An experimental arrangement has been developed to explore the thermoacoustic properties of stack materials. The apparatus may be viewed either as a thermoacoustic engine with two microphones mounted in the tube wall, or as a two-microphone impedance tube that accommodates thermoacoustic elements. A sample stack material is placed between two heat exchangers and specific acoustic impedance measurements are taken as a function of frequency and at low-drive amplitude (to avoid nonlinear effects). The design is such that nearly all physical elements and operational parameters can be varied fairly easily. The ‘‘tube’’ is composed of three (flanged) pipe sections of different length, which independently contain a loudspeaker, stack and heat exchangers, and a termination. The test is performed with and without a temperature gradient imposed across the stack. The tube may be pressurized—allowing for different gases held at various ambient pressures—while the relative positioning of the elements within the tube may be varied. Hence a very wide range of testing conditions may be achieved. [Work sponsored by ONR.]