DIGITAL HEARING AIDS: Hype, Hoax or Hope

Introduction :

This article, written for Audiology Online, reviews digital signal processing (DSP) hearing aids and discusses whether or not DSP provides advantages to the hearing impaired that can not be offered - or not offered as efficiently - by analog hearing aids. It is not the purpose of this article to indict analog technology or to unrealistically extol the virtues of DSP hearing systems. Rather, it is to present an objective overview of DSP contributions. For many of us, when new hearing aid technology is introduced there are academic and clinical challenges. They are in the form of questions such as: Does the new DSP technology perform tasks better or more efficiently than conventional analog technology? Does it provide control over more electroacoustic performance parameters than conventional technology? Do the benefits offered by the device or signal processing system outweigh the increase in cost to the patient?


Overview:

It may be instructive to explain how digital signal processing occurs and how it differs significantly from analog processing of acoustic signals. In analog processing systems, an acoustic signal is converted to its electrical analog at the microphone stage of the hearing aid circuit. That is, acoustic signals constantly vary in sound pressure. When converted to an electrical analog, the varying sound pressure is transduced to varying voltage. The amplitude of the voltage can be controlled so as to emphasize selected frequency regions, as well as carrying out other processing tasks. When the acoustic signal is converted to its electrical equivalent, a limited number of parameters can be individually controlled to meet the acoustic needs of the hearing impaired person.

When digital signal processing in hearing aids is used, a number of very interesting phenomena occur. The conventional wisdom goes something like this:

The acoustic signal is converted to its electrical analog at the microphone stage of the hearing aid system. After this conversion, a frequency filter is introduced to reduce possible distortion of the input signal. The signal is then "sampled" a given number of times per second. Normally, the sampling rate is 10,000 times per second, or greater.

The analog signal is then converted to its digital equivalent by the analog to digital (A/D) converter. Each samples receives a digital code. Binary numbers (O and 1) are used to represent the digital value of each sample. Following the digitization of the signal, the digital representations are processed by a central processing unit (CPU) or microprocessor. The digital values can be multiplied, divided, added, subtracted and grouped in defined ways. In the microprocessor are various algorithms. An algorithm is a system of instructions that operates in a manner determined by a set of mathematical rules and equations. If the algorithm is a dedicated one, it performs a specific task relative to the processing of the input signal. For example, one algorithm may control the frequency response of the instrument, another may control loudness growth, a third may function to enhance the speech signal in a background of noise, etc. After the microprocessor has performed its tasks, the digitized signal must be converted back to its analog equivalent. This is accomplished at the digital to analog (D/A) conversion stage. When the digitized signal is converted to its analog stage, it is frequency filtered again, to prevent signal distortion. It is then amplified in the conventional manner and submitted to the receiver (speaker) of the hearing aid. (Note: For some of the more recent DSP systems, the D/A conversion does not require a separate circuit. Rather, it uses the hearing aid receiver to accomplish digital to analog conversion. This process is referred to as a Direct Digital Drive.)

The obvious advantage of digital signal processing is that there are unlimited ways in which the signal can be manipulated. The number of parameters that can be utilized, and that can be utilized at the same instant in time, are significantly greater than those found in conventional analog systems. As such, the ability to manipulate the signal to more closely approximate the acoustic needs of the patient is greatly enhanced.

Soren Westermann of Widex Aps in Copenhagen, Denmark (personal communication) makes the following statement concerning the functional efficiency between analog and digital technology:

"Very briefly, analog signal processing is only practical for electroacoustic functions such as amplification, filtering and compression. Smart functions such as changing signal processing when environmental sounds change, removing, or significantly reducing feedback, making in-situ audiometric measurements and smart directional functions can only be made, practically, in digital technology.

Certainly, one can make many combinations of amplification, filtering and compression in analog technology, but if one designs very complex processing schemes, the analog technology will run into problems with internal electrical noise.

Unfortunately, this results in reduced (useful) dynamic range. Therefore, you can only employ analog signal processing to achieve a certain complexity of electroacoustic function. If one attempts to go beyond a given level of complexity, using analog processing, it will be drown in internal noise.

