Hearing Aid Microphone Research Articles

A comparison of two methods for estimating sound pressure level at the tympanic membrane at high frequencies for hearing aids.

For hearing aid fittings, it is important to know the delivered sound pressure level (SPL) at the tympanic membrane (TM) over a wide frequency range. Because it is infeasible to measure the SPL at the TM clinically, an indirect estimation based on measurements away from the TM is often used in practice. Furthermore, it is highly desirable that such a method does not require a clinician to use extra accessories to perform extra calibration in a clinic. Two such methods were proposed in the past. One used a set of empirically derived correction factors to estimate the SPL at the TM (Recker et al. (2009), the other used a lumped-element model to estimate the SPL at the TM [LoPresti et al. (2009)]. In this study, both methods were evaluated using actual hearing aid prototypes. An ITC hearing aid was built for each of ten participants. For each participant, the SPL in the ear canal was measured using a hearing aid microphone. The SPL at the TM was estimated using each method separately. The results were compared with the measured SPL at the TM. The relative strength and weakness of each method were discussed with the implications for clinical applications.

Electret condenser microphones in hearing aids.

As sensors small, high-gain devices, hearing aid microphones must be tiny, have high signal-to-noise ratios, be relatively insensitive to vibration from the nearby speaker, and operate at low hearing aid battery voltages. The subminiature backplate-electret condenser microphone was developed in the early 1970s specifically for hearing aids, with a very lightweight mechanical system for lower vibration sensitivity and somewhat improved signal-to-noise compared to previous designs. Although the design has been refined over several generations, the basic technology of electret films has proven to be very robust over the last 40 years, enabling continual size reductions while maintaining high signal-to-noise ratios. Noise generation and vibration sensitivity continue to be competitive dimensions for hearing aid microphones. Some of the mechanisms for noise generation and vibration pickup are intrinsic to the design of these microphones. Other mechanisms may be caused by interaction between the microphones and the hearing aid system of which they are a part. In this presentation, we will discuss both types of mechanisms and some design guidelines for minimal noise application of microphones in hearing aids.

Impact of Visual Cues on Directional Benefit and Preference: Part II—Field Tests

The field tests performed in the current investigation examined how visual cues impact the benefit provided by directional (DIR) microphone hearing aids in the real world. Specifically, the study tested the hypotheses that (1) the provision of visual cues would reduce the preference for DIR processing (re: omnidirectional [OMNI] microphone) and (2) laboratory audiovisual (AV) testing would predict real-world outcomes better than auditory-only testing. The same 24 hearing-impaired adults enrolled in the laboratory testing of this study compared microphone modes (DIR versus OMNI processing) in their everyday activities three times a day for 4 wk using paper and pencil journals. In each comparison, the participants were asked to identify an environment that favored DIR processing (e.g., the talker standing in front of the user and noise at his or her back), listen to speech amid noise via both the DIR and OMNI microphone modes, and then record the preferred microphone mode in the journal. To further understand what the listeners based their preference on, the participants were also asked to provide the reasons for their preferences. Microphone modes were compared when the listeners' eyes were either open or closed. The field results first suggested that OMNI processing was more frequently preferred over DIR processing. Visual cues were not found to have a significant effect on preference for DIR processing. Furthermore, the analysis revealed that when listeners indicated "louder" or "less internal noise" as the reasons for their microphone preference, the likelihood of preferring the OMNI mode increased significantly, suggesting that OMNI processing was preferred for its louder output and lower internal circuit noise level. Finally, the preference score obtained by the laboratory preference judgment task under the AV condition was shown to be the best predictor of microphone preference in the real world. The field data did not reveal the effect of visual cues on preference for DIR processing because preference was not altered by the availability of visual cues. This negative result may be due to the unsuccessful control of visual cues during the field trial. However, the finding that AV laboratory testing predicted field outcomes more accurately than did auditory-only testing demonstrates the role of visual cues in the real world and emphasizes the necessity of using AV testing in the laboratory to evaluate DIR microphone hearing aids. Furthermore, this study demonstrates the importance of factors other than DIR benefit-such as loudness and hearing aid internal noise-in determining preference for microphone mode in the real world.

