Microphone Array Measurements: Sound Source Localization by means of SODIX Stefan Funke, Henri Siller, Lars Enghardt Engine Acoustics, Institute of Propulsion Technology German Aerospace Center
Outline The German Aerospace Center (DLR) Institute of Propulsion Technology Engine acoustics Motivation Outdoor tests of aero-engines Indoor tests of aero-engines Sound source localization SODIX Methodology Validation and application Results Summary
DLR German Aerospace Center Research Institution Space Agency Project Management Agency
DLR: Sites and employees ~8.000 employees working in 33 research institutes and facilities at 16 Sites. Offices in Brussels, Paris and Washington. Sites of the Institute of Propulsion Technology Engine Acoustics - Berlin Stade Hamburg Bremen- Trauen Neustrelitz Berlin- Braunschweig Göttingen Jülich Köln Bonn Lampoldshausen Stuttgart Augsburg Oberpfaffenhofen Weilheim
Organisation of the Institute Institute of Propulsion Technology Reinhard Mönig Engine Andreas Döpelheuer Fan and Compressor Eberhard Nicke Combustor Christoph Hassa Turbine Frank Kocian Engine Acoustics Lars Enghardt Engine Measurement Systems Christian Willert Numerical Methods Edmund Kügeler Combustion simulation Francesca di Mare Combustion Test Christian Fleing
DLR.de Folie 6 Engine Acoustics - Core Competencies Turbomachinery sound sources Engine core sound sources Jet Noise Unsteady aerodynamics of turbomachinery Sound propagation in ducts Radiation of sound Sound source localisation Sound absorption / Liner Active flow/noise control Acoustic predesign Experimental facilities Methods & Tools
DLR.de Folie 7 Engine Acoustics - Core Competencies Turbomachinery sound sources Engine core sound sources Jet Noise Unsteady aerodynamics of turbomachinery Sound propagation in ducts Radiation of sound Sound source localisation Sound absorption / Liner Active flow/noise control Acoustic predesign Experimental facilities Methods & Tools
Motivation Picture: Rolls-Royce plc Development of analysis method finding an optimum model distribution of sources fitting measured array data Transfer of the analysis method to in-door measurements
State-of-the-art of source localization Establishment of cross-spectral matrix in the frequency domain Conventional beamforming Source: Blacodon und Élias (2004) Sound source deconvolution methods (DAMAS, CLEAN-SC) Inverse methods, MM > JJ No method available for accurate analysis of directed radiation from aircraft engine sound sources
State-of-the-art of source localization (cont d) Parametric optimization in literature - AFINDS - complex modelling of aircraft engine sources with low parameter count - SEM - distributed arrangement of monopoles as equivalent sources Conventional beamforming SEM Source: Blacodon und Élias (2004) Extension of SEM method taking into account the directivity of the sound sources
Methodology of SODIX SOurce DIrectivity Modeling in the Cross-Spectral MatriX Extension of SEM by modelling a free choice of source directivity SEM SODIX Determination of sound source strengths DD by means of iterative minimization of the cost function FF(DD) using a conjugative gradient method Consideration of side condition for positive source strengths by transformation DD = dd 22
Methodology of SODIX (cont d) - Startdistribution: energy-equivalent, constant source strengths - Plot of the equivalent source amplitudes as function of position and direction start ii solution ii ii = ii 200 100 = 50 52
Measurement of the same engine freefield and indoor engine far-field microphones microphone array Picture: Wolfram Hage
Measurement of the same engine freefield and indoor Rolls-Royce BR700 Picture: Rolls-Royce Deutschland
Validation of SODIX with farfield measurements The SODIX-Sourcemap depicts the directed source strengths of the axially distributed monopole sources Integration over areas of the linear source grid enables the discrimination of source regions inlet, nozzle and jet Extrapolation of the source strengths to the farfield positions with 1/rr The mean deviation between SODIX derived results and farfield measurements is in the order of 0.5 db
SODIX applied to the indoor configuration microphones
Determination of the sound power from the nozzle Low engine thrust - Integration over angular range from 85 to 115 - Small deviations up to 2.5 db at low engine thrust for frequencies above 200 Hz High engine thrust - Reproduction of spectral characteristics - Even better agreement for high engine thrust
Other aplications of SODIX QinetiQ NTF DLR AWB
DLR.de Chart 19 Summary SODIX is a universal source localisation method for microphone array data High spatial resolution (comparable to available deconvolution methods) High dynamic range (> 20dB, comparable to best deconvolution methods) Gives directly the highly resolved directivity of the sources Very robust No pre-calibration needed, no further assumptions Validated against beamforming and deconvolution methods SODIX allows for the comparison of in-door and free-field aero-engine acoustic measurement Targeted is the acoustic quantification of engine design changes by means of in-door engine tests