Thomas J. Moore

A note on the acoustic-phonetic characteristics of inadvertently clear speech

Abstract Even when speaking conditions and listener responses are very tightly controlled, some talkers are easier to understand than others. In a series of intelligibility tests using both English words without context and English sentences, both native and non-native listeners found one of five talkers difficult to understand. Since all talkers read similar materials and the task was the same for all listeners, the differences in intelligibility must have resulted from particular phonetic characteristics used by the talker. Spectrograms were made of all test words produced by the talkers and compared on selected acoustic-phonetic properties. In comparison with the more intelligible talkers, the least intelligible talkers produced test words at shorter durations; abbreviated vowel durations; used the least differentiated vowel space, as defined by the first two formants; used minimal cues for consonantal contrasts; and had the most varied amplitude of stressed vowels. These characteristics are similar to those distinguishing deliberately clear speech. That non-native and native listeners found the same talker difficult to understand suggests that the effect of clear speech, though different in degree, reflects the use of the same acoustic-phonetic information by both groups of listeners. Further, clear speech has similar acoustic-phonetic characteristics whether deliberately or inadvertently produced.

A Survey of the Mechanical Characteristics of Skin and Tissue in Response to Vibratory Stimulation

The possibility that the mechanical characteristics of skin and tissues may influence physiological and psychophysical measurements of tactile sensitivity is considered. A survey of selected literature indicating how certain mechanical characteristics of skin and tissue vary as a function of changes in variables known to influence physiological and psychophysical measurements of the tactile system is presented. Finally, certain physiological and psychophysical studies in which the physical properties of the area stimulated may have influenced the results are mentioned.

The Perception of Synthetic Speech in Noise

Although much information about synthetic speech has been acquired over past decades, we have been unable to find in the literature a systematic examination of the perception of synthetic speech in noise. Simpson [1] has reported that synthetic speech altitude callouts to airline pilots in widebody jet cockpit noise at a S/N of -10 dB for the first time were 99.1% intelligible and that synthetic speech voice warnings to helicopter pilots in simulated helicopter noise at a S/N ratio of -22 dB were 99.2% intelligible [2]. Nusbaum [3] has reported that perceptual confusions for synthetic CV and VC syllables were quite different than confusions observed for natural speech degraded by noise. Pisoni (personal communication) indicates that one of two synthetic speech systems with very high levels of segmental intelligibility in quiet, showed greater decrements in the intelligibility of CV syllables in noise than did the other system. Clark [4] reported little difference in the intelligibility of vowels in noise for synthetic and natural speech, whereas natural CV syllables were clearly superior to synthetic CV syllables under all noise conditions.

The Effect of Audio Bandwidth on Selected Digital Speech Coding Algorithms

The speech intelligibility in noise of three different digital speech coding algorithms was investigated at the standard audio bandwidth of 300 to 3400 Hz and a developmental bandwidth of 100-6000 Hz. Speech intelligibility was measured using a standardized instrument, the Modified Rhyme Test (MRT). Performance was investigated at three different levels of an acoustic noise environment. This report presents results, discussion, conclusions and recommendations for further research.

Research on Speech Processing for Military Avionics

The Biological Acoustics Branch of the Armstrong Aerospace Medical Research Laboratory (AAMRL) is engaged in research in a number of speech related areas. This paper will describe the approach used to conduct research in the development and evaluation of military speech communication systems, mention the types of studies done using this approach and give examples of the types of data generated by these studies. Representative data will also be provided describing acoustic-phonetic changes that occur when speech is produced under acceleration.