  In this paper, the effect on the ultrasonic attenuation of the grain size
heterogeneity in polycrystals is analyzed. First, new analytical developments
allowing the extension of the unified theory of Stanke and Kino to general
grain size distributions are presented. It is then shown that one can
additively decompose the attenuation coefficient provided that groups of grains
are defined. Second, the study is specialized to a bimodal distribution of the
grain size for which microstructures are numerically modeled by means of the
software Neper. The additive partition of the attenuation coefficient into
contributions coming from large and small grains motivates the derivation of an
optimization procedure for characterizing the grain size distribution. The
aforementioned approach, which is based on a least squares minimization, is at
last presented and illustrated on both analytical and numerical attenuation
data. It is thus shown that the method provides satisfying approximations of
volume fractions of large grains and modal equivalent diameters from the
frequency-dependent attenuation coefficient.
