Fairbanks, Grant. Experimental Phonetics – T09

A Physiological Correlative of Vowel Intensity * 1

Grant Fairbanks
University of Illinois

The intensity of a vowel is usually
A taken to involve three anatomical
systems: the breathing apparatus, the
larynx, and the resonance cavities. The
relative roles of these systems and the
interactions between them are not well
understood, but two specific variables,
the vocal fold tone and the modification
of the vocal fold tone by the resonance
system, may be discerned as contributing
to the acoustic end product. It is
with the second of these variables that
the present study is concerned. The
approach may be introduced by two
hypothetical vocal situations.

Assume that a speaker is instructed
to phonate a given vowel and vary its
intensity, keeping the vowel constant.
If it is granted that he kept the vowel
constant by maintaining the anatomical
resonators in a relatively fixed state, the
variation in intensity may be considered
to have resulted from alteration of the
vocal fold tone. The latter has been
produced, presumably, through the mutually
interdependent and balanced interplay
between the firmness of the
glottal closure and the quantity of infra-glottal
air pressure, with related
variations in physiological activity on
the part of the thoracic and laryngeal
musculature.

Assume, on the other hand, that the
speaker is instructed to keep this physiological
activity constant (granting for
the sake of illustration that this is possible)
and phonate several different
vowels. Variations in the vocal spectrum,
most experimental phoneticians
would agree, are produced by differential
adjustments of the anatomical resonators,
especially the pharyngeal oral
cavities, so that characteristic differences
in the selective modification of the vocal
fold tone result. The systematic
nature of these adjustments has been
demonstrated repeatedly. While the
problem is complex and may easily be
oversimplified, certain variables may be
identified with considerable confidence.
Manipulation of the cavities by the mandible,
tongue and lips, for example, are
known to all students of speech and
need no summary here. Cavity adjustments
are pertinent to the present discussion,
however, because they not only
affect the vocal spectrum, but some of
them should also affect the vocal intensity.
If the speaker, then, varied vowel
quality by adjustment of resonators,
physiological activity in the glottal region
constant, variations in vocal intensity
should take place. 12

In a recent study by Fairbanks,
House and Stevens 2 3most of the common
American vowels were found to be significantly
different from each other in
mean relative intensity. Measurements
were made in an experimental situation
which appears to permit the conclusion
that these differences resulted primarily
from changes in the supra-glottal cavities.
Briefly, the procedure was as follows:
Eleven vowels were studied, each
being represented by 10 monosyllabic
75words which included voiceless consonant
elements only; 10 adult males
served as subjects, each subject furnishing
acceptable pronunciations of all no
words; the words were printed on individual
cards, intermixed without reference
to vowels, and presented to the subjects
in random order, a different order
being used for each subject; instructions
were to read the words to the experimenter,
who faced the subject across the
microphone and exposed the cards one
at a time. Under these conditions of randomized
multiplication, by words and by
subjects, it seems reasonable to assume
that the physiological activity of the
thoracic and laryngeal structures varied
essentially at random, and was comparable
from vowel to vowel. In other
words, the assumption made in the second
hypothetical case proposed above,
namely, constant physiological activity
at the larynx, was approximately satisfied
on a statistical basis. 3 4If this premise is
accepted provisionally, an explanation of
the obtained intensity differences may be
sought by studying the characteristics of
the vocal cavities during vowel production.

Physiological measurements were not
made in the above investigation, although
such an experiment is in progress.
Meanwhile it is of interest to examine
data from other studies. The
best evidence appears to come from
X-ray photography. Most of the X-ray
investigations, however, are not .directly
comparable to the intensity experiment,
and all of the work on English vowels
known to this writer has been based on
lateral exposures which, since they reveal
the median plane only, have definite
limitations. The findings of Treviño
and Parmenter, 4 5Parmenter, Treviño
and Bevans, 5 6and Parmenter and
Bevans 6 7seem most applicable. The data
are shown in the form of tracings of the
original negatives by methods which are
described in detail and evidence unusual
care. The report by Parmenter and
Bevans presents a group of such tracings
for the same 11 vowels that were used
in the study of Fairbanks, House and
Stevens. The subject, a young, adult,
male college student, native to the Middle
West, also was comparable. For the
present study measurements of these
plates were made and correlated with
the intensity data.

Procedure

Results

In consideration of the various limitations,
it does not seem profitable to
carry on additional quantitative treatment
with the present data. The most
important aspect of the study would appear
to be its definite indication that
further experimentation along these
lines should be fruitful. The findings
also call attention to a conception of
the system of anatomical resonators
which is not, in the experience of this
writer, commonly expressed by writers
and teachers in the field of speech. The
connected cavities of the larynx, pharynx,
and mouth may be regarded as constituting,
in the case of vowels, a curved
acoustic conduit, tube, or pipe, irregular
in cross-sectional area and shape
along its length, and changing in these
respects from vowel to vowel. Laryngeal
activity constant, a relationship between
vocal power and the area of the
78mouth of such a conduit, as suggested by
the present study, is plausible, on acoustical
grounds and has implications for
vocal theory. Phoneticians have long
been interested in the spectral functions
of the anatomical resonance system. It
seems fair to suggest that the concentration
on these phenomena, the common
use of the term “cavities,” and the reproduction
of diagrams of the positions
of structures in the median plane have
led to an over-emphasis of the irregularities
of the channel and distracted
attention from a view of it as a conduit.

Summary

In an attempt to identify some physiological
basis for the significant differences
in the relative intensities of vowels
found by Fairbanks, House and Stevens,
data from an X-ray study by Parmenter
and Bevans were examined, and
measurements of the diameter of the vocal
channel in the median plane were
made at three points of constriction.
The correlation between relative power
of vowels and relative channel diameter
between the upper and lower incisor
teeth was found to be significantly positive.
On the basis of the assumption
that laryngeal activity was essentially
random in the intensity study, this correlation
was tentatively interpreted as reflecting
the effect of variations in the
area of the mouth of the vocal conduit
upon the power of output.80