Future Directions in Signal Processing Hearing Aids Digital techniques offer many advantages over conventional analog methods in the development of modern hearing aids. These include: Harry Levitt, PhD Center for Research in Speech and Hearing Sciences, Graduate School and University Center of the City University of New York, New York, New York

ABSTRACT Digital hearing aids offer many advantages over conventional analog hearing aids, such as programmability,memory, extremely precise, flexible control of electroacoustic characteristics, and advanced signal processing capabilities for noise reduction and speech enhancement. At the present stage of development, digital hearing aids are subject to severe practical constraints with respect to size and power consumption. Hybrid analog/digital hearing aids have been developed which combine some of the advantages of digital technology with the practicality of small, cosmetically acceptable instruments. Recent studies with all-digital and hybrid analog/digital hearing aids have identified trends which are likely to influence future hearing aid design.

Digital technology has had a profound impact on the development of modern hearing aids. A range of different instruments has been developed in recent years. These include hearing aids that are almost all digital, hearing aids that employ digital techniques to some extent, and hearing aids that are essentially all analog but which draw on techniques introduced by digital technology. In an all-digital hearing aid, the acoustic signal picked up by the microphone is first converted to an electrical waveform which is then digitized using an analog to digital (A/D) converter. The digital signal is typically represented by sets of binary digits (0 or l), each set representing the value of the waveform at a discrete point in time. The sets of binary digits are then processed numerically using techniques already developed for use in digital computers. The processed digital signal is converted back to an analog electrical waveform by means of a digital to analog (D/A) converter. The electrical signal, in turn, is converted back to an acoustic signal using a conventional hearing aid receiver. For a tutorial on digital hearing aids, see Levitt (1 987). Ear and Hearing, Vol. 12, No. 6, 1991

1. Programmability. The capability of programming the hearing aid to have different electroacoustic Characteristics and to perform different functions. A digital hearing aid, for example, can be programmed by an external computer to have a wide range of different frequency gain characteristics. 2. Memory. The capability of storing programs and other information. An example of the application of memory is that of a digital hearing aid which can have several different sets of frequency responses and compression characteristics, any one of which can be programmed into the hearing aid at the press of a button. The programs needed to change the hearing aid characteristics can be stored conveniently in digital memory. 3. Logical Operations. The capability of performing algebraic logic. Advanced digital hearing aids have been developed which can determine whether speech is present in a noisy environment and then automatically adjust the frequency gain characteristic so as to reduce the effects of the noise. These operations require the use of algebraic logic. 4. Speed and Precision. Digital signals can be processed at great speed and with extremely high precision. As a consequence, the electroacoustic characteristics (frequency response, gain, compression) of a digital hearing aid can be adjusted almost instantaneously and with a precision far exceeding that possible in a conventional analog hearing aid. 5. Eficient, Error-Free Information Transfer.Information in digital form can be stored, retrieved, and transmitted without error and with great efficiency. As a consequence, digital hearing aids can be adjusted and readjusted efficiently and with perfect repeatability. Most of the above features of a digital system can be implemented in an analog signal processing system (e.g., an analog hearing aid), but usually with great difficulty. It is also relevant to note that those features which can be implemented in a practical analog system were not used in modern hearing aids until after these concepts were introduced using digital techniques. 0196/0202/91/1206-01258$03.00/0 ' EARAND HEARING Copyright 0 1991 by Williams 8. Wilkins * Printed in the U.S.A.

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The many advantages inherent to the digital approach can be used effectively in developing hearing aids with powerful and unique signal processing capabilities. A major problem, at present, in using digital techniques for processing audio signals, is the relatively large power consumption, and hence, relatively large size, of major digital components (such as the A/D converter) relative to conventional analog hearing aid components. Major compromises thus need to be made in using digital technology in a practical, wearable hearing aid.

