Useful intensity: A technique to identify radiating regions on arbitrarily shaped surfaces

This work presents a new technique for the computation of the numerical equivalent to the supersonic acoustic intensity, for arbitrarily shaped sound sources. The technique provides therefore the identification of the regions of a noise source that effectively contribute to the sound power radiated into the far field by filtering non-propagating sound waves. The proposed technique is entirely formulated on the vibrating surface. The radiated acoustic power is obtained through a numerical operator that relates it with the superficial normal velocity distribution. The power operator is obtained by using the boundary element method. Such operator, possesses the property of being Hermitian. The advantage of this characteristic is that it has real eigenvalues and their eigenvectors form an orthonormal set for the velocity distribution. It is applied to the power operator the decomposition in eigenvalues and eigenvectors, becoming possible to compute the numerical equivalent to the supersonic intensity, called here as useful intensity, after applying a cutoff criterion to remove the non-propagating components. Some numerical tests confirming the effectiveness of the convergence criterion are presented. Examples of the application of the useful intensity technique in vibrating surfaces such as a plate, a cylinder with flat caps and an automotive muffler are presented and the numerical results are discussed.