The mechanisms inside hearing aids vary among devices, even if they are the same style. Today, almost all hearing aids have digital/programmable circuitry; though, the available features and the sophistication of particular aids varies considerably.  Typically speaking, the more sophisticated the hearing aid, the greater the cost. 

Digital/Programmable: The audiologist programs the hearing aid with a computer and can adjust the sound quality and response time on an individual basis. Digital hearing aids use a microphone, receiver, battery, and computer chip. Digital circuitry provides the most flexibility for the audiologist to make adjustments for the hearing aid. Digital circuitry can be used in all types of hearing aids. Today, almost all hearing aids are digital and programmable.                                  

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 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