrument in comparison with the ITE hearing aids when using omnidirectional microphones. No differences were found for directional benefit between compression and linear fitting schemes. CONCLUSIONS: No systematic relationship was found between the relative directional benefit and hearing aid style; however, the speech recognition performance of the subjects was somewhat predictable based on Directivity Index measures of the individual hearing aid models. The fact that compression did not interact significantly with microphone type agrees well with previously reported electroacoustic data.     

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.     

The WHO-DAS II: measuring outcomes of hearing aid intervention for adults

The World Health Organization's Disability Assessment Scale II (WHO-DAS II) is a generic health-status instrument that provides six domain scores and a total, aggregate score. Two of the domain scores, communication and participation, and the total score, have good validity, internal-consistency reliability, and test-retest stability in individuals with adult-onset hearing loss. As such, these two domain scores and the total WHO-DAS II score may be useful as generic outcome measures to assess the effectiveness of hearing aid intervention for this population. Before the use of the WHO-DAS II in hearing aid clinical trials, however, the responsiveness of the instrument and the short- and long-term outcomes to hearing aid intervention had to be determined. Responsiveness and outcomes were assessed in 380 veterans (approximately half received hearing aids and half served as controls) by examining group differences, effect-size estimates, and individual differences as a function of hearing aid intervention. For comparison, data also were obtained on two disease-specific measures, the APHAB and the HHIE. The WHO-DAS II communication domain and total scores were sufficiently responsive to hearing aid intervention for use in future studies in which group differences are to be detected. The WHO-DAS II participation domain was not sufficiently responsive to hearing aid intervention. The APHAB and HHIE, both disease-specific measures, were more sensitive to hearing aid intervention than the generic measure. The short- and long-term outcomes of hearing aid intervention were also examined in the present study. Group outcomes for hearing aid intervention can be expected to be stable for at least 6 months when measured by WHO-DAS II total score and for at least 12 months when measured by the WHO-DAS II communication domain scores. Effect-size estimates and examination of the number of individuals exhibiting change scores exceeding 90% critical differences for true changes in scores indicate that for clinical applications, disease-specific instruments are more useful than the WHO-DAS II. The findings of this study support the use of the WHO-DAS II as a generic measure in hearing aid trials research so as to allow for comparisons of health-status outcomes across different diseases or disorders.     

Unaided and aided performance with a directional open-fit hearing aid

Differences in performance between unaided and aided performance (omnidirectional and directional) were measured using an open-fit behind-the-ear (BTE) hearing aid. Twenty-six subjects without prior experience with amplification were fitted bilaterally using the manufacturer's recommended procedure. After wearing the hearing aids for one week, the fitting parameters were fine-tuned, based on subjective comments. Four weeks later, differences in performance between unaided and aided (omnidirectional and directional) were assessed by measuring reception thresholds for sentences (RTS in dB), using HINT sentences presented at 0 degrees with R-Space restaurant noise held constant at 65dBA and presented via eight loudspeakers set 45 degrees apart. In addition, the APHAB was administered to assess subjective impressions of the experimental aid. Results revealed that significant differences in RTS (in dB) were present between directional and omnidirectional performance, as well as directional and unaided performance. Aided omnidirectional performance, however, was not significantly different from unaided performance. These findings suggest for the hearing aids and experimental condition used in this study, a patient would require directional microphones in order to perform significantly better than unaided or aided with omnidirectional microphones, and that performance with an omnidirectional microphone would not be significantly better than unaided. Finally, the APHAB-aided scores were significantly better than unaided scores for the EC, BN, RV, and AV subscales indicating the subjects, on average, perceived the experimental aid to provide significantly better performance than unaided, and that aided performance was more aversive than unaided.     

Unaided and aided performance with a directional open-fit hearing aid

Differences in performance between unaided and aided performance (omnidirectional and directional) were measured using an open-fit behind-the-ear (BTE) hearing aid. Twenty-six subjects without prior experience with amplification were fitted bilaterally using the manufacturer's recommended procedure. After wearing the hearing aids for one week, the fitting parameters were fine-tuned, based on subjective comments. Four weeks later, differences in performance between unaided and aided (omnidirectional and directional) were assessed by measuring reception thresholds for sentences (RTS in dB), using HINT sentences presented at 0° with R-Space^TM restaurant noise held constant at 65dBA and presented via eight loudspeakers set 45° apart. In addition, the APHAB was administered to assess subjective impressions of the experimental aid.

Results revealed that significant differences in RTS (in dB) were present between directional and omnidirectional performance, as well as directional and unaided performance. Aided omnidirectional performance, however, was not significantly different from unaided performance. These findings suggest for the hearing aids and experimental condition used in this study, a patient would require directional microphones in order to perform significantly better than unaided or aided with omnidirectional microphones, and that performance with an omnidirectional microphone would not be significantly better than unaided. Finally, the APHAB-aided scores were significantly better than unaided scores for the EC, BN, RV, and AV subscales indicating the subjects, on average, perceived the experimental aid to provide significantly better performance than unaided, and that aided performance was more aversive than unaided.     

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.     

