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Hearing Journal:
doi: 10.1097/01.HJ.0000423563.14554.01
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Breaking News: The Path from Analog to Digital: 65 Years of Advances in Audiology

Levitt, Harry PhD

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Dr. Levitt is the director of research at Advanced Hearing Concepts, Inc., in Bodega Bay, CA.

Audiology has benefitted substantially from technological advances; think of how profoundly the invention of the electronic amplifier and audiometer affected the field. These inventions not only helped advance audiology but served as a catalyst for changing our thinking about fundamental issues in audiology.

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The use of an audiometer in place of a tuning fork to evaluate hearing loss requires a mindset change, but this change seldom occurs immediately. It was widely believed, when electronic amplification was first introduced, that the correct form of acoustic amplification was to provide gain equal to the hearing loss, such as mirroring the audiogram. Electronic amplification assisted in amplifying signals to an uncomfortably loud level, thought to demonstrate reduced dynamic range in sensorineural hearing loss. It took some time, however, before it was recognized that reduced dynamic range, rather than an elevated hearing threshold, was the core problem.

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Advances for APD

It also took time before auditory processing disorders were recognized as a specific disorder. Soon after the development of electronic audiometry there were many reports of cases where speech understanding was much worse than expected from the audiogram. APDs are now recognized as a specific disorder and not just another manifestation of hearing loss. Even though evidence of this disorder was apparent in the early days of electronic audiometry, it took almost half a century for it to be recognized as such with concomitant changes in our thinking regarding its treatment.

A related issue concerns the difference between auditory processing disorders and auditory processing deficits, which does not appear to be widely recognized. The disorders are usually identified in childhood while deficits are exhibited by almost everyone over 60. The distinction between them is important. A substantial body of experimental data shows that older people with audiologically normal hearing have significantly poorer spectral and temporal resolution than young people with normal hearing. Reduced spectral and temporal resolution is an auditory processing deficit, but because it is common in virtually all older people and regarded as a normal part of aging. A person who exhibits reduced auditory processing substantially worse than others of the same age, however, has an auditory processing disorder.

The majority of people who use or should use hearing aids are senior citizens with age-related deficits. A major challenge for this population is hearing aid development with appropriate signal processing, which requires new ways of thinking about the nature of age-related communication deficits. The focus of developing improved communication devices for senior citizens should be on the combination of age-related hearing loss and age-related auditory processing deficits.

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Limits Still Exist

Technological limitations have also served as a constraint on our thinking in the same way advances have served as a catalyst in stimulating new thinking to address old problems. It was relatively easy using electronic equipment during audiology's formative years to measure the frequency response of audiological instruments (amplifiers, filters, earphones). It is also possible to specify the transmission characteristics of these instruments in their impulse responses, but it was difficult to obtain measurements with the available electronic technology. We grew accustomed to thinking in terms of the instruments’ frequency characteristics and concomitantly of the frequency processing capabilities of the ear and the effect of hearing impairments on these capabilities.

The audiogram soon became the basic measure of hearing loss although it provided information on only one aspect — audibility as a frequency function. Reduced temporal processing is an important feature of age-related hearing loss, but it is not measured routinely in hearing clinics even though digital instrumentation for measuring and specifying deficits in auditory temporal processing is now available. What would audiological evaluations be like today had it been easier to measure the temporal, rather than frequency, characteristics of hearing using the technology available during this formative period?

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Living in the Digital Age

Digital technology has also altered our thinking. The computer was developed to analyze large bodies of data at high speeds. An early innovative application of the computer was to simulate analog methods signal processing, such as frequency filtering. The motivation for this research was to develop and evaluate advanced signal processing systems in software before undertaking the costly and time-consuming task of building the final system in hardware. Programming a computer to simulate an electronic system requires a different mindset than that required for building a hardware system.

The researchers who developed the first digital speech processing systems used computer simulations of conventional analog electronic components. The resulting systems were superior to conventional analog systems by a relatively small degree. Researchers, when they began thinking in digital terms, developed software algorithms for processing speech in ways that could not be implemented using conventional analog components. This mindset change resulted in unique digital implementations of speech processing systems that were far superior to systems using conventional analog technology.

The development of the digital hearing aid followed a similar pattern. The first digital hearing aids implemented digital simulations of signal processing techniques used in analog hearing aids at that time. The result was a digital hearing aid that was only slightly better than a conventional analog hearing aid. The next generation of digital hearing aids, however, included innovative methods of signal processing, such as adaptive feedback cancellation, which were beyond analog hearing aids’ capabilities. It was only at this stage that the superiority of digital hearing aids was clearly established.

The explosive growth of social networks such as Facebook represents an innovative application of wireless digital technology that has met with highly favorable responses. Aspects of this technology could be beneficial to people with hearing loss. The first step in exploring possible applications of these new, innovative techniques is to develop a mindset appropriate for wireless digital technology. The imperative is not to think in terms of what old technology did to improve old methods, but to think of what new technology can do that the old could not so new ways of doing things with their attendant advantages are created.

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© 2012 Lippincott Williams & Wilkins, Inc.

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