vowel modification theory 

 When they constructively interfere, there's a buildup of pressure, hence we need to modify our vowels. This opens the space, which changes the frequency of where that standing wave occurs. We then need to modify back to avoid another standing wave from occuring. So we should be constantly and smoothly modifying back and forth to avoid this buildup of pressure. 

 The vowel OO periodically opens up to OH then back to OO.  The vowel EE periodically opens up to EH then back to EE.  The vowel AH periodically opens up to AW, then UH, then OO, the back to UH, AW, then AH.  AH has more vowel modifications over a longer series of notes due to longer standing wavelengths, since it's the most open vowel. For me the OO/OH and EE/EH has a period of about an octave, whereas the AH mods have a period of 3 octaves. From OO at C2, UH at E2, AW at G2, AH at C3, AW at C4, UH at G4, and back to OO at C5.  reddit /vowel_modifications_arent_just_for_high_notes_the/