Full time directional versus user selectable microphone modes in hearing aids

OBJECTIVE: The purpose of this experiment was to systematically examine hearing aid benefit as measured by speech recognition and self-assessment methods across omnidirectional and directional hearing aid modes. These data were used to compare directional benefit as measured by speech recognition in the laboratory to hearing aid wearer's perceptions of benefit in everyday environments across full-time directional, full-time omnidirectional, and user selectable directional fittings. Identification of possible listening situations that resulted in different self reported hearing aid benefit as a function of microphone type was a secondary objective of this experiment. DESIGN: Fifteen adults with symmetrical, sloping sensorineural hearing loss were fitted bilaterally with in-the-ear (ITE) directional hearing aids. Measures of hearing aid benefit included the Profile of Hearing Aid Benefit (PHAB), the Connected Sentence Test (CST), the Hearing in Noise Test (HINT), and a daily use log. Additionally, two new subscales were developed for administration with the PHAB. These subscales were developed to specifically address situations in which directional hearing aids may provide different degrees of benefit than omnidirectional hearing aids. Participants completed these measures in three conditions: omnidirectional only (O), directional only with low-frequency gain compensation (D), and user-selectable directional/omnidirectional (DO). RESULTS: Results from the speech intelligibility in noise testing indicated significantly more hearing aid benefit in directional modes than omnidirectional. PHAB results indicated more benefit on the background noise subscale (BN) in the DO condition than in the O condition; however, this directional advantage was not present for the D condition. Although the reliability of the newly proposed subscales is as yet unknown, the data were interpreted as revealing a directional advantage in situations where the signal of interest was in front of the participant and a directional disadvantage in situations where the signal of interest was behind the listener or localization was required. CONCLUSIONS: Laboratory directional benefit is reflected in self-assessment measures that focus on listening in noise when the sound source of interest is in front of the listener. The use of a directional hearing aid mode; however, may have either a positive, a neutral, or a negative impact on hearing aid benefit measured in noisy situations, depending on the specific listening situation.     

Advantages of directional hearing aid microphones related to room acoustics.

In this study, two types of hearing aids were used. Both aids had the same frequency characteristics for frontal sound, but one employed an omnidirectional microphone and the other a directional microphone. The frequency characteristics of both hearing aids were measured for five azimuths on KEMAR and in situ in 12 normal-hearing subjects. For these subjects we also determined the speech reception threshold (SRT) with background noise in two rooms with different reverberation times. The direction of the speech stimuli was always frontal; the direction of the noise was varied. Additionally, directional hearing was measured with short noise bursts from eight loudspeakers surrounding the subject. In the less reverberant room, sounds coming from behind were less amplified by the hearing aid with the directional microphone than by the one with the omnidirectional microphone. In this room the monaural SRT values were largely determined by the level of the background noise. For the directional hearing aids there was an extra binaural advantage which depended on the direction of the background noise. Only for low-frequency noise bursts was directional hearing better with directional hearing aids. In the more reverberant room, no distinct differences between the frequency characteristics of the two hearing aid types were measured. However, a systematic difference between monaural SRT values measured through the two hearing aids was found. This difference was independent of noise azimuth. In conclusion, hearing aid(s) with a directional microphone showed no disadvantages and clear advantages under specific conditions.     

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.     

Acceptable noise level as a measure of directional hearing aid benefit

An acceptable noise level (ANL) procedure for measuring hearing aid directional benefit was compared with masked speech reception threshold (SRT) and front-to-back ratio (FBR) procedures. ANL is the difference between the most comfortable listening level and the maximum accepted background noise level while listening to speech. Forty adult subjects wearing their own binaural hearing aids were evaluated in omnidirectional and directional modes. The subjects were fitted with a variety of hearing aids by clinical audiologists, independent of the study. For each procedure, speech and noise were presented through loudspeakers located at 0 degrees and 180 degrees azimuth, respectively. Mean ANL (3.5 dB), SRT (3.7 dB), and FBR (2.9 dB) directional benefits were not significantly different. The ANL and masked SRT benefits were significantly correlated. The ANL appears to be a quick, clinician/user friendly procedure for measuring hearing aid directional benefit.     

Predicting directional hearing aid benefit for individual listeners

The fitting of directional microphone hearing aids is becoming increasingly more routine, and this fitting option has proven to be a successful method to improve speech intelligibility in many noisy listening environments. Data suggest, however, that some hearing-impaired listeners receive significantly more directional benefit than others. It is of interest, therefore, to determine if directional benefit is predictable from identifiable audiologic factors. In this report, we examined whether the slope of audiometric configuration, amount of high-frequency hearing loss, and/or the aided omnidirectional performance for a speech-in-noise intelligibility task could be used to predict the magnitude of directional hearing aid benefit. Overall results obtained from three separate investigations revealed no significant correlation between the slope of audiometric configuration or amount of high-frequency hearing loss and the benefit obtained from directional microphone hearing instruments. Although there was a significant, negative relationship between aided omnidirectional performance and the directional benefit obtained in one study, there was considerable variability among individual participants, and nearly all of the listeners with the best omnidirectional hearing aid performance still received significant additional benefit from directional amplification. These results suggest that audiologists should consider the use of directional amplification for patients regardless of audiogram slope, high-frequency hearing loss, or omnidirectional speech intelligibility score.