<i>In vivo</i> insights into irregular voice production as a complex nonlinear system—a case study

The human voice exhibits diverse expressions, primarily generated by the regular oscillatory movement of both vocal folds. However, further vocal tract structures can also oscillate, as observed in different singing styles, supraglottic voice substitutes and functional voice disorders. To describe the spectrum of irregular phonation types from a nonlinear systems perspective, we conducted <i>in vivo</i> investigations with a professional metal singer using transnasal high-speed digital imaging combined with electroglottography and acoustic data. Here, we analysed 10 distinct voice qualities. Some of these show oscillatory mechanisms characterized by nearly periodic patterns that differ in three key aspects: the ratio of glottal to supraglottic oscillations, their behaviour during the glissando and the oscillating structures primarily involved. Both reduced glottal adduction and increased glottal and supraglottic compression contribute to a greater degree of irregularity. The supraglottic oscillations can coexist with various glottal oscillation mechanisms. Based on the <i>in vivo</i> data, we identified various nonlinear phenomena of different categories that include synchronized oscillations with defined frequency ratios, bifurcations, biphonations and chaotic oscillations.