by Todd A. Ricketts

There has been explosion in the number of digital hearing aids on the market in the last five years. At last count, there are 22 manufacturers with digital hearing aids marketed under 40 different model names. Manufacturers are moving toward their third or fourth generation of digital products. The technology is here to stay--but are digital hearing aids really better?

Digital hearing aids first came to market in 1987 with two manufacturers introducing hearing aids with digital signal processing (DSP) before the end of the 1980s. While high-tech for their time, these hearing aids had little success and were soon abandoned due to their large size and high battery drain.

Nearly a decade later, two separate manufacturers once again introduced digital hearing aids. 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 were well received by clinicians and consumers. This early success, combined with the promise of highly advanced signal processing, ensured that digital hearing aid technology had come of age.

So how far have we come? What is the current "state-of-the art" technology in digital hearing aids? Are digital hearing aids really superior to their analog counterparts? To determine whether digital hearing aids are better for patients, it is important to focus on the superior processing and features of these instruments. Digital hearing aids can't be described as if they are a separate entity from analog hearing aids. "Digital" simply indicates that the analog waveform is converted into a string of numbers for processing; and unfortunately, there is nothing inherently magical about this process. A linear, output-clipping, digital hearing aid could easily be built that would provide sound quality and speech recognition inferior to many analog hearing aids. Therefore, digital isn't superior just because it's digital, but because DSP allows manufacturers to create hearing aids with enhanced processing and features.

The Digital Advantage

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.

Todd A. Ricketts is an assistant professor in the department of hearing and speech sciences at Vanderbilt University and director of the Dan Maddox Hearing Aid Research Laboratory. His research interest is focused on various high-tech aspects of hearing aids and their impact on listener's benefit from, and satisfaction with, amplification. Contact him by email at todd.a.ricketts@vanderbilt.edu.