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Estimates of aeroacoustic source strength of voiced sound are presented from aeroacoustic measurements in a physical model of the upper airway.   These measurements include unsteady transglottal pressure, acoustic pressure in the vocal tract and trachea regions, projected glottal area, and volume flow.  Aeroacoustic theory identifies vocal fold drag as the principal source mechanism.  We present evidence that transglottal pressure can be used as a surrogate for drag, enabling direct measurement of the source strength.  Experiments conducted using vocal fold models show how the source strength changes for qualitatively different vibration patterns.