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Computing Science and Mathematics

PhD Thesis Abstract

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Below is the abstract from my PhD thesis. The full document is available by clicking here (beware that it is 325 pages long and a 7meg file..download/read at your peril!).

"The mechanical properties of the lips are of crucial importance to the function of a brass instrument. The natural resonance modes must be able to usefully interact with the instrument air column in order to sustain oscillations. Mechanical frequency responses of human and artificial lips used to play a brass instrument were measured using a high-speed digital video technique in an attempt to classify the true nature of the lip- reed. The results revealed the presence of at least two lip modes that exhibited the characteristic outward-inward striking behaviour seen in many in vitro replica lip-reed measurements. The Q-values of the human lip resonances were considerably lower than those seen for the replica lips. Transverse mechanical response measurements were also performed on an in vitro lip-reed to investigate the coupling between the outward and inward striking modes. The two dimensional motion of the lips during full oscillations was investigated. It is shown that a computational four degree-of-freedom model would be required to fully simulate the observed mechanical motion.

The fluid behaviour downstream from an in vitro vocal fold model was investigated using particle image velocimetry (PIV). A 'free jet' configuration with no downstream acoustical coupling was first investigated. The measurements revealed an unsteady glottal jet flow, consisting of a high velocity jet core, a transitional region of high jet deceleration and a turbulent mixing region. The jet was consistently skewed at angles to the glottal centreline, and appeared to oscillate back and forth across the centreline during the glottal cycle. The behaviour of the jet core was investigated in detail. A temporal asymmetry was observed in the mean velocity across the jet core such that the highest jet velocities were encountered during the closing phase of the vocal folds. The overall jet behaviour also showed a strong turbulent asymmetry between the opening and closing phases. High levels of vorticity and turbulent motion encountered during the closing phase were associated with the deceleration of the jet.

Three vocal fold configurations that included static replicas of the ventricular bands were finally investigated with the aim of characterising the aerodynamic interaction between the ventricular bands and the vocal folds. A marked effect on the glottal jet was observed for all configurations. The most physically realistic configuration appeared to stabilise the glottal jet, leading to a reattachment of the jet to the ventricular bands and a subsequent secondary flow separation from the downstream end. The implications of the aerodynamic interaction is discussed, with particular note to its possible relevance to the lip-reed and mouthpiece interaction in brass playing."

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