Tsukasa Yoshinaga

Sibilant /s/ Simulator Based on Computed Tomography Images and Dental Casts

The sibilant /s/ is produced by raising the tongue against the roof of the mouth to form a narrow constriction, which is adjusted so that the airstream emerging from it impinges on the incisors. However, the location where the sibilant sound occurs is unclear, as are the details of the mechanisms of its generation. In this study, we used a realistically shaped replica produced with a three-dimensional printer and demonstrated that turbulent flow was generated in the oral tract near the incisors and lips and that sufficiently developed turbulent flow generated a sound source up to 20,000 Hz at 333, 500, and 667 cm3/sec, which agrees with the range of physiological flow rates typical for /s/. The characteristics of the sound spectra agreed with those of the sibilant /s/ sound emitted by our control individual. Such a physical perspective could yield knowledge useful for oral surgery and speech science – for example, to predict how the generation of sibilants may be occasionally affected by orthodontic and prosthodontic treatments.

Effects of tongue position in the simplified vocal tract model of Japanese sibilant fricatives /s/ and /ʃ/

The effects of tongue position on sound properties were investigated by using simplified models of sibilant fricatives /s/ and /ʃ/. These were constructed from medical images of a native Japanese-speaking subject who pronounced both fricative sounds. The sounds generated by the models were experimentally measured and compared with the subjects /s/ and /ʃ/. The position of tongue models altered the main peak frequency and spectral mean of the generated sound, which resulted in the reproduction of frequency characteristics of /s/ or /ʃ/ with the subjects tongue position.

Attraction of posture and motion-trajectory elements of conspecific biological motion in medaka fish

Visual recognition of conspecifics is necessary for a wide range of social behaviours in many animals. Medaka (Japanese rice fish), a commonly used model organism, are known to be attracted by the biological motion of conspecifics. However, biological motion is a composite of both body-shape motion and entire-field motion trajectory (i.e., posture or motion-trajectory elements, respectively), and it has not been revealed which element mediates the attractiveness. Here, we show that either posture or motion-trajectory elements alone can attract medaka. We decomposed biological motion of the medaka into the two elements and synthesized visual stimuli that contain both, either, or none of the two elements. We found that medaka were attracted by visual stimuli that contain at least one of the two elements. In the context of other known static visual information regarding the medaka, the potential multiplicity of information regarding conspecific recognition has further accumulated. Our strategy of decomposing biological motion into these partial elements is applicable to other animals, and further studies using this technique will enhance the basic understanding of visual recognition of conspecifics.

Experimental and numerical investigation of the sound generation mechanisms of sibilant fricatives using a simplified vocal tract model

The sound generation mechanisms of sibilant fricatives were investigated with experimental measurements and large-eddy simulations using a simplified vocal tract model. The vocal tract geometry was simplified to a three-dimensional rectangular channel, and differences in the geometries while pronouncing fricatives /s/ and /∫/ were expressed by shifting the position of the tongue and its constricted flow channel. Experimental results showed that the characteristic peak frequency of the fricatives decreased when the distance between the tongue and teeth increased. Numerical simulations revealed that the jet flow generated from the constriction impinged on the upper teeth wall and caused the main sound source upstream and downstream from the gap between the teeth. While magnitudes of the sound source decreased with increments of the frequency, amplitudes of the pressure downstream from the constriction increased at the peak frequencies of the corresponding tongue position. These results indicate that the sou...

A mechanical deformation model of the tongue during speech production considering personalized anatomical structure

