  Acoustic materials displaying coupling between pressure and momentum are
known as Willis materials. The simplest Willis materials are comprised of
sub-wavelength scatterers that couple monopoles to dipoles and {\it vice
versa}, with the interaction defined by a polarizability tensor. We propose a
method for retrieving the polarizability tensor for sub-wavelength Willis
acoustic scatterers using a finite set of scattering amplitudes. We relate the
polarizability tensor to standard T-matrix and S-matrix scattering formalisms.
This leads to an explicit method for retrieving the components of the
polarizability tensor in terms of a small set of scattered pressure data in the
near- or far-field. Numerical examples demonstrate the retrieval method for one
and two dimensional configurations.
