The Propagation Characteristics of Sound Tubes and Acoustic Filters

Abstract

The process of measuring the propagation characteristics of uniform sound tubes and acoustic filters is complicated by the reflections which may occur at the ends of these structures. The present paper applies some theoretical results on the effect of reflections, obtained in a previous ${\mathrm{p}\mathrm{a}\mathrm{p}\mathrm{e}\mathrm{r},}^{1}$ to the measurement of the propagation characteristics of tubes and acoustic filters. In making these measurements the device to be measured is inserted in an acoustic transmission system, and the resulting changes in the magnitude and phase of the transmitted wave are measured. The actual observations are made in electrical circuits connected with the terminals of the acoustic system by loud speakers. The impedance of the acoustic system at the point of insertion is made an acoustic resistance. For measurements on straight tubes, the acoustic resistances used are of such a value as to prevent any appreciable reflections from the ends of the tubes, and as a result, the propagation characteristics of an infinite tube are obtained. The results of the measurements on straight tubes indicate that the Helmholtz-Kirchoff law is valid, while the results of measurements on acoustic filters are in good agreement with the theoretical results obtained previously.