There has been explosion in the number of digital hearing aids on the market in the last five years. Digital hearing aids first came to market in 1987, but weren’t readily sold until the late 90’s. By this time, the technology had improved so that these hearing aids could be produced in a range of popular styles, from behind-the-ear (BTE) to completely-in-the-canal (CIC). Despite their higher cost, they offer superior processing and features.
"Digital" indicates that an analog waveform is converted into a string of numbers for processing, allowing manufacturers to enhance processing and add features.
One of the primary benefits is the potential for increased audibility of sounds of interest without discomfort resulting from high intensity sounds (compression). Expansion, the opposite of compression, has also been introduced in digital hearing aids, 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.
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