Quantification of frequency-dependent absorption phenomena

Of all fluid and solid properties, quantities that describe losses are among the most challenging to quantify. In part, this is due to superimposed dissipative mechanisms, such as diffraction effects from spatially limited sources. Inherent to all these phenomena, however, is a specific frequency dependence. The nature of the frequency dependence varies, resulting from the respective absorption mechanism. Pure fluids, for example, exhibit absorption of acoustic waves with quadratic frequency dependence^[1]. In solids, there are several absorption models that can be applied, each having different characteristics with respect to frequency. Other dissipative effects, such as diffraction, also show frequency dependence. In an approach using the raw moments of the signals from acoustic transmission measurements, a method to quantify absorption and dissipation phenomena with arbitrary frequency dependence is presented. The described method is applied to different absorption measurement problems. To verify that accurate results can be achieved under ideal conditions, the method is applied to signals generated using acoustic field simulation with different absorption models. To show its numerical stability, it is used qualitatively to evaluate the absorption of a fluid at different thermodynamic states.