Fairbanks, G. Voice and Articulation Drillbook. New York: Harper & Row, pp. 127, 1960.
Fletcher, S.G. Diagnosing Speech Disorders from Cleft Palate. New York: Grune & Stratton, 1978.
Hinchcliffe, R. and D. Harrison. Scientific Foundations of Otolaryngology. London: William Heinemann Medical Book, Ltd., pp. 523-529, 1976.
Kummer, A.W. Chapter 14, “Nasometry” in: Cleft Palate and Craniofacial Anomalies: Effects on Speech and Resonance, Second Edition. New York: Thomson Delmar Learning, 2008.