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The Acoustics of Fricative Consonants
Christine Helen Shadle
Technical Report 506
March 1985
Massachusetts Institute of Technology
Research Laboratory of Electronics
Cambridge, Massachusetts 02139
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The Acoustics of Fricative Consonants
Christine Helen Shadle
Technical Report 506
March 1985
Massachusetts Institute of Technology
Research Laboratory of Electronics
Cambridge, Massachusetts 02139
This work has been supported in part by the National Institutes of Health Grant 5 R01 NS04332
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THE ACOUSTICS OF FRICATIVE CONSONANTS
by Christine Helen Shadle
Submitted to the Department of Electrical Engineering and Computer Science on March
21, 1985. in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
ABSTRACT
The acoustic mechanism of fricative consonants was studied in the context of three domains:
speech, mechanical models. and theoretical models. All fricative configurations have in common
a small turbulence-producing constriction within the vocal tract. Thus, preliminary experiments
were conducted using a mechanical model having this basic configuration of a constriction in
a tube. Parameters such as constriction area. length. location, and degree of inlet tapering,
and presence of an obstacle, were varied. It was found that acoustically the most significant
parameters are the presence of an obstacle, the length of the front cavity, and the flowrate.
Therefore, configurations in which only these parameters were varied, referred to as the obstacle
and no-obstacle cases, were examined more thoroughly and modeled theoretically.
A source function for the obstacle case was derived from the far-field sound pressure mea-
sured when the obstacle was located in space, downstream of a constriction in a baffle. The
directivity pattern produced by the obstacle in this position was similar to that of a dipole,
as expected. A dipole source located inside a duct is equivalent to a pressure source in a
transmnission-lille model, when only the longitudinal modes of a duct are considered. The filter
function. corresponding to the effect of the duct on such a pressure source, was derived for
the transmission-line representations of two configurations in which the obstacle was located
inside a front cavity of nonzero length. The spectra predicted by this source-filter model, when
compared to the far-field pressure measurements of the equivalent mechanical models, provided
a very close match in both absolute sound pressure level and spectral shape. Thus in this
case the presence of the duct does not significantly alter the sound source, and a simple linear
source-filter model works well.
For the no-obstacle case, it was not possible to derive a source from a free-field measurement,
so the validity of a source-tract model could not be checked directly. Investigation of the pressure
versus flow-velocity power laws showed evidence of source-tract interaction for the no-obstacle
case, but none for the obstacle case.
Spectral measures were developed that characterized the acoustic differences caused by the
different types of sources for the obstacle and no-obstacle cases. Analysis of real speech in terms
of these spectral measures revealed that fricatives are more similar to the obstacle than to the
no-obstacle case. More complex speech-like mechanical models were developed and the acoustic
characteristics of the sounds generated by those models were compared to the characteristics of
speech sounds, again in terms of the spectral measures. Very good models of /s/ and // were
obtained by usilg an obstacle at right angles to the flow and varying the constriction location.
For the fricatives /, f. 0/. in which the constriction is located forward in the tract. the shape
of the constriction was crucial. Constrictions that allowed the jet tocome in contact with a
surface produced sounds that most closely resembled the analyzed eamnples of these fricatives.
For /x/ and // in which the constriction is located 4 to 6 cm back from the mouth, a surface
also caused the mechanical model spectra to become the most like the speech spectra. However,
in general it appears that the constriction shape affects the far-field spectrum less as the front
cavity is lengthened. The model for /s, 8/ appears to be that of a series pressure source,
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located at the teeth. For the fricatives other than /s,3/. a "distributed obstacle", modeled as a
distributed pressure source. may be the dominant acoustic mechanism.
The whistles generated by several configurations were also investigated. Orifice tones oc-
curred for untapered constrictions. at a greater range of constriction sizes than predicted by
previous studies. The frequencies were related to the constriction length. Edgetones occurred
for configurations including an obstacle, at frequencies related to the flowrate and distance to
the obstacle. The occurrence. frequency and amplitude of the whistle tones were affected by
the constriction shape and the presence of a tube surrounding the obstacle.
Edgetones were generated by the /s/- and //-like mechanical models that were similar
acoustically to the whistles produced by a subject with the /s/ and // configurations. An even
more striking parallel between the whistles produced by speech-like models and humans was
found for the typical bilabial whistles produced by //-like configurations. Two constrictions,
reproducing the role of tongue and lips, were necessary to model these whistles. The agreement
in flow range. frequency, and control parameters of the holetones thus produced and the whistles
generated by humans was good enough to suggest that the same acoustic mechanism occurred
in both the models and the human vocal tract.
