Experimental Mechanical and Fluid Mechanical Investigations of the Brass Instrument Lip-reed and the Human Vocal Folds
Newton, Michael James
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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 arti cial 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 lipreed. 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 uid behaviour downstream from an in vitro vocal fold model was investigated using particle image velocimetry (PIV). A `free jet' con guration with no downstream acoustical coupling was rst investigated. The measurements revealed an unsteady glottal jet ow, 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 con gurations that included static replicas of the ventricular bands were nally investigated with the aim of characterising the aerodynamic interaction between the ventricular bands and the vocal folds. A marked e ect on the glottal jet was observed for all con gurations. The most physically realistic con guration appeared to stabilise the glottal jet, leading to a reattachment of the jet to the ventricular bands and a subsequent secondary ow 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.