I made this recording straight onto my mobile phone while I was checking the tuning on my flugelhorn.
It's about the speed of the air, and that is manipulated by aperture control and tongue position. I play in the upper register with a more open aperture and with my tongue anchored behind my teeth. I am raising the front of the tongue rather than the back. Thing "shhh" rather then "eeeee".
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https://www.youtube.com/watch?v=cbXa3fRrz-U