With digital technology, one can choose the dynamic range and noise floor that is consistent with the performance desired. To accomplish this with digital signal processing, it requires more "bits" and calculations, but there are virtually no limits in this respect."

In the final analysis, DSP performance efficiency is greatly improved, without excessive battery current demands. In fact, DSP systems can have significantly longer battery life than analog hearing aid systems. The ability of DSP circuits to accomplish processing tasks impossible for analog systems expands the utility of DSP hearing aid systems. Additionally, the size of the hearing instrument can be greatly reduced without sacrificing performance efficiency.

The performance of DSP instruments is dependent upon their functional design. DSP is a tool that can be used in many ways. The design and efficiency of the device depends on the philosophical basis used in determining its performance characteristics.

Questions asked in the determination of design considerations may include the following: 1) the number of algorithms needed to perform specific signal processing tasks, 2) the number of independent channels used in processing 3) the number of tasks needing to be addressed, 4) current demands (battery drain) to perform the processing tasks 5) the physical size of the instrument, and, 6) the ease of manipulation of the instrument.

Basic Assumptions:

Two basic assumptions underlie the development of DSP hearing aids. Each suggests that DSP technology can accomplish tasks that conventional analog systems cannot. Or perhaps we can say that conventional analog systems cannot accomplish these tasks as well, or as efficiently, as DSP circuits.

The first assumption is FUNCTIONALITY. Functionality is defined as the ability for enhanced flexibility, which permits the DSP hearing aid system to perform signal processing and other tasks not possible in analog hearing aid systems.

The second assumption is TECHNICAL PERFORMANCE. Technical performance is defined as the capability of digital technology to perform the same signal processing tasks more efficiently than analog technology.

Real Issues:

The primary issue in the design of hearing aid systems is "What do you want to accomplish?" What must the hearing instrument do to provide the best possible amplified, acoustic signal to the impaired ear?

There are three areas of concern. First is the hearing impairment, which determines the need for amplification. Second is the environmental background in which listening takes place. Third is the hearing aid device. The hearing aid must meet not only electroacoustic requirements of the patient, but must adjust its performance as the environmental background noise changes. These issues must be addressed and provided in an instrument whose physical size is cosmetically acceptable.

Having reached consensus regarding what the hearing instrument should do, one must decide whether analog or DSP systems achieve desired goals more efficiently.

What are some of the electroacoustic tasks that should be accomplished to offer optimal benefit? The following are reasonable goals: 1) Signal audibility, 2) Compensation for loudness recruitment, 3) Compensation for excessive masking, 4) Reduction of internally generated noise, 5) Automatic control of hearing aid performance and 6) improved performance with the use of conventional telephones and cell phones.

The question is whether these goals can be accomplished more efficiently - or accomplished at all - by analog or digital technology. It is a matter of fact, not hyperbole, that digital signal processing devices can manipulate the signal in ways that are impossible for analog devices. When an analog signal is converted to its digital equivalent, there are unlimited possibilities as to how the digital representations can be processed. The degree of processing depends on the sophistication of the microprocessor of the DSP device and the number of dedicated algorithms processing the signal. When the analog signals are converted to digital ones, processing becomes a mathematical task adhering to specific rules.

Physical Size Advantage:

The physical dimensions of the DSP "chip" are smaller than the corresponding analog circuit performing the same or similar tasks. This, too, is a reality and not hyperbole. Most hearing-impaired individuals prefer an instrument that is not readily visible to the casual observer. Unfortunately, some patients have opted for the smallest of analog hearing aid devices, completely-in-the-canal (CIC), even though a physically larger instrument would provide much greater acoustic benefit. However, since the DSP chip is more efficient, greater acoustic gain is possible without increasing the physical size of the hearing aid. Thus, using DSP technology, greater losses can be accommodated without sacrificing performance. However, CIC instruments, even with DSP technology, cannot resolve the acoustic needs for all levels of hearing, despite their cosmetic appeal.

