The clarinet produces sound by blowing air across a reed, into the mouthpiece, down through the body of the instrument and by pressing keys or covering holes on the instrument to change notes. The notes change in this manner because it disrupts the airflow and the wavelength of the air moving through the instrument.
The reed that is positioned on the mouthpiece also affects how the sound is produces from the clarinet. The air moving across the reed causes vibrations, which can change depending on what type of reed is used and how tightly it is gripped by the musician's lips. The vibrations paired with the change of the airflow by the fingers and ports on the instrument gives it the ability to cover a wide range of notes.