An experimental system for the measurement of porosity, the volume fraction of air contained in a material, is described. Porosity is important as one of several parameters required by acoustical theory to characterize a porous material. As with the technique described by Beranek [J. Acoust. Soc. Am.13, 248–260 (1942)], the isothermal pressure change in a closed volume containing a sample material is measured for a known change in the volume. The volume of air contained in the sample, and hence the porosity, is inferred from these two quantities. The new system, though, avoids the use of liquids, either directly in the technique or for temperature stabilization. Instead, a piston of accurately known diameter is used to produce the change in volume, and the change in pressure is measured with an electronic pressure transducer. Model calculations and measurements on real materials confirm that porosity can be measured rapidly and conveniently with this apparatus, with an accuracy of better than 1% over a broad range of sample volumes and porosities.