Sidelobe suppression for phased array aeroacoustic measurements

Abstract

Phased array test techniques are becoming increasingly popular in aeroacoustics. This is due, in part, to the use of optimized array designs (e.g., spiral shapes) which can operate over usefully wide frequency ranges with acceptable resolution and sidelobe rejection characteristics with practical numbers of microphones. Sometimes constraints such as limited space for microphone installation prevent the use of optimum array designs, raising the prospect of high sidelobe levels. Several modifications to classical beamforming which may reduce sidelobe levels for imperfect arrays (and possibly improve the performance of optimized arrays) are available. The effectiveness of these techniques is explored by applying three of them to four array designs using simulated wind tunnel data. The three bearnforrning techniques are robust adaptive beamforming, cross spectral matrix element weighting, and the CLEAN algorithm. The array designs are a cross, an array of four crosses, a filled square, and a spiral array. NOMENCLATURE N = Number of microphones y n = Location of microphone n, n=l...N M = Number of point sources x m -location of source m, m = 1...M w(r) = Narrowband array data vector S m (t) = Narrowband source strength C(x) = Array steering vector for point x P(t) -Scattering matrix associated with turbulence in boundary layer W(t] = Gaussian noise vector ( ) = Time average, also assumed to give expected value A = Cross spectral matrix K = Normalization factor for C(x) Pnn> = yn~y* G = Decay constant for boundary/shear layer effect b b f = Beamform map function for classical beamforming b csmw = Beamform map function for beamforming with cross spectral matrix weighting b cl = Beamform map function for beamforming with the CLEAN algorithm b 0 = The highest level in a beamform map b psl = The peak sidelobe level in a beamform map R(n,n'} = Vector spacing redundancy index used in cross spectral matrix weighting v nn , = Weight for CSM element (n,n'} B = Result of weighting the elements of A w = Microphone weighting vector in minimum variance or adaptive beamforming, used like C in classical beamforming a = Loop gain parameter in CLEAN algorithm INTRODUCTION

Instability waves in a subsonic round jet detected using a near-field phased microphone array

“…To minimize contamination of the signal by acoustic waves and pressure fluctuations associated with smaller-scale turbulence, we introduce a beam-forming (or matched-field processing) technique. The approach is in many ways analogous to aerodynamic noise source identification techniques that have been successfully used in many applications (Gramann & Mocio 1995;Mosher 1996;Piet & Elias 1997;Dougherty & Stoker 1998;Suzuki & Butler 2002;Underbrink 2002;Brooks & Humphreys 2003;Venkatesh, Polak & Narayanan 2003). The idea in these previous studies is to detect the position and amplitude of a noise source that best matches a reference solution (typically a monopole source) in a least-squares sense (cf.…”

Effect of directional array size on the measurement of airframe noise components

“…For windtunnels, sophisticated array acquisition and processing systems were built for Boeing and NASA for closed 16 and open 17 tunnel facilities. Efforts have been made to optimize array design and processing 18,19,20 , particularly to suppress array sidelobe interference in order to increase signal-to-noise and to reduce ambiguity in array results. Processing presently uses classical beamforming approaches in the frequency domain using cross-spectral matrices and robust steering algorithm codes 16,17,21 .…”

A Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) Determined from Phased Microphone Arrays

“…(6), (7), (8), or (9), over a range (grid) of steering locations about the source region. For particular frequencies, contours of the response levels are plotted over planes where sources are know to lie, or over volume regions in some cases.…”

“…Several efforts include those to better account for array resolution, ray path coherence loss, and source distribution coherence 6 and for test rig reflections 7 . In a simulation study 8 of methods for improving array output, particularly for suppressing sidelobe contamination, three beamforming techniques were examined. These were a cross spectral matrix (CSM) element weighting approach, a robust adaptive beamforming, and the CLEAN algorithm.…”