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Digital Signal Processing and Hearing Aids

Digital technology has revolutionized everything. The list of digitally based improvements and alterations is vast. Simply - everything has changed. The way we work and the way we play, the way we listen to and create music, hand-held portable digital cell phones, all manner of data information collection and management, the way we pay bills, digital and satellite TV, digitally controlled transportation systems, computers in the car, computerized assembly lines to build the cars, on-line education and even digitally based medical care and management (from digital thermometers to digital radiological scanners) and of course, digital hearing aids!

Hearing aids were previously somewhat customized for an individual's hearing loss and physical preferences (style of hearing aid). However, the ability to address each individual's unique listening environments and specific, unique acoustic needs is a relatively new development. With digital signal processing (DSP), hearing aid technologies have advanced to the point where each individual fitting can be tailored to the individual's acoustic and environmental needs. Indeed, with DSP technology, the sound quality and sound processing ability of hearing aids is tremendous. Almost every DSP hearing aid has thousands and thousands of variations and circuit options, all contained within the final product. This flexibility allows the audiologist to "tune" the hearing aid to the most preferred sound quality and the most needed sound processing characteristics. Importantly, this flexibility allows the audiologist to "retune" the hearing aids.

Although some hearing losses appear similar, they are each unique. For example, many people have "normal" hearing. Imagine if ten people with "normal" hearing are in the same cafeteria, all listening to the same thing. Of course, they will each likely "hear" different things. Then, consider the variation that occurs when we add into the equation the vast range of human hearing, and the vast types and degrees of hearing loss. Each person has a different perception of sound and each person has a different hearing loss. Importantly, the "perceived" sound is not just a matter of the hearing loss. Rather, the sounds you hear are determined by your ears and brain working together - and the results are virtually limitless! Therefore, when you have ten people with similar hearing loss (for instance all with mild to moderate high frequency hearing loss), they will probably require different hearing aid settings to best fit and meet their desired perception of sound. Some people like sounds quieter, some like them louder, some can tolerate more noise, and some can tolerate less noise. DSP hearing aids allow the audiologist to alter the hearing aids to better fit the patient's needs.

Variations exist not only among individuals' hearing losses and how their brains process sounds, but also within an individual's listening environments. A single hearing aid setting will not accommodate every listening environment. For example, a circuit that is ideal in quiet conversation may be too loud in a noisy room. High quality hearing aids are able to react differently to sounds originating from a telephone than to voices in a large crowded room. Additionally, within that crowded room, there can be variable room acoustics, as well as changing quantities and qualities of background noise. The demand for flexible hearing instruments has been apparent for years and has driven the development of hearing aid technologies.

Indeed, hearing aids have evolved over the years. Hearing aid researchers and audiologists have realized that making sounds louder (a relatively simple task for today's electronic components) is not sufficient for meeting the listening requirements of most patients. DSP technology has allowed us to better address the needs of individuals across a variety of listening situations.

Several types of hearing aid technologies have emerged and been incorporated into hearing aids. Multi-channel hearing aids were developed to provide precise amplification levels for different frequencies (or pitches). For example, many individuals with high-frequency (high-pitch) hearing loss need circuits that only amplify high-pitched sounds. Wide dynamic range compression (WDRC) circuits were developed to compensate for the effects of damage in the cochlea (inner ear) by varying the amount of amplification for different levels of sound. This is a step beyond compensating for loss of sensitivity to sound (by making all sounds louder), because it also addresses loudness recruitment. Loudness recruitment is a phenomenon whereby the person with hearing loss cannot hear soft sounds, but hears loud sounds normally (Villchur, 1996). Directional microphones, which vary the amount of sound picked up from different directions, were first introduced in 1971 (Agnew, 1999). Directional microphones provide assistance in noisy listening environments (Hawkins & Yacullo 1984, Valente et al, 1999). Programmable analog circuits provided listeners with multiple programs for multiple listening environments, which allow a hearing aid wearer to change the performance of the circuit by pushing a button.

DSP, as applied to hearing aids, takes these technological advances one step further. Digital signal processing refers to complex, computer controlled, mathematical manipulation of sound, allowing more precise fine-tuning. One way of considering this technology is to think of a pencil drawing that can be changed by erasing and redrawing lines. If that same image is on a computer, it's possible to magnify the drawing into a series of dots, allowing unlimited changes to the picture. The changes can be minute, or enormous. Changes made on the computer can be more precisely accomplished than can be done with a pencil. In a hearing aid with DSP, a similar process occurs. DSP converts sound waves into a series of numbers, which are changed to accommodate the hearing loss and the listener's preferences, and then the numbers are re-converted into sound waves as the signal passes through the hearing aid.

A major advantage of DSP technology is that is can be adaptive, i.e., the performance of the hearing instrument can change according to the listening needs of the hearing aid wearer.

Adaptive DSP algorithms are used regularly in today's digital hearing instruments. As the name implies, "adaptive technologies" adapt to the sounds in the environment, and, in some cases, modify the hearing aid performance over time. These types of signal processing features include, but are not limited to, adaptive directional systems, adaptive feedback management, and noise reduction systems.

For example, an adaptive directional system can change the pattern of sensitivity to sounds to maximize hearing speech in a noisy environment; adaptive feedback management stops hearing aid whistling/squealing by mathematically analyzing the feedback path, and reducing the tendency to squeal (Dyrlund, et al, 1994); noise reduction systems reduce gain for signals recognized as non-speech signals.

