In 1962 Robert Weast did some studies with a machine that blew pressurized air through rubber lips that were under tension.

The amounts of perssure do NOT compare with what we encounter. But the general findings do.

Using 5 oz of air pressure (oz/si) these notes were produced:

Ab ( 1 oz of membrane tension),
A   ( 2 oz of membrane tension),
Bb ( 5 oz of membrane tension),
B   ( 6 oz of membrane tension),
C   ( 7 oz of membrane tension),
D   ( 8 oz of membrane tension).

The D could be produced with:

14 oz of air @ 1 oz of tension,
13 oz of air @ 2 oz of tension,
12 oz of air @ 5 oz of tension,
 9  oz of air @ 6 oz of tension,
 7  oz of air @ 7 oz of tension,
 5  oz of air @ 8 oz of tension.

The looser lip setting required more air pressure to play higher notes. Faster air means less lip tension is needed and more endurance.

Don't confuse tension with compression. Lips CAN be compressed but relaxed and they CAN be tense and open.

There are 5 main ways that this lip compression is obtained.

1. The entire chop setting is drawn toward the center. Corners pulled in and top and bottom lip pulled together. Like the drawstring example in the Farkas book or the making a fist in Jacoby's book or the diagram in Callet's book. Three different embouchures that all use the same method of lip compression.

2. Using the muscles of the chin to push the lower lip into the top lip. This creates a knot of muscle at the chin and it moves the center portion of the lower lip.
 
3. Using the muscles of a frown to compress the lips together. The Roy Roman bulldog face. A frown will pull the top lip down slightly as it pushes the center part of the bottom lip upward.
 
4. Using the jaw to assist register changes. This is the way Roy Stevens taught. He started with a very open jaw (tooth) position. That way he could bring the lips in toward each other in more compression by moving the jaw upward. (This is fine if you make sure to keep the teeth apart at all times.)
 
5. Is done by use of a pucker. The compression is partially created by the lips in their pucker and partly by the mouthpiece holding them in place.

Overall a very interesting thread.

Information about my trumpet & embouchure books.
http://www.BbTrumpet.com
 
Best wishes
Clint 'Pops' McLaughlin