Regarding the multiplicity of fitting rules available for digital (and even non-digital) instruments, how do I choose a particular fitting rationale?
First of all, nearly all advanced technology hearing aids are based on multi-channel, Wide Dynamic Range Compression (WDRC) processing, so I will confine my comments to the non-linear fitting rationales. When deciding, I think that there are four important questions for the audiologist to ask.
1. What is the core philosophy of the fitting rationale? Some rationales are imbedded in the fitting software of the hearing aid companies. Others are independent (to be discussed further below). The independent rationales are based on goals such as restoring normal loudness perception (e.g., IHAFF, Fig6), maximizing audibility (DSL i/o) or maximizing speech information transfer (NAL NL-1). In addition, most of the manufacturer's specific rationales are also based on a specific philosophy and the manufacturer has the responsibility to clearly state the basis of their approach. Whether choosing an independent or imbedded rationale, the audiologist needs to have a good feel for what the basic goals of the fitting are and then decide if it is consistent with the fitting goals for that patients being served.
2. Is the implementation understandable and easy to use? Whether independent or imbedded, the implementation must be sensible and practical for the working clinician. Manufacturer's specific software should not feel like a black box where the clinician does not have any good idea of how input audiometric and demographic data affects the prescribed setting of the device. If the software is independent, the prescribed fitting parameters need to match to the types of technical controls in available hearing aids. The NAL-NL1 is a good example of an independent software package that matches well to the range of technical settings available in many advanced technology hearing aids.
3. Is the validity of the rationale proven to meet my criteria? There is little information available comparing the effectiveness of the different fitting rationales. Thus, published clinical studies cannot easily form the basis of comparison. However, other criteria are reasonable. Many clinicians will decide on the effectiveness of a particular fitting rationale based on their own experience in fitting patients. Others will turn towards the advice of other professionals who have experience with a particular fitting approach. In any case, I would caution against one-trial learning. There are a variety of other factors that may account for the relative success or failure of individual fittings. It is only after several fittings that the clinician can begin to get a feel as to the effectiveness of the rationale.
4. Independent or Imbedded? The various independent rationales that have been developed over the past several years were created to give the clinician guidance when fitting non-linear amplification based on specific and clear psychoacoustic assumptions. These independent prescriptions are needed because, in some cases, advanced technology devices have been introduced to the market without specific guidance as to setting parameters such as gain and compression ratio. The obvious advantage of these options is that they can be applied to any of the many available multi-channel WDRC devices. However, one of the important limitations of these independent rationales is that they do not specify settings on parameters such as crossover frequency, attack time, release times, and compression kneepoint. With the growing complexity of advanced technology devices, the clinician using an independent rationale still needs to make a variety of decisions for which there is no guidance. In contrast, manufacturer's imbedded prescriptions will usually fully prescribe all of the technical settings of the relevant product. Not only will parameters of gain and compression ratio be specified, but also settings on dimensions such as attack time, release time, kneepoints, etc. In most cases, the manufacturer can provide clear explanations as to why certain settings are recommended. Although there may be philosophical differences between companies on important issues such as compression type (usually determined by release time), at least the core philosophy is clearly stated. In addition, manufacturers of advanced technology products will often update the details of their prescriptions during new software releases based on field experience and feedback on the accuracy and usefulness of their fitting rationales.
Donald J. Schum, Ph.D., is the Vice President of Audiology & Professional Relations, Oticon, Inc., Somerset, NJ.
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