Human speech production obeys the same acoustic principles as vocal production in other animals but has distinctive features: A stable vocal source is filtered by rapidly changing formant frequencies. To understand speech evolution, we examined a wide range of primates, combining observations of phonation with mathematical modeling. We found that source stability relies upon simplifications in laryngeal anatomy, specifically the loss of air sacs and vocal membranes. We conclude that the evolutionary loss of vocal membranes allows human speech to mostly avoid the spontaneous nonlinear phenomena and acoustic chaos common in other primate vocalizations. This loss allows our larynx to produce stable, harmonic-rich phonation, ideally highlighting formant changes that convey most phonetic information. Paradoxically, the increased complexity of human spoken language thus followed simplification of our laryngeal anatomy.

A foremost challenge in modern network science is the inverse problem of reconstruction (inference) of coupling equations and network topology from the measurements of the network dynamics. Of particular interest are the methods that can operate on real (empirical) data without interfering with the system. One of such earlier attempts [Tokuda et al., PRL, 2007] was a method suited for general limitcycle oscillators, yielding both oscillators' natural frequencies and coupling functions between them (phase equations) from empirically measured time series. The present paper reviews the above method in a way comprehensive to domain-scientists other than physics. It also presents applications of the method to (i) detection of the network connectivity, (ii) inference of the phase sensitivity function, (iii) approximation of the interaction among phase-coherent chaotic oscillators, (iv) experimental data from a forced Van der Pol electric circuit. This reaffirms the range of applicability of the method for reconstructing coupling functions and makes it accessible to a much wider scientific community.

Experimental study on noise-induced synchronization of crystal oscillators is presented. Two types of circuits were constructed: one consists of two Pierce oscillators that were isolated from each other and received a common noise input, while the other is based on a single Pierce oscillator that received a same sequence of noise signal repeatedly. Due to frequency detuning between the two Pierce oscillators, the first circuit showed no clear sign of noise-induced synchronization. The second circuit, on the other hand, generated coherent waveforms between different trials of the same noise injection. The waveform coherence was, however, broken immediately after the noise injection was terminated. Stronger perturbation such as the voltage resetting was finally shown to be effective to induce phase shifts, leading to phase synchronization of the Pierce oscillator. Our study presents a guideline for utilizing noise to synchronize clocks of multiple CPU systems, distributed sensor networks, and other engineering devices.

Ventricular folds are located in the supraglottal region above the vocal folds. Although the ventricular folds do not vibrate under normal vocalizations, they vibrate under certain conditions, e.g., throat singing or ventricular fold dysphonia. In throat singing, the ventricular folds vibrate at the same frequency as (or at integer ratios of) the vocal fold vibration frequency. In ventricular fold dysphonia, on the other hand, the ventricular folds interfere with the vocal folds, giving rise to a hoarse voice. In the present study, the synthetic larynx model was utilized to examine the vocal–ventricular fold oscillations. Our experiments revealed that the vocal and ventricular folds can co-oscillate at the same frequency with an out-of-phase relation. Compared to the control condition, under which no ventricular folds exist, the phonation threshold pressure was increased in the presence of the ventricular folds. Acoustic analysis indicated that jitter was reduced and vocal efficiency was increased by the ventricular folds. Distance between the vocal and ventricular folds did not alter these oscillation properties. A computational model was further simulated to elucidate the mechanism underlying the observed vocal–ventricular fold oscillations. It has been suggested that out-of-phase oscillations of the vocal and ventricular folds are important for sustaining periodic laryngeal vibrations.

Synchronization is the phenomenon of two or more autonomous oscillators oscillating with the same frequency due to mutual interactions or common external forces. In this study, we focused on crystal oscillators, which are utilized in a wide variety of electrical circuits. When independently oscillating crystals in each circuit are synchronized in-phase, it is conceivable that simultaneous information processing is possible. To observe synchronization, we designed a circuit containing two Pierce oscillators with a branching path for mutual interactions. Then, we show that in-phase oscillation of the Pierce circuits can be induced by controlling the coupling strength.