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Publications of Dr. Martin Rothenberg:
Interpolating Subglottal Pressure from Oral Pressure
by Martin Rothenberg
Syracuse University, Syracuse, New York 13210
Journal of Speech and hearing Disorders, No.
47, pp. 218-224, May 1982
(Received August 28, 1981; Accepted September 17, 1981)
Smitheran and Hixon (1981) recently used intraoral pressure during
an unvoiced stop as a measure of the subglottal pressure in adjoining vowels.
I would commend the authors for testing the applicability of this method in
a clinical situation, though I have a few suggestions that potential users might
want to keep in mind.
When using this technique in a previous study (Rothenberg, 1973), I suggested
that the consonant used be a geminated unvoiced-voiced pair, as /pb/ in the
sequence b V p b V p b V pb - - - for bilabial consonants and a vowel V. An
English-speaking subject can be instructed to repeat the syllable /b V p/. Because
of the added /b/, the vocal folds are adducted again at or just before the articulatory
release. This greatly reduces the drop in subglottal pressure that would be
caused by the aspiration of the /p/ release; and thus makes the peak intraoral
pressure during the closure more nearly equal to the subglottal pressure during
the following vowel. The error caused by the aspiration would naturally vary
with the degree of aspiration. This error may be small with the short periods
of aspiration shown in the example of their
Figure 1, but could be appreciable in other cases.
A potential user of this method should also be cautioned that the response time
of the entire pressure measurement system (the time required to respond to a
sudden change in pressure) including the pressure transducer. sholild be less
than the shortest oral closure time expected. For example, long catheters of
very narrow diameter can greatly lengthen this response time, especially if
there is a large air volume between the catheter and the transducer diaphragm.
The response time of the total system can be measured by recording the system's
response to a sudden change of pressure. The simplest methods for accomplishing
this usually involve the release, or driving quickly to zero, of a constant
pressure at the probe tip. For example, one can hold a pressure in the catheter
by means of a finger over the tip, and then quickly remove the finger, or the
syllable /pa/ can be produced with the probe tip between the lips, and the decay
in the system output after the release of the /p/ measured. The resulting response
can be recorded on a chart recorder or storage oscilloscope.
To illustrate the effect of too slow a response time, the oscilloscope traces
below show the simultaneous response of two transducers to a repetition of /pa/,
when each was connected to a catheter between the lips. The amplitude calibration
was the same for both transducers; however, because of different catheters,
one had a total response time, defined here as the time required for attaining
95% of a sudden change, of roughly 40 msec, while the other required about 1.80
msec. The slower system only attained 85-90% of the true oral pressure before
the release occurred for closure periods of 100 to 150 msec. The response of
the fast-responding transducer shows a flat or slightly rounded peak, that approximates
the subglottal pressure during the lip closure, while the slow-responding system
shows a sharp peak at the instant of articulatory release.
To measure the system response time, it is not sufficient to measure the response
to the electronics alone, as Smitheran and Hixon appear to have done. They are
not clear on this point, but the pressure curve in their
Figure 1. indicates a frequency restriction on the complete pressure system
much lower than the 30 Hz they mentioned. Though the time scale in the figure
permits only a very rough estimate, I would judge the time constant t
of the primarily system-determined pressure decay after each release (the time
required to reach 1/e of the initial value), to be roughly 25 msec. Using the
equation fc = 1/2tp which is
valid for simple dynamic systems of this type, I would estimate the total system
frequency response to be flat to only about 6 Hz. The total response time, as
defined above, is about three times the time constant for a simple, exponential
response, and so would be about 85 msec. With this restriction, the measurements
of peak oral pressure made from
Figure 1. would have been about 5% low.
I would also favor a syllable repetition rate somewhat higher than the 1.5 per
second that they chose, because of the possibility of the subject making separate
respiratory gestures for each syllable at rates slower than about 2 per second.
Variations of respiratory activity within a syllable tend to invalidate the
continuity assumption on which this method is based. Respiratory gestures within
the syllable can usually be detected by a significant variation of oral pressure
within the period of articulatory closure, especially a peak at or just before
the release, as shown in
Figure 2, made by repeating the syllable /b ae p/ with a relatively constant
subglottal pressure (top trace), and with separate respiratory gestures for
each syllable (bottom trace).
In order to see the true variation of oral pressure in the pressure tracing,
the system response time must be no greater than about 1/3 the closure period.
Since closure periods of as little as 90-100 msec can be expected, it would
be necessary for the pressure system response time to be no greater than about
30 msec, with no insignificant resonances or overshoot due to the transducer
diaphragm, the acoustic system formed by the catheter and air chamber over the
diaphragm, or the conditioning electronics. A resonance-free response in under
30 msec should be easily attainable with a number of transducers now available.
The system response in this case was about 10 msec, as can be verifed by the
return-to-zero speed after each release, and so the waveform during the period
of complete closure is a fairly good indicator of the subglottal pressure during
that period. The primary exception is the occasional small decrease in pressure
prior to the sharp drop signaling the articulatory release. This small decrease
in pressure is probably caused by the closure of the vocal folds as they come
to the position for voicing (for the /b/ during the period of articulatory closure,
and momentarily seal the oral cavity. Since the mandible is dropping for the
articulatory opening at this time, the volume of the oral cavity is increasing,
and so the oral pressure would show a decrease that may not be present in the
subglottal pressure. (This artifact does not occur when using only a /p/ for
the intervocalic consonant.) To clarify the relationship of oral and subglottal
pressures with a fast-responding sensing system, the dashed lines in Figure 2
were sketched to show what the underlying subglottal pressure variation
might have been for each case, neglecting the slight decrease in subglottal
pressure during the vowel caused by the airflow acting on the subglottal flow
resistance.
REFERENCES
ROTHENBERG, M. A new inverse-filtering technique for deriving
the glottal airflow waveform during voicing. Journal of the Acoustical Society
of America 1973,53, 1632-1645.
SMITHERAN, J., & HIXON, T., A clinical method for estimating laryngeal airway
resistance during vowel production. Journal of Speech and Hearing Disorders,
1981,46, 138-146.
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