Structural acoustic radiation prediction: Expanding the vibratory response on a functional basis

Abstract The theoretical basis of the modal method for acoustic radiation from structures is first presented. It consists of expanding structural responses on a functional basis, truncated for numerical computations. Generally the natural vibration modes of the structure in vacuo are considered, even if it is possible to use other functions. The physical signification of modes allows one to select the modes governing radiation and to truncate the series for efficient computations. In this optimized form, the modal method allows one to calculate radiation of large structures from low to high frequencies within a reasonable computation time. The understanding of the physical phenomena required for mode selection is presented. The characteristics of the fluid-structure coupling are the radiation impedances of modes. Radiation resistances introduce a loss mechanism for the structure associated with radiated power. Radiation reactances are interpreted as added masses. Several examples of radiation impedances are analysed. To be efficient, the modal method requires the knowledge of the modal basis in analytical form. It is thus well adapted to problems involving simple geometric structures, such as rectangular plates or cylindrical shells. It is, however, easy to take into account inhomogeneities such as stiffeners, masses and perforations. The last part of the paper presents the simulation of two complicated problems: the transmission loss of double panels (with absorbing material and mechanical links) separating two rooms, and the radiation, into water, from a ring-stiffened cylinder covered with a coating layer. Finally, the influence of structural defects on radiation, and comparison of theory and experiment are presented.