Implications for Modal Voice

We have argued that, for sopranos, a crucial factor in producing vocalization at high sound pressure levels without vocal abuse is the ability of the larynx and vocal tract, working together, to maintain a reasonably low average air flow at elevated levels of lung pressure, without the strain of excess vocal fold adduction. There is considerable evidence that at lower values of Fo, for both males and females and in speech as well as singing, a similar function can be performed by inertive vocal tract loading. It is now generally accepted that, when Fo is much smaller than F1, the inertance of the vocal tract air flow, by creating an appropriate variation in trans glottal pressure during the glottal cycle, suppresses the buildup of air flow during the glottal opening phase and maintains a higher air flow during the glottal closing phase, thus skewing the glottal air flow pulse to the right and concentrating an increased generation of high frequency energy near the instant of vocal fold closure. If lung pressure is held constant and vocal tract inertance varied, this type of source-tract acoustic interaction will lead to an increase in energy in the third formant region of about 5-10 dB, depending on the degree of interaction and the model assumed for the calculations (Fant, 1982; Rothenberg, 1981).

However, increasing vocal tract inertance also decreases the average air flow for a given lung pressure. If when calculating the effect of varying vocal tract inertance, we consider the average glottal air flow to be constant and average lung pressure as a dependent variable that assumes the value necessary to maintain average air flow, then the increase in higher-formant energy that can conceivably be caused by this type of interaction increases to as much as 20 dB {Rothenberg, draft manuscript). Thus, the large differences in vocal efficiency in modal voice that are observed among otherwise normal voices could be explained by interaction with vocal tract inertance. However, we must break away from the concept that voice is produced by a fixed reservoir of pressure, which is relatively constant between speakers for a given "vocal effort," and change to the concept that the respiratory system is a source of air flow, which is relatively constant between speakers for a given "vocal effort," with lung pressure being a secondary variable in voice production.

This new concept is at least as supportable physiologically as the concept of a fixed reservoir of pressure. Excessive air flow can dry out the mucosa, lead to too-frequent breath pauses, and possibly even hyperventilation in a speaker or singer who is vocalizing continuously over a long period of time. On the other hand, measurements with a manometer and a tube at the lips will easily show how easily attainable are lung pressures much higher than commonly reported for speech. Moreover, high values of lung pressure are produced by relatively large abdominal and intercostal muscles that usually are not felt to be fatigued, even in stressful vocalization situations, and that could be developed further, if necessary, by the professional vocalist.

What is more likely to be fatigued as subglottal pressure is increased are those elements of the laryngeal musculature that must hold the vocal folds sufficiently adducted to maintain an acceptable rate of air flow at the increased lung pressure. The parameter of adductory tension is undoubtedly an important contributor to the total concept of vocal effort and should not be confused with subglottal pressure, even though the two tend to covary in a single individual, for a given flow rate.

The question of which is the primary aerodynamic variable in the generation of voice-average lung pressure or average air flow-may not have a perfectly clear answer because of the difficulty in defining and equating among speakers the degree of "vocal effort." It is an important question, nevertheless. For example, a measure of vocal efficiency proposed by Isshiki(1981), namely the ratio of ac to dc air flow, would be supported by a conclusion that flow is primary. The question is surely worthy of future research and debate.

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