Speech synthesis using an aeroacoustic fricative model
Daniel J. Sinder, James L. Flanagan · PhDT · 1999
The essential result from aeroacoustic theory incorporated into this work is that of Howe. His result relates the motion of vorticity through a duct of changing cross-section to the plane wave sound field generated by that motion. This relation is used to compute the value of an acoustic pressure source in the duct. The aeroacoustic theory implicitly incorporates the source spectrum, level, impedance, and spatial distribution, assuming the behavior of vorticity and the vocal tract shape are known. Due to its complexity, obtaining detailed information about the vorticity distribution of any turbulent flow entails a high cost in time and resources, whether the approach is computational or experimental. Fortunately, this problem has received enough attention that it is possible to parameterize the essential features of the vorticity field in the vocal tract into a jet model which requires a minimum of computational effort. Such a jet model is presented here. It prescribes the motion of vorticity based upon criteria which determine the location of jet formation (flow separation) in the vocal tract, the geometry of the location where the jet is formed, and the local airflow speed at the jet formation location. The new jet model and aeroacoustic source description were incorporated into a transmission line model for duct acoustics. The result is an engineering solution for a new fricative model which combines low-cost computation with judicious application of fundamental physics. Two sets of validation studies were conducted to test the computational method. The first synthesized the sound produced by steady airflow in a pipe with axial area variations. The pipe geometry and jet speed were matched to those of a quiet aeroacoustic pipe flow facility. The pressure spectrum measured at the pipe exit compared favorably to the pressure spectrum computed for the simulated system. The second validation study tested the method by synthesizing unvoiced speech sounds, both in isolation and in a vowel context. The results show the strong potential for this approach to produce high quality unvoiced speech without the need to estimate source strength, spectra, or location for different vocal tract geometries. That is, the synthesis of unvoiced sounds is gained automatically from the articulatory description. (Abstract shortened by UMI.)