Some DSP Advantages and Limitations

With the advent of DSP hearing aids and the unlimited ways in which digitized signals can be processed, future application of this technology is very bright indeed. This does not mean that analog instruments cannot perform some similar electroacoustic tasks. They can. To perform the same functions, however, may require more electronic components, and more components mean a larger physical size. The more sophisticated the performance of analog devices, the greater the current demands. The greater the current demand, the shorter the battery life. Digital hearing aid systems perform the same signal processing functions more efficiently and with greater flexibility

Digital signal processing hearing aids are not a panacea for solving all problems experienced by the hearing impaired person. DSP systems have their limitations. DSP systems do not separate signal and noise, although such would be most desirable. DSP systems do not alter the pathophysiology or neurology of the impaired cochlea or auditory system. DSP systems may not always be the most appropriate instrument of choice for all levels of hearing impairment. The instrument of choice is often dependent on the severity of hearing loss, the acoustic environment in which individuals function and whether or not social needs are being met. If a person leads a very sedentary - or socially restricted life - sophisticated, DSP hearing aid devices may be not be necessary to meet their limited social and acoustic needs.

Most hearing aid users have sensorineural impairments and lead fairly active lives. As such, they have varying levels of recruitment and are exposed to a variety of listening environments. They need to hear and understand the spoken word in a multitude of background noises. They need to hear low level inputs to maximize speech understanding and to accommodate sounds that do not violate their sense of comfortable loudness. For these individuals, DSP hearing instruments offer a potentially more efficient interface between the environment and the hearing loss.

We are at the threshold of understanding the contributions of DSP hearing aid systems. As we learn more about the pathology of the human auditory system, we can manipulate the ways in which the signal is managed by the microprocessor to approach optimal performance. The possibility of meeting complex acoustic needs is enhanced through the application of DSP systems.

Perhaps one should not prognosticate. However, it appears certain that
DSP technology will continue to advance, perhaps in ways we cannot even imagine at this time. As this technology assimilates new discoveries of auditory behavior, or introduces solutions to current problems, it will provide improved methods of meeting the amplification needs of the hearing impaired.

Unresolved Problems

One of the clinical barriers audiologists' face is verifying or validating the advantages of DSP hearing aids. Audiologists have relied on measurement tools used to assess the performance of conventional analog systems. There have been several excellent studies in which digital and/or digital and analog systems have been evaluated. (Arlinger et al, 1998, Arlinger & Billemark 1998, Knebel & Bentler 1998, Kuk 1997, Levitt, et al. 1990, Ludvigsen & Topholm, 1997, Luner et al 1998, Newman & Sandridge, 1998, Valente et al 1998 Valente et al 1999, Westermann & Sandlin 1997) Although, in some instances, speech discrimination scores were not improved when digital processing systems were compared in identical situations, overall performance favored DSP devices

There is an overwhelming need for audiologists to develop new test batteries to assess the advantages of DSP hearing instruments. Such factors as physical comfort, ease of listening, reduction of listening fatigue, reduction of internal noise, satisfaction with amplification, word recognition scores at different input levels, minimization of distortion, improved ability to reduce or eliminate feedback that increases headroom (i.e., allowing greater useable gain before feedback), availability of adaptive or dynamic directional microphones and improved use with cell phones are among those to be considered.

We need to know what is important to the hearing impaired and what is acceptable or unacceptable in terms of hearing aid performance. The development of precise and "information bearing' measurement tools can provide greater objectivity in the assessment process.

DSP hearing aids should not be condemned because they fail to solve all problems associated with hearing aid amplification use. Conversely, one should not embrace, or exaggerate the advantages of DSP systems simply because they represent a new technology. Unwarranted claims of performance, whether DSP or analog, must be challenged by the professional community. In the final analysis, organized and unbiased research studies will provide us with answers. Based on the abundance of existing data, in which analog and digital systems have been compared, under a multitude of conditions and environments, the edge, in my opinion, goes to DSP systems.

