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In this paper an impedance tube technique is described and evaluated for the measurement of the characteristic impedance and propagation constant of porous acoustical materials. In this method, data are determined under dynamic conditions through the investigation of the standing wave patterns existing in a tube driven at one end by a sound source and terminated at the other end by (1) a sample backed by a rigid wall; (2) a sample backed by an air space, a quarter-wave long, itself terminated in a rigid wall. The validity of the method is assessed by comparing values of characteristic impedance and propagation constant for Owens Corning Fiberglas, type 705, density 6 lb/ft3, obtained in this way to values obtained by two other well-known techniques: Beranek's method and Scott's method. Data were obtained at 800, 1200, and 1600 Hz. The analysis performed on the data obtained by the impedance tube technique and Beranek's method are in agreement at all frequencies. Comparable analyses on the data obtained by Scott's method show agreement with data obtained by the other two techniques only for values of the propagation constant at 1200 Hz and 1600 Hz. The discrepancies observed are attributed to experimental difficulties inherent in Scott's method.
Simone L. Yaniv (Thu,) studied this question.