TechnicalAcoustics,http://www.ejta.org,2007,17.CopyrightTechnicalAcous - PDF document

TechnicalAcoustics,http://www.ejta.org,2007,17.CopyrightTechnicalAcoustics[ISSN1819-2408](2007)EEAA. AudibilityoftemporalsmearingandtimemisalignmentofacousticsignalsMilindN.KunchurDepartmentofPhysicsandAstronomyUniversityofSouthCarolina,Columbia,SC29208(Dated:19.07.2007[received];29.08.2007[published])Misalignmentintimingbetweendriversinaspeakersystemandtemporalsmearingofsignalsincomponentsandcableshavelongbeenallegedtocausedegradationof“delityinaudioreproduction. reproductionsincesuchdistortionscannotnaturallyariseinanaudiochain.Thepresentworkreportsthedetectionofatemporalde-layshorterthananythathasbeenpreviouslypublished,andonewhosethresholdunderceeds1.Further-more,themethodandtypeofthetemporalfeature„adisparitybetweenspatialpathdistancesfromtwoloud-speakerdrivers„isadistortionthatcanactuallybeman-ifestedinareal-lifeaudiosetup,sincemostspeakersys-temsconsistofmultipledriversandeventhosethatdontwillexhibitatemporalspreadbecauseofthe“nitedi-mensionsofthedriver.Sucharrival-timediscrepanciescanplayanevengreatersigni“canceinmulti-channelsurround-soundsystems.Thepresentresultprovidesascienti“cbasisfortheanecdotalclaimsbyaudiophilesthat“delityrequirestimeresponseinthemicrosecondrange,andprovidesasolidquantitativestandardforas-sessingthedeterioratingeectsoftemporaldelaysandsmearinginanaudiochain.1BACKGROUNDThecentralgoalinhigh-“delitysoundreproductionistoreproduceasoundwithsucientaccuracysuchthattheerrorsinalldomainsarebelowtheirthresholdsofde-tectability.Settingasidestereoandspatial-localizationaspects,monauralsoundscanbeperceivedasdierentbecauseof(a)dierentfrequencycomponents,(b)dif-ferentlevelsofcomponents,(c)dierentrelativetimingsandrisetimesofcomponents,and(d)dierentphasesoftheindividualcomponents.The“rsttwodierencesareoftencollectivelyreferredtoasspectralŽ;although,strictlyspeaking,theseshouldbereferedtoasdierencesintheamplitudespectrumŽorintensityspectrumŽ,sinceallalterations,includingthoserelatedtophaseandtime,canbedescribedthroughchangesinthecomplexFourierspectrum.Exceptinsimplelinearsystems,theinter-relationsbetweenfrequency,amplitude,time,andphasearenotstraighforward.Inaudiosystems,forex-ample,acrossovercanintroduceafrequencydependentphasedierencewithoutphysicallydelayingtheonsetofonefrequencybandwithrespecttoanother;however,theeectofhavingunequallistener-to-subwooferandlistener-to-satellitespeakerdistancesisbestdescribedbyanoveralldelaybetweenthetwofrequencybands.Similarly,thehearingmechanismtreatsphaseandtimedierencesonseparatefootingsanderrorsinthetwodonothaveequivalentconsequences.Inalinearcircuit,adelayinasinusoidalsignalisrelatedtoitsphaseshift.Inthehearingprocess,thesoundsignalisdecomposedintoseparatefrequencychannelsthroughanarrayofsensoryinnerhaircells(IHCs)tunedtodierentcharacteristicfrequencies(CFs)andarrangedtonotopicallyalongthebasilarmembraneinthecochlea.Thistonotopicallyseparatedinformationiscarriedbyau-ditorynerve“bers(ANFs)tothecochlearnucleus(CN).ThenerveimpulsesalongtheANFsfollowthephaseoftheircorrespondingacousticsignalsforfrequenciesuptoabout4kHz.However,thehearingmechansimlargelyabandonscross-frequencyphaseinformationleadingtothefamousOhms(second)law[5,6]wherebytheearisnotacutelysensitivetophaseshiftsbetweenwellsepa-ratedfrequencies(despitelargedierencesinwaveformshape).Thisfactishelpfulinthedesignoffrequency-cross-overcircuitswherephasedierencesbetweenlow-passandhigh-passoutputsareimportantmainlytotheextentthattheyaecttheamplituderesponse[3].Above4kHz,theANFsrespondapproximatelywithaplateauofactivityforthedurationofthetonewithnosynchro-nizationbetweenthe“ringpatternandthephaseoftheacousticsignal.Whilecross-frequencyphasecoherenceislessimportant,timecoherenceisadierentmatter.Theauditorysys-temisverysensitivetothesynchronicityintheonsetsofdierentfrequencies„asiswellknown,instrumentaltimbrebecomesambiguousiftheonsetsanddecaysofthenotesareremoved[7].Inthecochlearnucleus,fastrespondingoctopuscellsactassynchronousANDgates60inputseach)toconvergecoincidentANFsig-nalsfromdierentfrequencychannels[8,9].Thesecellsrespondsharplytowelltimedmultichromaticactivityattheonsetofasoundandthustheiroutputisagaugeofthestimulusslewrate[10].IftheinitialtemporaluncertainityintheANFsignalisrepresentedbyaGaus-sianprobability-densityfunction,thenscanbetakenasaroughestimateoftheini-tialtemporalspreadsincetheANFslosephaselockingwiththeacousticstimulusaround4kHzandrespondonlytothepositivehalfcycle[11,12].Theprobabil-ityforNsignalstoarrivesimultaneouslyinordertoexciteanoctopuscellisproportionaltotheproductoftheprobabilities.Thisgivesanoutputprobabilityfunc- withareducedtemporalspreadof .BesidestheinitialconvergencefactorofN60ateachoctopuscell,theoctopus-celloutputsundergoadditionalconvergenciesathigherneurallevels(e.g.,insphericalbushycellsinthelateralleminiscus).ItisnotcleartowhatextenttheseconvergenciesboostNandimprovethetimeresolution;however,itisclearthatthemaximumpossibleconvergencefactorcannotex-ceedthetotalnumberofIHCs,whichis4000.Thusfortransientstimuli,theauditorysystemstemporalacuitymaybeestimatedtobeinthe2…16srange,taking withN=60…4000.Noticethatthevalueofthishasverylittletodowiththehigh-frequencyaudibil-itylimit.Asdescribedearlier,theauditorysignalsoriginatefromhaircellsarrangedsuchthattheonesclos- 4 d FIG.1:Experimentalcon“guration.Speaker-to-listenerdis-tance=4.3m,aperturelength=1.5cm,andspeaker-centertospeaker-centerdistancecm.Misalignmentisvariable.Duringblindtrials,alistenertriestodistinguishbetweenthealigned(=0)andmisaligned(settingsforvaluesranging2…10mm(2METHODS2.1ApparatusThecon“gurationoftheexperimentisshowninFig.1.Twoloudspeakersarestackedverticallyontopofeachotherwiththeirfrontfacesparalleltoeachother.Thetopspeakerismountedonrailsandcanslidebackandforthbetweena“xedstop(forthealignedposition)andamicrometer-setscrewadjustablestop(forthedisplacedposition)throughasetdisplacement.Thelistenerisseatedatadistance3m,facingthespeakerswithearsataheightmidwaybetweenthetwospeakers.Thespeakersarelaterallycenteredw.r.t.(withrespectto)bothearssothatbothearsreceivethesamesignal.Theroomshapeisarectangularparallelepipedwithaheightof2.7m,awidthof3.6m,andalengthof5.8m.Thespeaker-listeneraxisliesalongthelongdimensionandiscenteredw.r.t.thesidewalls;thisaxisisataheightof1.1mabovethe”oor.The”oorandwallsoftheroomwerecoveredwithacousticalcarpetingandtheceilingcoveredwithacousticaltiles.Thesematerialshaveabsorptioncoecientsof7atthefrequenciesofinterest(7kHz).Inaddition,panelsmadefrom38mmthickglass-“berboards(forwhich95)wereplacedatcertainstrategiclocationstosuppressprincipalre”ections(thereweresixsuchpanelswithatotalareaofabout9mTheloudspeakersusedwereapairofAurumCantusG2Siribbontweeters(JinlangAudioCo.Ltd.,PenglaiCity,P.R.ofChina)whichhaveafrequencyresponseof2…40kHz,asensitivityof96dB/Wat1m,andanom-inalimpedanceof6.Bothspeakerswereconnectedinparalleltothesame7kHzsquare-wavesignalsource.Thissignalsourceconsistedofananalogsignalgenerator(model4001manufacturedbyGlobalSpecialtiesInstru-ments,Cheshire,Connecticutt)followedbyawidebandampli“er(witha3-dBpowerbandwidthof0…2.2MHz).Fig.2showsthevoltagewaveformatonespeakersinputterminalsmeasuredwithaLeCroymodelLT322(LeCroyCorporation,ChestnutRidge,NewYork)500MHzdig-italstorageoscilloscope,whichdigitizedthesignalatasamplingrateof200MS/s(millionsamplespersecond)anda12-bitverticalresolution;thissameoscilloscopewasusedinalltheotherwaveformandspectrummea-surements.Notethewellcontrolledresponsewithneg-ligibleringingandovershoot,andrise/falltimesofs.Themeasuredjitterinthissquare-wavesignalwas68ns(05%oftheperiod). FIG.2:Waveformofvoltageatloudspeakerterminals,recordedata200MS/ssamplingrate.A7kHzsquarewaveformwaschosenbecauseithasonlyoddharmonicsthat,otherthanthefundamental,arebelowtheirrespectivesingle-toneaudibilitythresholds.However,theacousticoutputfromaloudspeakerwillnotbeaperfectreplicaofitselectricalinput.Besidesalteringtheharmoniccoecientsbecauseofanincon-stantfrequencyresponse,subharmonicsandotherspuri-ousanharmoniccomponentsmaybegenerated[35]whenaspeakerisdrivenathighlevels,especiallywithinad-equatedamping.Inthepresentexperimentthedrivinglevelismodest(0.5Vpeakinputvoltageand69dBSPLsoundlevelatlistenerposition)andthedampingiseective(40msignal-sourceoutputresistance)topre-ventanharmonicdistortion.Thisabsenceofanharmonicdistortionwasveri“edbyspectrumanalyzingtheacous-ticoutputoftheloudspeakerusinganACOPaci“c(ACOPaci“c,Inc.,Belmont,California)model7016measure-mentmicrophoneanda4012preampli“erwitha40dBgainstage.Thefrequencyresponseofthemicrophonetogetherwithitspreampli“erwas”at(3dB)withina4Hz…120kHzband.ThispowerspectrumoftheacousticsignalfromtheloudspeakerisshowninFig.3(a).Panel(b)showsamagni“edviewofthefundamental()peakwithlinearaxes;thefull-widthhalfmaximum(FWHM)is0.77Hz0.01%of).Nosubharmonicpeakscouldbedistin-guishedfromnoise.Theabsolutesoundlevelofthenoise(includinginthe3.5kHzsubharmonicvicinity)wasdBSPL(i.e.,belowthedashedlineinFig.3[a]).Therewasalsonodetectableanharmoniccontentabove.Asexpected,thespectrumisdominatedbyoddharmonics(7,21,35,49kHz,...)whichextendwellintotheultra-sonicrange.Becausetheentiresignalchainisanalog,spuriousfrequenciesthatcanresultfromaliasingindig- italsystemswereavoided. FIG.3:Powerspectrumofthe(unaveraged)acousticoutputofoneloudspeakeratadistanceof0.7m.Thepowercoecientsarenormalizedw.r.t.thefundamentalpeak.(a)Log-linearplotofthe20Hz…80kHzwindowin20Hzstepstakenata2MS/ssamplingrate.Thehorizontaldashedlinecorrespondstotheabsolutesoundlevelof0dBSPL.(b)Linear-linearplotoftheregionnearthefundamentalpeakin0.5Hzstepstakenata50kS/ssamplingrate.2.2AcousticstimuliatthelistenerpositionReferringbacktoFig.1,thetwosignalsarriveatthelistenersearswithaprimaryrelativedelayofisthespeedofsound.Asmentionedearlier,thelistenerisseatedatadistance3m,facingthespeakerswithearsataheightmidwaybetweenthetwospeakers.Thealignmentoftheapparatusandlistenerpositionshouldbesuchthatthelinejoiningthemidpointbetweenthelistenersearsandthemidpointbetweenthetwospeakersshouldbeperpendiculartotheplanede“nedbythespeakerfrontsurfaceswhentheyareundisplaced.Thiswas“rstcheckedwithalaserbeamandthenthelistenerfurther“netunedhis/herheadcenteringbyear.Fromthispointonwardthelistenerheldhis/herheadstillinthis“xedposition.Thesoundlevelatthesubjectlocationwas69dBSPLforEq.1wouldbeexactandcompleteifthetwospeakerswerepointsourcesandtherewerenoroomre”ections.However,soundproducedbyanextendedsourcewillsuf-feranintrinsictemporalspread(becausewaveletsem-anatingfromdierentpointsoftheradiatingsurfacewillarriveattheearsatdierenttimes)whichtendstoreducethesensitivityoftheexperiment.Heresoundisgener-atedineachtweeterbya6cmlongverticalaluminumribbon.AsshowninFig.1,absorbentbaeswereusedtocuttheaperture(andeectiveribbonlength