Acoustical imaging and farfield radiation patterns in air and water using STSF

Abstract

The spatial transformation of sound fields (STSF) technique involves a planar scanning across the source under investigation. From each scan position, the cross spectrum is measured to every one of a set of references and, further, the cross spectrum is measured between each pair of the references. From the measured cross spectra, any descriptor of the nearfield (pressure, particle velocity, intensity, etc.) can be investigated by means of nearfield acoustic holography, while the more distant field can be determined by application of Helmholtz's integral equation. The cross spectrum formulation implemented in the STSF technique has the advantage of putting no restrictions on the coherence of the sound field. Only a certain stationarity of the source during the measurement procedure has to be assumed. This paper describes the use of the STSF measurement method in an investigation of radiation from dipoles both in air and in water. The discussion will briefly review the principle of the STSF method and then will present pressure, velocity, and active, and reactive intensity plots calculated in the plane of the dipole sources from data measured in the acoustic nearfield. Farfield radiation patterns calculated from the same nearfield measurement data will also be shown and compared with sound-pressure levels measured directly in the farfield. Some factors influencing the accuracy of STSF measurements of farfield radiated noise will be identified.