@phdthesis{Jackson_PhD00,
	AUTHOR	=	"Jackson, P. J. B.",
	TITLE	=	"Characterisation of plosive, fricative and aspiration
components in speech production",
	SCHOOL	=	"Dept.\ Electronics \& Comp.\ Sci.,
Univ.\ of Southampton",
	YEAR	=	"2000",
	MONTH   =	"May",
	ADDRESS	=	"Southampton, UK",
	ABSTRACT = 	
"This thesis is a study of the production of human speech sounds by acoustic 
modelling and signal analysis. It concentrates on sounds that are not produced 
by voicing (although that may be present), namely plosives, fricatives and 
aspiration, which all contain noise generated by flow turbulence. It combines 
the application of advanced speech analysis techniques with acoustic flow-duct 
modelling of the vocal tract, and draws on dynamic magnetic resonance image 
(dMRI) data of the pharyngeal and oral cavities, to relate the sounds to 
physical shapes.
%
Having superimposed vocal-tract outlines on three sagittal dMRI slices of an 
adult male subject, a simple description of the vocal tract suitable for 
acoustic modelling was derived through a sequence of transformations. The vocal-
tract acoustics program VOAC, which relaxes many of the assumptions of 
conventional plane-wave models, incorporates the effects of net flow into a one-
dimensional model (viz., flow separation, increase of entropy, and changes to 
resonances), as well as wall vibration and cylindrical wavefronts. It was used 
for synthesis by computing transfer functions from sound sources specified 
within the tract to the far field.
%
Being generated by a variety of aero-acoustic mechanisms, unvoiced sounds are 
somewhat varied in nature. Through analysis that was informed by acoustic 
modelling, resonance and anti-resonance frequencies of ensemble-averaged 
plosive spectra were examined for the same subject, and their trajectories 
observed during release. The anti-resonance frequencies were used to compute 
the place of occlusion.
%
In vowels and voiced fricatives, voicing obscures the aspiration and frication 
components. So, a method was devised to separate the voiced and unvoiced parts 
of a speech signal, the pitch-scaled harmonic filter (PSHF), which was tested 
extensively on synthetic signals. Based on a harmonic model of voicing, it 
outputs harmonic and anharmonic signals appropriate for subsequent analysis as 
time series or as power spectra. By applying the PSHF to sustained voiced 
fricatives, we found that, not only does voicing modulate the production of 
frication noise, but that the timing of pulsation cannot be explained by 
acoustic propagation alone.
%
In addition to classical investigation of voiceless speech sounds, VOAC and the 
PSHF demonstrated their practical value in helping further to characterise 
plosion, frication and aspiration noise. For the future, we discuss developing 
VOAC within an articulatory synthesiser, investigating the observed flow-
acoustic mechanism in a dynamic physical model of voiced frication, and 
applying the PSHF more widely in the field of speech research."
}