扬声器等效模型.docx

扬声器等效模型.docx

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扬声器等效模型

扬声器等效模型

Youcansimulatethebehaviourofaspeakeranditscrossoverbyprofessionalspeakerdesignsoftware.Incaseyoudonothaveaccesstothesesoftware,analternativewouldbetousetheSPICEsimulationsoftwaretodothespeakersimulation.HereIwilldemonstratehowtodothisusingOrCad.

ThereisademoversionofOrCadrelease10whichyoucandownloadfromOrcad'swebsitetotry.

Thiele-smalleqivalentcircuit

BeforeyoucansimulateyourdriverinOrcad,youneedtocalculatetheeqivalentcircuitforyourdriverunit.ThiscircuitcanbederivedfromtheThiele-smallparameters.

Forexample,ifadriverhas

thefollowingThiele-SmallParameters:

Thiele-Small Parameters 

Revc

5.8ohms

DCresistanceofvoicecoil

Levc

0.55mH

voicecoilinductance

Bl

6.5T.m

forcefactor

Qts

0.35

totalQ

Qes

0.45

electricalQ

Qms

1.55

mechanicalQ

Fs

37Hz

resonantfrequency

Mmd

0.014kg

massofcone+voicecoil+etc

Rms

2.08

resistanceofsuspension

Cms

1.34mm/N

complianceofsuspension

Sd

0.0136sq.m

effectiveconearea

Vas

0.0347cu.m

equivalentacousticvolume

Xmax

0.004m

lineartravelofvoicecoil

FR

37-5000kHz

frequencyresponse

Vd

0.0005cu.m

driverunitvolumedisplacement

Thenthisdrivercanbemodelledwiththefollowingcircuit:

Someofthecircuitvaluesabovearealreadyobvious.Vegrepresentstheamplifierandisassumedtohavenooutputresistance.Theremainingvalueswerecalculatedfrom,

Cmes=Mmd/（Bl*Bl）=electricalanalogofdrivermechanicalconemass

Lces=Cms*Bl*Bl=electricalanalogofdrivermechanicalsuspensioncompliance

Res=Bl*Bl/Rms=electricalanalogofdrivermechanicalsuspensionresistance

Cmef=8*po*Ad*Ad*Ad/（3*Bl*Bl）=electricalanalogofairloadonthedriverunit'scone

where:

po=airdensity=1.18kg/cu.m

Ad=effectiveradiusofthedriverunit'scone=SquareRootof（Sd/3.14）

Someoftheseparameterscanbelookupfromthedriver'sspecification.ThismissingonecouldbecalculatedusingtheUniboxexcelfile.Uniboxisafreetoolthatcanhelpyoutodesignspeakerenclosure.

SpeakerDriverUnitSimulation

Afteryoucreatedyourdrivermodels,youcouldstartbytestingifyourdrivermodel'simpedanceresponseissimilartoyourdriver'sactualimpedanceresponse.NowyouneedtofamiliaryourselfwithOrCadifyouhavenotuseditbefore.HereisagreatOrcadtutorialthatIsuggestyoutoreadifyouhavenotusedOrcadbefore.

YoucansavesometimebydownloadingmyOrcadprojectfileforasingledriverunit,justchangetheRLCvaluesandyoucanstarttryingOrCadsimulationrightaway.

Let'sdothewooferasanexample.First,startupOrcadCaptureandcreateanewemptyproject.ThenintheschematicdrawthedrivermodelusingPlace->Part...command.TheR,LandCareallundertheANALOGlibrary.（Youmayneedtoaddalllibrariesfirstifyouhavenotdoneit.）ThenaddtheACvoltagesource（calledVACundertheSOURCElib.）andalsoaddagroundreferencebyPlace->Ground...->0/Source.ConnectallcomponentsbyPlace->Wire.Youshouldnowhaveacircuitsimilartotheonebelow:

Let'sdosomesimulation:

AddanewsimulationprofilebyselectPSpice->NewSimulationProfile,typeinaname.Asimulationsettingsdialogwillpopup.IntheAnalysistab,setAnalysistypetoACSweep/Noise.SelectLogarithmic,startfrequencyof10Hz,EndFrequencyof20kHz,Points/Decadeto50.ClickOK.

