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.     

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.     

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.     

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.     

Performance of directional microphones for hearing aids: real-world versus simulation

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.     

Low-frequency gain compensation in directional hearing aids

Hearing aids currently available on the market with both omnidirectional and directional microphone modes often have reduced amplification in the low frequencies when in directional microphone mode due to better phase matching. The effects of this low-frequency gain reduction for individuals with hearing loss in the low frequencies was of primary interest. Changes in sound quality for quiet listening environments following gain compensation in the low frequencies was of secondary interest. Thirty participants were fit with bilateral in-the-ear hearing aids, which were programmed in three ways while in directional microphone mode: no-gain compensation, adaptive-gain compensation, and full-gain compensation. All participants were tested with speech in noise tasks. Participants also made sound quality judgments based on monaural recordings made from the hearing aid. Results support a need for gain compensation for individuals with low-frequency hearing loss of greater than 40 dB HL.     

方向性麦克风技术在助听器中的应用

引言 目前，助听器的广泛使用已经使广大听力损失患者在很大程度上改善了他们的生活质量，但这些助听器用户在日常生活中还是会有一些聆听上的困难，尤其是在噪声环境下与人交谈。方向性麦克风系统（directional microphones system）也译为指向性麦克风，是用于提高助听器言语信噪比的一项重要技术，尤其能改善在噪声环境下的言语清晰度（可懂度）。本文通过介绍方向性麦克风的原理、作用以及实际应用方面等相关知识来帮助听力从业人员、技术人员和用户更好地了解和使用这项技术。     

The effects of changes in head angle on auditory and visual input for omnidirectional and directional microphone hearing aids

Improving the signal-to-noise ratio (SNR) for individuals with hearing loss who are listening to speech in noise provides an obvious benefit. Although binaural hearing provides the greatest advantage over monaural hearing in noise, some individuals with symmetrical hearing loss choose to wear only one hearing aid. The present study tested the hypothesis that individuals with symmetrical hearing loss fit with one hearing aid would demonstrate improved speech recognition in background noise with increases in head turn. Fourteen individuals were fit monaurally with a Starkey Gemini in-the-ear (ITE) hearing aid with directional and omnidirectional microphone modes. Speech recognition performance in noise was tested using the audiovisual version of the Connected Speech Test (CST v.3). The test was administered in auditory-only conditions as well as with the addition of visual cues for each of three head angles: 0 degrees, 20 degrees, and 40 degrees. Results indicated improvement in speech recognition performance with changes in head angle for the auditory-only presentation mode at the 20 degrees and 40 degrees head angles when compared to 0 degrees. Improvement in speech recognition performance for the auditory + visual mode was noted for the 20 degrees head angle when compared to 0 degrees. Additionally, a decrement in speech recognition performance for the auditory + visual mode was noted for the 40 degrees head angle when compared to 0 degrees. These results support a speech recognition advantage for listeners fit with one ITE hearing aid listening in a close listener-to-speaker distance when they turn their head slightly in order to increase signal intensity.     

Comparison of performance across three directional hearing aids

This study compared the speech recognition performance of 12 hearing-impaired listeners fit with three commercially available behind-the-ear hearing aids in both directional and omnidirectional modes. One digitally programmable analog and two "true digital" hearing aids were selected as test instruments. Testing was completed in both "living room" and anechoic room environments. Speech recognition was examined using modified forms of the Hearing in Noise Test and the Nonsense Syllable Test. The single competing stimuli of these tests were replaced with five uncorrelated competing sources. Results revealed a significant speech recognition in noise advantage for all directional hearing aids in comparison to their omnidirectional counterparts. Maximum performance of the directional hearing aids did not significantly vary across circuit type, suggesting that processing differences did not affect maximum directional hearing aid performance. In addition, the results suggest that performance in one reverberant environment cannot be used to accurately predict performance in an environment with differing reverberation.     

Totally implantable hearing aids: the effects of skin thickness on microphone function

With the advent of totally implantable hearing aids, the question of the impact of the choice of implantation site on microphone function has yet to be fully addressed. We investigated the effects of skin thickness on microphone function over a 2-week period in a porcine model. Sound attenuation was found to be directly proportional to skin thickness within the range of 0.5 to 3.0 mm. A decrease in resonant frequency of 500 Hz was noted after implantation under a skin thickness of 3.0 mm. The general shape of the frequency response curves was maintained across all thicknesses tested. Attenuation (dB loss) across the range of 500 to 8,000 Hz appeared to be linear over the various skin thicknesses measured, with a regression coefficient (r = .99 at 1 kHz and r = .99 at 3 kHz). To minimize attenuation and exploit the sound-collecting qualities of the external ear, the deep meatal skin appears to be a favorable position for implantation.     

Speech understanding and directional hearing for hearing-impaired subjects with in-the-ear and behind-the-ear hearing aids

With respect to acoustical properties, in-the-ear (ITE) aids should give better understanding and directional hearing than behind-the-ear (BTE) aids. Also hearing-impaired subjects often prefer ITEs. A study was performed to assess objectively the improvement in speech understanding and directional hearing afforded by ITE aids versus BTEs. In 28 hearing-impaired subjects, who visited our Centre for a check-up of their ITEs, the following parameters were measured: (a) thresholds for third-octave bandpass noises between 0.25 and 4 kHz, (b) speech reception thresholds for short Dutch sentences in quiet and with background noise, (c) directional hearing. All three experiments were done binaurally with the subjects wearing their ITEs, BTEs, and no hearing aid. With the 2-cc coupler, the gain used by the subject was measured. The SRT values for the ITE were significantly lower than those for BTE. More gain at 2 and 4 kHz in the ITE proved to be a responsible factor for this improvement. Directional hearing was not improved by wearing an ITE. Large interindividual differences were found between functional gain and the 2-cc coupler measurements. The mean functional gain at 4 kHz for an ITE is higher than the gain measured in a 2-cc coupler. For a BTE, the functional gain at 2 and 4 kHz is lower than the 2-cc coupler gain.     

The occlusion effect in unilateral versus bilateral hearing aids

The benefit of bilateral hearing aids is well documented, but many hearing-aid users still wear only one aid. It is plausible that the occlusion effect is part of the reason for some hearing-aid users not wearing both hearing aids. In this study we quantified the subjective occlusion effect by asking ten experienced users of bilateral hearing aids and a reference group of ten normal-hearing individuals to rate the naturalness of their own voice while reading a text sample aloud. The subjective occlusion effect was evaluated in the unilateral versus bilateral condition for a variety of vent designs in earmolds and in a custom hearing aid. The subjective occlusion effect was significantly higher for bilateral hearing aids with all vent designs with the exception of a non-occluding eartip option. The subjective occlusion effect was reduced with the more open vent designs in both the unilateral and bilateral conditions. Assuming that the occlusion effect is a barrier to bilateral hearing aid use, these results indicate that open-hearing-aid fittings can help promote the use of two aids.     

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.