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What is the Optimum Number of Hearing Aid Channels?

Lynda Clark, MS, CCC-A

January 9, 2006


Question

Are there any studies/research done regarding the effect of multiple bands and/or channels to increase the benefits received through amplification? If so, what is optimal? Also is there any research done on 'how long the hearing aids will last'?

Answer

Numerous studies have attempted to address the question concerning the benefits of multiple bands and/or channels and what is optimal. The difficulty in answering that question arises from the fact that hearing aids designed with the same number of bands/channels do not perform the same. Bands/channels can be used in different ways for different purposes. One manufacturer may use the bands/channels solely for frequency shaping, while another may implement independent signal processing schemes in a channel specific manner designed to enhance listening comfort. The shape or steepness of the channels/bands, the speed of the signal processing, and the overall bandwidth of the hearing aid all affect performance and contribute to sound quality.

The question of benefit is another difficult area to define. Benefit can refer to an objective, measurable improvement on a listening task; a user's perceived or subjectively reported improvement in hearing; and/or their perceived impressions in regards to comfort, ease of use and pleasantness of sound. Depending on how the study is constructed and the design of the hearing aids being studied, the optimal number of channel/bands will vary greatly.

The goal of a well designed hearing aid is to maximize speech understanding and sound quality. Furthermore, a well designed hearing aid should be able to fit persons with a variety of hearing loss configurations. With these goals in mind, Rickert and colleagues at Starkey Laboratories designed a study to answer the question, "how many independent signal processing channels are required to maximize speech audibility and to match a non-linear fitting formula for a variety of audiograms?" They began with 1156 audiograms and from those they identified 15 representative audiometric configurations. The fifteen audiograms were computationally fit to maximize speech audibility [as quantified by the Articulation Index (AI), ANSI S3:5] and to match targets from the Cambridge non-linear fitting formula using a range of channels varying from 1 to 18. The results, shown in the figure below, demonstrate that moving from 1 to 2 channels provides a statistically significant improvement in Articulation Index (AI), as does moving from 2 to 3 channels and from 3 to 4 channels. Although there is an improvement as the number of channels is increased to 5 or 6, the improvement is not statistically significant. After 6 channels there is no further improvement in AI at all.


Figure 1: Audibility quantified by improvement in Articulation Index as a function of the number for gain and compression channels. Each curve represents data from individual audiograms.


One might surmise from this study that the optimal number of channels is four. Four is the optimal number of channels if the channels are configured as they were in the study. In the Rickert, et al, study, the channels were independently configurable for gain and compression with adjustable crossover frequencies. There was no additional frequency shaping control. If additional frequency shaping (such as high and low pass filters) was available, 2 channels with an adjustable crossover frequency would have been sufficient. Without channel crossover frequency adjustments, more than 6 channels would have been needed to reach an optimal setting across the range of audiometric configurations.

The optimal number of channels and bands cannot be determined without considering the other design elements in a hearing instrument. For instance, advanced features such as noise management are often implemented on a channel-specific basis. Although I know of no research to confirm or deny the superiority of multi channel noise management, it is reasonable to assume that conservative noise management implemented on a channel specific basis which preserves audibility would be preferable to an overall gain reduction in the presence of background noise. However, in the absence of clinical validation studies, one should not assume that more channels or bands will equate to better performance or more benefit.

To answer the question of how long hearing aids will last, I would refer the reader to The Consumer Handbook on Hearing Loss & Hearing Aids: A Bridge to Healing, by Richard Carmen, Au.D., Editor. In the chapter written by Thayne Smedly, PhD and Ronal Schow, Ph.D entitled "Problem-Solving and Extending the Life of Your Hearing Aids", the writers state,
"You may have asked, 'How long will my hearing aids last?' Just as hearing aids will malfunction on occasion...it follows that they won't last indefinitely. This is true even for very expensive ones. For various reasons, cost being one of them, some wearers expect their instruments to last 10 to 15 years or more. Hearing aids that remain in useful service for this long are the exception rather than the rule. In fact, research has demonstrated that the typical hearing aid gets replaced about every five years."
Additionally, I poled a group of audiologist involved in fitting hearing instruments. The conventional wisdom of these professionals also indicates that hearing aids are generally replaced after three to five year of use. The three to five year number represents the buying habits of hearing aid wearers. The reader should bear in mind that hearing aids are replaced for a variety of reasons (aside from malfunctioning) such as the introduction of newer technology, the changing hearing needs of the user, or the ability to be fit with smaller instruments.

The factors that affect the life of a given hearing aid depend greatly on the care given. I contacted Starkey Laboratories Director of Quality Engineering Doug Link for more information on this mater. According to Doug, there are three primary environmental factors that will greatly affect the performance of a hearing instrument - Wax, Moisture and Mechanical Shock.

Wax from the ear can block the receiver and/or microphone ports. Daily cleaning is required to keep these ports open and prevent wax from migrating into the case of the hearing aid. Wax blocking devices in or on the receiver opening are effective in protecting the hearing aid. Individual hearing aid users would be well advised to consult a health professional about managing excessive wax build up in their own ears.

Exposure to moisture in the form of humidity, perspiration, and water all contribute to degrading the performance of a hearing aid over time. Hearing aids are designed and tested to withstand significant exposure to perspiration and humidity. Tests performed on sample products before market release include salt fog and damp heat testing. In salt fog testing, the instrument operation is tested in a warm (95F) salt fog chamber for multiple days. In damp heat testing, the aid operates for 21 consecutive days in a hot/humid environment of 40C & 95% relative humidity. The hearing aid is expected to function properly throughout both of these tests. It is important to realize that no instrument is immune to the ill effects of excessive moisture over time. Drying the hearing aid daily in one of the many available drying systems is highly recommended to ensure the longevity of the instrument.

Finally, permanent damage to a hearing aid can occur from being dropped, thrown, or stepped on. The damage may be visibly obvious, like a cracked case, or may be visually unnoticeable, like damage to the receiver resulting in distortion.

The bottom line is that proper care and maintenance can extend the useful life of hearing aids and enhance the listening enjoyment that they provide.

References
  1. Rickert M. Van Tasell DJ. Woods WS: Compression bands: How many are needed? Presented at the American Auditory Society, April 2000, Phoenix.

  2. ANSI S3.5: Method for Calculation of the Articulation Index. New York: American National Standards Institute, 1969.

  3. Moor BCJ, Alcantara JI, Stone MA, Glasberg BR: Use of a loudness model for hearing aid fitting: II. Hearing aids with multi-channel compression. Brit J. Audiol 1999; 33: 157-70.

  4. Carmen R (1998). The Consumer Handbook on Hearing Loss and Hearing Aids. A Bridge to Healing. Sedona, AZ: Auricle Ink Publishers.
Lynda Clark has been an audiologist for 19 years. She is a Product Design Audiologist at Starkey Laboratories and resides in Plymouth, MN. Lynda can be reached at lynda_clark@starkey.com.

For more information on Starkey, visit www.starkey.com


lynda clark

Lynda Clark, MS, CCC-A


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