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Two approaches have been pursued in the development of digital hearing aids. The first has been to develop experimental digital hearing aids without regard to size or complexity in order to investigate the potential capabilities of digital techniques in acoustic amplification. This approach has led to the development of digital master hearing aids, referred to as DMHA (Levitt, Neuman, Mills, & Schwander, 1986). These instruments are typically too large to be wearable, but are extremely flexible with substantial signal processing capabilities. DMHA have proven to be invaluable tools for research and development and have paved the way for the development of smaller, more practical instruments for personal use. The second approach has focused on the development of practical, wearable digital hearing aids. In so doing, many significant engineering compromises have been made. The most common compromise is a hybrid analog/digital hearing aid in which the audio signal is processed by analog means, but the parameters of the system (gain, frequency response, compression characteristics) are controlled by digital means. Most of the programmable hearing aids currently on the market are of this type. The two approaches are illustrated in Figures 1 and 2. Figure 1 shows a block diagram of a digital master hearing aid first introduced by Levitt (1982) and used in several exploratory investigations on digital techniques in acoustic amplification (Levitt et al, 1986; Levitt, Sullivan, Neuman, & Rubin-Spitz, 1987; Levitt, Neuman, & Sullivan, 1990; Levitt & Neuman, 1991). Note that two computers are used. One computer is a high-speed array processor dedicated to the task of signal processing. The second computer, which is smaller, slower, and draws less electrical power, is used to control the high-speed array processor. The controlling computer makes heavy use of memory, logic, information transmission, and other noncomputational computer operations. Wearable all-digital hearing aids have also been developed (Cummins & Hecox, 1987; Engebretsen, Morley, & Popelka, 1987; Nunley, Staab, Steadman, Wechsler, & Spencer, 1983). Thus far, these instruments have

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been relatively large body worn units or experimental prototypes. A modified version of the system described by Cummins and Hecox (1987) was subsequently developed into a commercial product (the Nicolet Phoenix hearing aid), but it is no longer being marketed. The second approach, that of using smaller, powerefficient analog components for the signal processing path, has resulted in commercially viable instruments. Figure 2 shows a typical hybrid analog/digital hearing aid. The audio signal is processed by analog components consisting of a preamplifier, filter, power amplifier, and limiter. These components are essentially the same as those used in a conventional hearing aid. Each analog component, however, is controlled by a master digital unit (the digital controller). The power consumption of the digital controller is very low and, as a consequence, both the analog and digital components of this hearing aid can be packaged in a small, cosmetically acceptable unit. Although the flexibility and signal processing capabilities of the hybrid analog/digital unit are limited, this new type of hearing aid does embody many of the features inherent to digital technology, such as programmability. It should also be noted that the basic structure of the hybrid analog/digital hearing aid is not very different from the all-digital master hearing aid shown in Figure 1. That is, in both cases, a small programmable digital unit is used to control other unit(s) in the system dedicated to the task of signal processing. TRENDS IN EXPERIMENTAL EVALUATIONS

A very attractive possibility with digital hearing aids is that of using powerful new signal processing techniques for noise reduction and speech enhancement. As a consequence, a range of different noise reduction techniques for hearing aid use have been developed and Ear and Hearing, Vol. 12, No. 6, 1991

evaluated. The results, thus far, have been mixed. The initial thrust in investigating noise reduction techniques for hearing aids focused on the traditional single-microphone hearing aid (Graupe, Grosspietsch, & Basseas, 1987; Levitt et al, 1986; Neuman & Schwander, 1987). Although some hearing-impaired subjects showed small gains in their ability to understand speech, the general trend did not show significant improvements in intelligibility, even with the most modern signal processing techniques. These techniques, however, did reduce background noise levels, and many of the subjects reported corresponding improvements in overall sound quality. Typically, the subjects preferred listening to the processed speech in noise, although there was no significant gain in speech intelligibility. In contrast, significant gains in both speech intelligibility and overall sound quality have been obtained using two or more microphones. Two particularly promising forms of multimicrophone noise reduction use electronic beam forming (Peterson, Durlach, Rabinowitz, & Zurek, 1987) and adaptive noise cancellation (Brey, Robinette, Chabries, & Christiansen, 1987; Chabries, Christiansen, Brey, Robinette, & Hams, 1987; Chazan, Medan, & Shvadron, 1987). These techniques depend on differences between the speech and noise among the different microphones that are used. The net effect is similar to that of using a highly directional microphone to focus on the speech source, and attenuating interfering sounds coming from other directions. These techniques only work well when there is some spatial separation between the speech and noise sources. As in the case of a directional microphone, none of these techniques works well in a highly reverberant sound field. The use of multimicrophone techniques poses severe practical problems for hearing aid use. The signals picked up by each microphone need to be transmitted to a central processor. It is inconvenient to do this using Future Directions in Signal ProcessingHearing Aids