Directional Microphones

It is well-documented (Valente 2000, Kochkin 1993).) that one of the major problems experienced by the hearing impaired is that of understanding the intended message in a background of noise. It is of clinical interest to note that most, if not all, hearing impaired persons have difficulty understanding the intended verbal message in a noise filled background. From a technological point of view, what role do omni and directional microphones play in the processing of speech in a milieu of noise?

Studies of DSP systems using directional microphone technology suggest that a more favorable signal to noise ratio (SNR) can be achieved when compared to omni-directional microphones. The greater the signal to noise advantage, the greater the probability of increased intelligibility of the message. Valente (2000) reviews the advantages for adults and Kuk et.al; (1999) discusses the advantages for children. The reader should be aware that improved recognition of speech in noise, using directional microphones, can also be said for hearing aids with analog signal processing. However, with digital signal processing, the electroacoustic sophistication of noise reduction is enhanced.

With the anticipated contributions of beam forming microphones, coupled with sophisticated digital signal processing, word recognition in noise may be greatly enhanced.

Non-technical problems associated with DSP hearing instruments

More recently, audiologists have given greater attention to self-assessment measurements such as satisfaction, acceptance and benefit. It is fairly common knowledge among dispensing audiologists that meeting specific target gains dictated by a given fitting formula does not guarantee acceptance, perceived benefit or satisfaction.

Although the hearing aid response (digital or analog) may provide the patients with improved ability to respond more appropriately to his or her acoustic environment, if the hearing aid does not meet the patient's expectations it may be rejected entirely. In order to achieve acceptance of the hearing aid a well organized, patient management strategy is needed to gain a sense of benefit and satisfaction. Without appropriate counseling, during which the patient gains greater insight into his or her hearing problem, little progress may be realized. Hosford-Dunn & Huch (2000) have reported on the importance of hearing aid benefit, acceptance and satisfaction. Huch & Hoford-Dunn (2000) offer an excellent overview of the various self-assessment tests to determine patient satisfaction, benefit and acceptance.

Some thoughts about the retail cost of DSP hearing instruments.

The question that the audiologist must address is; Have the retail costs of hearing aids been over-inflated by the introduction of DSP technology and other high performance hearing aid?. (Sandlin 2000)

There are those who would answer this question by suggesting that setting the retail cost of DSP hearing aids is the responsibility of the individual dispenser and not some local, national or international body. Others may argue that the benefits of DSP should be available to all with hearing impairment and that high cost is contrary to the social conscience.

We live in a capitalistic society, which defends the rights of individuals to make decisions affecting their financial practices and security, within the confines of legality. If one embraces "free enterprise," the cost of a product is dictated by what the provider determines is fair and reasonable. Cost is determined, also, by what the market will bear.... Therefore, since the DSP hearing aids are in demand, there is a tendency to charge more for them than for analog instruments (Purdy, 2001, Ross and Beck, 2001).

Ideally, hearing impaired patients should have access to the latest hearing aid technology, if such technology is superior to all others. However, it is sheer fantasy to believe that this benevolent approach is based on "reality thinking." In almost all societies there are products and services that can only be afforded by those with sufficient capital. Whether it is the British Jaguar, the American Cadillac or the Japanese Lexus, they are not produced for the masses, but for those individuals who can afford them. Such is true for the current DSP hearing aid devices.

One is not discounting completely the possibility of less expensive DSP systems. As the cost of microprocessors drop and the manufacturer's production costs are reduced, the retail cost of the most recent DSP technology may be lowered as well.

Additionally, it is well within reason to suggest that "more economical" versions of DSP systems will become available without all of the sophisticated functions of the most expensive models. As a matter of fact, a major manufacturer of hearing aid systems has introduced a 100% DSP hearing aid at a cost substantially less than the more sophisticated DSP systems.

There is no magical solution to the relatively high cost of DSP hearing aids. Every dispensing audiologist and every manufacturer of DSP instruments must decide what position to take regarding electroacoustic sophistication versus cost.

