The severity of voice disorders is often described in terms of roughness in the laryngeal sound source. Much research has been undertaken to describe and quantify this roughness with perceptual, acoustic, and visual imaging methodologies. While these approaches provide a useful description of the final outcome of a disordered system, they say little about the internal system interactions that lead to this outcome. Self- sustained vocal fold vibration in an airway is an example of a highly interactive (nonlinear) process that has the ability to self-organize. If interaction is strong enough, with energy flowing bi-directionally between components, a structurally disorganized system can become functionally organized. Two asymmetric vocal folds with independent and unrelated natural vibration patterns can synchronize their combined motions as long as energy is allowed to flow between them. Thus, synchronization in coupled oscillators has become an important part of the physics and biology of self-organization. New opportunities will open up for clinicians who re-structure or re-program an impaired vocal system.
The first aim i s designed to improve the computational fluid dynamics of the voice simulator VoxInSilico. Critical shapes of an asymmetrical 3D glottis (airspace between the vocal folds) will be examined that include a variety of contact patterns on the medial surfaces of the vocal folds. The critical shapes will then define the airflow channels, for which isobar contour lines will be derived. To maintain both speed and fidelity of computation, analytical approximations will be developed for the isobars so that interpolations can be made between time steps in a glottal cycle during later time-dependent simulations. Further validation of the isobar contours will be obtained with three-dimensional computer assisted design (CAD) of physical models of the glottal shapes. Pressure taps will be placed so that gradients of pressure can be determined in coronal and horizontal planes.
The second aim i s to explore synchronization for asymmetric vocal folds with modified airway structures above the vocal folds. The clinical condition known as hyper-function will be addressed. The laryngeal and pharyngeal regions of the vocal tract will be systematically widened and narrowed for decreased and increased source-filter interaction. Vocal efficiency and periodicity measures will be compared to non-interactive baseline cases. The remaining two aims address source-filter interaction in connected speech, where both prosodics and articulation vary continuously. Contours for sound pressure level and fundamental frequency (SPL- fo), as well as contours in vowel space (F1 - F2), will be obtained from adult speakers using limited, normal, and heightened speech gestures. Synchronization between harmonics nfo and formants F1 and F2 will be determined. Vocal efficiency and periodicity will be compared.
The proposed research introduces the concept of self-organization and synchronization into the field of clinical voice, showing that the effects of structural asymmetry between two vocal folds can be minimized with source-filter interaction. The specific aims are directed toward finding the relevant physiologic parameters that allow regions of stability and efficiency to be exploited. Applications are anticipated in patients for whom full symmetry between vocal folds cannot be surgically restored and patients with hyperfunction who have adopted inefficient coping strategies for a weak voice.
