Supersonic dispersion in gases

Abstract

The rapid degradation of intensity suffered by compressional waves of high frequency in gases was first observed by pierce in carbon dioxide for frequencies in the neighbourhood of 2.10 5 cycles/sec. Although in the last few years a considerable number of measurements of the velocity of supersonic waves have been made, less experimental work has been done on the absorption . This present paper describes some work aimed at elucidating the mechanism of the phenomenon. Consideration was first given to the establishment of a source of vibrations of the requisite frequency. The possible apparatus reduces itself to four types: (1) edge-tones, (2) electric sparks, (3) small resonators of gas, (4) solid resonators; both of the latter types to be maintained by oscillating circuits incorporating valves. The frequency of an edge-tone depends directly on the velocity of the blast, and inversely on the distance from the blower to the edge, so that it should be possible to produce supersonic waves by making the former very large and the latter very small; in fact, Hartmann has already used such a source. The difficulty of maintaining constant blast velocity and the complications which the blast introduces in the propagation of such waves would, however, have made such a source unmanageable in the present work. Nekle-pajev has used sparks as sources in the examination of the absorption in air. But here again the frequency is difficult to measure or to maintain constant. Some success was obtained by the author with gaseous resonators consisting of short brass tubes terminated at one end by a brass stopper, and at the other end by a soap film, the distance between the two being half the wave-length at the frequency of excitation. The resonator was maintained in vibration by a valve oscillator, of which the plate was connected to a point electrode just above the soap film, while the grid was connected to the brass tube itself. Response of the resonator due to electrostatic attraction of the film was observed by the image of a glowing filament reflected from the slightly concave film on to a scale. By varying the tuning of the oscillator the response curve of the little resonator could be obtained.