Comparative Analysis of Different Prescriptive Formulae Used in the Evaluation of Real Ear Insertion Gain for Digital Hearing Aids

Various prescriptive procedures have been developed, for calculating the real ear insertion gain of a digital hearing aid based either on loudness equalization or speech intelligibility. But the problem for audiologists lies in selecting an appropriate procedure for particular types of hearing impairment. The aim of this work is to analyze various linear and nonlinear prescriptive procedures, by performing various audiological investigations on hearing impaired subjects, so as to select an optimum procedure for a specific type of impairment. An audiological test was carried out on 237 hearing impaired subjects in the age group of 23 to 75 years, and they were given appropriate hearing aids with gain recommendations, using different formulae by audiologists. The degrees of success of the recommended formulae were analyzed, based on the speech discrimination score. Subjects with mild and moderate losses were found satisfied with the gain suggestions of linear prescriptive procedures, in addition to procedures based on loudness equalization NAL-RP (National Acoustics Laboratory-Revised Profound), POGO II (prescription of Gain Output), and DSL (Desired Sensation Level). Subjects with sensorineural loss were satisfied only with the gain recommendations of non linear prescriptive procedures NAL-NL1 (National Acoustics Laboratory-Non Linear), NAL-NL2 and DSL I/O (Desired Sensation Level Input/Output). An appropriate prescriptive procedure for a specific audiogram could be successfully suggested in future based on the results obtained from the total population on whom the study was carried out. In the case of those subjects who were not satisfied with the suggested procedure, a revised formula would be arrived at, by suitably varying the gains by the audiologists