Conclusion

The hearing health professional, whether physician, audiologist or dispenser, should be current with technological advances in hearing aids to offer guidance to those needing hearing aid amplification. DSP instruments are very sophisticated and offer a wealth of advantages and options, not available in standard technology.

Hopefully, this article can assist the hearing health professional to evaluate the contributions of DSP systems. Unquestionably, there has been much hyperbole associated with DSP hearing aids. Nonetheless, they do offer significant benefit to those with hearing impairment and portend great hope and promise for the future as we gain greater insights into the functioning of the human auditory system.

REFERENCES

Arlinger S, (1998). Clinical assessment of modern hearing aids. Scand Audiol 27 (Suppl 49): 50-53.

Arlinger S. Billemark E, Oberg M, Lunner T, Hellgren J. (1998). Clinical trial of the Oticon Digifous hearing aid. Scand Audiol 27:127-135.

Hosford-Dunn, H, & Huch, J. (2000). Acceptance, benefit, and satisfaction measures of hearing aid user attitudes. In R. E. Sandlin (Ed.) Textbook of Hearing Aid Amplification. San Diego, CA: Singular Publishing Group. 467-488.

Huch, J. & Hosford-Dunn, H. (2000). Inventories of self-assessment measurements of hearing aid outcome. In R.E. Sandlin (Ed.) Textbook of Hearing Aid Amplification. San Diego, CA: Singular Publishing Group. 489-556.

Knebel S, Bentler R. (1998). Comparison of two digital hearing aids. Ear Hear 19(4):280-289.

Kuk F. (1997). Technical Improvements to enhance the performance of CIC instruments. Hear Rev: High Performance Solutions 2:20-22

Kuk F, Kollofski C, Brown S, Melum A, Rosenthal A. (1999) Use of a digital hearing aid with directional microphones in school age children. J Am Acad Audiol 10; 535-548.

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Ludvigsen C, Topholm J. (1997). Fitting a wide dynamic range hearing instrument using real ear threshold data: a new strategy. Hear Rev: High Performance Solutions 2:37-39.

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Purdy, J.K. (2001). Roles in Successful Hearing Aid Fitting: Consumers, Audiologists and Manufacturers. Audiology Online. April 30, 2001. See www.audiologyonline.com, article archives.

Ross, M. and Beck, D.L. (2001). Expensive Hearing Aids: Investing in Technology and the Audiologist's Time. Audiology Online. March 26, 2001. See www.audiologyonline.com, article archives.

Sandlin R. (2000). Observations and future consideratons. In R. Sandlin (Ed.) Textbook of Hearing Aid Amplification. Singular/Thompson Learning, San Diego. pp 727-748.

Valente M, Fabry D, Potts L, Sandlin R. (1998). Comparing the performance of the Widex Senso digital hearing aids with analog hearing aids. J Amer Acad Audiol 9:342-360.

Valente M, Sweetow R, Potts L, Bingea B. (1999). Digital versus analog signal processing : effect of directional microphone. J Amer Acad Aduiol 10:133-150.

Valente M, (2000), Use of microphone technology to improve user performance in noise. In R. Sandlin (Ed.) Textbook of Hearing Aid Amplification. Singular-Thompson Learning, San Diego. pp 247-284.

Westermann S, Sandlin R. (1997). Digital signal processing: benefits and expectations. Hear Rev High Performance Hearing Solutions 2:56-59.

About the author:

Dr. Sandlin is an adjunct professor of Audiology at San Diego State University, San Diego, California. He has published over 100 articles and edited four textbooks relating to the hearing aid sciences. He served on the American Tinnitus Association's Scientific Advisory Board for over 20 years. He is in private practice at the following address:

Robert E. Sandlin, Ph.D.
728 Robinson Ave
San Diego, CA. 92103

Phone: 619 295 3243
Fax: 619 295 3378
e-mail: rsandlin@mail.sdsu.edu

Acknowledgement

The author wishes to than Dr. Michael Valente and Dr. Francis Kuk for their review of this article. Their suggestions regarding content and structure were invaluable. I thank them most sincerely.