Impact of Visual Cues on Directional Benefit and Preference: Part I—Laboratory Tests

The purpose of the laboratory tests of the current investigation was to examine how visual cues impact directional (DIR) benefit and preference for the DIR microphone hearing aid (re: the omnidirectional [OMNI] microphone). Specifically, three hypotheses were examined: (1) the presence of visual cues would improve OMNI-aided performance to ceiling levels and therefore reduce DIR benefit and preference, (2) DIR benefit measured in the audiovisual (AV) condition could not be predicted by that measured using auditory-only (AO) testing, and (3) with visual cues, listeners with greater lipreading skills would perceive less DIR benefit than did listeners with lesser lipreading skills. Twenty-four adults with sensorineural hearing loss were recruited. Their speech recognition performances were measured in two hearing aid microphone modes (DIR and OMNI), at various signal-to-noise ratios (SNR, -10 to +10 dB in 4-dB steps) and under two presentation conditions (AV and AO) by using the AV version of the Connected Speech Test. Microphone preference (DIR versus OMNI) was also assessed with and without visual cues at each of the SNRs by using the same Connected Speech Test sentences. Lipreading skills were measured using the Utley test. The speech recognition data revealed that the participants obtained significantly less DIR benefit in the AV condition because their AV performances were at the ceiling level. Consistent with this, the likelihood of preferring DIR processing was significantly reduced when visual cues were available to the listeners. Further, DIR benefit measured in the AV condition was not correlated with that measured in the AO condition while being significantly and negatively correlated with lipreading skill. These results suggest that AO laboratory testing overestimates the DIR benefit and preference for DIR processing that hearing aid users may have in most face-to-face conversations in typical SNR, real-world environments. Additionally, because the DIR benefit measured in the AV condition cannot be predicted by the benefit score obtained from traditional AO laboratory testing, the effect of visual cues should be considered in establishing a valid laboratory/clinical environment and protocol for DIR microphone hearing aid testing. In light of these findings, consultations should be provided for hearing aid users with superior lipreading skills to help them adjust their expectations regarding the possibly small perceivable benefit gained from DIR microphone hearing aids.

Effect of Age on Directional Microphone Hearing Aid Benefit and Preference

Despite the recognition that the directional microphone hearing aid (DMHA) is an important intervention aimed at helping older hearing-impaired adults understand speech in noisy environments, there is little evidence that older listeners can actually benefit from directional processing. The objective of this study was to determine if older and younger adults can obtain and perceive comparable benefit afforded by DMHAs. Twenty-four hearing-impaired adults aged 36 through 79 yr were fit with switchable-microphone hearing aids and tested in the laboratory and the field. In the laboratory, the listeners' directional benefit and preferences for microphone modes (directional vs. omnidirectional) were assessed using various speech-recognition-in-noise tests. In the four-week field trial, a paired-comparison technique and paper-and-pencil journals were used to determine the benefit provided by directional processing. The effects of age on directional benefit/preference were analyzed using generalized linear models with controlling for the effect of hearing loss. The results revealed that age did not have a significant effect on directional benefit and preference as measured in the laboratory. However, the field data showed that older age was significantly associated with a lower preference for the directional mode. These results indicate that although listeners of different ages may obtain and perceive comparable benefit from DMHAs in laboratory testing, older users tend to perceive less benefit than do younger users in the real world. The implications of these findings are discussed.

Methods, devices, and computer program products for operating a mobile device in multiple signal processing modes for hearing aid compatibility

An audio signal received at a mobile device is processed using a first set of equalization parameters in an acoustic coupling mode of operation, and is processed using a second set of equalization parameters in an inductive coupling mode of operation. The processed audio signal is transmitted to a speaker associated with the mobile device for acoustic coupling to a human ear and/or a hearing aid microphone, or for inductive coupling to a hearing aid telecoil.

