The Core Features of Modern Hearing Aids

Donald J. Schum, Ph.D./CCC-A
Vice President, Audiology & Professional Relations
Oticon, Inc.

Introduction
Advanced technology hearing aids by Oticon and others, generally include many important, diverse and pragmatic features in their premium products. Although these features have a variety of brand and model names, it is important for audiologists to have a clear understanding of the basic benefits associated with each feature.

Therefore, the aim of this paper is to describe essential core features that help define modern amplification, each of which is worthy of consideration to fulfill the needs and desires of the hearing impaired patient. Unfortunately, no single feature in isolation meets the full range of patient needs associated with various degrees and configurations of hearing loss.

1. Multi-channel, Nonlinear Amplification
To account for the frequency-specific loss of sensitivity and the growth of loudness in the hearing impaired ear, multi-channel, nonlinear amplification is the amplification strategy of choice. The essential goal of this approach is to provide the sensorineural hearing loss patient access to nearly the same range of inputs as would be available to a person with normal hearing. Enough gain is provided to soft and moderate speech sounds to make them audible, with emphasis in the mid to high frequency regions. At the same time, gain is reduced as input increases to insure acceptable loudness for higher inputs.

2. Adaptation Managemen
Most fitting algorithms are based, wholly or in part, on estimates of loudness perception common to patients with similar degrees of sensorineural hearing loss. However, most commercial algorithms do not take into account the fact that new hearing aid users will need time to adjust to newly audible sound. 

In 1996, we (Oticon) introduced a fitting concept for fully automatic hearing aids called "The Adaptation Manager." The Adaptation Manager facilitates a smooth transition from hearing loss to full adaptation, by slowly and purposefully altering the adaptation process into three steps.

Step 1 is slight amplification, Step 2 is almost the full hearing aid prescription, Step 3 is the full prescribed algorithm setting. The Adaptation Manager allows the patient to adapt slowly and over time to the full prescription hearing aid fitting.

3. Intelligent, Fully Automatic Operation
Over a decade ago, Oticon was among the first companies to actively promote fully automatic hearing aids. As multi-channel, nonlinear amplification was emerging, we emphasized the potential for hearing aids which the user did not have to manually control. We explored and made available fully automatic hearing aids for patients that preferred them, and for fittings that did not allow manual controls, such as CICs. Of course, some people prefer user controls, and we have maintained user controlled VCs and other functions, for patients that prefer them. 

With the release of Syncro in 2004,  a new era in digital technology emerged, namely, the use of Artificial Intelligence (AI) in hearing aids. Based on a parallel processing architecture, AI allows simultaneous evaluation and multiple permutations, while automatically selecting and delivering  the maximal speech signal at a given point in time. This architecture allows maximal integration of all automatic features within a device (compression, adaptive directionality, noise reduction, feedback cancellation, etc.).  AI allows intentional optimization across functions while avoiding potential negative interactions. Importantly, AI allows selection of device settings based on a confirmed analysis of the output signal, rather than a prediction based on input data.

4. Intelligent Use of Directionality
Directionality is the only proven technology in hearing aids to improve the signal-to-noise (S/N) ratio.  However, directionality needs to be accomplished in a manner that makes the most sense for the patient and that reflects the true value of the technology.  Manually controlled directionality provides excellent signal enhancement when environmental conditions are appropriate. However, manually controlled directionality requires direct intervention by the user.

With our current generation of AI-based products, we have implemented automatic directionality switching and have pioneered multi-band, adaptive directionality. The multi-band directional system has allowed us to introduce an important new concept in directionality: split directionality.

Although directionality is essentially a positive feature, there are times when it is not advantageous for the patient. Additionally, patients rarely prefer the sound quality of a hearing aid in its directional setting in challenging acoustic environments. 

To that end, we have designed and implemented "Split Directionality." With Split Directionality, low frequencies (below 1000 Hz.) are kept in omni-directional, while higher frequencies are in directional mode. Split Directionality allows a good balance between speech intelligibility and sound quality.

