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This paper combines the physics of sound propagation in a real, homogeneous and absorptive fluid medium with relevant concepts in linear systems theory to obtain a linear-filter representation of the signal transmission process in diagnostic applicationl of ultrasound. The model is based on the fact that energy absorption in soft mammalian tissue results from relaxation phenomena associated with the medium macrostructure and on the engineering approximation that the signal paths through the medium respond linearly over the dynamic range of illuminating waveforms, The beamwidth and bandlimiting effects of the coupling electroacoustic transducer are accommodated by means of a suitably defined aperture function. It is demonstrated that, apart from the interfaces in the propagation medium, the transducer and the signal path between the electronic signal source and any point in the medium jointly resemble a linear network characterized by an appropriately defined transmission function. The detailed form of thi...