MusictoYourEars:NewTransducersMeetElectrostaticHeadphonesAnaudiotechnologystartupdeliversnewmanufacturabletransducersforhigh-endelectrostaticheadphonesandreduceslow-endroll-off.

ByJenniferHand

Serioushi-fienthusiastsgetexcitedaboutthemusicalexperiencedeliveredbyelectrostaticheadphones.Producinganatural,airysound,theyprovidegreaterclarity,lessdistortion,andextendedbandwidthwhencomparedtoothertypesofheadphoneswherehighresolutionaudiosourcesareinvolved.

Mostelectrostaticspeakersapplyanelectricchargeonathinelasticmembranesituatedbetweentwoconductiveplates.Thechargedmembranemovesindirectresponsetotheelectricalinput,generatingthesoundwavesthatourearsandbraininterpretasmusic,andmovingustojoyandtears.

Despitetheirhighqualityandaccurateaudioreproduction,electrostaticspeakerscanbeprohibitivelyexpensive,sometimesfragile,anduntilrecently,werehandmadebecauseofmechanicalprecisionrequirements.Seeinganeedforaffordable,high-qualityheadphonesthatcouldbemanufacturedmoreeasily,WarwickAudioTechnologiesLimited(WAT)designedtheHigh-PrecisionElectrostaticLaminate(HPEL)transducer,apatentedtechnologybasedonanultrathindiaphragmandasingleconductiveplateinsteadofapair.WithitsoriginsatWarwickUniversityintheUK,WAThasdevelopedalightweightlaminatemembraneonly0.7mmthickthatisperfectlysuitedforelectrostaticheadphones.

ThenewHPELsarelightweightthin-filmstructuresmanufacturedthroughacontinuousrollprocess.“Thetechnologywe’vedevelopedisunique,”explainsMartinRoberts,CEOofWAT.“TheHPELtransducerismadeupofametallizedpolypropylenefilm,apolymerspacerwithhexagonalcells,andaconductivemesh"(Figure1).

Figure1.Lefttoright:WAT’sHPELtransducers;singlelaminate,assembled,andexplodedviewsofa

finishedHPELtransducer.AlllaminatesaremadeintheUK.

IInthetypicalsetup,directcurrent(DC)biasvoltageisappliedtotheelasticmembraneandalternatingcurrent(AC)drivesignaltothesurroundingplates.WAT'sone-sidedspeakerinvolvesboththeDCbiasandtheACdrivesignalappliedtotheelasticmembrane,withasinglewiremesh(plate)positionedoppositethemembraneasagroundplane.

Thefabricationmethodmakesitpossibletoreproducethetransducersatasignificantlylowercostthantraditionalelectrostaticspeakers.Thismeansthatforthefirsttime,electrostaticsmaybe

consideredacommerciallyviablehigh-resaudiooptionacrossawiderangeofdevicetypesandmarketsegments.

SimulatingAcousticPlaybackTodevelopatransducerlikethis,whichcanbeeasilymanufacturedandinexpensivewithout

compromisingsoundquality,theWATteamthoroughlyinvestigatedtheinfluenceofmanydesignelementsbeforesettlingonafinalversion.“Wehaddevelopednumerousprototypesthatclearlyperformed.Thebigissuewasthatwewerenotentirelysurehowvaryingindividualmaterialanddesignparametersaffectedthetransducer’sperformance,”Robertssays.

ThedynamicsoftheHPELaredependentontheextremelycomplexinterplaybetweenmembranetension,ACsignallevel,speakergeometry,elasticanddielectricmaterialproperties,thermoacousticlosses,andtheaddedmasseffectsoftheairnexttotheopensideofthemembrane.Thedesignerswantedtoimprovebassperformancebyreducinglow-endroll-off,minimizingdistortion,andmaximizingthesoundpressurelevelforagivenelectricalinput.Buttheydiscoveredthatsmallchangestoanycomponentgreatlyaffectedtheacousticoutput.

AlthoughWAThadsignificantmechanical,electrical,andacousticexpertise,theyhadnoin-housesimulationcapabilitytohelpthemunderstandthisinterplay.InordertoperformavirtualoptimizationoftheHPELtransducerdesigntheyenlistedthehelpofXiEngineering,aCOMSOLCertifiedConsultantthatspecializesincomputationalmodeling,designrecommendations,andsolvingnoiseandvibrationproblemsinmachineryandothertechnology.

Dr.BrettMarmo,technicaldirectorofXiEngineering,oversawthedevelopmentofthe

COMSOLMultiphysics®softwaremodelstheyusedtoanalyzethebehavioroftheHPEL.COMSOL®software,whichallowedXiEngineeringtomodelnonlineareffectsthatwouldarisewithamendmentstotheHPEL’sasymmetricaldesign.

“Wekepttheearlymodelsimple,focusingonspecificsthatinfluencesoundquality,forexamplekeepingthefirstharmonicaslowaspossibletounderstandtheacoustic-structureinteractionandtheHPEL’sperformanceatlowfrequency,”Marmoexplains,describingtheirpreliminarytests.“Ourmodelshowedhowappliedvoltageaffectssignallevels,whichhelpedusunderstandsounddistortionforaninitialcase.”

