Table Of ContentTHE EFFECT OF SELECTIVE NARROW-BAND
FILTERING ON THE PERCEPTION OF
CERTAIN ENGLISH VOWELS
JANUA LINGUARUM
STUDIA MEMORIAE
NICOLAI VAN WIJK DEDICATA
edenda curai
CORNELIS H. VAN SCHOONEVELD
STANFORD UNIVERSITY
SERIES PRACTICA
XIII
1964
MOUTON & CO
LONDON • THE HAGUE • PARIS
THE EFFECT OF SELECTIVE
NARROW-BAND FILTERING ON
THE PERCEPTION OF
CERTAIN ENGLISH VOWELS
by
WILLIAM E. CASTLE
1964
MOUTON & CO
LONDON • THE HAGUE • PAWS
© Copyright Mouton & Co., Publishers, The Hague, The Netherlands.
No part of this book may be translated or reproduced in any form, by print, photoprint, microfilm,
or any other means, without written permission from the publishers.
Printed in the Netherlands by Mouton & Co., Printers, The Hague.
ACKNOWLEDGEMENTS
The author wishes to extend his sincerest appreciation to Professor Dorothy A.
Huntington of Stanford University for her counsel and guidance with respect to his
study. More importantly, he is grateful for the interest which she has instilled in him
in the speech sciences by way of her own heart-felt interest.
Appreciation is also due Professors Richard F. Dixon and Alphonse G. Juilland,
also from Stanford University, for their helpful criticism and suggestions.
In addition, the author would like to acknowledge the love offered him by his wife,
Diane, throughout the writing struggle.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS 5
I. INTRODUCTION AND STATEMENT OF HYPOTHESIS 9
II. BACKGROUND OF THE PROBLEM 12
The Importance of Steady State Formants for Perceptual Distinctions among
Vowels 12
The Importance of Factors other than Steady State Formants for Perceptual
Distinctions among Vowels 21
Summary 30
III. PROCEDURES 31
Procedure I for Preparation and Presentation of Stimuli 31
Procedure II for Preparation and Presentation of Stimuli 36
Additional Procedures 44
IV. CHARACTERISTICS OF INSTRUMENTATION AND SPEAKERS 47
Comparison of the Filter Systems 47
Formant Frequencies for Speakers 67
V. PRESENTATION AND DISCUSSION OF RESULTS 74
Identification of Unfiltered Stimuli 74
Identification of Filtered Stimuli 76
Special Points of Discussion 138
VI. SUMMARY 155
Procedures 155
Results and Conclusions 157
APPENDIXES 161
BIBLIOGRAPHY 207
I
INTRODUCTION AND STATEMENT OF HYPOTHESIS
Researchers have been interested for some time in determining what it is in the acous-
tic nature of spoken vowels which allows them to be categorized phonemically regard-
less of speaker, speech rate, phonetic environment, position in the word, and presence
or absence of distortion. As Curtis (12)1 pointed out in 1954, the problem is not simply
that of describing the acoustic characteristics of spoken vowel sounds. It is just as
important to determine which of these characteristics are significant cues to vowel
perception and which are incidental.
It is a common hypothesis that the significant acoustic correlates of the perception
of a given vowel phoneme can be specified with a description of those regions in the
frequency spectrum in which energy is most predominant. Such regions of energy
predominance are assumed to be directly related to the resonances which occur in the
vocal tract of the speaker as he produces samples of the vowel phoneme. These energy
regions and their center frequencies are often referred to as "formants" and "formant
frequencies", respectively. In the present paper this terminology will be employed.
It has often been assumed that a complete description of the necessary and sufficient
acoustic cues for the perception of a vowel, whether isolated or in context, is obtained
by determining the formant frequencies that occur "over a single fundamental period,
or over a short interval including at most a few cycles of the fundamental frequency"
(87, p. 1). It has often further been assumed that such measurements need only be
made for the so-called "steady state" portion of the vowel. For sustained, isolated
production of a given vowel, the energy vs. frequency distribution is considered to be
essentially steady state throughout the production, since the formant positions remain
nearly the same during all fundamental periods except those at the onset and the ter-
mination. For context productions of the same vowel by the same speaker, those por-
tions "where the rates of formant change are slowest" (6, p. 161) often show energy
vs. frequency distributions which approximate that of the isolated production. Such
portions of the context productions of the vowels are, therefore, sometimes defined as
the steady state portions; for the purposes of this investigation, this definition has
been adopted.
While it has often been assumed that some form of invariance inheres in the steady
1 Numbers between parentheses refer to the Bibliography, pp. 207-209.
10 INTRODUCTION AND STATEMENT OF HYPOTHESIS
state formant structure of all vowel samples from the same phoneme category, there is
not general agreement about how many or which of the formants define the assumed
invariance. Most of the available evidence suggests that for the perception of any
vowel the first two formants are the most important acoustic cues. Some evidence
suggests, however, that for back vowels only a single formant may be important.
There is evidence, too, which suggests that three or more formants are important to the
perception of front vowels at least and perhaps to the perception of all vowels.
Along with the assumption that there may be inherent invariants in the steady state
formant structure of a vowel, it is sometimes suggested that these invariants are the
absolute values of the formant frequencies present. There are a number of facts which
do not support this suggestion, however. For instance, vowels produced by members
of a relatively homogeneous group of speakers (e.g., adult males) are sometimes per-
ceived as different vowels even though the same set of steady state formant frequencies
was used by the speakers involved (20, 30, 75). Also, productions of the same vowel
phoneme by adult males, adult females, and children are usually perceived as the same
phoneme though the respective average formant frequencies for the three groups of
speakers are quite different (8,24,31,46, 69, 72). Moreover, several productions of the
same vowel phoneme by a single speaker are perceived correctly though the steady
state formant frequencies for the various productions are sometimes very different
(20,30, 75).
It is also sometimes suggested that the invariance assumed to inhere in the steady
state formant structure of a given vowel phoneme is based upon a ratio which exists
between the first and second formant frequencies for all productions of that phoneme,
regardless of speaker (1, 61). This suggestion is not supported, however, by the fact
that the same ratio may occur for several vowel productions which are perceived as
different vowels (20).
Chiba and Kajiyama (8) have described the assumed invariance for the steady state
formant structure of a vowel in terms of ranges of variation. According to their de-
scription, whenever the formant frequencies present in a vowel are "situated within
certain frequency regions fixed for a given vowel (i.e., characteristic frequency regions)"
(8, p. 194), it is that vowel which will be perceived, regardless of speaker.
Certain supporters of the distinctive feature theory of phoneme perception (25, 31,
43, 44) believe that it is the relationship which the steady state formant frequencies
bear to one another that is of primary importance to vowel perception. They do not
support a simple ratio hypothesis, however. Instead, they propose that the assumed
invariants consist of a set of binary decisions which the listener makes with regard
to the steady state energy vs. frequency distribution for each production of a given
vowel phoneme. For English vowels, for instance, the listener is presumed to decide
whether the first formant is "maximally low" or not (31, p. 126), whether the first for-
mant is "maximally high" or not (31, p. 127), whether there is a "lowering of all for-
mants" or not (31, p. 127), and whether the second formant is closer to the first for-
mant than to the third formant or vice-versa (43, p. 30). Theoretically, whenever a