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wires, and the use of radio transmission presents new problems in terms of power consumption and various forms of radio frequency interference. A possible solution to this problem is to use different types of microphones (unidirectional and omnidirectional) mounted on the same ear and to process the differences between the speech and noise at these microphones as if the microphones were placed at separate locations. This technique has been used successfully in conjunction with a two-microphone adaptive noise canceler (Schwander & Levitt, 1987). Signal processing for speech enhancement is still at an early stage of development. Promising results have been obtained for profoundly hearing-impaired subjects using feature extraction techniques in which important speech features are emphasized. One example is the SiVo hearing aid in which only the voice fundamental frequency is amplified (Rosen, Walliker, Fourcin, & Ball, 1987). The device has proven to be a useful supplement for lipreading. Another promising form of processing is to exaggerate phonetic features that may not be audible to severely hearing-impaired persons. Revoile et a1 (1986, 1987), for example, have obtained improved speech recognition scores by exaggerating the phonetic distinctions between voiced and unvoiced consonants. Implementation of these techniques in a wearable personal hearing aid, however, is unlikely in the near future because of the many practical problems involved. A particularly dificult problem is that of processing the audio signal so as to automatically recognize the phonetic features of speech. Investigations of advanced methods of amplitude compression have yielded disappointing results. Despite the theoretical appeal of multiband amplitude compression, experimental evaluations of this technique as well as related forms of amplitude compression have yielded mixed results (Bustamante & Braida, 1987a,b; Levitt, 1991). As in the case of single-microphone noise reduction, some improvement was obtained for some hearing-impaired subjects. The best results obtained thus far have been with a two-channel compression hearing aid (Moore, 1987). Other forms of compression are currently being investigated. A phonetically based form of amplitude compression is that of adjusting the consonant-vowel ratio. This technique is discussed in greater detail in the article by Preves, Fortune, Woodruff,and Newton ( 199 1). A form of signal processing that is particularly well suited for digital techniques is that of feedback cancellation using phase controlled signals. Whereas it is very difficult to control the phase characteristics of analog circuits, both amplitude and phase can be controlled with relative ease and a high degree of precision in digital systems. Figure 3 shows the block diagram of a hearing aid with feedback cancellation using digital filters in which both amplitude and phase are carefully controlled. When there is no acoustic feedback, the audio signal picked up by the microphone is amplified and filtered ~

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as in any conventional hearing aid. In this mode of operation, points A and B (see Fig. 3 ) are connected by a switch controlled by the microprocessor. The audio signal picked up by the microphone passes through the preamplifier, filter F1, and power amplifier before being delivered to the hearing aid receiver, which generates the acoustic output. Filter F1 and the power amplifier are controlled by the microprocessor so as to provide the desired frequency gain characteristic. The level of the incoming signal is monitored by the microprocessor. If the monitor signal shows a sudden increase in level at a single frequency, indicative of the onset of acoustic feedback oscillations, then the microprocessor breaks the connection AB so as to remove the input signal, and inserts a probe signal to filter Fl instead. Filter F1 is left unchanged, but filter F2 is adjusted so as to produce an electrical feedback signal that is equal in amplitude but opposite in phase to the acoustic feedback signal. Consequently, when the electrical feedback signal is added to the output of the preamplifier, the acoustic feedback signal is canceled. The microprocessor then removes the probe signal and reconnects points A and B so that the system functions once again as a conventional hearing aid, but without acoustic feedback. The above method of canceling acoustic feedback has been implemented successfully in a digital master hearing aid (Levitt, Dugot, & Kopper, 1988). Methods of implementing the technique or variations of it in a practical, wearable hearing aid are currently being explored (Bustamante, Worrall, & Williamson, 1989; Engebretsen, French-St. George, & O’Connell, 199 1; Kates, 1990, 1991). Experimental evaluations of hybrid analog/digital hearing aids comparable in size and cosmetic appeal to conventional analog instruments have yielded consistently good results (Johnson, Kirby, Hodgson, & Johnson, 1988; Moore, 1987; Pluvinage & Benson, 1988; Rapisardi, 1989). Although these instruments do not have the powerful signal processing capabilities of alldigital hearing aids, they do embody many of the advantages offered by digital systems, such as programmability and memory. An obvious advantage of interchangeable memory in a digital or hybrid analog/digital hearing aid is that the electroacoustic characteristics of the instrument can be changed rapidly and conveniently depending on the user’s needs. Data obtained by Sullivan, Levitt, Hwang, and Hennessey (1988) and others show that there is no single frequency gain characteristic that is optimal for all conditions of use. It is, thus, very useful for the hearing aid user to be able to select an appropriate set of electroacoustic characteristics depending on the acoustic environment (quiet, noisy, reverberant) and type of sounds to be amplified (speech, music). Similarly, an obvious advantage of programmability is that appropriate sets of electroacoustic characteristics can be determined individually for each user and for different acoustic environments.

