1. Embouchure: The musician's lips, shaped into a specific embouchure, create an opening when they vibrate against the mouthpiece of the instrument. This controlled vibration disrupts the airflow and creates a turbulent airstream.
2. Airflow: The turbulent airstream enters the tube of the instrument. The shape of the mouthpiece, the diameter of the tube, and the length of the tube all influence the characteristics of the sound produced.
3. Standing Waves: As the turbulent air travels through the tube, it interacts with the boundaries of the tube and forms standing waves. These are waves that appear to be stationary within the tube. The frequency of these waves determines the pitch of the sound.
4. Resonance and Harmonics: The tube's length and shape cause certain frequencies to resonate, while others are dampened. This results in the production of specific notes or pitches. Each instrument has a fundamental pitch, and depending on the embouchure, fingering techniques, and other factors, musicians can produce different notes (harmonics) that are multiples of the fundamental frequency.
5. Radiation: The vibrating air inside the tube generates pressure changes that cause the instrument's bell to vibrate. These vibrations are radiated into the surrounding air as sound waves, resulting in the sound we hear.
So, the sound in a tube of a brass instrument is produced by the combination of the musician's embouchure, the airflow, the formation of standing waves within the tube, and the radiation of sound waves through the instrument's bell. These factors work together to create the distinctive and rich sound of brass instruments.