Experimental and theoretical assessment of flow asymmetries in normal and pathological speech

Abstract

Speech is initiated when a critical pressure is achieved in the lungs, forcing the vocal folds apart. As the air is pushed up the trachea and through the larynx, the resulting aerodynamic forces, coupled with the vocal fold tissue properties, excite self-sustained oscillations of the vocal folds, which form the basis for vocalized speech. Voiced speech is a complex process which involves the inter-dependency of fluid flow, tissue properties, and acoustics. The speech process becomes even more complicated by the introduction of neurogenic and structural pathologies which may disrupt vocal fold motion and alter fluid behavior and/or the acoustical response. Recurrent laryngeal nerve paralysis is the most common neurogenic pathology resulting from damage to the vagus nerve which innervates all of the muscles of the larynx except the cricothyroid. Recurrent laryngeal nerve paralysis usually results in the complete immobility of the damaged vocal fold. Unilateral polyps, characterized by large growths on the medial surface of the vocal folds, are a common structural pathology that most often results from misuse and abuse of the voice. Flow through 7.5 times scaled-up driven vocal fold models in a pressure-driven flow facility was investigated for both normal and pathological speech conditions over a range of physiologically relevant flow rates. The glottal jet trajectory was resolved during the divergent portions of the phonatory cycle. The glottal jet assumed a bi-modal trajectory for normal speech. Vocal fold paresis and paralysis were modeled by limiting the motion of one vocal fold wall, which resulted in a stable, stationary attachment of the flow to the impaired vocal fold wall. The presence of a unilateral polyp introduced (1) large spatial variations in the flow in both the anterior-posterior and the inferior-superior directions and (2) fluctuations in the flow separation points that were atypical compared to those found during normal vocal fold motion. A theoretical solution for flow over an infinite flat plate that is translating and rotating at constant velocity in a direction normal to the freestream velocity was developed for application to intraglottal flows. The solution addresses the role of boundary conditions on flow stability. It is shown that downstream of the point of rotation, the influence of a rotating boundary acts as a favorable pressure gradient, stabilizing the flow field. A theoretical self-similarity solution was derived in the rotating reference frame for flow over the rotating and translating flat plate, allowing direct application of the solution to flow within the glottis. Reasonable agreement (∼ 5%) between the theory and the experimental results was found. A new procedure for applying the theoretical solution to multi-mass models of speech is proposed, with comparison to the often-employed, but inappropriate, Bernoulli flow assumption