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A less obvious, but important advantage of both memory and programmability is that these features can be used effectively in the prescriptive fitting of hearing aids. As noted earlier, there are large individual differences in the relative effectiveness of modern signal processing techniques for noise reduction and speech enhancement. These observations add a new dimension to the problem of hearing aid prescription. In addition to considerations such as frequency gain characteristic, saturation sound pressure level, and other traditional variables, it may also be necessary to determine which form(s) of signal processing are best suited for each individual user. This is a difficult problem which can be addressed using adaptive paired comparison techniques (Neuman, Levitt, Mills, & Schwander, 1987). Programmable hearing aids with memory provide the means for implementing powerful new prescriptive techniques of this type. A practical problem encountered in the development of digital and hybrid analog/digital hearing aids is that of controlling the many variables that need to be adjusted. This problem is becoming increasingly more severe with the growing complexity of modern hearing aids and the ongoing trend towards instruments of smaller and smaller size. Modern methods of information transmission and control have been used in addressing this problem. One outcome has been the use of remote controls in adjusting hearing aid settings. For older hearing aid users with poor manual dexterity, the use of large remote controls can be very helpful. The use of digital telemetry also opens up the possibility of developing “intelligent” remote controls that have been programmed to simplify the adjustment procedure. CONCLUSIONS

The trends observed in experimental evaluations of digital and hybrid analog/digital hearing aids serve as a revealing signpost for future directions. Data obtained thus far indicate that digital techniques offer many advantages over conventional analog methods. These include programmability, convenient memory storage, use of logical operations, error-free information transfer, and high-speed, precise operation. Although some of these features are not unique to digital systems, they have been implemented in modern hearing aids as a result of the introduction of digital techniques. Much af the research on experimental nonwearable digital master hearing aids has focused on the development and evaluation of advanced digital signal processing strategies for reducing background noise and improving sptxxh wkhgdxhty. Although no dramatic breakthroughs have been achieved, as yet, the results obtained have identified directions in which significant improvements are likely. The most promising results, thus far, relate to the use of multimicrophone techniques for noise reduction. The implementation of these techniques in a practical hearing aid, b o w w r , remains a formidable problem. Research is currently in Ear and Hearing, Vol. 12, No. 6, 1991