Method for obtaining real ear measurements using a hearing aid

The invention regards a method for measuring audio response of a real ear using the microphone of a hearing aid of an end user. The following sequence of steps is performed: provide a sound tight connection between the hearing aid microphone and a first end of a soundguiding flexible tube, disable any further microphones in the hearing aid, place a second end of the tube at the location wherefrom sound is to be captured, provide an audio sound signal at the area of the second end of the tube and record the sound guided through the tube to the microphone of the hearing aid, process the recorded sounds to retrieve sound level values. Hereby real ear measurements may be gained with a hearing aid with only one microphone without any additional measuring equipment.

Method and device for matching the phases of microphone signals of a directional microphone of a hearing aid

The phase differences of microphones of a hearing aid microphone are to be reduced. To do this, the level of an output signal of a directional microphone is compared with an omnidirectional signal. If the level of the output signal of the differential directional microphone is above the level of the omnidirectional signal, this level difference is minimized by an adaptive, frequency-selective transit time compensation in individual frequency bands and phase matching of the microphones is thus achieved. By means of an alternative method, microphone matching is achieved in that the measurable delay of the two microphone signals is adaptively limited in individual frequency bands to a maximum value corresponding to the sound transit time between the microphones. Phase matching without knowing the position of a sound source can thus be achieved.

Improved Noise Power Spectrum Density Estimation for Binaural Hearing Aids Operating in a Diffuse Noise Field Environment

The current generation of digital hearing aids allows the implementation of advanced noise reduction schemes. However, most current noise reduction algorithms are monaural and are therefore intended for only bilateral hearing aids. Recently, binaural in contrast to monaural noise reduction schemes have been proposed, targeting future high-end binaural hearing aids. Those new types of hearing aids would allow the sharing of information/signals received from both left and right hearing aid microphones (via a wireless link) to generate an output for the left and right ear. This paper presents a novel noise power spectral density estimator for binaural hearing aids operating in a diffuse noise field environment, by taking advantage of the left and right reference signals that will be accessible, as opposed to the single reference signal currently available in bilateral hearing aids. In contrast with some previously published noise estimation methods for hearing aids or speech enhancement, the proposed noise estimator does not assume stationary noise, it can work for colored noise in a diffuse noise field, it does not require a voice activity detection, the noise power spectrum can be estimated during speech activity or not, it does not experience noise tracking latency and most importantly, it is not essential for the target speaker to be in front of the binaural hearing aid user to estimate the noise power spectrum, i.e., the direction of arrival of the source speech signal can be arbitrary. Finally, the proposed noise estimator can be combined with any hearing aid noise reduction technique, where the accuracy of the noise estimation can be critical to achieve a satisfactory de-noising performance.

Real-world benefit from directional microphone hearing aids

This article summarizes data from a 3-year, double-blinded study of directional hearing aid benefit. Ninety-four subjects in three hearing loss groups, all previous users of omnidirectional output-compression hearing aids, completed all aspects of the study. Participants were fit with new hearing aids for 1 month in a directional mode and 1 month in an omnidirectional mode. Following 1 month of use, subjects completed a number of objective and subjective measures of hearing aid outcome. Objective and subjective data were analyzed across hearing aid and hearing loss conditions. Subjects in all hearing loss groups exhibited better performance in the directional conditions for objective speech-in-noise measures; however, subjective data did not indicate a clear advantage for directional amplification. Results and clinical implications are discussed.

Real‐world effectiveness of directional microphone hearing aids

Single, dual, triple, and array microphone designs that are static, automatic and/or adaptive are now available in marketed hearing aids. The cost of designing and producing these more sophisticated systems is significant; yet, studies of user benefit are often equivocal, particular those involving self‐report or field‐obtained measures. In a series of studies we have attempted to characterize (model) the function of a number of these designs in simple and complex environments in an effort to explain the results obtain in both laboratory and field studies. Rather than directivity limitations, factors such as internal noise, loudness preference and visual cues have all been implicated in the limited user benefit, and will be discussed in this presentation.

Using a signal cancellation technique to assess adaptive directivity of hearing aids

The directivity of an adaptive directional microphone hearing aid (DMHA) cannot be assessed by the method that calls for presenting a "probe" signal from a single loudspeaker to the DMHA that moves to different angles. This method is invalid because the probe signal itself changes the polar pattern. This paper proposes a method for assessing the adaptive DMHA using a "jammer" signal, presented from a second loudspeaker rotating with the DMHA, that simulates a noise source and freezes the polar pattern. Measurement at each angle is obtained by two sequential recordings from the DMHA, one using an input of a probe and a jammer, and the other with an input of the same probe and a phase-inverted jammer. After canceling out the jammer, the remaining response to the probe signal can be used to assess the directivity. In this paper, the new method is evaluated by comparing responses from five adaptive DMHAs to different jammer intensities and locations. This method was shown to be an accurate and reliable way to assess the directivity of the adaptive DMHA in a high-intensity-jammer condition.

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.

Predictive Measures of Directional Benefit Part 2: Verification of Different Approaches to Estimating Directional Benefit

In this investigation, the relation between various directivity measures and subject performance with directional microphone hearing aids was determined. Test devices included first- and second-order directional microphones. Recordings of sentences and noise (Hearing in Noise Test, HINT) were made through each test device in simple, complex, and anisotropic background noise conditions. Twenty-six subjects, with normal hearing, were administered the HINT test recordings and directional benefit was computed. These measures were correlated to theoretical, free-field, and Knowles Electronic Manikin for Acoustic Research (KEMAR) directivity index (DI) values, as well as front-to-back ratios (FBR), in situ signal-to-noise ratios (SNR), and a newly proposed Db SNR, wherein a predictive value of the SNR improvement is calculated as a function of the noise source incidence. The different predictive scores showed high correlation to the measured directional benefit scores in the complex (diffuse-like) background noise condition (r = 0.89 to 0.97, p < 0.05) but not across all background noise conditions (r = 0.45 to 0.97, p < 0.05). The Db SNR approach and the in situ SNR measures provided excellent prediction of subject performance in all background noise conditions (0.85 to 0.97, p < 0.05). None of the predictive measures could account for the effects of reverberation on the speech signal (r = 0.35 to 0.40, p < 0.05). For environments that included a discrete number of noise sources, the in situ SNR and Db SNR estimates were most predictive of subject performance. No predictive approach was indicative of the directional benefit achieved when the speech was also subjected to reverberation (temporal distortion). This finding has implications for real-world estimates of directional benefit.

Micromachined microphones with diffraction-based optical interferometric readout

A diffraction-based optical method for integrated interferometric detection of micromachined microphone diaphragm displacement is described. With multichip optoelectronics integration, this approach yields highly sensitive optical microphones in mm-cube volumes. Since the microphone sensitivity does not depend on capacitance, this method changes the paradigm for the backplate and gap structure design. As a result, one can use millimeter size diaphragms to achieve wide frequency response and low thermal mechanical noise levels characteristic of precision measurement microphones. Furthermore, the electrical port of the device, which is freed by optical detection, is used for electrostatic actuation of the microphone diaphragm to tune microphone sensitivity and to generate self-characterization signals. Prototype optical microphone structures have been fabricated using Sandia National Laboratories’ silicon based SwIFT-LiteTM process. Measurements on these diaphragms show an A-weighted diaphragm displacement noise of 2.4 pm and flat electrostatic response up to 20 kHz. These results indicate the feasibility of realizing measurement microphones with 1.5-mm-diam diaphragms, 15-dBA internal noise, and 40-kHz bandwidth. Application of the detection method in a bio-inspired directional microphone for hearing aids is also discussed. [Work partially supported by NIH Grant 5R01DC005762-03, Sensing and Processing for Hearing Aids.]

