Johan Sundberg – “Vocal Tract resonance in singing”. 1987. https://www.nats.org/_Library/Kennedy… Manuel Garcia – “Hints on Singing”,1894. pgs 12-13. Douglas Stanley – “Your Voice” “Applied science and vocal art. Singing and speaking” . 1945. pg 61 William Vennard – “Singing the mechanism and the technic” 1967. pg 82, paragraph 297.