“Quiming” asks from Germany:
What’s interesting for me is to know: during the lips vibration, if in all the registers and different colors of sound the lips always have a moment of completely closed? Easily to say, if the vibration is “closed-open” or “less open – more open”?
There are many ideas out there about what the embouchure aperture does while playing, but the best way to understand how it functions is to take a look at videos that either use high speed filming or a strobe light to slow down the vibrations. There have been a number of researchers who have done this, and they all show that the aperture opens and closes many times per second while a brass performer is playing. Although some players swear that they are playing with an “open” aperture, this is almost certainly not true. The lips always spend a moment completely closed while playing.
When talking about the “size” of the aperture, what many people are referring to is the position of their lips while they are most open in the cycle of vibrations. This is influenced primarily by two factors, the pitch being played and how loud it is being played. The lower the pitch being played the longer the aperture remains open and the larger it is at its most open position. When playing a crescendo on a single pitch the speed of the vibration remains the same, so the lips open and close at the same rate, but the aperture will get larger the louder it is played.
Lloyd Leno’s film, Lip Vibration of Trombone Embouchures, shows a very clear view of embouchure apertures in slow motion. Using high speed filming, Leno shows how the lips open and close, spending less time open for the higher notes and more time open for the lower pitches. It’s also interesting for the view of seeing how the lips vibrate differently for different embouchure types.
Another resource to see how the aperture moves in slow motion are the videos (some of the articles are in German) available at this IWK Brass Research page. The videos you can watch there were made with a strobe light to simulate the slow motion filming and include both trumpet and trombone embouchures.
That said, there are many brass players and teachers who believe that you should play with an open aperture. I’ve heard from some who swear that their lips aren’t completely closing when they play. Others simply describe their aperture as being “more open” to communicate a playing sensation they feel gets a desired sound or other effect, although most people haven’t actually looked at a video of their aperture to know whether it actually is more open or just feels that way. This description is common enough that as an analogy it could have some merit. That said, I personally err on the side of understanding how our bodies actually work to play our instrument.