On the Retrieval of the Directional Scattering Matrix from Directional Noise

The crosscorrelation of ambient acoustic noise observed at two receivers yields the impulse response between these receivers, assuming that the noise field is diffuse. In practical situations the noise field exhibits directionality, which imprints the angle-dependent correlation function. For the situation of a directional scatterer in a directional noise field, the correlation function contains the product of the directional scattering matrix and the directional noise. This seemingly underdetermined problem can be resolved by exploiting a relation between the causal and acausal parts of the correlation function. For a given pair of receivers, the causal and acausal parts of the correlation function contain the same element of the scattering matrix (by reciprocity) but different elements of the directional noise field. This property can be used to estimate the directionality of the noise (apart from an undetermined scaling factor) and, subsequently, of the scattering matrix.