The fixed reed and fairly uniform diameter of the clarinet give the instrument an acoustical behavior approximating that of a cylindrical stopped pipe. Recorders use a tapered internal bore to overblow at the octave when the thumb/register hole is pinched open, while the clarinet, with its cylindrical bore, overblows at the twelfth. Adjusting the angle of the bore taper controls the frequencies of the overblown notes (harmonics). Changing the mouthpiece's tip opening and the length of the reed changes aspects of the harmonic timbre or voice of the clarinet because this changes the speed of reed vibrations. Generally, the goal of the clarinetist when producing a sound is to make as much of the reed vibrate as possible, making the sound fuller, warmer, and potentially louder.