Microphone Array Measurements: Sound Source Localization by means of SODIX

Stefan Funke, Henri Siller, Lars EnghardtEngine Acoustics, Institute of Propulsion TechnologyGerman 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

DLRGerman Aerospace Center

Research InstitutionSpace AgencyProject Management Agency

Köln

Lampoldshausen

Stuttgart

Oberpfaffenhofen

Braunschweig

Göttingen

Berlin-

Bonn

Trauen

Hamburg Neustrelitz

Weilheim

Bremen-~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

DLR: Sites and employees

Jülich

Stade

Augsburg

Institute of Propulsion TechnologyReinhard Mönig

CombustorChristoph Hassa

TurbineFrank Kocian

Combustion TestChristian Fleing

Engine AcousticsLars Enghardt

Engine Measurement SystemsChristian Willert

EngineAndreas Döpelheuer

Fan and CompressorEberhard Nicke

Numerical MethodsEdmund Kügeler

Combustion simulationFrancesca di Mare

Organisation of the Institute

DLR.de • Folie 6

Engine Acoustics - Core Competencies

Turbomachinerysound sources

Engine core soundsources

Jet Noise Unsteadyaerodynamics ofturbomachinery

Sound propagationin ducts

Radiation of sound Sound sourcelocalisation

Sound absorption / Liner

Active flow/noisecontrol

Acoustic predesign Experimental facilities

Methods & Tools

DLR.de • Folie 7

Turbomachinerysound sources

Engine core soundsources

Jet Noise Unsteadyaerodynamics ofturbomachinery

Sound propagationin ducts

Radiation of sound Sound sourcelocalisation

Sound absorption / Liner

Active flow/noisecontrol

Acoustic predesign Experimental facilities

Methods & Tools

Engine Acoustics - Core Competencies

Picture: Rolls-Royce plc

• Development of analysis method finding an optimum model distributionof sources fitting measured array data

• Transfer of the analysis method to in-door measurements

Motivation

Establishment of cross-spectral matrix in the frequency domain

Conventional beamforming

Sound source deconvolution methods (DAMAS, CLEAN-SC)Inverse methods, 𝑀𝑀 > 𝐽𝐽

No method available for accurate analysis of directed radiation from aircraftengine sound sources

State-of-the-art of source localization

Source: Blacodon und Élias (2004)

Parametric optimization in literature

- AFINDS - complex modelling of aircraft engine sources with low parameter count

- SEM - distributed arrangement of monopoles as equivalent sources

Extension of SEM method taking into account the directivity of the sound sources

Conventional beamforming SEMSource: Blacodon und Élias (2004)

State-of-the-art of source localization (cont‘d)

Methodology of SODIX

SOurce DIrectivity Modeling in the Cross-Spectral MatriXExtension of SEM by modelling a free choice of source directivity

SEM

SODIX

Determination of sound source strengths 𝐷𝐷 by means of iterative minimization ofthe cost function 𝐹𝐹(𝐷𝐷) using a conjugative gradient methodConsideration of side condition for positive source strengthsby transformation 𝑫𝑫 = 𝒅𝒅𝟐𝟐

- Startdistribution: energy-equivalent, constant source strengths- Plot of the equivalent source amplitudes as function of

position and direction

start solution𝑖𝑖 = 2𝑖𝑖 = 5𝑖𝑖 = 10𝑖𝑖 = 20𝑖𝑖 = 50𝑖𝑖 = 100𝑖𝑖 = 200

Methodology of SODIX (cont‘d)

Measurement of the same engine freefield and indoor

microphone array

engine

far-field microphones

Picture: Wolfram Hage

Rolls-Royce BR700

Picture: Rolls-Royce Deutschland

Measurement of the same engine freefield and indoor

Validation of SODIX with farfield measurementsThe SODIX-Sourcemap depicts the directed source strengths of the axiallydistributed monopole sources

• Integration over areas of the linear source grid enables the discrimination ofsource 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

- Integration over angular range from 85° to 115°

Low engine thrust

High engine thrust

- Small deviations up to2.5 dB at low enginethrust for frequenciesabove 200 Hz

- Reproduction ofspectral characteristics

- Even better agreementfor high engine thrust

QinetiQ NTF DLR AWB

Other aplications of SODIX

• 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 acousticmeasurement

• Targeted is the acoustic quantification of engine design changes bymeans of in-door engine tests

Summary

DLR.de • Chart 19