McKinney, James C. (1994). The Diagnosis and Correction of Vocal Faults. Nashville, Tennessee: Genovex Music Group. ISBN978-1-56593-940-0. LCCN97209575.
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Vennard, William (1967). Singing: the Mechanism and the Technic (4th ed.). New York: Carl Fischer. ISBN978-0-8258-0055-9. OCLC1011087.