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Features
Fortunately, for both dispensing audiologists and patients, there are
features and advanced signal processing schemes available in current
digital hearing aids that do have significant advantages over those
found in analog instruments. Potential digital advantages include those
related to:
 Gain
Processing. One of the primary benefits associated with
flexible gain-processing schemes is the potential for increased
audibility of sounds of interest without discomfort resulting from high
intensity sounds. While this is more generally a benefit of compression
rather than digital processing per se, the greatly increased flexibility
and control of compression processing provided by DSP--such as input
signal-specific band dependence, greater numbers of channels, and
kneepoints with lower compression thresholds--can lead to improved
audibility with less clinician effort. Expansion, the opposite of
compression, has also been introduced in digital hearing aids. This
processing can lead to greater listener satisfaction by reducing the
intensity of low-level environmental sounds and microphone noise that
otherwise may have been annoying to the user.
Digital
Feedback Reduction (DFR). The most advanced feedback
reduction schemes monitor for feedback while the listener is wearing the
hearing aid. Moderate feedback is then reduced or eliminated through the
use of a cancellation system or notch filtering. DFR can substantially
benefit users who experience occasional feedback, such as that
associated with jaw movement and close proximity to objects.
Digital
Noise Reduction (DNR). This processing is intended to
reduce gain, either in the low frequencies or in specific bands, when
steady-state signals (noise) are detected. Although research findings
supporting the efficacy of DNR systems are mixed, they do indicate that
the DNR can work to reduce annoyance and possibly improve speech
recognition in the presence of non-fluctuating noise. DNR is sometimes
advocated as complementary processing to directional microphones. While
directional microphones can reduce the levels of background noise
regardless of its temporal content, they are limited to reducing noise
from behind or to the sides of the user.
Digital
Speech Enhancement (DSE). These systems act to increase
the relative intensity of some segments of speech. Current DSE
processing identifies and enhances speech based either on temporal, or
more recently, spectral content. DSE in hearing aids is still relatively
new, and its effectiveness is largely unknown.
Directional
Microphones and DSP. The ability of directional hearing
aids to improve the effective signal-to-noise ratio provided to the
listener is now well established. In some cases, however, combining DSP
with directional microphones can act to further enhance this benefit. In
some hearing aids, DSP is used to calibrate microphones, control the
shape of the directional pattern, automatically switch between
directional and omnidirectional modes, and through expansion, reduce
additional circuit noise generated by directional microphones.
Digital
Hearing Aids as Signal Generators. Since digital hearing
aids have a DSP at their heart, they are able to generate--as well as to
process--sound. Current digital hearing aids use this capability to
perform loudness growth and threshold testing in order to obtain fitting
information specific to an individual patient's ears in combination with
a specific hearing aid. Sound levels also can be verified through the
hearing aid once it is fit. This technology has the potential both to
increase accuracy of hearing aid fittings and potentially streamline the
fitting process by reducing the need for some external equipment.
Current digital hearing aids are certainly exciting, and the future
possibilities are endless. Before long, digital hearing aids will
replace their analog counterparts altogether. We must, however, present
this technology to patients in an informative and educational manner.
Like many other high-tech devices, high expectations often accompany
digital hearing aids. Counseling patients about appropriate expectations
will continue to be more--not less--important as technology continues to
advance.
References
Excerpts from May 1999, NIH Pub. No. 99-4340
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