The technology explosion of the last 10 years has affected every facet of our daily and professional lives. As audiologists and hearing instrument dispensers, we fit hearing instruments made of integrated circuits programmed through sophisticated computer software. Digital hearing aids and digitally-based hearing instrument fitting systems are here and they will, in all likelihood, soon dominate the market. Currently, approximately one-third of all hearing instruments sold are digital signal processing (DSP) units (Strom, 2002).
Audiologists and other researchers have invested tremendous time and effort developing innovative ways to best fit the amplification needs of the hearing impaired patient. Specifically, digital signal processing (as it applies to hearing aid technology) and multi-media fitting software both play significant roles in the maximal customization of the hearing instrument fitting.
Traditionally, one of the goals of hearing aid fittings has been to customize the instrument's response to maximally fit an individual's hearing loss and life-style requirements. The evolution of hearing aid fitting technologies has improved our ability to reach this goal, particularly with the introduction of DSP in hearing aids. DSP instruments with adaptive capabilities can respond quickly and specifically to acoustic changes in the environment. Additionally, DSP instruments can actively engage more than one circuit option at a time, to better manage the acoustic environment with respect to the patient's desires and needs.
Of course, we all realize that variations exist among and between individuals with respect to their type and degree of hearing loss. However, room acoustics vary from room-to-room, and within a given room, as do various quantities and qualities of background noise. In the final analysis, there are almost limitless permutations of acoustic environments and hearing impairments. With DSP adaptive capabilities, hearing aid technologies have advanced to the point of being able to react to real-time changes in listening environments, while better addressing the individual's acoustic and environmental listening needs.
Hearing aid researchers and audiologists have known for many years that simply making sounds louder (a relatively simple task for today's electronic components) is not sufficient to address the listening requirements of most patients. Several non-DSP hearing aid technologies emerged over the last few decades to address specific hearing loss and environmental needs.
Analog-based multi-channel hearing aids addressed the need for frequency response shaping for different hearing losses. For example, many individuals with high frequency hearing loss, might need circuits that only amplify high frequency sounds. Wide dynamic range compression (WDRC) circuits were developed to compensate for the effects of outer hair cell damage in the cochlea by varying the amount of amplification provided, based on the loudness of the sound as it entered the hearing aid circuit. This was a step beyond compensating for loss of sensitivity to sound (making all sounds louder) because it also addressed loudness recruitment, a phenomenon whereby the person with hearing loss cannot hear soft sounds in a normal fashion, but does indeed hear loud sounds normally (Villchur, 1996).
Directional microphones were first introduced in non-DSP hearing aids in 1971. Directionality in hearing aids provided assistance in noisy listening environments (Agnew, 1999; Hawkins & Yacullo, 1984). Programmable analog circuits provided listeners with multiple programs for multiple listening environments, which allowed the hearing aid wearer to change the response of the circuit by pushing a button.
A. Digital Signal Processing (DSP)
Today, DSP capabilities are taking these technologies even further. DSP has provided the opportunity to address additional consequences of hearing impairment beyond the loss of sensitivity, such as poor frequency resolution and degraded signal-to-noise ratio. DSP can manipulate sound in ways that were not previously possible.
Adaptive signal processing algorithms are available in today's DSP hearing instruments. Adaptive signal processing features include, but are not limited to, directional microphone systems, feedback management, and noise reduction systems.
For example, adaptive directional systems can change the pattern of sensitivity to sounds to maximize hearing speech in a noisy environment; adaptive feedback management addresses hearing aid whistling/squealing by mathematically analyzing the feedback path and reducing the tendency to squeal (Dyrlund, et al, 1994; Kates, 1999; Edwards, 2000); adaptive noise reduction systems reduce gain for signals recognized as non-speech signals.
Adaptive Directional Systems
"Directionality" with regard to hearing aids, indicates that the hearing aid treats sounds coming from one location (for example, the front) differently than it does sounds originating from a different location (for example, the rear). Effective directional systems traditionally provided the listener with an improved signal-to-noise ratio for sounds coming from the front, as compared to sounds originating from the rear (Hawkins & Yacullo, 1984; Valente, et al, 1999). Maintaining a positive SNR is important in maximizing audibility in the presence of noise (Killion, 1997).
Directionality can be achieved using a single directional microphone with multiple ports, or with a dual microphone system. A potential advantage of digital dual microphone systems is that multiple patterns of directionality can be applied to the same instrument (Mueller & Ricketts, 2000). Different listening environments demand different patterns of sensitivity to acoustic stimuli to achieve optimum results.
Many directional systems are static, i.e., they use fixed patterns of microphone sensitivity. Because the adaptive system changes directionality patterns based on the acoustic environment, the adaptive system is more likely to provide optimum directionality for multiple listening situations.
Many factors impact the amount of benefit derived from a static directional system. Some of these factors are location of target noise sources, head shape, device style, microphone placement, and microphone drift (degraded performance over time), which may occur in any dual microphone system (New Digital Hearing Instrument Technology, 2001).
Further, with many "fixed" directional systems, the hearing aid user must be aware of when to choose to listen through the directional system. In other words, success is dependent not only on the technology, but also on patient awareness, dexterity, comprehension and training in the use of a directional microphone system.
These issues led to the development of adaptive directional microphone systems that can adapt in and out of their directional mode based on environmental sounds - the patient does not have to choose to activate the adaptive directional system.
