Non-negative intensity for a heavy fluid-loaded stiffened plate

Localisation of sound sources on vibrating structures is a critical part of the design in many engineering applications. In structures with stiffeners, so-called Bloch-Floquet waves are generated due to the interaction between the flexural waves in the host structure and the flexural/torsional waves of the stiffeners. It is known that the Bloch-Floquet waves have a significant contribution to the radiated sound. However, it is not understood which area of the vibrating stiffened structures contributes significantly to the radiation in the far-field. Non-negative intensity (NNI) is a powerful tool developed recently to locate the surface regions on structures that can contribute to the radiated sound power. Although NNI has been used for several distinct structures under different excitations, it has not been considered for analysing structures with stiffeners. In this work, NNI is evaluated for an infinite fluid-loaded stiffened plate subjected to a point force to localise the sources of sound and to shed light on the mechanism involved in the far-field radiation. An analytical model formulated in the wavenumber domain is presented to carry out fast calculations of the plate vibroacoustic responses and the NNI maps. A parametric study is then performed by comparing the vibroacoustic responses with the NNI maps to highlight the capability of NNI for sound source localisation. This is achieved by analysing the results for stiffened/unstiffened structures with excitation on/between the stiffeners, and at frequencies either in a passband or in a stopband. Moreover, the NNI maps are further explained and interpreted using identified Bloch-Floquet radiating bands.