Patric, with t and k you have no air flowing and therefore no sound. ooh and ah are the just vowels to visualize what the tongue is doing. You better not be singing any note into your horn. Vocal chords that "sing along" or grunt while playing can make your sound very dirty! Tooh (Tu) is the most open and is used by Arban and other masters as the preferred articulation to start with. The variations later for a more legato do not change the vowel setting, only the consonant at the beginning. Other consonants are d, g, l, and r. If you check out doodle tonguing, you will see that the OOH is also very prevalent in the jazz world too. If you check your anatomy, it is really tough to change the size of your throat. You can change the tension on the outside all the way down to the shoulder muscles and that is what most people perceive as "constricted" or open. It has nothing to do with an "open" or "closed" throat. If you do what most people do when seemingly "opening" the throat, you are approaching a "yawn". Try it -yawn and try and then pronounce an AH - you will see how the tongue moves up and limits the airflow. Tooh on the other hand keeps the larger oral cavity intact. I do not know where you read about this AAH stuff. I do not recommend working with that method in any case! It is simply wrong.