Plate acceleration and sound transmission due to random acoustic andboundary-layer excitation

Abstract

The 'APEPS statistical energy analysis model is used in the frequency range of 315-5000 Hz to predict the acceleration response and sound transmission of plates that are exposed to random acoustic or turbulent boundary-layer excitation. Good agreement was found with measured noise reduction data for aluminum, acrylic, and graphite-epoxy plates with length and width larger than the acoustic wavelength. The VAPEPS predicted acceleration response of a 12 X 8 X 0.040-in. plate, installed in a 1-in. thick rigid plate inside a wind tunnel, agreed well with measured data for a stationary (random acoustic) as well as for a convected (turbulent boundary layer) pressure field. It was found that lowest order plate modal frequencies increase most with free flow velocity U. The overall acceleration level as well as the transmitted sound pressure level due to turbulent boundary-layer excitation were close to a Us.' dependence. Excitation by a laminar boundary layer resulted in a U3.6 dependence. Measured noise reduction data for random acoustic excitation of the plate in the wind tunnel fell between the VAPEPS predicted curves for resonant and nonresonant sound transmission showing the limitations of statistical energy analysis to accurately predict sound transmission at low modal densities. The noise reduction of the plate, when backed by a shallow cavity and excited by a turbulent boundary layer, was predicted using a simplified theory based on the assumption of adiabatic compression of the fluid in the cavity. VAPEPS predicted plate acceleration response was used as input in the noise reduction prediction, and reasonable agreement was found with the measured noise reduction in the frequency range of 315-1000 Hz.