Hironori Takemoto

Measurement of temporal changes in vocal tract area function from 3D cine-MRI data.

A 3D cine-MRI technique was developed based on a synchronized sampling method [Masaki et al., J. Acoust. Soc. Jpn. E 20, 375-379 (1999)] to measure the temporal changes in the vocal tract area function during a short utterance /aiueo/ in Japanese. A time series of head-neck volumes was obtained after 640 repetitions of the utterance produced by a male speaker, from which area functions were extracted frame-by-frame. A region-based analysis showed that the volumes of the front and back cavities tend to change reciprocally and that the areas near the larynx and posterior edge of the hard palate were almost constant throughout the utterance. The lower four formants were calculated from all the area functions and compared with those of natural speech sounds. The mean absolute percent error between calculated and measured formants among all the frames was 4.5%. The comparison of vocal tract shapes for the five vowels with those from the static MRI method suggested a problem of MRI observation of the vocal tract: data from static MRI tend to result in a deviation from natural vocal tract geometry because of the gravity effect.

Acoustic analysis of the vocal tract during vowel production by finite-difference time-domain method

The vocal tract shape is three-dimensionally complex. For accurate acoustic analysis, a finite-difference time-domain method was introduced in the present study. By this method, transfer functions of the vocal tract for the five Japanese vowels were calculated from three-dimensionally reconstructed magnetic resonance imaging (MRI) data. The calculated transfer functions were compared with those obtained from acoustic measurements of vocal tract physical models precisely constructed from the same MRI data. Calculated transfer functions agreed well with measured ones up to 10 kHz. Acoustic effects of the piriform fossae, epiglottic valleculae, and inter-dental spaces were also examined. They caused spectral changes by generating dips. The amount of change was significant for the piriform fossae, while it was almost negligible for the other two. The piriform fossae and valleculae generated spectral dips for all the vowels. The dip frequencies of the piriform fossae were almost stable, while those of the valleculae varied among vowels. The inter-dental spaces generated very small spectral dips below 2.5 kHz for the high and middle vowels. In addition, transverse resonances within the oral cavity generated small spectral dips above 4 kHz for the low vowels.

Acoustic roles of the laryngeal cavity in vocal tract resonance

The acoustic effects of the laryngeal cavity on the vocal tract resonance were investigated by using vocal tract area functions for the five Japanese vowels obtained from an adult male speaker. Transfer functions were examined with the laryngeal cavity eliminated from the whole vocal tract, volume velocity distribution patterns were calculated, and susceptance matching analysis was performed between the laryngeal cavity and the vocal tract excluding the laryngeal cavity (vocal tract proper). It was revealed that the laryngeal cavity generates one of the formants of the vocal tract, which is the fourth in the present study. At this formant, the resonance of the laryngeal cavity (the 1/4 wavelength resonance) induces the open-tube resonance of the vocal tract proper (the 3/2 wavelength resonance). At the other formants, on the other hand, the vocal tract proper acts as a closed tube, because the laryngeal cavity has only a small contribution to generating these formants and the effective closed end of the whole vocal tract is the junction between the laryngeal cavity and the vocal tract proper.

Mechanism for generating peaks and notches of head-related transfer functions in the median plane

It has been suggested that the first spectral peak and the first two spectral notches of head-related transfer functions (HRTFs) are cues for sound localization in the median plane. Therefore, to examine the mechanism for generating spectral peaks and notches, HRTFs were calculated from four head shapes using the finite-difference time-domain method. The comparison between HRTFs calculated from the whole head and the pinna-related transfer functions calculated from the segmented pinna indicated that the pinna determines the basic peak-notch pattern of the HRTFs. An analysis of the distribution patterns of pressure nodes and anti-nodes on the pinna computed in the steady state for sinusoidal excitations confirmed that the first three peaks correspond to the first three normal modes of the pinna. The analysis also revealed that at the first spectral notch frequencies, one or two anti-nodes appeared in the cymba and the triangular fossa, and a node developed in the concha. Furthermore, according to changes in the instantaneous pressure distribution patterns on the pinna, three types of mechanisms were hypothesized for inducing the node in the concha depending on the source elevation angle.

Visualisation of hypopharyngeal cavities and vocal-tract acoustic modelling

The hypopharyngeal cavities consist of the laryngeal cavity and bilateral piriform fossa, constituting the bottom part of the vocal tract near the larynx. Visualisation of these cavities with magnetic resonance imaging (MRI) techniques reveals that during speech, the laryngeal cavity takes the form of a long-neck flask and the piriform fossa takes the form of a goblet of varying shapes: the former diminishes greatly in whispering and the latter disappears during deep inhalation. These cavities have been shown to exert significant acoustic effects at higher frequency spectra. In this study, acoustic experiments were conducted for male and female mechanical vocal tracts with the results that acoustic effects of those cavities determine the frequency spectra above 2 kHz, giving rise to peaks and zeros. An acoustic model of vowel production was proposed with three components: voice source, hypopharyngeal cavities and vocal tract proper, which provides effective means in controlling voice quality and expressing individual vocal characteristics.

