주파수중요기능(Frequency Importance Function, FIF) 은 언어를 인지하기 위한 공헌도를 1/3 또는 1 옥타브 주 파수대역(octave frequency band) 별로 0에서 1까지의 숫자 또는 백분율로 나타낸 것으로 어음명료지수(SpeechIntelligibility Index, SII)의 산출에 있어서 청력역치와 더불어 중요한 요소이다. 초기의 FIF 관련 연구는 다양한 전기음향적인 조건에서 어음 신호가 전화 등의 통신기기를 통하여 어떻게 전달되는지를 예측하기 위한 것으로 미국의 Bell Telephone Laboratories에서 개발하였으며(French & Steinberg, 1947; Fletcher & Galt, 1950), 미국국가 표준기구(American National Standard Institute, ANSI) 는 1969년에 FIF를 미국의 국가표준으로 제정하였다 (ANSI S3.5, 1969). 이후 ANSI는 1997년에 FIF와 관련 된 용어인 \'Articulation index\'를 \'Speech intelligibility index\'라는 용어로 개정하여 현재까지 사용하고 있다 (ANSI S3.5, 1997; ANSI S3.5, 2007). SII의 용도는 난청인의 청력 역치에 따른 어음이해능력을 예측하거나 보청기 착용 전후의 어음인지능력, 즉 보청기의 수행능력평 가(Hou & Thornton, 1994; Pavlovic, 1994; Amlani et al., 2002), 혹은 난청인의 보청기적합에 이용하거나 보청 기적합공식의 산출에도 유용하게 사용할 수 있다(Byrne & Dillon, 1986; Souza et al., 2000; Byrne et al., 2001).
Speech intelligibility index (SII) can be utilized for predicting speech recognition abilities of the hearing impaired based on the pure tone thresholds. For calculation of SII, frequency importance function (FIF) has to be produced considering the language characteristics. The purpose of this study was to derive FIF at 1/3 and 1 octave frequency bands using recently developed Korean standard monosyllabic word lists for adults (KS-MWL-A). KS-MWL-A speech materials were presented to normal hearing 17 adults, age ranging from 19 to 25, after filtering with 7 high pass filters (HPFs) and low pass filters (LPFs) combined with Korean speech shaped noise. The cut-off frequencies (COFs) of the LPF were 224, 447, 891, 1413, 2239, 3548, 5623 Hz and the HPF were 447, 891, 1413, 2239, 3548, 5623 Hz. Signal-to-noise ratios (SNRs) were -10 to +8 dB by 2 dB steps. Relative speech intelligibility index (R-SII) were derived by word recognition scores (WRSs) as a function of COFs at each SNR. Finally, FIFs were calculated by cumulative curves as function of frequencies. Regression equation of R-SII and WRS as a function of SII showed cubic function as y = 41.77x3-159.01x2+208.31x-4.08 (R2 = 0.992, p < .000). As Korean R-SII increased 0.0 to 0.5, WRS increased rapidly comparing to English R-SII. Maximum FIFs of KS-MWL-A showed 0.2190 at 800 Hz by 1/3 octave band frequency and 0.4507 at 1,000 Hz by 1 octave band frequency. Crossover frequencies of KS-MWL-A showed 1,518-1,941 Hz and increased as SNR increased. All the results were different from English monosyllabic words. The results of this study can be utilized for calculating hearing aid fitting formula, for determining aims, and effective process of auditory training. However, further study should be performed using various speech materials such as Korean sentence and short passages and etc.
INTRODUCTION MATERIALS AND METHODS RESULTS CONCLUSIONS REFERENCES
