Evidence-based occupational hearing screening II: validation of a screening methodology using measures of functional hearing ability.

OBJECTIVE: Validate use of the Extended Speech Intelligibility Index (ESII) for prediction of speech intelligibility in non-stationary real-world noise environments. Define a means of using these predictions for objective occupational hearing screening for hearing-critical public safety and law enforcement jobs. DESIGN: Analyses of predicted and measured speech intelligibility in recordings of real-world noise environments were performed in two studies using speech recognition thresholds (SRTs) and intelligibility measures. ESII analyses of the recordings were used to predict intelligibility. Noise recordings were made in prison environments and at US Army facilities for training ground and airborne forces. Speech materials included full bandwidth sentences and bandpass filtered sentences that simulated radio transmissions. STUDY SAMPLE: A total of 22 adults with normal hearing (NH) and 15 with mild-moderate hearing impairment (HI) participated in the two studies. RESULTS: Average intelligibility predictions for individual NH and HI subjects were accurate in both studies (r2 ≥ 0.94). Pooled predictions were slightly less accurate (0.78 ≤ r2 ≤ 0.92). CONCLUSIONS: An individual's SRT and audiogram can accurately predict the likelihood of effective speech communication in noise environments with known ESII characteristics, where essential hearing-critical tasks are performed. These predictions provide an objective means of occupational hearing screening.

The effect of interleaved filters on normal hearing listeners' perception of binaural cues.

OBJECTIVES: Hearing-impaired individuals often have difficulty in noisy environments. Interleaved filters, where signals from neighboring frequency regions are sent to opposite ears, may benefit those individuals but may also reduce the benefits of spatial cues. This study investigated the effect of interleaved filters on the use of spatial cues. DESIGN: Normal-hearing subjects' sound localization abilities were tested with and without interleaved filters. RESULTS: Participants' localization performance was worse with interleaved filters but better than chance. Interleaving in high-frequency regions primarily affected interaural level difference cues, and interleaving in low-frequency regions primarily affected interaural time difference cues. CONCLUSIONS: Interleaved filters reduced but did not eliminate the benefits of spatial cues. The effect was dependent on the frequency region they were used in, indicating that it may be possible to use interleaved filters in a subset of frequency regions to selectively preserve different binaural cues.

Determining the relevance of different aspects of formant contours to intelligibility.

Previous studies have shown that "clear" speech, where the speaker intentionally tries to enunciate, has better intelligibility than "conversational" speech, which is produced in regular conversation. However, conversational and clear speech vary along a number of acoustic dimensions and it is unclear what aspects of clear speech lead to better intelligibility. Previously, Kain et al. [J. Acoust. Soc. Am. 124 (4), 2308-2319 (2008)] showed that a combination of short-term spectra and duration was responsible for the improved intelligibility of one speaker. This study investigates subsets of specific features of short-term spectra including temporal aspects. Similar to Kain's study, hybrid stimuli were synthesized with a combination of features from clear speech and complementary features from conversational speech to determine which acoustic features cause the improved intelligibility of clear speech. Our results indicate that, although steady-state formant values of tense vowels contributed to the intelligibility of clear speech, neither the steady-state portion nor the formant transition was sufficient to yield comparable intelligibility to that of clear speech. In contrast, when the entire formant contour of conversational speech including the phoneme duration was replaced by that of clear speech, intelligibility was comparable to that of clear speech. It indicated that the combination of formant contour and duration information was relevant to the improved intelligibility of clear speech. The study provides a better understanding of the relevance of different aspects of formant contours to the improved intelligibility of clear speech.

Hybridizing conversational and clear speech to determine the degree of contribution of acoustic features to intelligibility.

Speakers naturally adopt a special "clear" (CLR) speaking style in order to be better understood by listeners who are moderately impaired in their ability to understand speech due to a hearing impairment, the presence of background noise, or both. In contrast, speech intended for nonimpaired listeners in quiet environments is referred to as "conversational" (CNV). Studies have shown that the intelligibility of CLR speech is usually higher than that of CNV speech in adverse circumstances. It is not known which individual acoustic features or combinations of features cause the higher intelligibility of CLR speech. The objective of this study is to determine the contribution of some acoustic features to intelligibility for a single speaker. The proposed method creates "hybrid" (HYB) speech stimuli that selectively combine acoustic features of one sentence spoken in the CNV and CLR styles. The intelligibility of these stimuli is then measured in perceptual tests, using 96 phonetically balanced sentences. Results for one speaker show significant sentence-level intelligibility improvements over CNV speech when replacing certain combinations of short-term spectra, phoneme identities, and phoneme durations of CNV speech with those from CLR speech, but no improvements for combinations involving fundamental frequency, energy, or nonspeech events (pauses).