Measuring the long-term SNRs of static and adaptive compression amplification techniques for speech in noise.

BACKGROUND: Multichannel wide-dynamic-range compression (WDRC) is a widely adopted amplification scheme in modern digital hearing aids. It attempts to provide individuals with loudness recruitment with superior speech intelligibility and greater listening comfort over a wider range of input levels. However, recent surveys have shown that compression processing (operating in the nonlinear regime) usually reduces the long-term signal-to-noise ratio (SNR). PURPOSE: The purpose of this study was to determine the long-term SNR in an adaptive compression-ratio (CR) amplification scheme called adaptive wide-dynamic-range compression (AWDRC), and to determine whether this concept is better than static WDRC amplification at improving the long-term SNR for speech in noise. DESIGN AND STUDY SAMPLE: AWDRC uses the input short-term dynamic range to adjust the CR to maximize audibility and comfort. Various methods for evaluating the long-term SNR were used to observe the relationship between the CR and output SNR performance in AWDRC for seven typical audiograms, and to compare the results with those for static WDRC amplification. RESULTS: The results showed that the variation of the CR in AWDRC amplification can maintain the comfort and audibility of the output sound. In addition, the average long-term SNR improved by 0.1-5.5 dB for a flat hearing loss, by 0.2-3.4 dB for a reverse sloping hearing loss, by 1.4-4.8 dB for a high-frequency hearing loss, and by 0.3-5.7 dB for a mild-to-moderate-sloping high-frequency hearing loss relative to static WDRC amplification. The output long-term SNR differed significantly (p < .001) between static WDRC and AWDRC amplification. CONCLUSIONS: The results of this study show that AWDRC, which uses the characteristics of the input signal to adaptively adjust the CR, provides better long-term SNR performance than static WDRC amplification.

Development of a mandarin monosyllable recognition test.

OBJECTIVE: This study aimed to develop a set of 50- and 25-item word lists that exhibited familiarity, homogeneity, and phonemic balance for a Mandarin monosyllable recognition test (MMRT). DESIGN: To achieve the design goal of high subject familiarity with the test material, we selected the 700 most frequently occurring monosyllables to be the test material. The homogeneity of the test material was achieved by evaluating five psychometric characteristics of these 700 monosyllables to obtain 348 homogeneous monosyllables with similar psychometric functions for constructing the word lists. The phonemic balance of the 50-item word lists was achieved by deriving the desired numbers of initials, finals, and tones in these lists according to their occurrence frequencies in 4733 monosyllabic words. The phonemic balance of the 25-item word lists was achieved by equally dividing the desired numbers of initials, finals, and tones in the 50-item word lists into two groups, called half-A and half-B lists. Three half-A lists and three half-B lists were constructed from the 348 homogeneous monosyllables, and they could be paired to form nine 50-item word lists. Accordingly, all of the MMRT word lists are familiar, homogeneous, and phonemically balanced. RESULTS: The homogeneity of the MMRT word lists was examined by evaluating the interlist equivalence, interitem variability, and intersubject variability. A chi test was used to evaluate the interlist equivalence among the MMRT word lists; that is, to determine whether the values of percent correct recognition were distributed identically at 12 different presentation levels among the six 25-item word lists and nine 50-item word lists. No statistically significant differences were found among the word lists: chi(55) = 12.004 and p = 1.000 for the six 25-item word lists, chi(88) = 7.695 and p = 1.000 for the nine 50-item word lists, and chi(154) = 19.699 and p = 1.000 for the six 25-item word lists plus nine 50-item word lists. The chi tests indicated that the six 25-item word lists and nine 50-item word lists exhibited interlist equivalence. The mean interitem variability (2.0 dB) and mean intersubject variability (2.3 dB) are lower for the MMRT word lists than for previously reported word recognition tests. CONCLUSION: This study applied an innovative approach to design MMRT word lists from familiar and homogeneous monosyllables, in which the familiarity, homogeneity, and phonemic balance of the six 25-item and nine 50-item word lists were strictly controlled. These word lists exhibit interlist equivalence with respect to their psychometric functions and five psychometric characteristics; moreover, their interitem and intersubject variability are lower than those of previously reported lists. Future clinical experiments should examine whether such a design approach can improve the reliability and diagnostic sensitivity of word recognition tests for hearing-impaired listeners.