A simplified theoretical model based upon phonon diffusion is proposed in this paper for the evaluation of the intensity level of ultrasonic waves in inhomogeneous media. This model shows large similarities with well‐known energy transfer theories and leads to particularly simple analytical results applicable to the case where the incoherent wave level predominates over the coherent one. A characteristic length beyond which acoustic image reconstruction is no longer feasible is defined here and a very simple analytical formula for this important parameter is derived. This model is then applied to the acoustic wave propagation in concrete. A reasonable fit is found between theory and experiments in this case.