Nowplaceavoltageprobeandacurrentprobeatinputoftheoftheresistor,byselectingPSpice->Markers->VoltageLevel,andthenpointtheprobeatthelinebetweentheVACandtheresistor.SelectPSpice->Markers->CurrentintoPin,andselecttheinputpinoftheresistor.

SelectPSpice->Runtostartsimulation.Afterthesimulationisdone,theresultwillpopupinanotherPSpiceA/Dwindow.Nowyoucanseethevoltageandcurrentthatgointo

thedriveratdifferentfrequency.Togetaimpedanceplot,youhavetoaddatrace,byselectingTrace->AddTrace,thentypeV（V2:

+）/I（R1）intheTraceExpressioninputline,thenOK.Nowyoushouldseethedriverunit'simpedancevsfrequency,liketheonebelow.

Youcancomparetheresulttotheimpedanceplotfromthedriverunit'sdatasheet,orbetteryet,compareittoyourmeasuredimpedance.Iftheylooksimilar,thenthisshouldbeagoodmodeltouse,ifnot,tuneyourcircuit'sRLCvalueuntiltheyarecloseenough.

Crossover+DriversSimulation

Afteryouhavedoneyourwooferandtweetersimulation,youcanaddyourcrossovertotheschematic.Iseparatedthedriverunitsandthecrossoverinto2pages,youcandothisbyusingthePlace->Off-Pageconnectortoconnectthe2pages.Myspeakerhas2woofersconnectedinparallel,asshownintheschematicbelow.

YoucanusethedbVoltageprobetocheckthefrequencyresponseofeachindividualdriveunit.Ifyouwanttochecktotalresponse,youneedtosumthe2outputsofthecrossover.

Insomecrossoverdesign,youwillhavetoreversethepolarityofthetweeter.YoucansimulatethisinOrcadbyeitherchangingthepolarityofthevoltagesourceofthetweetercircuit（SetthephasepropertyoftheVACto180degree）,oryoucanaddaFTABLEparttochangethephaseofthesignalbeforeitgoesintotheSUMpart.

BelowisthefrequencyofthedBvoltageresponseofcrossover+tweeter+woofer

Youcanalsocheckthephaseresponsebyputtingaphaseofvoltageprobetotheoutput.

Nowyouhavesimulatedaperfectmodelofaspeakerdriver,butrealspeakerunitsdonotgivesmoothandflatresponse.Isimulatedthepeaks,dipsandnaturalrolloffofaspeakerbyusingtheFTABLEparttoaddresponsevariationatdifferentfrequencies.

YoucouldalsouseFTABLEtoadjustthesensitivityofthetweeter/wooferunitbyadding/subtractingafewdBacrosstheentirefrequencyrange:

AfterIaddedtheFTABLEtoboththetweeterandwooferIranthesimulationagain:

AfterIaddedtheFTABLEtoboththetweeterandwooferIranthesimulationagain:

Whenyoucomparethesimulatedresulttomymeasuredresultinmyspeakerpage,youwillfindthemquitesimular.

Updated5thOct,2005-

Youcanactuallysimulatethephaseshiftduetothemis-alignmentofthetweeterandwooferacousticcenter.Inmycase,thetweeterisabout30cmclosertothelisteningpositionthanthewoofer.Sothephaseshift（tweeterleading）atvariousfrequencycanbedeteminedbythefollowingformula:

v=f*λ

Phaselead=TweeterWoofer

Acousticcenteroffset/λ*360

where:

v=speedofsound=345M/s

f=frequency

λ=wavelength

Inmycase,theacousticcenteroffset=0.03M

Phaselead@1000Hz=0.03/（345/1000）*360=31.3degree

Phaselead@2500Hz=0.03/（345/2500）*360=78.3degree

ByenteringthesephaseleadinthetweeterF-table,youcandoamoreaccuratesimulation.