So, how can you be sure that the opening or aperture is correct due to the fact that it is not visible to the eye? That is a more important question than most people think.
How to be sure your lip aperture is correct for playing trumpet-
1. Can you start a note without tonguing it?
You should be able to start any note simply by moving the air through the opening in your lips. Sounds simple but many players find this difficult if not impossible. If you are unable to accomplish this, you need to evaluate the size of the opening. In most cases, if the opening is too small, the lips are not positioned to freely vibrate. On the other hand, if the aperture is too open, air passing between your lips will not allow the lips to vibrate also.
2. If I can’t see the size of the opening, how can I tell if it is too wide or too small?
Stick the tip of your tongue between your lips and into the cup of your mouthpiece. By placing the tip of your tongue into the mouthpiece, you are able to “feel” the size of the opening. Before you old school purists start yelling that you should never stick the tip of the tongue between the lips, please take note-
“I am not advocating that you tongue with the tip in the mouthpiece or even between your lips!
THIS EXERCISE IS USED ONLY TO DETERMINE THE SIZE OF THE OPENING AND NOT BE USED WHEN TONGUING NOTES!
By extending the tip of your tongue into the cup of your mouthpiece, you will have a very good idea as to how open the aperture is between your lips. I have been using this trick most recently when I finished the spring season of a show in town and took a couple days off to get away from the horn. When I restarted practicing, I noticed that I was having problems with notes closing on me above the staff. After a day of analyzing my problem, I began to thrust the tip of my tongue into the bottom of my mouthpiece and discovered that I was starting my notes with an opening much too small.
If the aperture size between your lips is too small or too large, all the strength in the world will not compensate for this error.