The Vocal Tract - Vocal Resonance.

Johan Sundberg - “Vocal Tract resonance in singing“. 1987. 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.