While there’s really two parts of developing the high register, they both have one thing in common: extending the high range is 80% mental and only 20% physical.
Part one is that there’s really not a fast way to learn to make everything work so efficiently that you can play in the upper register with facility and versatility of tone and dynamics. It takes years and years.
So I suggest you go at it from the strategy of play in your regular range using good, strong fundamentals, and slowly work to extend your range with those good fundamentals intact.
(By good fundamentals, I am referring to a solid embouchure with good “meat” – whether pucker, corners in or corners down – and a strong-cored air column that has it’s bottom at the base of the gut. Pushing from the bottom, and focusing a clean, coherent air stream through massed lip tissue is the heart of strong trombone-playing fundamentals)
The second factor is the important concept of focusing on modulating the airstream to the proper size, intensity and shape for the range you are playing in, and pushing from the bottom to create an air column that has the large, stable muscles of the torso doing the hard work.
Push From the Bottom
“Air support” is important for two reasons. The first of which is because air pressure from the bottom of the gut, combined with the small aperture described by Richard Corliss, = an air stream with the size and speed needed to buzz the lips for high register playing.
Also, solid air support from the bottom of the air column means you are using the big muscles to do the heavy lifting, and it means you are taking in and using more air, which increases the oxygen content in the bloodstream and means the blood getting to the facial muscles will be charged with oxygen, thus increasing the amount of time the face can support the higher mouthpiece pressure required by intense, high playing.
So strong air support from the bottom = increased endurance.
Air Focus at the Top
Lest those who have a different approach take offense, I should say there are quite many ways to go about funneling or focusing the airstream for the upper register.
First I’ll tell you how I do it.
The closest analogy I can make to describe what I do is to just whistle and observe the size and shape of your mouth and the position of the tongue as you go back and forth between low and high whistling. That is the general movement of my mouth.
The big difference between the whistling and the actual playing is that the *front* of my tongue is not quite as elevated when I play.
Now the way I usually start younger students on focusing their air is the way Jake taught me, which is to push out with your air, and as you slur upwards, say past the , you begin to change the syllable a bit from perhaps an “AUHHH” to an “AHHH” or even an “EHHH”.
Some have trouble with these syllables because they want their mouth to be in an “O” shape. I have decided that all this means is that they tend to pull their cheeks in in addition to dropping their jaw which is fine. This is actually what I do (as in the whistling analogy) is try to envision blowing air through a straw without hitting the sides (!).
Randy Purcell told me the number one thing he does to achieve a focused air stream is put your hand about 8 inches in front of your face and blow into it, trying to make the airstream hitting the hand the size of a quarter.
I like to employ the analogy of a ball player. A professional hitter in baseball (cricket for you brits) doesn’t hit with their arms. A player that uses their arms can barely clear a ball out of the infield. Power hitters and even finesse hitters use the large muscles of the legs, hips and torso to effect the proper biomechanics to have a swing with enough power to hit a ball hard; even the high average hitters use that approach because they can take a, 85% effort swing and have absolute bat control.
The same goes for playing trombone. The big muscles pushing the airstream from the bottom are where the power and intensity of airstream come into play; combining that with modulating the size and shape of the airstream in the mouth and you get great sound (the tone chamber goes deep into the body instead of just in the chest), a powerful ability to play at the dynamic extremes (obvious for the louds, but using the big gut muscles to drive a pianissimo sound produces a big and steady soft sound), a tremendous ability to make big interval jumps (a little more effort in the gut combined with a precise airstream adjustment in the mouth beats the heck out of trying to muscle it up with the little chop muscles) and good range with control and sound.