Development of novel biologically inspired directional microphones

The development of novel directional microphones for hearing aids is described. The mechanisms underlying the design of these unusual microphones were inspired by our earlier study of the ears of the parasitoid fly Ormia ochracea [Miles, et al., J. Acoust. Soc. Am. 98, 3059–3070 (1995)]. The structure of Ormia’s ears inspired new approaches to the design of directional microphones that have the potential to be more sensitive and have lower thermal noise than typical miniature microphones. The mechanisms for directional hearing in this animal are discussed along with the engineering design concepts that they have inspired. Microphones have been fabricated out of silicon that employ either capacitive sensing or optical sensing to convert the diaphragm motion into an electronic signal. Measured results indicate that the directivity of these microphones is very similar to that of an ideal first-order differential microphone. In addition, novel microphone diaphragms have been fabricated that posses a second-order directional response. These can be used to achieve a significant reduction of unwanted background acoustic noise in hearing aid applications. [Work supported by NIH Grant 5R01DC005762-03, Sensing and Processing for Directional Hearing Aids, to RNM.]

Evaluation of a Second-Order Directional Microphone Hearing Aid: II. Self-Report Outcomes

This clinical trial was undertaken to evaluate the subjective benefit obtained from hearing aids employing automatic switching second-order adaptive directional microphone technology, used in conjunction with digital noise reduction, as compared to a fixed directional microphone or omnidirectional microphone response with the same digital noise reduction. Data were collected for 49 participants across two sites. Both new and experienced hearing aid users were fit bilaterally with behind-the-ear hearing aids using the NAL-NL1 (National Acoustics Laboratory-Nonlinear version 1) prescriptive method with manufacturer default settings for various signal processing (e.g., noise reduction, compression parameters, etc.). During ten days of hearing aid use, participants responded to daily journal questions. Subjective ratings for each of the three hearing aid responses (omnidirectional, automatic-adaptive directional, and automatic-fixed directional) were similar. Overall preference for a microphone condition was equally distributed between no preference, omnidirectional, and automatic adaptive and/or fixed directional.

Evaluation of a Second-Order Directional Microphone Hearing Aid: I. Speech Perception Outcomes

This clinical trial was undertaken to evaluate the benefit obtained from hearing aids employing second-order adaptive directional microphone technology, used in conjunction with digital noise reduction. Data were collected for 49 subjects across two sites. New and experienced hearing aid users were fit bilaterally with behind-the-ear hearing aids using the National Acoustics Laboratory-Nonlinear version 1 (NAL-NL1) prescriptive method with manufacturer default settings for various parameters of signal processing (e.g., noise reduction, compression, etc.). Laboratory results indicated that (1) for the stationary noise environment, directional microphones provided better speech perception than omnidirectional microphones, regardless of the number of microphones; and (2) for the moving noise environment, the three-microphone option (whether in adaptive or fixed mode) and the two-microphone option in its adaptive mode resulted in better performance than the two-microphone fixed mode, or the omnidirectional modes.

