In Fig. 30 will be found another type of Diaphone in which the tone is produced through the medium of a number of metal balls, covering a series of holes or openings into the bottom of a resonator or pipe, and admitting intermittent puffs of air.
Fig. 30. Diaphone Producing Foundation Tone
The action is as follows. Air under pressure enters the chamber B through the pipe foot A, and passing up the ports C, C1, C2, etc., forces the metal balls D, D1, D2, etc., upwards into the chamber E; the bottom end of the resonator or pipe. The pressure of air above the balls in the resonator E, then rises until it equals or nearly equals the pressure of air in chamber B. This is owing to the fact that the column of air in the pipe or resonator E possesses weight and inertia, and being elastic, is momentarily compressed at its lower end. This increased pressure above the balls allows them to return to their original position, under the influence of gravity. By the time they have returned to their original position, the pulse of air compression has traveled up the pipe in the form of a sound wave, and the complementary rarefaction follows.
The cycle of movement will then be repeated numerous times per second, with the result that a very pure foundation tone musical note will be produced.
The Diaphone is tuned like ordinary flue pipes and will keep in tune with them; the pressure of wind (and consequently the power of the tone) may be varied without affecting the pitch. The form of the pipe or resonator affects the quality of the tone; it may be flue-like or reedy in character, or even imitate a Pedal Violone, a Hard and Smooth Tuba, an Oboe, or a Clarinet.
In closing this chapter, the writer desires to express indebtedness for much of the material therein to the comprehensive "Dictionary of Organ Stops," by James Ingall Wedgwood, Fellow of the Society of Antiquaries, Scotland, and Fellow of the Royal Historical Society (published by the Vincent Music Co., London, England). Although the title is somewhat forbidding, it is a most interesting book and reveals an amount of original research and personal acquaintance with organs in England and the Continent that is simply marvelous. It ought to be in the library of every organist.
[1] Broadhouse, J., "Musical Acoustics," p. 27.