| Summary: | 博士 === 國立陽明大學 === 生物醫學工程學系 === 103 === Binaural hearing is an important mechanism of human auditory system, which involves using information relating to the differences between the signals that arrive at the two ears. Binaural hearing plays an important role in enhancing the ability to detect signals in noisy environments, which can be illustrated by measuring the binaural masking-level difference (BMLD). The BMLD phenomenon refers to the difference in the just-audible test-tone level when the phases of the maskers are the same at the two ears comparing to the phase of the signal is zero at one ear (S0N0) and inverted at the other (SπN0). The sensorineural hearing loss (SNHL) is the largest group of hearing loss, which is characterized as decreasing audibility and decreasing ability of speech recognition in noise. The most common method for hearing compensation of SNHL is the use of hearing aids; however reduced intelligibility of speech in noise is a common complaint from SNHL listeners who wear hearing aids. The advent of joint processing technology between bilateral hearing aids has allowed the signals supplied to the two hearing aids to be manipulated to be interaurally in antiphase, which may improve the listening performance of the hearing aid wearer in noisy conditions. In addition, the performances of BMLD in SNHL were varied in the previous studies. Therefore, this study proposed a method to measure the BMLD in SNHL listeners at various frequencies using of two hearing aids. Mandarin is one of the most commonly used languages. However, there is no material based on Mandarin speech for the clinical evaluations of BMLD or BILD. Mandarin is a tonal language with one of four basic tones allotted per syllable. Mandarin tones carry not only lexical meaning but also acoustic feature. Therefore, this study also aimed to investigate the BMLD and BILD of Mandarin tones by using different tones of Mandarin speech.
There were two experiments designed in this study: (1) first experiment proposed a method and a platform using two hearing aids to compensate the hearing audibility and to study the BMLD in SNHL with various frequency stimuli; (2) second experiment proposed a study to investigate the BMLD and BILD using Mandarin tones. It finally aimed to propose the method and materials for the BMLD measurement of Mandarin speakers.
(1) Seven SNHL subjects participated in the first experiment. The BMLDs were measured based on the detection threshold differences for pure tones of 125, 250, 500, 1000, and 2000 Hz in the presence of white noise when presented interaurally in phase (S0N0) and interaurally in antiphase (SπN0). (2) Twenty normal hearing subjects participated in the second experiment. The BMLDs of Mandarin tone detection were measured based on the detection threshold differences for the four tones of the voiced vowels /i/ (i.e., /i1/, /i2/, /i3/, and /i4/) and /u/ (i.e., /u1/, /u2/, /u3/, and /u4/) in speech-spectrum noise when presented interaurally in phase (S0N0) and interaurally in antiphase (SπN0). The BILD of Mandarin tone recognition in speech-spectrum noise were determined as the differences in the target-to-masker ratio (TMR) required for 50% correct tone recognitions between the S0N0 and SπN0 conditions.
The results of this study showed that (1) When using white noise as a masker, the average detection thresholds for the target signals were significant lower in the SπN0 condition than in the S0N0 condition (p<0.05), and the SNHL subjects had mean BMLDs of 3.7, 4.6, 4.1, 6.0, and 4.0 dB at 125, 250, 500, 1000, and 2000 Hz, respectively. (2) The detection thresholds for the four tones of /i/ and /u/ differed significantly (p<0.001) between the S0N0 and SπN0 conditions. The BMLDs ranged from 7.3 to 11.5 dB, and the mean BMLD is 9.1 dB. The TMR for 50% correct Mandarin tone recognitions also differed significantly (p<0.001) between the S0N0 and SπN0 conditions, at –13.4 and –18.0 dB, respectively, with a mean BILD of 4.6 dB.
This study provided a preliminary support that SNHL listeners may hear target signals easier in noisy environments when the signals were presented in antiphase via two hearing aids. It implied that SNHL listeners may benefit from the application of the BMLD phenomenon in noisy environments when using two hearing aids. In addition, this study also showed that the thresholds of Mandarin tone detection and recognition in the presence of speech-spectrum noise were significantly improved when phase inversion was applied to the target speech. Therefore, this study suggested that the proposed BMLD measuring method and materials may be clinically applied for the BMLD measurement of the Mandarin speakers, and the related result may provide information for further applications of the BMLD phenomenon on bilateral hearing aids.
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