Esling, J. H. “Pharyngeal Voice Qualities: Auditory Change With Pitch, and Fibreoptic Filming of Aryepiglottic Articulations”. Canadian Acoustics, vol. 24, no. 3, Sept. 1996, pp. 22-22, https://jcaa.caa-aca.ca/index.php/jcaa/article/view/996.