Thesis supervisor: Kenneth N. Stevens
Title: Clarence J. LeBel Professor of Electrical Engineering
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To
my father
my mother
and my grandmother
for giving me the desire to learn
and
for making it possible
4
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Acknowledgments
Doing this thesis has been of necessity a solitary task. yet. paradoxically. a time most richly
populated with people. It is with delight that I acknowledge those who helped me in such a
variety of ways.
First. I would like to thank Ken Stevens for teaching and advising me patiently and thought-
fully. His willingness to let me go off on tangents increased immeasurably the intellectual
pleasure I derived from my work; lo and behold, some of the tangents did finally turn into a
thesis.
Lou Braida, my graduate counsellor, has also been an indispensable figure of the last six
years. Many thanks to him for popping in unexpectedly with interesting and relevant problems
and advice, and for concrete assistance in setting up my lab.
The members of my committee, Bill Rabinowitz. Victor Zue. Sheila Widnall, and Uno
Ingard, spanned beautifully orthogonal areas of interest and expertise. I thank them for their
help and advice, and for reading all those drafts.
I am deeply grateful to Max Mathews, Charles Thompson, and Gunnar Fant. for helping me
in a most difficult phase. that of problem definition. Their comments in the beginning stages,
as well as later on, greatly influenced my approach.
I wish to thank Rich McGowan, Toni Quatieri., Louis Goldstein, Rich Goldhor, John Wyatt,
and Shinji Maeda. who, when asked specific technical questions, took the time to discuss them
thoroughly, and taught me a great deal as a result.
Thanks to Patrick Hosein, for his careful work in revising TBFDA; Peter Vitale, for pro-
gramming the LSI-11; and Dick Lyon, for his help in designing my muffler. Thanks to Joe
Perkell, Larry Frishkopf, and Bob Hillman, for their generosity in lending equipment for "a
year" that became two or three. And thanks to Keith North, for explaining the intricacies of
tape recording and sound level meters, and finding oscillators at a moments notice. Thanks to
Jim Byrne, for teaching me to use a machine shop, and Manny Cabal, for machining all those
constrictions.
Special thanks go to the members of the Communication Biophysics Group, for help with
an unfamiliar computer system, and for my extensive use of their facilities.
I thank Phyllis Vandermolen and Amy Hendrickson, for their help with figures and text
formatting (see, for example, Figure 2.3, and Table 4.2).
Thanks to KS, PP, EM, AS and G for serving as subjects. (Note: upon reading this thesis,
you will wonder, who was AS? AS was a subject for a recording session so preliminary that it
did not appear in these pages, but for whose time I am nevertheless grateful.)
Many thanks to NIH and the LeBel Foundation. for financial support.
Finally, I want to thank the people who have supported, encouraged and expressed their
faith in me so long and well. Among these, I would like especially to thank: Rich Goldhor and
Stephanie Seneff. for our thesis-encounter-group lunches that. among other things, helped me to
keep a sense of humor: Marie Southwick, Patti Jo Price. and Corine Bickley. for sharing so much
with me. and convincing me that there is life during as well as after thesis; Anne Black, John
Wyatt, Kent Pitman. Adele Proctor, and Stefanie Shattuck-Hufnagcl. for celebratory dinners,
a sense of perspective when I needed it, cookies in the lab at 3 A.M.. and the friendship
that made all of that possible; Janet Koelnke, Lorraine Delhornle, Neil Macmillan, Rosalie
Uchanski, Diane Bustaniante. Dan Leotta. Tom Lee, Nat Durlach. and the other members of
the Communication Biophysics Group at M.I.T., who responded to my use of their facilities
5
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with friendship, encouragement, and philosophical discussions, and in general made me feel
like an honorary CBG-er; Rosalind Fine. Lois Eichler. John Rust. Yehuda and Joy. for helping
me make the most out of the process of being a graduate student: Max Mathews. Sandra
Pruzansky, Moise Goldstein, Marcia Bush. Gary Kopec. Karen Landahl. and Emnel Gokcen. for
help in spite of being miles away: and my family, Paul. Elinor. Anna and Paula Shadle and
Sally and Roger Gottlieb. who found so many ways to tell me to hang in there.
6
Description:The acoustic mechanism of fricative consonants was studied in the context of For the fricatives other than /s,3/. a "distributed obstacle", modeled as a.
