Measures of inferred subglottal air pressure, glottal airflow waveform characteristics, sound pressure level (SPL) and the acoustic spectral slope were studied for individual speakers with normal voices. Combinations of different levels of sub-glottal air pressure and varying glottal configurations could result in the same SPL. Relatively high air pressure levels were associated with a steep spectral slope, reflecting a more sinusoidal glottal waveform and a relatively abducted membranous glottis, which would result in damping of F1. Data suggested that the interarytenoid glottal opening could vary without systematically affecting SPL or voice quality. The results indicate that the principles of production-related economy of effort and physiological, acoustic and perceptual constraints may apply to voice production.