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THE NATURE OF SOUND . 299
WIND INSTRUMENTS
The wind instruments include the brass wind and the wood wind. In the former class we find the cornet, trombone, bass horn, French horn, saxophone, and others. In the wood wind class we find the flute, clarinet, oboe, and others. In both classes we find instruments with cupped mouthpieces and instruments with reeds. In all cases there is an air column which acts as a resonator.
RESONANCE OF AIR COLUMNS AND VOLUMES
Usually, in order to get a loud clear tone, it is necessary that the action of the original source of vibrations be supplemented by a resonator. Although a resonator does not in itself add or create any energy, it usually does act in such a way as to allow the vibrator to generate a greater amount of energy by giving it an impedance to work against. Thus the air in the horn of a loud speaker acts as a "load" on the diaphragm and enables us to put a greater amount of energy into the system than if the diaphragm vibrated in free air. Resonators may consist of sounding boards like those in pianos or the bodies of violins (in which cases they respond almost uniformly to any frequency which may be impressed on them) or they may consist of columns or volumes of air.
Air columns, if narrow, respond to vibrations of different frequency depending on the length of the column.
If the column is open at one end and closed at the other it will respond to tones of such frequency that the length of the column is 14 of the wave length. But it also responds to tones such that the length of the column is 3/4, 5/4, 7/4, etc., of other wave lengths. If the column is open at both ends, as in the case of most musical instruments, the response will be to tones for which the column length is 1/2 wave length, 2/2, 3/2, 4/2, etc. Thus, any air column may respond to many different frequencies and will do so simultaneously if the various frequencies are present in the original vibrations.
Thus we again find the results containing a "fundamental" plus a series of "overtones."
The simple motion just given is modified by "end corrections," by sloping sides and flared ends, and by other conditions.
Air masses, such as occur in the human throat, mouth, and nasal cavities also may act as resonators. The action is here