Near-field sound radiation from a line-driven fluid-loaded infinite flat plate with periodic and nonperiodic attached rib stiffeners.

Abstract

The near-field acoustic pressure radiated from a line-driven, fluid-loaded, rib-stiffened plate has been obtained using numerical integration techniques. The plate has been configured to have two sets of rib stiffeners, though the formulation given may be extended to include additional rib stiffener sets. The stiffeners composing a given set are identical and are spaced periodically with distance l. However, one set of stiffeners is shifted by an amount Δ from the other set. In this manner, portions of the plate may be configured with repeating sections having nonperiodic rib spacing. The stiffeners exert reactive forces upon the plate, but not angular moments. Results are presented that show the variation of near-field acoustic pressure along the surface of the plate at a given stand-off distance and at a fixed frequency. At certain frequencies, the investigation revealed that the wavelength of the stiffened plate’s near-field acoustic oscillations−due to the propagating flexural wave−was much greater than that given by the unstiffened plate’s bending wavelength. The effect may be due to the stiffened plate’s flexural response being split into two components that are close together in wave number. The wavelength of acoustic oscillation appears to result from taking the difference of the two spectral components.