Page 40 - Jul2009

Standards
CLARIFICATION OF SOME HEARING AID TERMINOLOGY: BANDS AND CHANNELS VERSUS HANDLES; FIRST-ORDER VERSUS SECOND-ORDER MICROPHONES
Ruth A. Bentler
University of Iowa, Speech and Hearing Center Iowa City, Iowa, 52242
and
Barbara Kruger
Kruger Associates Inc. 37 Somerset Drive Commack, New York, 11725
A little history
The digital hearing aid was first introduced to the US in the late 1980s. Due, in part, to its large size and high power consumption, the first marketing attempt was not success- ful. In the mid-90s ear-level digital hear- ing aids made their way to the market and now account for nearly all hearing aids sold. In fact, many manufacturers have discontinued their analog products in favor of these higher-tech, easier-to- manufacture options.
How they work
The digital hearing aid performs its
complex operations in one or more dig-
ital signal processors that are packaged
in miniature integrated circuits (ICs).
Sound received by a microphone is con-
verted to analog electrical voltages and then converted to a digital format by analog-to-digital (A-to-D) converters. This process is reversed at the output when electrical voltages are directed to a transducer (usually called a receiver) to gener- ate sound. The digital hearing aid is typically fitted by adjust- ing programmable microprocessors which control the hear- ing aid’s smart internal micro-signal processors using “con- trols” or “handles” on the computer’s graphic user interface (GUI) that is provided by each digital hearing aid manufac- turer. Signal processing in a digital hearing aid is governed by algorithms, i.e., step-by-step series of instructions deter- mined by sets of mathematical formulae or rules to perform a specific task. For example, the noise reduction algorithm may use a set of rules to judge if the input signal is speech or noise and react to it accordingly. Because most of the hearing aid’s functions are controlled by numerical computations, the number of discrete components such as resistors and capaci- tors are minimal in today’s digital hearing aids, thereby increasing its functionality and minimizing its size. With di- gital signal processing, the size of the microprocessor is much
“Hearing aid manufacturers, through the standardization process, develop and confirm methods to describe the spatial properties of the sound field with different types of directional microphone systems (ANSI S3.35) and report the performance to the clinicians.”
smaller than any analog circuit previ- ously used to perform even part of the same task. In fact, many of the process- ing tasks we presently take for granted were not even possible with analog hearing aid circuitry. The digital hearing aid typically performs more sophisticat- ed processing such as very steep filtering of signals, blocking loud sound impuls- es, and differentially controlling the dynamics of different parts of the audio spectrum. With appropriate designs, algorithms can filter digital signals, extract frequency content of those sig- nals, obtain statistical information on the signals, and so on. This sophisticat- ed filtering has benefited the hearing aid user in a number of ways, including sig- nificantly increasing control of acoustic feedback.
Is it a band, a channel, or a handle?
Among the many advantages of the digital hearing aid is that of providing multiple bands, more than one channel, and the availability of handles for the clinician to use in the pro- gramming or fitting stage. Filters may be used to divide the frequency range into sections. A frequency band is a section, or region, of frequencies within which one may shape or adjust the gain of the hearing aid with linear processing inde- pendent of input level. However, if a signal processing algo- rithm is performed, such as compression or expansion, so that more than gain is adjusted within a frequency region, it is called a channel. A channel may include several or many bands. Such a group of contiguous bands is called a frequen- cy channel when it is used for specific, non-linear processing, including control of gain or output sound pressure level (SPL) as a function of input level, thereby resulting in changes in compression and/or expansion. Each group of bands within a channel shares one or more digital algo- rithms. Within a single hearing aid there may be a different
Standards 39