Fluid flow in a dynamic mechanical model of the larynx
A dynamic mechanical model of the human larynx and vocal tract has been developed to investigate its acoustic and fluid dynamic behaviour during sustained vowel production. The model comprises a cylindrical duct, open at one end, with a controlled air flow introduced at the other. The flow entering the duct is modulated by the periodic opening and closing of pair of electro-mechanically driven shutters. Far field measurements of the radiated pressure have shown that the model generates sound which has a spectral distribution that corresponds, at low frequencies, to that of an open vowel. However the spectral amplitudes were somewhat lower than voiced speech sounds normally generated at the same low rate. The addition of an orifice plate to reduce the duct exit area was found to increase the level of the radiated sound and to modify the spectral distribution somewhat. The flow distribution throughout the model duct has been measured using hot wire anemometry. The velocity distribution measured in the model was found to correspond to that measured in the oral cavity of four live subjects. Calibrate pressure measurements at the duct wall have been used define the associated pressure field within the duct. The pressure distribution found within the model corresponded to that measured in vivo by other researchers. The velocity within the duct was shown to be associated with contributions from three separate velocity fields, the rotational acoustic particle velocity and a rotational velocity field due to vortex development at the exit to the shutters. It was shown that the rotational velocity disturbance convected along the duct at approximately the local mean flow velocity. Comparison of prediction with measurement of the radiated sound fields showed that the presence of a rotational velocity field at the duct exit made a significant contribution to the radiated sound pressure level. A discussion is included as to whether acoustic sources, associated with the rotational flow, exists at area discontinuities in the vocal tract in addition to the generally accepted acoustic source due to fluctuating mass flow at the glottal exit. The influence of the Rotheberg Mask on the flow and acoustic behaviour of the model was investigated in some detail. Measurements show that the velocity field was almost unaltered, but the fluctuating pressure amplitudes were greatly reduced. Corresponding reductions were found in the radiated acoustic power with further reductions apparently due to the suppression of the rotational flow sources by the mask.