Becausethetransducerisone-sided,theelectrostaticforcevarieswiththepositionofthevibratingmembrane,decreasingwiththesquareofthedistancebetweenthemembraneandthemesh.Oncetheyunderstoodtheresultingnonlineardistortionandwereabletopredictitseffects,theWATengineerscouldthencancelanyrelateddistortionselectrically.

PerfectingtheHPELTransducerDesignInamoreextensivesimulationthatinvolvedastructural-MEMS-acousticcoupling,heexamined

theimpactofadjustingparameterslikethesizeofthehexagonalcellsinthewiremesh,thicknessofthewires,membranetension,spacingbetweenmembraneandmesh,andmaterialpropertiesofeachcomponent.MarmoandhiscolleaguesalsostudiedtheeffectsofdifferentDCbiases,whichareoftenresponsiblefordistortionatlowfrequencies,andlookedatconductivityalongtheplatetodiscernwhethervoltageswerehigherinoneareathananother.TheythenusedCOMSOLtostudythethermoacousticlossesandmodelthedisplacementofthemembranefordifferentfrequencies(Figure2).

Figure2.Simulationplotshowingthesoundpressurelevel(thermalcolorsurface)indBandthedisplacementofthemembrane(rainbowcolorsurface)inmmfromafullycoupledacoustics-MEMS

modelsolvedinthefrequencydomain.Left:solutionat5,000Hz.Right:solutionat5,250Hz.

“Wefoundthatthistypeofsimulationwastheonlyaccuratewaytotrulymodelplanarelectrostatictransducers,”Marmocontinues.“Forthiscase,lumpedparametermodelingcancharacterizelimitedaspectsofperformance,suchaslow-frequencyamplituderesponse.Oneparametermightbeexcellentbuttheremaybesignificantdistortioncreatedelsewhere.Multiphysicsmodelingencompassesalldimensionsthataffectourperceptionofsound,suchasthetime-domainresponseandnonlineardistortion.”

ThesimulationsmadeitpossiblefortheengineersatWATtotweakdesignparametersinordertooptimizeoverallperformance.Ultimately,theywereabletopredictwhatwascausingspikesinthefrequencyresponseandsmoothoutthesignalforbetterfidelity.

“Thisrepresentedahugecostandtimebenefitforus,”saysRoberts.“Wewentfrommakingmultipleprototypesbyhandeachweektosimplydialingupanewoneinthesoftware.Inadditiontosettlingonafinaldesignwe’reveryhappywith,itisnoweasyforustocustomizeourtransducersforclients’individualrequirements.”

Marmo’steamcomparedeachmodelwithphysicalmeasurementsprovidedbytheWATdesignteam.“Thesimulationresultswereastoundinglyclosetothephysicalmeasurements,”commentsDanAnagnos,CTOatWAT.“Thatwasprobablythemostexcitingaspect,seeingthesimulationcometolifeandknowingitwasgivingusanaccuratepictureofhowthespeakerwouldperform.”

FreedomandFlexibilitywithaSimulationAppWithsimulationresultsverifiedandWATsatisfiedwiththeirdesign,thenextstepwasforXi

EngineeringtoputWATincontroloffurthermodeling.TheApplicationBuilderavailableinCOMSOLsoftwareenabledMarmo’steamtobuildanappfromtheirsimulationandhostitonline.

Theapp’sinterfaceallowsuserstochangecertaininputstotestchangestoanumberofparameters,suchastheDCbias,ACsignallevel,frequencyrangeandresolution,materialproperties,speakersize,wiremeshshapeandsize,andspacerplacement(Figure3).Theoriginalmodelsetupisnotaccessiblefromtheapp;instead,itallowsuserstorunfurthertestswithoutneedingtolearnthesoftware.

“ProvidingWATwithasimulationappremovedtheneedforthemtopurchasethesoftwareorappointanexperienceduser,”Marmosays.“Simulationappsputourcustomersincharge,sotheydon’thavetocomebacktousforsmallchangesandtheycantestexactlywhattheywant.Italsofreesustoexplorenewchallenges,ratherthanworkingonvariationsofthesameproblem.”XiEngineeringexpectstousecomputationalappsmoreandmoreinthecourseofitsworkforothercustomers.

Figure3.Bottom:TheappdevelopedbyXiEngineeringallowsengineerstovaryparametersrelatedtofrequency,electricalinput,speakerdimensions,andpropertiesofthemembrane,spacer,andwiremesh.Resultsgivethesoundpressurelevelsfordifferentcases,membranedisplacement,frequencyresponsetodifferentDCbiases,andacomparisonofthesimulateddesignagainstexperimentalresults.

Top:TheappissharedthroughtheCOMSOLServer™productandaccessiblefromawebbrowser.

WATisdoingthesame,sharingtheappwiththeirowncustomers—companieswantingtofindtheHPELtransducerbestsuitedtotheirparticularheadphonedesigns.“TheteamatXiEngineeringhavebeensuperb.Theyhavedeepexpertiseandhelpedtounpackthecomplexityofourproduct,”addsRoberts.“TheintuitiveappthatXidevelopedforusisanadditionalbonus.Withoutrevealinganyintellectualpropertywecangiveourownclientsaccesstoourdesignthroughtheapp,sotheycantestandincorporatethetechnologyintotheirownhigh-endheadphones.”