Investigation of the Sound Transmission into an Advanced Grid-Stiffened Structure

Abstract

The noise transmission behavior of an advanced grid-stiffened (AGS) composite structure has been investigated by combining numerical and experimental methods. Structural-acoustic coupling was found to be light, permitting separate analysis of the structure and acoustic cavity. Finite element analysis permitted the resonant frequencies of acoustic cavity and structure to be calculated, which play an important role for noise transmission through the structure. Acoustic mode shapes permitted internal coincidence frequencies to be estimated and provided insight into modal pressure distributions, when considering payload location. Experimental structural and acoustic modal analysis permitted the resonant frequencies and damping ratios for the structure and cavity to be determined, which in turn were used to corroborate the FEA model. Finally, direct measurement of the noise transmission was performed based on noise reduction spectrum (NRS), which is calculated from spatial averages of the RMS acoustic pressures inside and outside of the shell. It was found that the NRS was dominated by acoustic resonances, which were marked by sharp dips in the NRS curve. Internal coincidence of the axial wavenumbers was also found to be a significant mechanism for noise transmission. External coincidence and ring frequencies were found to provide less of an impact on the overall NRS for the structure.