Adaptive Directionality

"Directionality," with regard to hearing aids, means that the hearing aid treats the sound from one location (for example, the front) differently than it does sounds that occur from another location (for example, the rear). So in brief, the goal is usually to achieve the most audibility for sounds coming from the front of the listener. This provides the listener with an improvement in signal-to-noise ratio (loudness of speech compared to loudness of noise) for sounds coming from the front compared with sounds coming from other directions (Agnew, 1999). Maintaining a positive signal-to-noise ratio (SNR) is important in maximizing speech audibility in the presence of noise (Killion, 1997). This is particularly useful in noisy environments such as a restaurant or cocktail party. Different listening environments demand different patterns of sensitivity to sound to achieve optimum results.

Directional systems can be static, meaning they use a fixed pattern of sensitivity to sound, or adaptive, meaning they automatically change in response to varying sound levels. A fixed system is not as likely to provide the optimum directionality for every listening situation, whereas an adaptive system can change directionality patterns to suit the environment. Furthermore, the hearing aid user must be aware of, and may want control of, when to choose to listen with the directional system. Patient success is dependent not only on technology, but on patient awareness, dexterity, and training in the use of a directional microphone system. Some people are able to use and control directional hearing aid technology, some need automatic systems.

These issues led to development of adaptive directional microphone systems that can adapt the directional mode based on noise levels - the patient listens, and the hearing aid adapts, based on the sounds in the environment. Adaptive directionality techniques can choose the best directional pattern for any given situation. Some systems can sample the incoming sound at a rate of 250 times per second. Obviously, these automatic systems can react far more quickly to an ever-changing sound environment than can any person.

An additional benefit to hearing aid wearers from DSP technology is the implementation of adaptive microphone matching techniques. An adaptive microphone matching system helps insure that the dual-microphone directional system will provide the optimal directional pattern and benefit over time.

Adaptive Feedback Management

The management of acoustic feedback ("whistle" or "squeal") has changed significantly with new DSP technology. Feedback occurs when the amplified sound from the hearing aid travels back to the microphone and is re-amplified ("feedback path"). Hairstyle, hats, hugs, style and length of shirt and coat collars, scarves, earwax build-up, physical fit and style of the hearing aid, loudness of the hearing aid, and other factors all impact the feedback path.

Digital technology can account for these and other changes in the feedback path by actively monitoring and adapting to them to minimize or eliminate feedback. With DSP, estimations of the feedback path can be made, and this information can be used to subtract out the feedback portion of the hearing aid output, preserving the original signal and desired output. This yields automatic feedback elimination without gain reduction or instrument modification, which are the traditional methods of eliminating feedback.

Noise Reduction

Listening in the presence of noise has been a problem for many hearing aid wearers. No wonder so much time and effort has been spent developing noise reduction technologies! Noise reduction systems have become almost "common-place" in DSP hearing aid circuits. The majority of noise reduction systems are based on identifying patterns of sound, and reducing the loudness of certain sound signatures. That is, we know that speech has a variety of patterns. Therefore, if the sound pattern appears to the DSP hearing aid to be speech, the goal is usually to preserve the gain applied to the signal. However, if the pattern is not like speech, with respect to loudness, pitch and timing characteristics, the goal is usually to reduce the gain applied to that signal. These noise processing models may provide the listener with improved comfort in noisy listening environments (Boymans & Dreschler, 2000; Walden, et al, 2000).

As is true in many other fields, digital technology is making a tremendous impact on developing better hearing aids. Acoustic problems that seemed almost insurmountable a few years ago are very near solution. There is lots of good news for those who can benefit from wearing hearing aids. Today's DSP technology allows more people than ever before to benefit from hearing aids, and enjoy fuller, more satisfying listening experiences.

REFERENCES:

Villchur, E. "Multichannel Compression in Hearing Aids," in Hair Cells and Hearing Aids, C. Berlin ed., Singular Publishing Group, Inc. San Diego, 1996.

Agnew, J. "Challenges and some solutions for understanding speech in noise," High Performance Hearing Solutions, Vol 3, Hearing Review Supplement, vol. 3, Jan, 1999.

Hawkins, D. & Yacullo, W., "Signal to Noise Ratio Advantage of
Binaural Hearing Aids and Directional Microphones Under Different Levels of Reverberation", JSHD, 1984, 49:278-286.

Valente, M, Sweetow, R, Potts, LG, Bingea, B (1999). Digital Versus Analog Signal Processing: Effect of Directional Microphone. Journal American Academy of Audiology 10:133-150.

Dyrlund O, Henningsen LB, Bisgaard N, Jensen JH, "Digital feedback suppression (DFS). Characterization of feedback-margin improvements in a DFS hearing instrument," Scand Audiol 1994;23(2):135-8.

Killion, M, "SNR loss: I can hear what people say, but I can't understand them," Hearing Review, 1997, vol 4 (12) pp. 8,10,12,14.

Boymans, M, Dreschler, WA, Field trials using a digital hearing aid with active noise reduction and dual-microphone directionality.
Audiology, 2000 Sep-Oct:39(5):260-8.

Walden, BE, Surr RK, Cord MT, Edwards B, Olson L, "Comparison of benefits provided by different hearing aid technologies," J Am Acad Audiol. 2000 Nov-Dec;11(10):540-60.

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