  In a communication scheme, there exist points at the transmitter and at the
receiver where the wave is reduced to a finite set of functions of time which
describe amplitudes and phases. For instance, the information is summarized in
electrical cables which preceed or follow antennas. In many cases, a random
propagation time is sufficient to explain changes induced by the medium. In
this paper we study models based on stable probability laws which explain power
spectra due to propagation of different kinds of waves in different media, for
instance, acoustics in quiet or turbulent atmosphere, ultrasonics in liquids or
tissues, or electromagnetic waves in free space or in cables. Physical examples
show that a sub-class of probability laws appears in accordance with the
causality property of linear filters.
