An apparatus, system, program product, and method are disclosed for determining the microstructure and properties of materials using acoustic signal processing. An apparatus includes a one or more sensors for sensing information describing a multiphase material using sound waves. The apparatus includes a processor operably coupled to the one or more sensors and a memory that stores code executable by the processor. The code is executable by the processor to receive sound-wave input from the one or more sensors, perform one or more quantitative analyses on the received sound-wave input in the frequency domain, and determine a microstructure of the multiphase material based on results from the one or more quantitative analyses.