Digital Hearing Aids
The following are a few advantages digital hearing aids have over the more common hearing aid:
Gain Processing. 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, 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.
Digital Feedback Reduction (DFR). Moderate feedback is 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 noise is detected. Research findings indicate that the DNR can work to reduce annoyance and improve speech recognition in the presence of non-fluctuating noise. DNR is sometimes advocated as complementary processing to directional microphones.
Digital Speech Enhancement (DSE). These systems act to increase the relative intensity of segments of speech. Current DSE processing identifies and enhances speech based either on temporal, or more recently, spectral content.
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, 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 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.