The purpose of digital hearing aids is to enhance the speech intelligibility and provide the informative sound including speech with good sound quality by setting a reasonable target gain to individual hearing impaired person. The attributes of a good sound quality here include the clarity and naturalness of speech, music, warning signal, and other sounds in daily life. One can achieve this goal by utilizing the measured optimal spectral gain of each patient which is needed to compensate for the hearing loss: this is called the "fitting" process.
Existing fitting methods which employ a pure tone stimulus, such as the Fig6 fitting method or the POGO2 method, yield the same target gains when individual hearing thresholds are identical. Due to this fact, they cannot take into account the loudness perception of an individual and the fitting procedures become very time-consuming for detailed adjustment. Fitting methods using 1-octave, 1/2-octave or 1/3-octave band tone stimulus frequently results in excessive target gains at low frequencies and they require a number of loudness measurements. A new method called PAFA (Psycho-Acoustic Fitting Algorithm) is suggested here, which reflects the subjective perception and the characteristics of speech communication: The suggested method would offer natural listening environment and it would be appropriate in collecting the data with confidence and in an economical manner. The method starts from the selection and modification of critical bands of human auditory system in the frequency range from 0.1 to 9.5 kHz. The modified 14 critical band tones considering the speech intelligibility were fed to 43 subjects with normal hearing. The magnitudes of sample sounds were ranged from the hearing threshold level to the discomfort level. In this method, five classes of loudness sensation category were employed in the subjective test and the results were statistically processed to be used as the reference data for the prescription of target gains for the patient. The same procedure was applied to the impaired individual to obtain the target gain: the loudness normalization process was practiced by comparing the loudness sensation of average normal hearing with that of an impaired person. The proposed PAFA fitting method was compared with the previous methods and it was clearly observed that the present method yielded for better results: in other words, a well-matched gain to the hearing sensitivity could be provided to the impaired individual that profoundly reduced the dissatisfaction compared to the existing fitting algorithms. Also, it is observed that the COSI questionnaires to evaluate the degree of satisfaction with the PAFA showed that the result of PAFA was felt well in conversation in quiet places than that of re-adjusted Fig6 method, in which the latter method was taking into account of the experience of an audiologist and the sensation of a subject. The PB-word test for checking the speech intelligibility in an objective manner did not show any meaningful differences between PAFA method and re-adjusted Fig6 method. All the test results suggest that the proposed PAFA method can be directly used in the digital fitting of hearing aids, of which the auditory performance is far better than the existing fitting algorithms and is at least equal to or slightly better than the empirically re-adjusted fitting technique.
