Subject: Aperture shapes:
Apertures fall in 3 types flat causing a thin shrill sound. Oval causing the full sound we all seek. And Round causing a dull too dark sound. Aperture is controlled by the mouth corners. If they pull out from normal then it is flat. If they stay put or come in slightly it is oval. And if they draw in to make a fish face it is round. When you draw the corners to make a more rounded or oval aperture then you must use a little more vertical lip compression or the lips separate.
These aperture shapes as well as the size are completely controlled by the muscles forming the embouchure and are independent of tongue level. There is a study involving a floroscope machine done in the 60's that was published in the Instrumentalist. The arch of the tongue (tongue level ) can affect the direction of the airstream and certainally the airspeed but not the shape or size of the lip aperture.
Finally aperture size can be affected in two directions height and length. It is the ability to control the length that most people ignore. There are some remarkable results to be found here by those willing to work on this. Jake said it very simply "think of making a fist with your mouth". It draws in from all sides.
A collection of some past posts & information about my book.