The two principal sources of sound in speech, voicing and frication, occur simultaneously in voiced fricatives as well as at the vowel-fricative boundary in phonologically voiceless fricatives. Instead of simply overlapping, the two sources interact. This paper is an acoustic study of one such interaction effect: the amplitude modulation of the frication component when voicing is present. Corpora of sustained and fluent-speech English fricatives were recorded and analyzed using a signal-processing technique designed to extract estimates of modulation depth. Results reveal a pattern, consistent across speaking style, speaker, and place of articulation, for modulation at f0 to rise at low voicing strengths and subsequently saturate. Voicing strength needed to produce saturation varied 60–66dB across subjects and experimental conditions. Modulation depths at saturation varied little across speakers but significantly for place of articulation (with [z] showing particularly strong modulation) clustering at approximately 0.4–0.5 (a 40%–50% fluctuation above and below unmodulated amplitude); spectral analysis of modulating signals revealed weak but detectable modulation at the second and third harmonics (i.e., 2f0 and 3f0)
The two distinct sound sources comprising voiced frication, voicing and frication, interact. One effect is that the periodic source at the glottis modulates the amplitude of the frication source originating in the vocal tract above the constriction. Voicing strength and modulation depth for frication noise were measured for sustained English voiced fricatives using high-pass filtering, spectral analysis in the modulation (envelope) domain, and a variable pitch compensation procedure. Results show a positive relationship between strength of the glottal source and modulation depth at voicing strengths below 66 dB SPL, at which point the modulation index was approximately 0.5 and saturation occurred. The alveolar [z] was found to be more modulated than other fricatives.
In speech, the combination of amplitude-modulated (AM) noise and a low-frequency periodic source characterizes voiced fricative sounds. To investigate the modulation’s contribution to fricative auditory quality, AM white noise, with simultaneous sinusoidal component at the modulating frequency, provided stimuli for perceptual tests. Two AM detection-threshold experiments were conducted to establish the effect of varying the relative amplitude and phase of the tone. In the first experiment, tone and noise stimuli were separated within each trial by short pauses; in the second, the tone played continuously throughout the trial. AM detection thresholds when the tone was absent were similar to previously published thresholds for AM noise with no tone (averaging approximately −22 dB). The effect of the quieter tones (10–30 dB tone-to-noise ratio, TNR) was dependent on test condition. In the broken-tone experiment, a slight increment in threshold occurred for all relative phases. The constant-tone experiment revealed impaired detection when tone and noise modulation were in phase, and the converse when in antiphase (5-dB difference in threshold at 30-dB TNR). As TNR increased further (40–50-dB TNR), thresholds rose in all cases. Future work is needed to evaluate how this detectable AM percept influences quality and categorization of fricatives.
As an approach to understanding the characteristics of the acoustic sources in voiced fricatives, it seems apt to draw on knowledge of vowels and voiceless fricatives, which have been relatively well studied. However, the presence of both phonation and frication in these mixed-source sounds offers the possibility of mutual interaction effects, with variations across place of articulation. This paper examines the acoustic and articulatory consequences of these interactions and explores automatic techniques for finding parametric and statistical descriptions of these phenomena. A reliable and consistent set of such acoustic cues could be used for phonetic classification or speech recognition. Following work on devoicing of European Portuguese voiced fricatives [Jesus and Shadle, in Mamede et al. (eds.) (Springer-Verlag, Berlin, 2003), pp. 1–8]. and the modulating effect of voicing on frication [Jackson and Shadle, J. Acoust. Soc. Am. 108, 1421–1434 (2000)], the present study focuses on three types of information: (i) sequences and durations of acoustic events in VC transitions, (ii) temporal, spectral and modulation measures from the periodic and aperiodic components of the acoustic signal, and (iii) voicing activity derived from simultaneous EGG data. Analysis of interactions observed in British/American English and European Portuguese speech corpora will be compared, and the principal findings discussed.
The aperiodic noise source in fricatives is characteristically amplitude modulated by voicing. Previous psychoacoustic studies have established that observed levels of AM in voiced fricatives are detectable, and its inclusion in synthesis has improved speech quality. Phonological voicing in fricatives can be cued by a number of factors: the voicing fundamental, duration of any devoicing, duration of frication, and formant transitions. However, the possible contribution of AM has not been investigated. In a cue trading experiment, subjects distinguished between the nonsense words 'ahser' and 'ahzer'. The voicing boundary was measured along a formant-transition duration continuum, as a function of AM depth, voicing amplitude and masking of the voicing component by low-frequency noise. The presence of AM increased voiced responses by approximately 30%. The ability of AM to cue voicing was strongest at greater modulation depths and when voicing was unavailable as a cue, as might occur in telecommunication systems or noisy environments. Further work would examine other fricatives and phonetic contexts, as well as interaction with other cues.
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