In addition, adaptive directionality techniques exist which can choose an optimal directional pattern for many noisy situations, based on an analysis which occurs 250 times per second. This can help address the effects of head-shadow and microphone placement on the directional pattern. Rapidly changing background noise sources are maximally managed with fast-acting directional pattern selections. Obviously, these automatic systems can react far more quickly than would be the case if the patient had to switch manually into the optimal directional listening mode.
An additional benefit to hearing aid wearers utilizing digital technology is the implementation of adaptive microphone matching techniques. Hearing aid microphones may drift in their sensitivity over time due to the build up of debris, cerumen, skin flakes, and basic "wear and tear." Even a slight mismatch between microphones in a dual microphone directional system can reduce the effectiveness of the directionality. An adaptive microphone matching system helps insure that the dual microphone directional system will provide the optimal directional pattern and benefit over time (New Digital Hearing Instrument Technology, 2001).
Adaptive Feedback Management
The management of acoustic feedback has changed significantly with digital signal processing techniques. External acoustic feedback occurs when amplified sound from the receiver travels back to the microphone and is re-amplified. Traditional methods of eliminating feedback include reducing the gain, reducing the vent size, or tightening the fit of the prosthetic in the ear canal, so less amplified sound from the receiver reaches the microphone.
With digital technology, mathematical estimations of the feedback path can be made and used to compensate for the feedback, essentially without affecting the input signal, while ideally preserving the desired output. This method provides an added 6-10 dB average headroom improvement and possibly avails more useable gain (Edwards, 2000; Olson, et al, 2001).
The acoustic feedback path is another factor that varies not only among individuals, but also within the same individual, depending on various factors. Changes in hairstyle, wearing a hat or other head covering, the style and length of shirt collars, head and body positions, cerumen build-up, and insertion characteristics of the instrument all affect the feedback path. DSP technology can account for these and other changes by actively monitoring and adapting to them (Kates, 1999). In addition, estimation of the feedback path can help predict the maximum gain for each frequency before acoustic feedback occurs, allowing further customization of the fit. This technology addresses the exact fit of a given instrument on a given ear (New Digital Hearing Instrument Technology, 2001).
Listening in noise is a major problem with many hearing aid fittings, and for good reason. It has been reported that one of the most common reasons that hearing aid users do not wear their hearing aids is "poor performance in background noise" (Kochkin, 1999). It is no wonder that much time and effort has been spent on developing noise reduction technologies. Noise reduction systems have become almost "common-place" in digital hearing aid technology.
The majority of systems are modulation based. That is, highly modulated signals (with changing patterns of high and low frequency, and soft and loud sounds) such as speech are preserved, while steady-state background noise is reduced. These processing schemes may provide the listener with improved comfort in noisy listening environments (Boymans & Dreschler, 2000; Walden, et al, 2000). Different amounts of noise attenuation can be selected, further customizing the hearing aid performance for each individual.
B. Multi-Media Technology
In addition to digital signal processing, hearing health care professionals have at their disposal multi-media fitting software. The combination of these two technologies holds great promise for bringing a higher level of customization to the hearing instrument fitting and fine-tuning process.
Listening environments similar to a hearing aid user's real-life listening situations can be artificially, but realistically, recreated using "surround sound" recordings. This technology may be used to verify and program the customized and adaptive signal processing schemes provided by DSP instruments. By placing the patient in a calibrated, surround-sound environment and presenting high-quality recordings of sound scenarios and individual sounds, a precise, personalized fine-tuning of the patient's hearing instruments can be accomplished (for a review of these technologies, please see Russ and Olsen, 2001).
Hearing aid fitting strategies are becoming more concerned with individual patient needs and desires. Multi-media fitting software allows the hearing healthcare practitioner to better reach these goals. Referred to as a "Patient-Driven Approach," (Lindley et al, 2000) this protocol of caring for hearing-impaired patients advocates focusing less on the "bells and whistles" of a given hearing instrument or circuit, and more on addressing specific patient requirements for listening and understanding speech.
Furthermore, fitting software programs offer simulations of hearing loss, allowing the patient's spouse, family or significant other to better appreciate the difficulties hearing loss can cause (Russ & Olsen, 2001).
Multi-media fitting software that provides real-world sound representations is a powerful tool for hearing health care professionals, allowing them to address and perhaps replicate specific listening requirements for the individual patient (Meskan & Robinson, 2000). For example, when verifying comfortable speech levels, sound samples are available that present male or female, children or adults, with and without background noises. Thus, the hearing healthcare professional can choose the sound sample that is most relevant to the patient's complaint (e.g., "I can't hear my grandchildren"). Such real-life presentations can also help in counseling the patient and setting realistic expectations for listening situations that are particularly meaningful each individual patient.
Researchers continue to evaluate ways to use the adaptive DSP features discussed earlier, combined with multi-media technology, to further improve and customize hearing instrument fittings.
Future research considerations include; exploring multi-media as a means to demonstrate DSP features, using multi-media as an aural rehabilitation tool to develop anticipatory coping strategies, and potentially to re-train auditory systems to 'listen smarter'. We can look forward to future reports on these topics.
As is true in so many other fields, digital technology has become the conduit through which sophisticated technical solutions have occurred. Better hearing instruments and more customized hearing aid fittings have resulted from DSP technology. Acoustic problems that seemed almost insurmountable a few years ago are very near solution. Current DSP techniques, coupled with computer-driven software and multi-media fitting systems, provide the platform for hearing health care professionals and patients to benefit the most from today's fitting technologies.
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DSP & Multi-Media Technologies: Addressing Patient NeedsDSP & Multi-Media Technologies: Addressing Patient Needs
March 11, 2002
This article is sponsored by ReSound.
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