The important acoustical characteristic common to members of the musical brass instrument family is not the material of construction, but the way in which the note is sounded by vibrating the lips against the rim of a mouthpiece. The lips act as a valve, open and closing periodically to modulate the flow of air into the instrument. The resulting pressure changes in the mouthpiece set up standing waves in the air column contained by the walls of the instrument. A small fraction of the sound energy stored in the standing waves is radiated at the bell of the instrument, and this is the sound heard by a listener. The frequency and pitch of the sound depend both on the resonant frequencies of the air column and on the natural resonant frequency of the lips, which can be controlled by the muscles of the player's mouth.