In the previous studies, several tongue models have been developed to investigate the relationship between the contraction of tongue muscles and the deformation of the tongue during speech. However, the tongue deformation is highly subject-specific and their modellings are insufficient to uncover the detailed mechanism. In this study, a mechanical tongue model considering personalized anatomical structure of tongue myofiber is proposed. A set of reference geometries of tongue and jaw is constructed from the medical images of computer tomography (CT) and the magnetic resonance imaging (MRI) for a Japanese male adult. In addition, we reflect the information of tongue myofiber orientations evaluated from the medical images of diffusion tensor imaging (DTI) on the model using a smoothing extrapolation technique. The tongue deformation is calculated by solving a force-equilibrium equation using a continuous Galerkin finite element method with a hyperelastic material, where the deformation is driven by active contraction of the tongue muscles. We examined the tongue deformations under several contraction conditions during speech and confirmed reasonable numerical reproductions of actual tongue motion and shape. These results indicate that the proposed model reflecting personalized information including myofiber orientations has a potential to identify the tongue muscle contractions in speech production.In the previous studies, several tongue models have been developed to investigate the relationship between the contraction of tongue muscles and the deformation of the tongue during speech. However, the tongue deformation is highly subject-specific and their modellings are insufficient to uncover the detailed mechanism. In this study, a mechanical tongue model considering personalized anatomical structure of tongue myofiber is proposed. A set of reference geometries of tongue and jaw is constructed from the medical images of computer tomography (CT) and the magnetic resonance imaging (MRI) for a Japanese male adult. In addition, we reflect the information of tongue myofiber orientations evaluated from the medical images of diffusion tensor imaging (DTI) on the model using a smoothing extrapolation technique. The tongue deformation is calculated by solving a force-equilibrium equation using a continuous Galerkin finite element method with a hyperelastic material, where the deformation is driven by active c...

Model-based inverse estimation for active contraction stresses of tongue muscles using 3D surface shape in speech production

This paper presents a novel inverse estimation approach for the active contraction stresses of tongue muscles during speech. The proposed method is based on variational data assimilation using a mechanical tongue model and 3D tongue surface shapes for speech production. The mechanical tongue model considers nonlinear hyperelasticity, finite deformation, actual geometry from computed tomography (CT) images, and anisotropic active contraction by muscle fibers, the orientations of which are ideally determined using anatomical drawings. The tongue deformation is obtained by solving a stationary force-equilibrium equation using a finite element method. An inverse problem is established to find the combination of muscle contraction stresses that minimizes the Euclidean distance of the tongue surfaces between the mechanical analysis and CT results of speech production, where a signed-distance function represents the tongue surface. Our approach is validated through an ideal numerical example and extended to the real-world case of two Japanese vowels, /ʉ/ and /ɯ/. The results capture the target shape completely and provide an excellent estimation of the active contraction stresses in the ideal case, and exhibit similar tendencies as in previous observations and simulations for the actual vowel cases. The present approach can reveal the relative relationship among the muscle contraction stresses in similar utterances with different tongue shapes, and enables the investigation of the coordination of tongue muscles during speech using only the deformed tongue shape obtained from medical images. This will enhance our understanding of speech motor control.

Experimental Validation of Sound Generated from Flow in Simplified Vocal Tract Model of Sibilant /s/.

The coupled numerical simulation of flow and sound generation in a simplified vocal tract model of sibilant /s/ were validated with experimental measurements. The simplified model consists of incisors and four rectangular channels representing a throat, constriction, space behind the incisors, and lips. Velocity distribution and far-field sound were measured by a hot-wire anemometer and an acoustic microphone, respectively. Simulated amplitude of velocity fluctuation at the flow separation region was stabilized by increasing the grid resolution, and agreed with those of the measurement. Amplitude of sound pressure simulated by the low-resolution grids was larger than that of the high-resolution grids, indicating that calculation accuracy of velocity fluctuation at the separation region is required to simulate sound generation of the sibilant /s/.

Effects of the Position of Tongue on the Sound Generation of Sibilant/S/

The sibilant /s/ is one of the fricative sounds produced by raising the tongue against the frontal palate of the mouth to form a constriction of the airflow. Thus, the movement of the tongue plays important roles in the sound generation of sibilant /s/. In this study, we investigated the effect of the tongue positions on the airflow and generation of the sibilant /s/ using an in vitro vocal tract model implementing a movable tongue. A replica model of vocal tract was produced by a 3D printer from the CT images of a subject who pronounced sibilant /s/. Steady airflow was given to the vocal tract model by compressor and the sound generated was measured with a microphone. The time variations of airflow velocity in the vocal tract were also measured with a hot wire anemometer. Our results showed that the power spectrum density of the sound generated by the vocal tract model quantitatively agreed with that of the sibilant /s/ from the subject by adjusting the position of the tongue. The flow measurements revealed that the flow fluctuation increased in the vicinity of the incisors rather than the constriction where the flow velocity was the highest. We found that the sound source of sibilant /s/ was produced by turbulence enhanced by the impingement of the airstream emerging from the constriction on the downstream incisors.