progress on practical methods for processing signals received by two or more microphones without the need for unwieldy microphone connections. If this research effort is successful, practical noise-reducing hearing aids using two or more microphones could become a reality. Research on wearable, experimental hearing aids has also identified areas in which small but significant improvements can be obtained. In particular, relatively simple methods of changing the frequency gain characteristic as a function of signal level or background noise have proved beneficial. These techniques are only slightly more complex than conventional amplitude compression (e.g., the use of two-band rather than single-band compression). Practical hearing aids of this type have already been developed using hybrid analog/ digital techniques, and it is anticipated that variations of this approach will be used in more advanced signal processing hearing aids. Another area in which digital techniques appear promising is that of feedback reduction. in this case, however, the processing involved is fairly complex, and it remains to be seen if these techniques can be incorporated into a practical, wearable instrument. The primary problem in developing practical digital hearing aids is the relatively large size and power consumption of the current generation of digital chips. Cosmetic considerations are of great importance to the majority of hearing aid users and the current trend, for successful marketing, is to make hearing aids as small as possible. Whereas considerable progress has been made in the past few years in reducing the size and power consumption of digital components for hearing aids, much progress has also been made in reducing the size of analog hearing aid components. As a consequence, both conventional analog and experimental digital hearing aids have become progressively smaller. The relative rates of progress in the miniaturization of digital and analog circuit chips is such that digital technology is expected to catch up with analog technology in the not too distant future. At present, the most practical approach to implementing digital techniques has been in the form of hybrid analog/digital hearing aids. It is anticipated that increasingly greater use will be made of digital technology in these hybrid instruments, but that it will still be some time before all-digital personal hearing aids with powerful signal processing capabilities become commercially viable. An important consequence of the increasing use of advanced signal processing techniques in hearing aids is that auctiofogical tecmques for the prescriptive fitting of hearing aids need to be upgraded. Substantial individual differences have been observed in the effects of signal processing for noise reduction and speech enhancement. Thus, in addition to considering conventional electroacoustic variables such as frequency response, gain, and saturation sound pressure level, hearing aid dispensers will also need to consider which forms of signal processing are best suited for each individual client. Future Directionsin Signal Processing Hearing Aids

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Perhaps the most important result to follow from the introduction of digital techniques is that our way of thinking about hearing aids has changed. Whereas in the past, hearing aids were viewed primarily as devices consisting of amplifiers, filters, and related hardware, the modern concept of a hearing aid involves both hardware and software. The design and evaluation of modern hearing aids, thus, involve considerations of not only the means by which the audio signal is processed (the hardware), but also the rules or programs by which the various components of the hearing aid are controlled (the software). This change has forced all concerned in the hearing aid field to reconsider the basic function of a hearing aid and how it can best be implemented. REFERENCES Brey RH, Robinette MS. Chabries DM, and Christiansen RW. Improvement in speech intelligibility in noise employing an adaptive filter with normal and hearing-impaired subjects. J Rehabil Res Dev 1987;24(4):75-86. Bustamante DK and Braida LD. Multiband compression limiting for hearing impaired listeners. J Rehabil Res Dev 1987a:24(4): 149160. Bustamante DK and Braida LD. Principal-component amplitude compression for the hearing impaired. J Acoust SOC Am 1987b;82(4):1227-1242. Bustamante DK, Worrall TL, and Williamson MJ. Measurement of adaptive suppression of acoustic feedback in hearing aids. Proceedings of the 1989 International Conference on Acoustics Speech, and Signal Processing, Glasgow, Scotland I989:20 17-2020. Chabries DM, Christiansen RW, Brey RH, Robinette MS, and Hams RW. Application of adaptive digital signal processing to speech enhancement for the hearing impaired. J Rehabil Res Dev 1987;24(4):65-74. Chazan D, Medan Y, and Shvadron U. Evaluation of adaptive multimicrophone algorithms for hearing aids. J Rehabil Res Dev 1987;24(4):1II-I 18. Cummins KL and Hecox KE. Ambulatory testing of digital hearing aid algorithms. In Steel R D and Gerrey W, Eds. Proceedings of the 10th Annual Conference on Rehabilitation Technology. Washington, DC: RESNA-Association for the Advancement of Rehabilitation Technology, I987:398-400. Engebretsen AM, French-St. George M, and OConnell MP. Adaptive feedback stabilization of hearing aids. Second International Workshop on Hearing Impairment and Signal-Processing Hearing Aids, Ciba Foundation, London, England, June 1991. Engebretsen AM, Morley RE, and Popelka GR. Development of an ear-level digital aid and computer-assisted fitting procedure: An interim report. J Rehabil Res Dev 1987;24(4):55-64. Graupe D. Grosspietsch JK, and Basseas SP. A single-microphonebased self-adaptive filter of noise from speech and its performance evaluation. J Rehabil Res Dev 1987;24(4):I 19- 126. Johnson JS, Kirby VM, Hodgson WA, and Johnson LJ. Clinical study of a programmable, multiple memory hearing instrument. Hear Instrum l988;39( I1):44-46. Kates JM. Feedback cancellation in hearing aids. Proceedings of IEEE International Conference on Acoustical Speech and Signal Processing, Albuquerque, NM, 1990:1 125- 1 125. Kates JM. The problem of feedback in hearing aids. J Commun

