Models for representing the acoustic radiation impedance of the mouth

The acoustic radiation impedance of the mouth is important in the production of speech in that it affects the amount of acoustic power radiated away from the mouth and it influences the acoustic response of the excited vocal tract. Five existing methods of modeling the acoustic radiation impedance of the mouth are: 1) the pulsating sphere, 2) the circular piston in an infinite baffle, 3) the circular piston in a spherical baffle, 4) an approximation by Fant, and 5) an approximation by Flanagan. A comparison has been carried out between the acoustic impedances as a function of frequency predicted by these models for mouth aperture areas of 0.9 and 5.0 cm². The basis for comparison was the piston-in-a-spherical-baffle model. This model was chosen as being the most physically representative one which is computationally tractable, although it is by far the most computationally difficult of the models considered. Special precautions were needed to prevent errors due to arithmetic overflow. On the basis of the comparison, a new approximation is proposed. This new model is based on the piston-in-an-infinite-baffle model, but includes more terms than the Flanagan model, resulting in increased accuracy for higher frequencies and larger mouth apertures.