Cyclicity of laryngeal cavity resonance due to vocal fold vibration

Acoustic effects of the time-varying glottal area due to vocal fold vibration on the laryngeal cavity resonance were investigated based on vocal tract area functions and acoustic analysis. The laryngeal cavity consists of the vestibular and ventricular parts of the larynx, and gives rise to a regional acoustic resonance within the vocal tract, with this resonance imparting an extra formant to the vocal tract resonance pattern. Vocal tract transfer functions of the five Japanese vowels uttered by three male subjects were calculated under open- and closed-glottis conditions. The results revealed that the resonance appears at the frequency region from 3.0 to 3.7 kHz when the glottis is closed and disappears when it is open. Real spectra estimated from open- and closed-glottis periods of vowel sounds also showed the on-off pattern of the resonance within a pitch period. Furthermore, a time-domain acoustic analysis of vowels indicated that the resonance component could be observed as a pitch-synchronized rise-and-fall pattern of the bandpass amplitude. The cyclic nature of the resonance can be explained as the laryngeal cavity acting as a closed tube that generates the resonance during a closed-glottis period, but damps the resonance off during an open-glottis period.

Single-matrix formulation of a time domain acoustic model of the vocal tract with side branches

Although it has been found that the piriform fossae play an important role in speech production and acoustics, the popular time domain articulatory synthesizer of [Maeda, S., 1982. A digital simulation method of the vocal-tract system. Speech Comm. 1 (3-4), 199-229] currently cannot include any more than one side branch to the acoustic tube that represents the main vocal tract. To overcome this limitation, in this paper we extended Maedas (1982) simulation method, by mathematical reformulation in terms of a single-matrix equation having a system matrix that is both sparse and symmetric. Using vocal tract area functions measured by MRI, the simulation results showed that the piriform fossae suppress the energy in the higher frequencies by introducing spectral zeros around 4-5kHz, and also tend to lower the second formant of vowels. These spectral changes agree with results produced using a well-tested frequency domain transmission-line method, thus validating our new formulation of the time domain synthesizer. The reformulation can be easily extended to accommodate any number of vocal tract side branches, thus enabling more realistic, physiologically correct acoustic simulation of speech production.

Effects of side cavities and tongue stabilization: Possible extensions of the quantal theory

Abstract This article provides experimental evidence for two hypotheses, the “interdental-space effect” and the “stabilization effect”, that may help to understand quantal acoustic properties of certain vowels. The interdental-space effect has its basis on the hypothesis [Stevens, K. N., & Perkell, J. S. (1977). Speech physiology and phonetic features. In M. S. Sawashima, & F. Cooper (Eds.), Dynamic aspect of speech production (pp. 323–341). Tokyo: University of Tokyo Press] that the gap between the upper and lower dental arches is involved in abrupt changes of oral cavity volume between low and high vowels. Our magnetic resonance imaging (MRI) data indicate that this side cavity of the vocal tract is fused with the oral cavity in low vowels such as [a] but forms a bilateral side-branch in non-low vowels such as [i] and [e]. This abrupt change in the cavitys affiliation to the vocal tract contributes a discontinuity in otherwise smooth formant transitions. The stabilization effect [Fujimura, O., & Kakita, Y. (1979). Remarks on quantitative description of the lingual articulation. In B. Lindblom, & S. Ohman (Eds.), Frontiers of speech communication research (pp. 17–24). London: Academic Press] is the hypothesis that co-contraction among bundles of the genioglossus muscle stabilizes the formant pattern for [i], which was supported by analysis of electromyographic data. Our MRI-based measurement of tongue muscle geometry suggests an additional cause of vowel stabilization for [a]: posterior bulging of the tongue base for this vowel by the hyoglossus and styloglossus is mechanically limited by the parapharyngeal tissue that surrounds the extra-lingual styloglossus.

Computer Simulation of HRTFs for Personalization of 3D Audio

To give listeners a vivid sense of 3D spatial audio, virtual auditory display technology relies crucially on head related transfer functions (HRTFs). However, as each person has unique morphological characteristics of their head and ears, for a realistic auditory experience it is important to use personalized HRTFs. Our approach to HRTF personalization is first to measure a listeners head and ear morphology, currently by magnetic resonance imaging (MRI); then to use the 3D morphological data in computer simulation of sound wave propagation, by the finite difference time domain (FDTD) method. This paper summarizes our methods and recent improvements, which have led to obtaining more faithful, personalized HRTFs by FDTD simulation.

Acoustic interaction between the right and left piriform fossae in generating spectral dips.

It is known that the right and left piriform fossae generate two deep dips on speech spectra and that acoustic interaction exists in generating the dips: if only one piriform fossa is modified, both the dips change in frequency and amplitude. In the present study, using a simple geometrical model and measured vocal tract shapes, the acoustic interaction was examined by the finite-difference time-domain method. As a result, one of the two dips was lower in frequency than the two independent dips that appeared when either of the piriform fossae was occluded, and the other dip was higher in frequency than the two dips. At the lower dip frequency, the piriform fossae resonated almost in opposite phase, while at the higher dip frequency, they resonated almost in phase. These facts indicate that the piriform fossae and the lower part of the pharynx can be modeled as a coupled two-oscillator system whose two normal vibration modes generate the two spectral dips. When the piriform fossae were identical, only the higher dip appeared. This is because the lower mode is not acoustically coupled to the main vocal tract enough to generate an absorption dip.