Speech Synthesis Using an Aeroacoustic Fricative Model

		       by

               Daniel J. Sinder


Progress in advanced computer speech interfaces is limited
in part due to incomplete knowledge of the physics of speech
production.  Unvoiced speech sounds such as fricatives are an
important example.  These sounds are produced by ``turbulent'' air
motion in the vocal tract.  A proper understanding of how unvoiced
sounds are produced is thus far lacking because the speech community
has for the most part limited its physical picture of air motion in
the vocal system to only acoustic motion. In particular, the
characteristics of the source (spectrum, level, impedance, spatial
distribution) as a function of vocal tract shape, lung pressure, and
other speech parameters is not at all clear.  

A considerable body of work has been produced on the subject of
aeroacoustics, which is the study of the interaction between sound and
non-acoustic air motions such as turbulence.  The purpose of this
dissertation is to apply ideas from aeroacoustics and unsteady
aerodynamics to produce a model of the aeroacoustic source associated
with turbulent flow in the vocal tract.  Particular emphasis has
been given to producing a model suitable for articulatory speech synthesis.
This requirement led to the development of reduced-complexity modeling
of turbulent flow such that the computational requirements are
not far in excess of those needed for existing transmission-line
computations of speech signals.

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.
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