)tocm(with=4.2cm),thusreducingthetemporalspreadwhichissmallcomparedwiththedelaysprobedintheexperiment.Theverticalseparationof9cm,betweenthecentersoftheupperandlowerspeakers,re-sultsinanangularseparationbetweenthesesourcesofatthelistenerposition.Aswiththetemporalspreadswithinthesources,roomre”ectionscanalsodiminishthetemporalde“nitionofthedelay.Whileprecautionsweretakentominimizere”ectedenergy,eventhebestanechoicchamberwillnothaveperfectabsorption.Thereforeitisnecessarytoquantitativelyassesstheeectofre”ectedenergyontheexperiment.Thesignalatthelistenerpositionwillconsistofthedirectradiationfrombothspeakersplusthesumofallre”ections.Everyre”ectedpathoriginat-ingfromonespeakercanbeassociatedwithamatchingre”ectedpathoriginatingfromtheotherspeakerandallre”ectionscanthusbeconsideredinsuchmatchedpairs.Labellingeachsuchre”ectionpairby=0cor-respondingtothepairofdirectsignals),aspeakermis-alignmentintroducesanincrementalpathdierencebetweenthemembersofapair.Itisobviousfromthege-ometrythatinallcases.Infactforthe“rstre”ec-tionsfromthesidewalls,dD/ istheroomwidthtransversetothespeaker-listeneraxis)andforthe“rstre”ectionsfromthe”oorandceiling,dD/ +(2istheverticaldistancefromthespeaker-listeneraxistothe”oororceil-ing).ThesescorrespondtodelaysThedirectsignalandthere”ectionfromthebackwall(behindthelistener)bothhaveandcorrespondexactlytotheprimarydelayof.Higher-orderre”ec-tionswillhaveprogressivelydiminishingdelays.Besideshavingashorterincrementaldelaybetweenthetwospeakersignals,apairofre”ectedsignalscanalsohaveanextrainitialgeometricalpathdierencebe-tweenpairmembers.Forpathsthatundergoasinglere”ectionfromeitherthe”oorortheceiling, D22 D2Š D2 andforpathsthatundergoasinglere”ectionfromany ),where7kHz.ThesecoecientsaregiveninTableIIandhavebeennor-malizedw.r.t.the“rst-harmoniccoecientofthecontrolwaveform(i.e.,by[0]).Columnsforharmonicswherevaluesfallwellbelowthenoise”oor(0.005)havebeenexcluded(thisisthecaseforallevenharmonicsex-ceptfor=2,whichisontheborderline).Thephaseofeachharmonicisspeci“edrelativetothephaseofthefundamentalforthatsamevalue(i.e.,w.r.t.w.r.t.d]);theabsolutephaseandthephasedierenceacrossdierentvaluesofareofcourseinconsequential.Besidesthenoiseerror,thevaluesofthenon-zerocoecientsvarytosomeextentdependingonthepositionofthemicrophonebecauseofsmalllocalvariationsinintensitycausedbyweakpartialstandingwaves.Thispositionalsensitivityproducesanuncertainityofabout015inthenon-zerocoecients,whichisequivalenttoanerrorinthemeasuredattenuationsinthe13to16dBrange.Inaperfectsquarewaveform,theharmoniccoecientsaregivenbyforodd=0foreven.ItcanbeseenfromTableIIthattheevenindeednegligibleandthattheoddfollow1wellupto=3.Beyondthatthehigh-frequencyfalloofthespeakersresponseisevidentandthecoecientsaresmallerthantheir1theoreticalvalues.Thetotalsoundlevelat=0is69dBSPL.Most(88%)ofthispoweriscontainedinthe7kHzfundamental.Thelev-elsofallharmonicsbeyond7kHzatall(forexampleexample12dBandand60dB)fallbelowtheirthresholdsofaudibility[35…37].ThelastTABLEII:Harmoniccontentsofacousticsignals.Coe-areexpressedasafractionof.Phases,inradians,areexpressedrelativetotheforthesamevalue.Thelasttwolowercolumnsgivethepowerattenu-ations,indB,inthetotalrmsstrengthsand“rst-harmoniccomponents()relativetotheirundisplaced()con-trolvalues.Thenoise”ooranderrorbarforthecoecientsare0.005and0.015respectively. =7kHz =14kHz =21kHz =35kHz 1 1 C2 2 C3 3 C5 5 0 0.00 0.006 2.9 0.00 0.006 6.2 0.00 0.005 0.816 0.00 0.004 =49kHz =63kHz =77kHz Attenuation 7 7 C9 9 C 1 0 0 0.24 0.60 10.3 0.007 1.76 