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1、8ActiveStructuralAcousticControl.IPlateSystems8.1IntroductionInnumerousindustrialapplications,structurallyradiatednoiseisapersistentproblemwhichisoftenpoorlyalleviatedbypassivemeans,particularlyatlowfrequencies.Thepossibilityofactivenoisecontrolwassuggestedover50yearsago(Lueg,1936)butitsimplement
2、ationisarelativelyrecentdevelopmentduetoadvancesinfastmicro-processorsfordigitalsignalprocessing.Inthischapterwediscusstheactivecontrolofsoundradiationfromdistributedvibratingstructures.Twoformsofcontrolsourcesaregenerallyavailable.Theuseofacousticcontrolsourceshavebeeninvestigatedbypreviousworke
3、rs(DeffayetandNelson,1988andFulleretal.,1991).Ingeneralithasbeenshownthatwhenthesoundsourceiscomplexordistributedovermultiplesurfaces,manyacousticcontrolsourcesarerequiredinordertoprovideglobalcontrol.AnalternativeapproachasembodiedintheresearchofFullerandhisco-workers(Fuller,1985a,1987,1988),ist
4、ousecontrolinputsapplieddirectlytothestructureinordertoreduceorchangethevibrationdistributionwiththeobjectiveofreducingtheoverallsoundradiation.ThistechniquehasbeentermedActiveStructuralAcousticControl(ASAC),anabbreviationthatconformswiththegenerallyacceptedterminologiesofActiveNoiseControl(ANC)a
5、ndActiveVibrationControl(AVC).Figure8.1showsageneticarrangementofadistributedelasticsystemexcitedbyanoscillatingdisturbance.Soundradiationoccursasaresultofthecontinuityofparticledisplacementattheinterfacebetweenthestructureandthesurroundingcompressiblemedium.Theobjectiveistoreducethesoundradiatio
6、n.Obviouslycompletelyreducingtheoverallstructuralresponsewithactivevibrationcontrolwouldleadtoanattenuationofthesoundradiation.However,asshownlater,variousmodesofvibrationhavedifferingradiationefficienciesandsomearebettercoupledtotheradiationfieldthanothers.Thissuggeststhatinordertoreducesoundrad
7、iation,onlyselectedmodesneedtobecontrolled,ratherthanthewholeresponse.Inaddition,iftherelativephasesandamplitudesofamulti-modalresponsecanbeadjustedsothattheydestructivelyinterfereintermsofradiatedsound,thentheradiatio
