| A hearing aid is a device used to help the | | | | Gain Processing. One of the primary benefits |
| hard-of-hearing distinguish sounds and converse | | | | associated with flexible gain-processing schemes is |
| normally. In the past, hearing aids consisted of a funnel- | | | | the potential for increased audibility of sounds of |
| like amplification cone, called an ear trumpet. Bigger | | | | interest without discomfort resulting from high intensity |
| aids used a desk with a built-in amplifier into which a | | | | sounds. While this is more generally a benefit of |
| microphone and earphones were plugged; these | | | | compression rather than digital processing per se, the |
| worked better than passive ear trumpets, however | | | | greatly increased flexibility and control of compression |
| they could be only kept at one place. | | | | processing provided by DSP--such as input |
| Digital hearing aids first came to market in 1987 with | | | | signal-specific band dependence, greater numbers of |
| two manufacturers introducing hearing aids with digital | | | | channels, and knee points with lower compression |
| signal processing (DSP). While high-tech for their time, | | | | thresholds--can lead to improved audibility with less |
| these hearing aids had little success and were soon | | | | clinician effort. Expansion, the opposite of compression, |
| abandoned due to their large size and high battery | | | | has also been introduced in digital hearing aids. This |
| drain. | | | | processing can lead to greater listener satisfaction by |
| Nearly a decade later, two separate manufacturers | | | | reducing the intensity of low-level environmental |
| once again introduced digital hearing aids. By this time, | | | | sounds and microphone noise that otherwise may |
| the technology had improved so that these hearing | | | | have been annoying to the user. |
| aids could be produced in a range of popular styles, | | | | The most advanced feedback reduction schemes |
| from behind-the-ear (BTE) to completely-in-the-canal | | | | monitor for feedback while the listener is wearing the |
| (CIC). Despite their higher cost, they were well | | | | hearing aid. Moderate feedback is then reduced or |
| received by clinicians and consumers. This early | | | | eliminated through the use of a cancellation system or |
| success, combined with the promise of highly | | | | notch filtering. DFR can substantially benefit users who |
| advanced signal processing, ensured that digital hearing | | | | experience occasional feedback, such as that |
| aid technology had come of age. | | | | associated with jaw movement and close proximity to |
| So how far have we come? What is the current | | | | objects. |
| "state-of-the art" technology in digital hearing aids? Are | | | | Digital Noise Reduction (DNR). This processing is |
| digital hearing aids really superior to their analog | | | | intended to reduce gain, either in the low frequencies |
| counterparts? To determine whether digital hearing | | | | or in specific bands, when steady-state signals (noise) |
| aids are better for patients, it is important to focus on | | | | are detected. Although research findings supporting the |
| the superior processing and features of these | | | | efficacy of DNR systems are mixed, they do indicate |
| instruments. Digital hearing aids can't be described as if | | | | that the DNR can work to reduce annoyance and |
| they are a separate entity from analog hearing aids. | | | | possibly improve speech recognition in the presence of |
| "Digital" simply indicates that the analog waveform is | | | | non-fluctuating noise. DNR is sometimes advocated as |
| converted into a string of numbers for processing; and | | | | complementary processing to directional microphones. |
| unfortunately, there is nothing inherently magical about | | | | While directional microphones can reduce the levels of |
| this process. A linear, output-clipping, digital hearing aid | | | | background noise regardless of its temporal content, |
| could easily be built that would provide sound quality | | | | they are limited to reducing noise from behind or to the |
| and speech recognition inferior to many analog hearing | | | | sides of the user. |
| aids. Therefore, digital isn't superior just because it's | | | | Many theatres and lecture halls are now equipped with |
| digital, but because DSP allows manufacturers to | | | | assistive listening systems that transmit the sound |
| create hearing aids with enhanced processing and | | | | directly from the stage; audience members can |
| features. | | | | borrow suitable receivers and hear the program |
| The Digital Advantage | | | | without background noise. |
| Fortunately, for both dispensing audiologists and | | | | Providing good and high quality hearing aid can |
| patients, there are features and advanced signal | | | | enhance learning for the hard of hearing and give |
| processing schemes available in current digital hearing | | | | equality to all. This must be kept in mind while designing |
| aids that do have significant advantages over those | | | | schools and other entertainment places. Digital hearing |
| found in analog instruments. Potential digital advantages | | | | equipment do not cost much and should be acquired to |
| include those related to: | | | | give access to all. |