The work is inspired by thermo-and photoacoustic imaging, where recentefforts are devoted to take into account attenuation and varying wave speedparameters. In this paper we derive and analyze causal equations describingpropagation of attenuated pressure waves. We also review standard models, likefrequency power laws, and the thermo-viscous equation and show that they lackcausality in the parameter range relevant for biological photoacoustic imaging.To discuss causality in mathematical rigor we use the results and concepts oflinear system theory. We present some numerical experiments, which show thephysically unmeaningful behavior of standard attenuation models, and therealistic behavior of the novel models.
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