When we raise the arms from the side and percuss the thorax between the folds of the axillae, where the serratus magnus muscle alone intervenes between the ribs and the skin, the pulmonic sound will answer clearly.
"Surgical Anatomy"
Joseph Maclise
I will first produce the vowel sound oo and proceed with the vowel sounds to i; you will observe that the pitch rises an octave; that this is due to the changes in the form of the resonator is shown when I percuss the resonator in the position of the different vowel sounds.
"The Brain and the Voice in Speech and Song"
F. W. Mott