Brass instruments rely on vibrations created by the movement of a musician's lips at the opening of the instrument. As the vibrations pass through the mouthpiece into the tube of the instrument, a unique sound is created, which can be manipulated into various volumes, notes and tones according to the movements of a musician's lips. The valves and sliding mechanisms on brass instruments are also used to manipulate sounds.
Brass instruments built with a series of valves, such as trumpets, rely on the buttons to change the way sound moves through the body of the instrument. When a valve is pressed by a musician, it lengthens the distance the sound must travel before it exits the end of the instrument, which is called the bell. As the musician presses and releases these buttons, the notes emanating from the instrument change. A similar action takes place in brass instruments that have slides, such as trombones. Slides allow a musician to lengthen and shorten the bore of the instrument, and they also have the unique added effect of creating a sliding sound between notes. The note changes created from slides are not as rapid as the modifications that valve instruments are capable of creating. The use of valves and slides, combined with the musician's varying lip movements, are the two main factors that actively control sound from brass instruments. The size of the instruments and their openings also determine the quality and pitch of the sounds they produce.