Digisonix, Inc., 908 Stewart St., Madison WI 53713
An apparatus has been built that can measure the acoustical impedance (Z) of a variety of acoustical objects including membranes, plates, and porous materials. It is also possible to use the technique to measure the complex bulk modulus (B) of air within a porous material. When B is combined with Z, a complete linear description of the bulk properties of porous material is obtained. The apparatus is very simple, consisting of two volumes, one of which is driven by a loud speaker at frequencies low enough so that the two volumes can be accurately modeled as lumped elements. It is shown that Z and B primarily depend upon on the pressure transfer function between the two volumes, which can be measured very quickly and accurately with modern analyzers. Due to the simplicity of the apparatus, it is felt that this technique is superior to methods previously used [J. D. McIntosh et al., J. Acoust. Soc. Am. 88, 1929--1938 (1990)], however it is limited to lower frequencies (<500 Hz with the current apparatus). Measured impedance data of membranes, plates, ports, perforated plates, and porous material is presented as well as bulk properties of open cell foam.