Biomedical Engineering Reference
In-Depth Information
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 may have been annoying to the user.
Digital feedback reduction (DFR).
The most advanced feedback reduction schemes mon-
itor for feedback while the listener is wearing the hearing aid. Moderate feedback is then
reduced or eliminated through the use of an out-of-phase cancellation system or notch
filtering. DFR can substantially benefit users who experience occasional feedback, such
as that associated with jaw movement and 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. Al-
though research findings supporting the efficacy of DNR systems are mixed, they do indi-
cate that the DNR can work to reduce annoyance and possibly improve speech recognition
in the presence of nonfluctuating noise. DNR is sometimes advocated as complementary
processing to directional microphones. While directional microphones can reduce the lev-
els 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 whether it is particularly effective is 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, to control the shape
of the directional pattern, to automatically switch between directional and omnidirectional
modes, and through expansion to reduce additional circuit noise generated by directional
microphones.
Frequency shifting.
A new innovation has been to shift some of the high-frequency
components of the audio signal into a lower-frequency band where the patient is still able
to hear. One of the Phonak hearing aids uses a nonlinear frequency compression technique
to downshift the 3 kHz to 6 kHz band down to the band spanning 1.5 kHz to 2 kHz. Though
this process takes some time to get used to, it offers significant advantages in regard to
speech comprehension.
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 to obtain fitting
information specific to an individual patient in combination with a specific hearing aid.
Sound levels also can be verified through the hearing aid once it is fitted. This technology
has the potential both to increase accuracy of hearing aid fittings and to streamline the
fitting process by reducing the need for external equipment (Ricketts, 2008).
Other capabilities include improved frequency shaping with fine control of up to
16 overlapping channels, binaural processing, pinna and ear canal filtering, as well as
