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Hearing Aid Designs
Technological advances in the areas of electronics and digital signal processing have been incorporated by the hearing aid industry, leading to impressive strides in the design of hearing aids.
Two major trends in the design of hearing aids can be identified;
1) long-term, toward miniaturization and 2) more recent, involving the use of digital technologies for rapid programming of hearing aids and advanced processing capabilities.
CIC Hearing Aids
The trend toward miniaturization has been driven largely by the desire for less visable hearing aids. Within the last several years, completely-in-the-canal (CIC) hearing aids have become available and should meet the cosmetic needs of most users. These hearing aids fit deeply into the ear canal and are virtually invisible unless one peers directly into the canal. They are available from several manufacturers.
Candidates for CIC hearing aids are persons with mild to severe hearing loss. Patients with severe to profound hearing loss are not candidates for the CIC hearing aids because of limitations in the physical gain possible with these aids. Other potential hearing aid wearers may not be candidates for CIC's because of the nature of their hearing loss or because they lack the manual dexterity to fit and remove such a small hearing aids. Others have ear canals, which are too small to accommodate even the CIC hearing aid style and some hearing aid users can not tolerate long ear canal molds because of discomfort. Thus ear canal anatomy and individual manual skills play a part in determination of candidacy for the CIC hearing aid.
The impression used to make the hearing aid shell for the CIC must be taken with considerable accuracy and conform to the shape of the canal or the hearing aids can slip out or move when chewing. Some persons complain that the CIC is uncomfortable and in these instances the impression must be repeated.
Because the CIC fits deep into the canal, the output is closer to the eardrum and thus the amount of amplification available is greater for the same amount of power as in other hearing aids. Some patients report that the CIC sound is clearer than their previous aid and others find listening on the telephone is easier especially if they previously had to remove their hearing aids. Because the CIC fits so snugly in the canal, acoustic feedback or the all too common whistling sound familiar to hearing aid users, may be eliminated. Wind noise is also reduced because of the placement of the microphone inside the ear canal.
Unfortunately, you will discover that CIC's are more expensive than the more conventional in-the-ear hearing aids.
Programmable Hearing Aids
An increasingly popular trend in hearing aid design is the use of digital technology in the design and programming of hearing aids. Slightly more than a decade ago, digitally programmable hearing aids were introduced and have found a firm and growing niche in the market. These hearing aids still contained amplifiers, filters, microphones, etc., as in the available more conventional analog hearing aids but they can be programmed rapidly for the individual user using software programs. Even greater attention has been given to the more recent availability of "true digital" hearing aids, that is, hearing aids which contain a microprocessor which governs their daily operation.
Confusion Between Programmable and True Digital Aids
There is sometimes confusion between analog hearing aids, which are can be programmable and true digital hearing aids. Both are often referred to as "digital." To clarify, some hearing aids on the market are programmable or can be fitted using digital techniques, but the aid contains conventional analog devices for its daily operation. True digital hearing aids contains a microprocessor, which controls the daily operation of the instrument. The latter hearing aids are digitally programmable and also use digital technology for day-to-day operation. Of course, even a true digital hearing aid utilizes a conventional analog microphone to "pick up" the signal and a receiver to output the amplified sound into the ear canal of the wearer.
Digital Hearing Aids
True digital hearing aids became available in the late 1980's but these early instruments were housed in body-worn shells with high power consumption. These obstacles were overcome and in the mid-90's true digital hearing aids became available not only in behind-the-ear shells, but recently for in-the-ear and CIC models. Because of the nature of digital techniques, digital hearing aids can be quickly programmed and reprogrammed with a wide variety of characteristics.
While digital hearing aids do not currently eliminate feedback, several of the current instruments are more effective in reducing feedback problems than the more common filter systems found in conventional aids. There is no question that digital hearing aids have increased flexibility and precision, but, the important issue is whether or not they provide greater benefit to users than a well-fitted more conventional analog instrument.
Data suggests that objective performance improvements, as measured by speech recognition in quiet or noise, are somewhat ambiguous when comparing digital instruments to well-fitted conventional aids. Some reports have suggested that subjective performance is improved, but these results have been difficult to document using objective techniques. Presently, the noise reduction systems available in digital hearing aids have not been demonstrated to significantly improve listening to speech in noisy backgrounds or other voices, a problem of extreme importance to almost all hearing aid users. In this regard, a limitation to the successful application of digital techniques to hearing aids is a fundamental lack in understanding of the best way to process speech in order to make it more intelligible to persons with hearing loss.
All digital hearing aids contain one or more useful characteristics such as multiple bands for fine tuning, adaptive compression to keep constantly varying speech signals within the audible range and directional microphones to reduce background noise levels while maintaining the speech level. However, many programmable hearing aids with conventional analog parts may also have these often, desirable characteristics.
Impressive strides, however, have been made over the past two decades in the digitization of hearing aids. Digital hearing aids are available in small, powerful packages that no doubt surpass the expectations that might have been anticipated some 20 years ago. The clinical advantages of such instruments may take longer to demonstrate than the obvious technical advantages.
The popularity of digital hearing aids, suggest that many patients prefer the sound quality and overall performance of these aids relative to previously worn conventional aids. Competition is also increasing as more and more manufacturers are making digital hearing aids available. These digital hearing aids are not all the same, some manufacturers have developed "in-house" processing schemes which differ from others.
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