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Disord 1991:24:223-235. Levitt H. An array-processor computer hearing aid. Asha 1982;24:805. Levitt H. Digital hearing aids: A tutorial review. J Rehabil Res Dev 1987:24(4):7- 19. Levitt H. Advanced signal processing techniques for hearing aids. In Studebaker G, Bess F. and Beck L, Eds. The Vanderbilt Hearing Aid Report 11. Monkton. MD: York Press, 1991. Levitt H, Dugot R. and Kopper, KW. Programmable digital hearing aid system. U.S. Patent #4.731,850, 1988. Levitt H and Neuman A. Evaluation of orthogonal polynomial compression. J Acoust SOCAm 199 1 ;90:24 1-252. Levitt H, Neuman A, Mills R, and Schwander T. A digital master hearing aid. J Rehabil Res Dev 1986;23(1):79-87. Levitt H, Neuman A, and Sullivan J. Studies with digital hearing aids. Acta Otolaryngol l990;469(Suppl):57-69. Levitt H, Sullivan JA, Neuman AC, and Rubin-Spitz JA. Experiments with a programmable master hearing aid. J Rehabil Res Dev 1987;24(4):29-54. Moore BCJ. Design and evaluation of a two-channel compression hearing aid. J Rehabil Res Dev 1987:24(4):181-192. Neuman AC and Schwander TJ. The effect of filtering on the intelligibility and quality of speech in noise. J Rehabil Res Dev 1987:24(4):127-134. Neuman AC, Levitt H. Mills R, and Schwander T. An evaluation of three adaptive hearing aid selection strategies. J Acoust SOCAm 1987:82: 1967-1976. Nunley J, Staab W, Steadman J, Wechsler and Spencer B. A --wea-abIie digital hearing aid. Hear J B 8 3 October:29-3 1,34-35. T f e E o 3 PM, Durlach NI, Rabinowitz WM, and Zurek PM. Multimicrophone adaptive beamforming for interference reduction in hearing aids. J Rehabil Res Dev 1987:24(4):103-110. Pluvinage V and Benson D. New dimensions in diagnostics and fitting. Hear Instrum 1988;39(8):28-29,39. Preves DA, Fortune T, Woodruff B, and Newton J. Speech enhancement strategies for hearing aids. Ear Hear 1991;12(suppl):139s153s. Rapisardi D. Bridging the gap between product R & D and the consumer. Hear Instrum 1989;40(10):20-2 1. Revoile SG, Holden-Pitt L, Edward D, and Pickett JM. Some rehabilitative considerations for future speech-processing hearing aids. J Rehabil Res Dev 1986;23(1):89-94. Revoile SG, Holden-Pitt L, Edward D, Pickett JM, and Brandt F. Speech-cue enhancement for the hearing-impaired: Amplification of burst/murmur cues for improved perception of final stop voicing. J Rehabil Res Dev 1987;24(4):207-216. Rosen S, Walliker JR, Fourcin A, and Ball V. A microprocessorbased acoustic hearing aid for the profoundly impaired listener. J Rehabil Res Dev 1987;24(4):239-260. Schwander TJ and Levitt H. Effect of two-microphone noise reduction on speech recognition by normal-hearing listeners. J Rehabil Res Dev 1987;24(4):87-92. Sullivan JA, Levitt H, Hwang JY. and Hennessey AM. An experimental comparison of four hearing aid prescription methods. Ear Hear 1988:9:22-32.

Acknowledgments: Preparationof this paper was supported by Grant #H133E80019 from the National Institute on Disability and Rehabilitation Research. Address reprint requests to Harry Levitt, Center for Research in Speech and Hearing Sciences, Graduate School and University Center of the City University of New York, 33 West 42nd St., New York, NY 10036.

Ear and Hearing, Vol. 12, No. 6, 1991, Supplement

Future directions in signal processing hearing aids.

Digital hearing aids offer many advantages over conventional analog hearing aids, such as programmability, memory, extremely precise, flexible control...
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