Everyone has given a correct answer. So I offer this to explain why.
In 1962 Robert Weast did some studies with a machine that blew pressurized air through rubber lips that were under tension.
The amounts of pressure do NOT compare with what we encounter. But thegeneral 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).
Although both open and closed produce notes. Open needs more air than closed. Closed with the same airspeed as open gives a higher note.
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.
He had NO way to add lip compression.
He could make the hole bigger and smaller by tension alone.
The looser lip setting required more air pressure to play higher notes.
Don't confuse tension with compression.
Lips CAN be compressed but relaxed and they CAN be tense and open.
There are a few real world things missing here.
Volume, lip abuse (endurance) and sound quality.
We understand volume more air means stronger vibrations and louder notes. This study didn't say if the volume was more or less using different amounts of lip tension or air pressure.
Lip abuse (endurance) when we rely on the lips endurance suffers. More
air speed helps to cushion the lips and prevent them from giving out due
to mouthpiece pressure.
Sound quality. The Static exercises from the Stevens book are lip squeaks. Very little air lots of compression. This is the extreme of using lip and not air speed. Nobody confuses these with being musical. So there is a point where the air has to take over for the lips. I can't tell you where that is in a letter. You can however; hear where that is.
Some people play the trumpet in a way that makes it sound like 3 or
4 different instruments. Low G to low C has one sound, the next octave
a different sound.....
Others maintain the same sound through the entire range of the horn. I have found that the main offender is air speed.
People who maintain one or 2 airspeeds change tone quality. Those who have many levels of air speed keep one sound from top to bottom.
Finally everyone thinks they breathe and support the air correctly.
However; most players do not. I did not before I studied with Jake. If
you really did you would be playing like Maynard or Bud.
Clint 'Pops' McLaughlin