Walter P. Sjursen

Satisfying patient needs with nine fixed acoustical prescription formats

INTRODUCTION Conventional hearing aid fittings are usually accomplished by providing a set of predetermined electroacoustic characteristics to the patient’s ear based on some fitting prescription or rationale. The desired characteristics may then be preset in a hearing aid and verified by 2-cc coupler measurements or programmed while in the patient’s ear. The prescription can then be verified by insertion or functional gain measures. In both procedures, the hearing aids may be adjusted further to meet any subjective loudness, quality, or clarity requirements of the patient. This approach may lead to confusion on the part of the dispensing professional when attempting to adjust programmable or trimmer settings regarding what effect each adjustable parameter has on performance. There are differing opinions on how best to compensate for a specific type and severity of hearing loss. Some fitting prescriptions attempt to predict desired gain functions from threshold data.1 Others calculate target characteristics from direct loudness scaling from individual patients.2 The rationale behind some prescriptive techniques is to ensure loudness equalization (amplifying components of speech to contribute equally to loudness at different frequencies), whereas the goal of others is to perform loudness normalization (to restore the overall loudness of sounds to normal loudness and to restore the relative loudness of different frequency components). Because of the inherent differences among these fitting procedures, if they are applied to a single patient, the electroacoustic outcome of each may sound different to the patient depending on the formula selected by the fitter. Although there has been no single formula sufficiently compelling to be embraced by the majority of clinicians, the consensus is that most are beneficial. In addition to the acoustic variances resulting from the selection of one fitting formula versus another, there is considerable interaction between the prescription used and subjective performance and preference. Further, sound quality and clarity are not simply subjective in nature, but are influenced by factors associated with the pathologic changes in the individual’s ear, such as loss of hair cells and impaired function of the peripheral and central neural pathways. These factors cannot be compensated for by using any single fitting formula, nor can the considerable differences in subjective sound quality, clarity, or loudness be accurately predicted. Thus, an important step in hearing aid fittings is fine-tuning or changing electroacoustic characteristics to improve performance or subjective preference. Recent developments of non-linear fitting formulas, along with improved compression circuitry, have resulted in more natural perception of soft, medium, and loud sounds by patients wearing hearing aids. As with linear formulas, each of the non-linear hearing aid prescriptive formulas is based on a particular fitting rationale which results in differences in the final fitting.