Directional microphone hearing aids (2003)

FigureOtto Hassler received the first U.S. patent for the directional microphone in 1969 (application dated December 1966). His company, Willco of Germany, was affiliated with Maico Hearing Instruments. The first directional microphone was introduced with the Willco “Diskriminator 1H45” in 1969 and, in May 1971, Maico released its “Mark 100” with the same microphone. With the typical front and rear sound ports, this microphone was reported to contain “a tiny honeycomb array of hundreds of microscopic sized glass tubes (8.5 mm long x 0.1 mm diameter).”1 The tubes served to delay slightly the sounds coming into the rear port, thus creating a phase reversal with the sound entering the front port. Research done at that time documented that a directional microphone built into a hearing aid and mounted on the side of the head lost some of its directional characteristics, a design and measurement issue still faced today. Similarly, the early research acknowledged the negative impact of reverberation on directional benefit, another challenge that continues to confront us. Nonetheless, the design was successfully marketed and, by the early 1980s, had found its way into in-the-ear instruments. Yet, only a limited number of companies manufactured directional microphone hearing aids, and eventually interest in this feature waned with the introduction of even smaller hearing aids and a multitude of programmable features. In the mid-90s, the introduction of the two-microphone design seemed to generate renewed interest and increased acceptance of this “higher tech” version of the old design. And, as the quest for the perfect signal processing scheme intensified in the late 90s, it became increasingly clear that the one scheme that did work was the directional microphone. Horse hair and camel hair were originally used as the “delay” element for early prototypes to provide acoustic resistance to sounds from the background. Modern versions now employ anything from fine metal mesh screen to electrical communication networks to achieve the same outcome, that is, to reduce the level of the background interference inputs relative to the front-facing inputs. Add to that the innovative algorithms for adapting the placement of the null(s) with additional noise-reduction solutions, and we find a myriad of options available to the consumer. Interest in this feature continues to grow as researchers, clinicians, and consumers strive to better understand the benefits and limitations of the technology. AN OVERVIEW OF the TECHNOLOGY In this special issue of The Hearing Journal, a group of experts has provided an overview of design and applications that should serve as a valuable resource for practitioners in their clinical decision making, as well as for students preparing to begin their careers in audiology. Steve Thompson begins with a tutorial on design issues. Whether the hearing aid implements a single-microphone design or a multiple-microphone design, issues of low-frequency compensation, sensitivity drift, internal noise, and even wind noise reduction need consideration. Andrew Dittberner follows with an overview of the problems associated with quantifying the directivity of the directional microphone system. Laboratory measures of the directivity index (DI) offer an acoustical method of quantifying reduction for signals arriving off-axis or angles other than those directly in front. Clinical measures of front-to-back ratio (FBR) provide a simplified measure of gain reduction, as a function of frequency. But where and how that measurement of directivity is made can have a direct impact on the apparent effectiveness of the directional microphone system; further, how the dispenser interprets the quality of the product based on those measures can have direct implications for the ultimate outcome and satisfaction of the hearing aid user. Todd Ricketts provides an excellent explanation of factors that can also affect the measured benefit. From microphone port orientation to earmold venting, from environmental to human differences, what we measure in the laboratory is not always realized in real-world use. On that topic, Brian Walden, Rauna Surr, and Mary Cord provide some provocative data from their research group at Walter Reed. That “…directional microphones work better in the test booth than they do in everyday listening situations…” might not be a great surprise to many. But, along with those findings, the authors offer important recommendations to clinicians on how to help patients realize the potential benefits that do exist with current designs. And, lest we limit our knowledge base to current schemes, Jan Wouters introduces some additional signal processing applications that deserve consideration. Finally, Andrew Dittberner and I speculate on the future trends of this directional microphone option. Whether it is in the casing, in the spacing, or in the algorithm design, directional microphone options continue to evolve. All the contributors to this special edition have spent considerable time compiling information and data to support their points of view. The result is an up-to-date compendium of those issues and efforts relative to directional microphone hearing aids that can be used by every hearing healthcare provider. Some say we are limited only by our imagination in this field. We have terrific signal processing potential and continue to expand our possibilities with each generation of hearing aid design. At the same time, the ears (and the head, for that matter) have size and spatial orientation limitations that will require consideration, as we move forward to increase the likelihood of improved speech understanding in adverse listening conditions. As we understand the human auditory system more completely and find an increasing number of hardware and software solutions for the problem of listening in noise, those of us involved in hearing healthcare have reason to be optimistic. ABOUT THE EDITOR Ruth A. Bentler, PhD, guest editor of this special issue of The Hearing Journal, is one of the nation's leading hearing aid researchers, as well as a practicing clinician at the University of Iowa. Dr. Bentler, a professor in the Department of Speech Pathology and Audiology, is also director of the university's Hearing Aid Laboratory for Basic and Applied Research, which boasts one of the few anechoic chambers in the U.S. Our guest editor has received numerous grants from non-profit organizations and manufacturers to conduct investigations at her lab of new amplification technology. One current study, commissioned by the Retirement Research Foundation, is particularly relevant to this special issue, since its topic is Effectiveness of Directional Microphone Designs. Along with researching and fitting hearing aids, our guest editor has written extensively about them, including in four book chapters and in many of her dozens of articles in refereed journals. She has spoken at more than 25 international conferences around the world and more than 120 national meetings. For each of the past several years, she has gone to China to teach university courses on hearing aids in Chengdu and Hong Kong. Ruth Bentler was a member of the American Speech-Language-Hearing Association Ad Hoc Committee on Hearing Aid Selection and Fitting and, for the past 2 years, was in charge of amplification papers at the ASHA Convention. Currently, she is an editorial board member or consultant for Journal of the American Academy of Audiology, Ear & Hearing, Trends in Amplification, Journal of the Acoustical Society of America, Journal of Speech and Hearing Research, and the International Journal of Audiology. Among the honors she has received is the Editors Award from ASHA, where she was also named a Fellow.