Non-negative intensity and back-calculated non-negative intensity for analysis of directional structure-borne sound

Non-negative intensity (NNI) is an approach to identify the surface areas of a structure that contribute to sound power. NNI is evaluated in terms of the acoustic impedance matrix obtained directly at the structural surface and as such can only identify surface contributions to sound power at a far-field receiver surface that fully circumscribes the structure. In contrast, back-calculated NNI is evaluated in terms of the acoustic impedance matrix obtained at a far-field receiver surface, and hence can identify surface contributions to sound power at a far-field receiver surface that does not fully circumscribe the structure. In this work, NNI and acoustic intensity obtained numerically using the boundary element method and experimentally from near-field acoustic holography measurements are compared for different modes. Back-calculated NNI evaluated for full and partial receiver surfaces is also compared with acoustic intensity for the different modes. Results for back-calculated NNI show that different regions on the plate surface contribute sound to different receiver locations.