Porous materials are critical engineering components in many process industries. Although porous materials have been successfully utilized in many areas, characterization of porous structures is still a significant problem limiting the applications of porous materials, especially when the application involves the change of porous structure. Ultrasonic techniques have been reported for successful applications on material characterization, including porous materials. This research utilized an acoustic technique for permeability measurement by measuring the critical wave number of Biot’s slow longitudinal wave. The slow longitudinal wave can serve as an important tool for investigating the ability of fluids to penetrate into porous materials. Since the slow longitudinal wave is associated with out-of-phase movement of the pore fluid relative to the matrix structure, it is very sensitive to the permeability of the porous formation. In Biot’s theory the slow wave is highly attenuated below a critical frequency. The critical wave number can be directly related to the permeability of porous materials. The measurement of the critical wave number provides a unique method to obtain the permeability measurement, which can be apply to structure monitoring, quality control, etc.