These student aid tools show at a glance when the mouthpiece is too high or too low on the lips, whether the lower jaw recedes, whether the tongue protrudes during the attack, and many other embouchure faults.
Visualizers are also a great way to practice buzzing with out making a lot of noise. They are just as much a practice tool as a teaching aid.
Comes in three styles; for Trumpet, Trombone and as a Double that accommodates a trumpet on one end and a trombone on the other