Ultrasonics in an aluminum foam

Open celled foamed aluminum typically consists of an irregular frame of solid struts with typical lengths of the order of millimeters and thicknesses of a fraction of a millimeter. Total aluminum volume fractions are of the order of 10%. Quoted values of static modulus and mass density allow one to infer long wavelength (much greater than strut length) ultrasonic wave speeds. These predictions are compared with the velocities observed in 100 kHz transient signals. At higher frequencies, where wavelengths are comparable with strut lengths, ultrasound can be transported only diffusively. Wave speeds are unmeasurable and of little meaning anyway. We therefore also report on ultrasonic measurements appropriate for the high-frequency regime. These include diffusivity, absorption and modal density. Recent literature has reported measurements of diffusivities and absorption in other materials, particularly in metallic polycrystals. Measurements of modal densities are new.