The inherent amplitude fluctuations in narrow bands of noise may limit the ability to detect gaps in the noise; 'dips' in the noise may be confused with the gap to be detected. For people with cochlear hearing loss, loudness recruitment may effectively magnify the fluctuations and this could partly account for the reduced ability to detect gaps in noise bands that is usually found for such people. Previously, we tested these ideas by processing the envelopes of noise bands to alter the amount of envelope fluctuation. We showed that instantaneous compression, implemented via processing of the Hilbert envelope, led to smaller (that is, better) gap detection thresholds for subjects with cochlear hearing loss. In the present experiment, we determined whether fast-acting compression of the type sometimes used in hearing aids could also lead to improved gap detection. A behind-the-ear (BTE) digital hearing aid was programmed to implement multi-band compression, either fast-acting or slow-acting (control condition). A reference condition using unaided listening was also used. Stimuli were delivered via an earphone placed over the hearing aid. Overall stimulus levels at the output of the hearing aid were similar across conditions. Thresholds for detecting gaps in noise bands centred at 4 kHz were measured as a function of noise bandwidth (10-500 Hz). To prevent the detection of spectral changes introduced by the gap, stimuli were presented in a broad-band background noise. Three normally hearing subjects and three subjects with bilateral cochlear hearing loss were tested. Gap thresholds varied non-monotonically with noise bandwidth, being maximal around 50 Hz. Gap thresholds were generally higher for the hearing-impaired than for the normally hearing subjects. For the latter, gap thresholds were similar for the three conditions. For the hearing-impaired subjects, gap thresholds were similar for the unaided condition and the condition using slow compression. However, fast compression led to smaller gap thresholds, especially for noise bandwidths up to 50 Hz. The results show that fast compression can improve the ability of hearing-impaired subjects to detect gaps in sounds with slowly fluctuating envelopes.

Noise reduction systems have been implemented in hearing aids to improve signal-to-noise ratio and listening comfort. The aim of this study was to evaluate the efficacy of hearing aid noise reduction for Mandarin speakers. The results showed a significant improvement in acceptable noise levels and speech reception thresholds with noise reduction turned on. Sound quality ratings also suggested that most listeners preferred having noise reduction turned on for listening effort, listening comfort, speech clarity, and overall sound quality. These results suggest that the noise reduction system used in this study might improve sentence perception in steady-state noise, noise tolerance, and sound quality, although not all listeners preferred aggressive noise reduction. However, due to large interindividual variation, clinical application of the results should be on an individual basis.

Objectives: Lexical tone information provides redundant cues for the recognition of Mandarin sentences in listeners with normal hearing in quiet conditions. The contribution of lexical tones to Mandarin sentence recognition in listeners with hearing aids (HAs) is unclear. This study aimed to remove lexical tone information and examine the effects on Mandarin sentence intelligibility in HA users. The second objective was to investigate the contribution of cognitive abilities (i.e., general cognitive ability, working memory, and attention) on Mandarin sentence perception when the presentation of lexical tone information was mismatched. Design: A text-to-speech synthesis engine was used to manipulate Mandarin sentences into three test conditions: (1) a Normal Tone test condition, where no alterations were made to lexical tones within sentences; (2) a Flat Tone test condition, where lexical tones were all changed into tone 1 (i.e., the flat tone); and (3) a Random Tone test condition, where each word in test sentences was randomly assigned one of four Mandarin lexical tones. The manipulated sentence signals were presented to 32 listeners with HAs in both quiet and noisy environments at an 8 dB signal to noise ratio. Results: Speech intelligibility was reduced significantly (by approximately 40 percentage points) in the presence of mismatched lexical tone information in both quiet and noise. The difficulty in correctly identifying sentences with mismatched lexical tones among adults with hearing loss was significantly greater than that of adults with normal hearing. Cognitive function was not significantly related to a decline in speech recognition scores. Conclusions: Contextual and other phonemic cues (i.e., consonants and vowels) are inadequate for HA users to perceive sentences with mismatched lexical tone contours in quiet or noise. Also, HA users with better cognitive function could not compensate for the loss of lexical tone information. These results highlight the importance of accurately representing lexical tone information for Mandarin speakers using HAs.

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The present study examined the change in spectral properties of Mandarin vowels and fricatives caused by nonlinear frequency compression (NLFC) used in hearing instruments and how these changes affect the perception of speech sounds in normal-hearing listeners. Speech materials, including a list of Mandarin monosyllables in the form of /dV/ (12 vowels) and /Ca/ (five fricatives), were recorded from 20 normal-hearing, native Mandarin-speaking adults (ten males and ten females). NLFC was based on Phonak SoundRecover algorithms. The speech materials were processed with six different NLFC parameter settings. Detailed acoustic analysis revealed that the high front vowel /i/ and certain compound vowels containing /i/ demonstrated positional deviation in certain processed conditions in comparison to the unprocessed condition. All five fricatives showed acoustic changes in spectral features in all processed conditions. Fourteen Mandarin-speaking, normal-hearing adult listeners performed phoneme recognition with the six NLFC processing conditions. When the cut-off frequency was set relatively low, recognition of /s/ was detrimentally affected, whereas none of the NLFC processing configurations affected the other phonemes. The discrepancy between the considerable acoustic changes and the negligible adverse effects on perceptual outcomes is partially accounted for by the phonology system and phonotactic constraints in Mandarin.