Reproduction of random pressure fields based on planar nearfield acoustic holography

This article discusses an open-loop approach based on planar nearfield acoustic holography (P-NAH) for the reproduction of random pressure fields, mainly intended for the measurement of vibroacoustic properties of plane panels. The main application is the simulation of turbulent boundary layer excitation in a laboratory environment, as an alternative to in-flight or wind tunnel experiments. The problem under study is the synthesis of random pressure distributions on a plane reproduction surface using acoustic monopoles distributed on a plane source surface facing the reproduction surface. The problem of reproducing a pressure distribution on a plane surface is addressed using the theoretical framework of P-NAH, which is extended to random pressure fields with corresponding imposed cross-spectral density functions. Results of numerical simulations are presented for the reproduction of a diffuse acoustic field, and a subsonic and supersonic turbulent boundary layer. The influence on the reproduction accuracy of the respective sizes of the two planes, their separation and the reproduction source separation are studied. The reproduction approach shows to be effective for the reproduction of diffuse acoustic field and turbulent boundary layer, but with different requirements in terms of plane separation and reproduction sources separation. In the specific case of subsonic turbulent boundary layer and associated sub-wavelength correlation scales reproduction, possible improvements of the method are suggested.