The animation below is an excerpt from a lecture presentation given by Brad Story at the Sixth Joint Meeting of the Acoustical Society of America and the Acoustical
Society of Japan in Honolulu, Hawaii, USA, on December 4, 2025, in Special Session – 4aMU, Multiphonics in Vocalization.
The title of the presentation was “Controlling the convergence of vocal tract resonances to generate apparent multiphonic effects”
The animation shows a simulation of overtone (harmonic) singing based on a sound production system representative of an adult female singer/talker. The top row can be considered the “time domain” variables – glottal flow generated by vocal fold vibration (based on nonlinear interaction of flow and acoustic pressure in the vocal tract), time-varying vocal tract configuration, acoustic pressure radiated at the lips (analogous to a microphone signal). The bottom row shows the “frequency domain” counterpart to each variable in the top row – glottal flow spectrum, vocal tract frequency response (peaks are resonances), and radiated pressure spectrum (prominences of energy are the formants).
The primary points of interest are the vocal tract configuration (top middle), the associated frequency response (bottom middle) and the radiated pressure spectrum (bottom right). At the beginning (and end) of the animation, the vocal tract is in a neutral configuration with resonance peaks roughly evenly spaced in frequency. The vocal tract then shifts into a new shape that merges the second and third resonance; their collective effect enhances the amplitude of the nearest harmonic. As the animation plays, the 10th harmonic is initially enhanced, and then vocal tract shape changes such the successive harmonics with decreasing frequency are selectively enhanced. For most of the harmonics, the enhanced amplitude exceeds the amplitude of their neighboring harmonics by more than 10 dB (for the sixth harmonic the difference is more than 20 dB). The auditory effect is an enhanced harmonic that is heard as a tone nearly separate from the other harmonics.
The vocal tract configurations were derived with the technique described in this article:
Story, B. H. (2006). Technique for “tuning” vocal tract area functions based on acoustic sensitivity functions. The Journal of the Acoustical Society of America, 119(2), 715-718. https://doi.org/10.1121/1.2151802