Air Column Instruments

The woodwind instruments use only the first few resonances of air columns and depend upon opening holes in the sides of the air columns to ascend in pitch. The brass instruments employ a large number of resonances (harmonics) of their air columns and make use of valves or slides to lengthen the air columns for a downward progression of pitches.

Air Column Excitation

While the frequencies produced by an air column instrument depend upon the type of column, the timbre of the sound is significantly influenced by the method of excitation of the air column. The flute and the recorder, as well as diapason organ pipes, are excited by blowing air over an edge. This edgetone can be altered by changing airstream velocity or the distance between slit and edge, so that an upper register is possible.

Other woodwinds may use a single reed (clarinet, saxophone) or a double reed (oboe, bassoon) to excite the air column. Such instruments achieve their upper register with the use of a register key which destroys the lowest mode of vibration of the air column and forces the column to vibrate at its next highest harmonic. For the clarinet this upper register is the third harmonic, but for the other woodwinds, it is an octave up because they are conical air columns.

Brass instrument air columns are excited by the action of the player's lips on the mouthpiece.

Air Column Registers

The playing range of woodwind instruments is extended by the existence of upper registers which allow them to excite upper harmonics of the air columns. In the flute the process is called overblowing. The upper register is an octave up in such instruments since they act as open-ended cylinders.

Other woodwinds achieve their upper register with the use of a register key which destroys the lowest mode of vibration of the air column and forces the column to vibrate at its next highest harmonic. For the clarinet this upper register is the third harmonic, but for the other woodwinds, it is an octave up because they are conical air columns.

The Brass Mouthpiece and the Players Lips

The brass player's lips are the source of excitation for the air column of a brass instrument. As described by Scott Whitener in his "A Complete Guide to Brass", "The sound of a brass instrument is created by the vibration of the lips initiating and maintaining a longitudinal standing wave in air enclosed within the instrument's tubing. ... The player's embouchure may be seen as a flow-control valve acting on the steady airflow coming from the lungs. The closed lips are blown apart, setting them into vibration. Puffs of air are thereby emitted into the mouthpiece, setting in motion a sound wave that eventually reaches the instrument's expanding bell."

As Whitener points out, the sound waves reflect from the open end at the bell and travel back and forth between the mouthpiece and players lips and the bell, producing a longitudinal vibration of the air column at the frequency of the player's lips. This efficient standing wave can be produced only at or near the resonant frequencies of the instrument, giving the characteristic sequence of harmonics of the instrument.