The mouthpiece turns lips into a controllable valve
A brass player does not simply blow air through an open tube. The player brings the lips together loosely at the mouthpiece and pushes air through the narrow opening. Air pressure makes the lips part; their elasticity and the changing pressure help pull them closed again. This rapid opening and closing creates a repeating pulse of air—the buzz.
The player controls that pulse by adjusting lip tension, opening, and airflow. These adjustments affect which pitches the lips can produce and how steadily they sound. The mouthpiece gives the lips a place to vibrate against and passes their pulses into the instrument.
The tube helps select and strengthen pitches
Inside the instrument, the pulsing air interacts with the column of air in the tubing. At certain frequencies, that column resonates: pressure waves reinforce one another, making some pitches easier to sustain and louder than others. The tubing therefore helps shape and project the sound, but it does not create a musical tone from an unmodulated stream of air on its own.
Players change the available resonances by changing the effective length of the air column. Trumpet valves redirect air through extra tubing; a trombone slide moves to add or remove tubing. The player’s lips then settle into a buzz that fits one of the instrument’s resonant pitches.
That is why a trombone can produce a breathy rush when the player blows without buzzing. The air is moving, but it is not supplying the regular pulses the tube can organize into a clear note.