5. Intelligibility-based Noise Reduction
The purpose of noise reduction circuits in hearing aids is to improve comfort and acceptability of the hearing aid fitting in noisy environments. Although modulation-based noise reduction systems allow identification and spectral analysis of noise in the environment, they can potentially reduce important speech information, if applied too aggressively. In modulation-based systems, speech signals can be protected if the system operates in a level dependent manner in which the magnitude of the gain reduction is proportional to the input level, and if gain reductions are applied cautiously from approximately 1000 to 3000 Hz. 

Alternately, "Synchrony Detection" (an Oticon feature) can be implemented as part of a noise reduction system. Synchrony Detection allows accurate identification of speech -- even at very poor S/Ns, allowing implementation of a noise management strategy that protects the speech signal anytime speech is present. Synchrony Detection can be implemented alone or in tandem with modulation analysis. The combination allows protection of important speech signal elements while allowing comfort based noise reduction across non-essential low, and extremely high frequencies if speech is present, or across the entire bandwidth when speech is not present.

6. Device Identity Options
Over the past few years, advanced digital signal processing hearing aids have implemented many sophisticated compression, directional, noise management and feedback management systems. These systems, although beneficial in and of themselves, have typically been applied "independently" across hearing aid products. There have not been clear, elucidated strategies as to how these systems should work together.

To present our sub-system combinations in a comprehensive manner, we created the concept of "Identities." An Identity is a discrete group of hearing aid settings covering a broad range of device dimensions, designed to operate maximally together. In general, when moving from one end of the Identity continuum to the other, the speed and amount of reaction to a change in the environment varies. On one end of the continuum, the instrument will have a subtle, slowly adapting reaction to a change in the acoustic environment, while the other end of the continuum allows a more rapid and aggressive change to occur.

7. Open Ear Acoustics (OEA)
Traditional hearing aid fittings generally closed off the ear canal (via occluding ear molds) to control unwanted acoustic feedback. Although closing off the ear canal allowed more sound pressure to reach the tympanic membrane, an uncomfortable "plugged-up" sensation known as "occlusion" regularly occurred.  Digital Signal Processing (DSP) technology allows feedback cancellation systems with large, non-occluding vents.  Therefore, two of the main problems faced by hearing aid wearers  (feedback and occlusion) have been effectively managed. OEA provides patients with a more natural and acceptable hearing aid experience from the very beginning of the fitting process, allowing greater acceptance and comfort.

Evolving the Process
Modern hearings aids are designed to allow hearing impaired patients to function as effectively as possible across a multitude of acoustic environments.  Employed features (described above) and the intelligent combination of these same features provide patients multiple benefits not previously possible.

Of course, learning to incorporate the new functionality available in modern hearing aids takes time and effort - and it is absolutely worth the investment! 

The different features described above assume relative importance depending on the specific needs and characteristics of each patient and the communication situations at a given time.

In 2005, it is not enough to focus solely on one area (i.e., directionality) as paramount in a given hearing aid fitting.  Rather, a comprehensive, integrated approach using the available features maximally, allows the highest levels of patient performance and satisfaction, and allows "feature synergy" to occur.

Final Thoughts
Regardless of the technology, the fitting of amplification has to be seen as part of a larger, patient-centered process. 

A successful patient-centered process includes a complete audiometric evaluation, a patient interview/case history, to reveal the patient's associated medical and audiologic history, and their motivation, needs and expectations. Audiometric test results, questionnaires, a physical exam of the ear, and a discussion about realistic expectations, appropriate counseling and a patient-centered one-on-one or group aural rehabilitation (AR) experience will maximize the hearing aid fitting.  Important support tools such as the COSI, multimedia educational software and eCAPS can provide valuable guidelines and help define realistic goals and their use is encouraged. By integrating excellent technology into a process that is constructed from the patient's point of view, the potential benefit from advanced technology is maximized.

COSI - Client Oriented Scale of Improvement - Dillon H, James A, Ginis J. (1997). The Client Oriented Scale of Improvement (COSI) and its relationship to several other measures of benefits and satisfaction provided by hearing aids. J Am Acad Audiol 8:27-43

eCAPS - Electronic Client Assisted Product Selection - Oticon Inc.