Sound transmission through finite rib-stiffened and orthotropic plates

Rib-stiffened plates are commonly employed for reasons of weight and static stiffness. Unfortunately, attaching stiffeners to a plain plate generally increases its radiation efficiency and reduces its airborne sound insulation. This contribution aims at gaining insight into the sound insulation performance of finite-sized rib-stiffened plates through numerical modeling, with a particular focus on finding a physical explanation for the experimental observation that the narrow-band sound transmission loss may oscillate heavily with frequency in the mid-frequency range. To this end, finite element models of a rib-stiffened plate with various levels of detail are constructed and coupled to a reverberant model of the adjoining rooms within the hybrid finite element-statistical energy analysis framework. In this way, the room models are computationally very efficient, the coupling loss factors are computed in a rigorous, straight forward way, and the model predictions are robust with respect to random wave scattering in the rooms. The plate models are calibrated by low-frequency modal testing and the room-plate room models are validated against narrow-band laboratory measurements. It is shown that the low-frequency calibration can result in more accurate predictions across the entire building acoustics frequency range, that the mid-frequency oscillations of the sound transmission loss are caused by the modal behavior of the plate, that the oscillations can also occur in finite orthotropic plates and that the oscillation dips appear at specific natural frequencies of the plate, where (semi-discrete) coincidence or (semi-discrete) semi-coincidence occurs.