Ultrasonic methods and their implementation for evaluating porous materials

In this article application of direct and indirect ultrasonic methods for evaluating and measuring porous materials are reviewed. Ultrasonic waves, due to their physical properties and wide frequency range can successfully be applied when evaluating the porosity of materials. Ultrasonic methods have many advantages when comparing them with other, non-acoustic measurement methods, which are also briefly reviewed in this article. We examine application of the proposed acoustic echolocation method when evaluating porous materials directly and indirectly. The possibilities to apply Lamb waves for evaluation of porous structures are also examined. The application of ultrasonic echolocation measurement method to evaluate porous structures indirectly is presented in depth, along with the process description and various possible implementations. The basic principle along with advantages and shortcomings of such methods are explained. Physical-mechanical properties of porous materials are also described, along with mathematical equations, which are necessary for their theoretical analysis. The ability to determine porosity of various materials is necessary to insure the quality of the final product. We also present a working real-world system, which implements an indirect ultrasonic porosity evaluation method. For indirect porosity determination, we use a very accurate ultrasonic echolocation-based distance meter. Block diagram for such unit is presented. The most important component in the acoustic porosity evaluation system is the electro-acoustic transducer. We describe the most suitable transducers for use in this case, along with acoustic antennas constructed using such transducers. Antennas, designed for measurements in air, consist of transducers vibrating in a flexural mode, which give the best possibility to match acoustic impedances between air and the transducer. Specific type of transducers for acoustic antennas is described, along with their schematic diagrams. The necessary expressions for calculating radiations patterns are also supplied. Schematic diagrams of actual antennas, along with their directivity patterns are presented. A method for eliminating peripheral radiation of these antennas is also described.