SUBGLOTTAL AIR PRESSURE
In normal speech production, we generate and maintain a steady subglottal air pressure (Ps). This steady Ps ranges from ~4-9cmH20; perhaps an average of 4cmH20 for a given speaker, and 9cmH20 for another speaker. The pressure typically varies +/- 1.0cmH20 around the speakers average Ps.
Ps is maintained at a steady level throughout the breath group. The breath group is defined as the number of syllables produced on one exhalation.
The direct measure of Ps is from a hypodermic needle inserted between tracheal rings and placed beneath the vocal folds. Fortunately, there are two methods to estimate Ps that are noninvasive: (1) Ps during syllable repetitions, and, (2) using a U-tube water manometer.
Ps during Syllable Repetitions
Ps is obtained from an air pressure tube placed behind the lips. The tube is connected to a transducer whose output is amplified. When the subject produces a series of [pa] 5-7 syllables at conversational rate and loudness, the air pressure behind the lips during the [p] stop phase equals Ps given three conditions:
(1) the lips are completely sealed,
(2) the velopharynx is closed, and
(3) the vocal folds are sufficiently open such that the air pressure in the mouth equilibrates to the Ps. This situation resembles the pressure in an inflated balloon when the balloon tip opening is squeezed closed.
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Using a U-tube Water Manometer
In the video clip, the speaker is using a straw as a “leak tube”; simulating an average air flow escape during conversational speech. The straw is only an approximation of this average flow. For more precise estimates of the flow, see the leak tube dimensions in Netsell & Hixon*
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* Netsell, R. & Hixon, T.J. A noninvasive method for clinically estimating subglottal air pressure. Journal of Speech & Hearing Disorders, 326-330 (1978).