Changing pitch     While the reed initially produces the sound, the mouth piece can't be the only thing involved in note production. After all, by simply changing which notes are pressed down on the clarinet, the pitch can change. Furthermore, by blowing only into the mouthpiece, it is obvious the pitch produced is way higher than anything that can be produced on the clarinet. What causes these changes in pitch?    Unlike many instruments, the clarinet is a closed pipe meaning the sound can't exit from one end (the end with the mouthpiece). Furthermore, the end opposite the mouthpiece is open, which implies that the pressure just outside the end of the clarinet must be pretty near the pressure just inside the clarinet. Since the region of highest pressure is in the mouthpiece and the region of no pressure is at the end, the entire body of the clarinet must be 1/4th the frequency (since the highest point of a sin wave to the point where the wave is 0 represents a fourth its total period). Therefor, by changing the length of the clarinet, the pitch can be raised or lowered. In fact, clarinetists tune by separating or pressing the pieces of the clarinet together. http://www.physicsclassroom.com/class/sound/Lesson-5/Closed-End-Air-Columns