For healthier hearing aids and happier patients, use a warm-air drying system

The first author, an audiologist with Newport Audiology Centers, recently completed a 4-year study on the benefits of patients using the Dry & Store drying system. He and his colleagues found that patients who used the moving warm-air drying system averaged a repair every 27.2 months, while patients who did not averaged a repair every 9.7 months. The researchers expected to see fewer repairs in the group using the drying systems. However, they were surprised to find an almost threefold difference between the two groups. In some cases where the drying kits were not used, they noticed a reduction of 4 to 6 dB in the high-frequency response of recently fitted hearing aids—some as soon as 6 months after the fitting. Doug's findings are consistent with what both of us have experienced in our practices. And, if readers are frustrated with the number of repairs you see, we suggest you recommend warm-air drying systems to your patients. THE ROOT OF THE PROBLEM When moisture and cerumen accumulate in hearing aid microphones and receivers, they cause intermittency, increased distortion, reduction in gain (especially high-frequency), and an overall deterioration of performance. The reduction in gain causes speech to be less clear and reduces word understanding. Eliminating or greatly reducing the moisture in hearing aids allows the instruments to provide high-frequency amplification. The microphones in directional hearing aid systems take a huge amount of abuse from moisture. The directionality of these systems depends on the microphones being perfectly calibrated or very close to it. The microphones must be in phase so the difference in “arrival time” between microphones can be determined. Debris and moisture in microphone screens quickly destroy the mics' function. Using a warm-air drying system helps maintain the effectiveness of directional-microphone systems and reduces the frequency of repairs. WHERE THE MOISTURE COMES FROM The human body is composed primarily of water. We excrete large amounts of moisture from the skin each day. In addition, many people exacerbate the problem by letting water enter and become trapped deep in the ear canal when they are taking a bath or shower. The humidity in and around the ear is high, in the 85%-88% range, and this moisture collects in the microphone, receiver ports, and screens, as well as in the battery and amplifier compartments. When the moist, warm air in the ear canal moves outward toward the cooler metal parts of the hearing aids, it deposits microscopic droplets of water, which accumulate over time. Constant exposure to moisture destroys the integrity of the transducers, causing corrosion or rust in the battery compartment and leading to intermittency or shorts in the amplifier. Moisture and debris also penetrate into the volume control, wreaking further havoc. HOW DEBRIS AND MOISTURE INTERACT We all see lots of debris in the microphone and receiver openings of our patients' hearing aids. This debris is composed of dead skin flakes, lint from clothing, contamination from hair and skin products, and other miscellaneous material. The debris absorbs moisture like a sponge. When a wearer removes his or her hearing aids for the night, a sufficient amount of the moisture in the debris usually evaporates that the hearing aids work the next morning. But, as soon as they are placed in (or on) the ears the next morning, the debris at the entrance to the microphone or exit port of the receiver begins absorbing moisture again, resulting either in gain being reduced or amplification stopping altogether. When hearing aids are sent in for repair, the damaged and moisture-laden transducers are usually replaced. Often they feed back when they are refitted on the patient. This occurs because the repaired instruments are providing considerably more amplification in the higher frequencies now that the microphones and receivers are working correctly. BENEFITS OF A DRYING SYSTEM We have found that using moisture-absorbing products such as the Dry & Store offers many benefits, including: greater gain, especially in the crucial high frequencies, clearer sound quality, improved word understanding, including in noise, better localization of sound, reduced need for repairs, less irritation to the ear canal because the UV light in the system kills spores, bacteria, and fungi, a 5%-10% increase in battery life, and fewer patient complaints. In our practices, we urge our patients to purchase and use a warm-air drying system. We recommend that you do so too.

Comparison of adaptive noise reduction algorithms in dual microphone hearing aids

In this paper, a physical and perceptual evaluation of two adaptive noise reduction algorithms for dual-microphone hearing aids are described. This is the first comparison between a fixed directional microphone on the one hand, and an adaptive directional microphone and an adaptive beamformer on the other hand, all implemented in the same digital hearing aid. The adaptive directional microphone is state-of-the-art in most modern commercial hearing aids. The physical evaluation shows the importance of an individual calibration procedure for the performance of the noise reduction algorithms with two microphone hearing aids. The directivity index calculated in anechoic conditions and intelligibility-weighted polar diagrams measured in reverberant conditions show that all the noise reduction strategies yield an improved signal-to-noise ratio (SNR), but that the adaptive beamformer generally performs best. From the perceptual evaluation, it is demonstrated that the adaptive beamformer always performs best in single noise source scenarios. In a more complex noise scenario, there is still a SNR improvement with all the techniques, however the effect is the same for all the strategies.