Abstract
Ultrasound can deliver speech information when it is amplitude-modulated with speech and presented via bone conduction. This speech-modulated bone-conducted ultrasound (SM-BCU) can also transmit prosodic information. However, there is insufficient research on the recognition of vowel duration in SM-BCU. The aim of this study was to investigate the categorization of vowel durational changes in SM-BCU using a behavioral test. Eight Japanese-speaking participants with normal hearing participated in a forced-choice behavioral task to discriminate between “hato” (pigeon) and “haato” (heart). Speech signal stimuli were presented in seven duration grades from 220 ms to 340 ms. The threshold at which 50% of responses were “haato” was calculated and compared for air-conducted audible sound (ACAS) and SM-BCU. The boundary width was also evaluated. Although the SM-BCU threshold (mean: 274.6 ms) was significantly longer than the ACAS threshold (mean: 269.6 ms), there were no differences in boundary width. These results suggest that SM-BCU can deliver prosodic information about vowel duration with a similar difference limen to that of ACAS in normal hearing.
Highlights
Ultrasound of frequencies higher than approximately 20–24 kHz [1] are not audible to humans via air-conduction
There was no difference in boundary width for the categorization of “hato” and “haato,” the speech-modulated bone-conducted ultrasound (SM-bone-conducted ultrasound (BCU)) threshold was significantly longer than the air-conducted audible sound (ACAS) threshold
These results suggest that SM-BCU can deliver prosodic information about vowel duration, and that individuals with normal hearing can categorize short- and long-duration vowels in SM-BCU with a similar difference limen to that of ACAS
Summary
Ultrasound of frequencies higher than approximately 20–24 kHz [1] are not audible to humans via air-conduction. When it is presented via bone-conduction, humans can perceive ultrasound up to approximately 120 kHz [2] as an auditory sensation. This phenomenon was first reported by Gavreau in 1948 [3]. The pitch of bone-conducted ultrasound (BCU) is similar to that of high frequency air-conducted audible sound (ACAS) (approximately 8–16 kHz) [2,4,5], but the just noticeable frequency difference is worse than that of ACAS [6,7]. There are several differences in the perceptual characteristics between BCU and ACAS
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