twolowercolumnsinTableIIgivetheattenuationsinthetotalrmspower()andinthefundamentalcomponent(presumablytheonlyaudi-bleFouriercomponent)relativetotheirvaluesfor=0.Thesemeasuredattenuationsinthefundamentalcom-ponentareplottedinFig.5andareseentomore-or-lessagreewiththetheoreticalcurveofEq.7andcorrespond-ingcalculatedupperboundsofTableI. FIG.5:Powerattenuationofthe7kHzfundamentalcom-ponentasafunctionofspeakerdisplacement.Thesymbolsshowthemeasuredattenuationintheacousticsignalatthelistenerposition.ThesolidlineshowsthetheoreticalcurvecorrespondingtoEq.7.Comparedtotheabovemeasuredacousticwaveforminair,thewaveformattheeardrumwillbepoorerinhigher-harmonicamplitudesbecauseof“lteringbytheearcanal.So,forexample,thethird-harmonicto“rst-harmonicratiosattheeardrumwillbelowerthanthemeasured)ratiosofTableII.However,thefrac-tionalchangeineachFourieramplitudeattheeardrumwillbeexactlythesameasthecorrespondingmeasured(0)ratioinTableII.Thustheattenuationsinthefundamentalattheeardrumwillbeexactlythesameasthemeasuredvaluesgiveninthelastlowercolumnofthetableandthetotalrmsattenuationattheeardrumwillbemarginallylowerthanthesecond-lastlowercol-umnofthetable.2.3ProcedureInthisexperiment,subjectsareseatedinfrontofthetwocloselyspacedspeakers(Fig.1).Forthecontrolcondi-tion,thespeakerswerealignedandequidistantfromthelistenersears;forthetestcondition,thespeakersweremisalignedbyadisplacement.Theacousticwaveformattheearsbecomesprogressivelytemporallysmeared(Fig.4andEq.6)andtheharmonicsincreasinglyat-tenuated(Eq.7)asisincreased.Thecontrolsound=0)wasperceivedtohaveasharperorbrightertim-brethanthedisplacedsetting(=0),untilbecametoosmalltomakeadierence.Thegoalwasto“ndthethatcouldbarelybediscriminated.Intheblindtest,thesubjecttriestojudgewhetheranunknownsoundcorrespondstothecontrolordisplacedsettingfordierentvaluesof.Itwasfoundthatsubjectstypicallyneedtolistentothesoundsforafewsecondstoforma 9 FIG.6:Summaryofresultsasafunctionofthetimede-lay(averagedacrossallsubjects).Eachdatapointconsistsof50blindtrials.(a)Thepercentageofcorrectjudgements.(b)Chi-squaredvalue.Thedashedline,correspondingtothecriticalvalueof3.84,intersectsthedatacurvearoundbetweenmultipledrivers)introducetemporalsmearinganddelays.Inthelastcase,thetemporalsmearingcanbeenormous.Forexample,Eq.2indicatesthatadipoleloudspeakerwithasingleelectrostaticpanelofheight=1.5mataspeaker-listenerdistanceof=5m(withthelistenersearathalfspeakerheight)willhaveatem-poralspreadof65ms.Whatthismeansisthateveniftheentirechainhadanotherwiseunlim-itedbandwidth,adelta-function(narrowimpulse)inputsignalwillgetspreadoutovera650slongrectangu-larwindowatthelistenerposition.Thusaloudspeakerthatsubtendsalargeangleatthelistenerpositionmustnecessarilycompromise“delity,perhapsexplainingwhysmallspeakerstendtohaveasubjectivelycleanerandmorecoherentsound(althoughtheymaybede“cientintheirlow-frequencyresponse).Thepresentworkprovidesthebestcurrentquantitativeassessment(s)astowhatextentsuchtemporalerrorsmakeanaudibledierence.Thevastmajorityofpreviouspsychoacousticexperiments(summarizedintheBackgroundsection)thatprobedthisquestion,usedequipmentwhoseownowntemporalresponsemayhavebeenamajorlimitation.Mostofthatresearchusedrathercoarsedigitalsynthesisforthesignalsource,usedampli“cationofinsucientintrinsicresponsespeed,andtransducerswithlimitedbandwidthsthatweredrivenwithinadequatedamping.Thepresentworkusesanana-logchaininwhichthesquare-wavesignalpresentedtothetransducer(includingtheresponseofboththesignalgen-eratorandampli“er)hasrise/falltimesthatareabout100timesfasterthan48-kHzsampling-ratedigitalsyn-thesis.Theunsuallylow(40m)outputimpedancethatsourcesthetransducersprovidesexceptionaldampingandconsequentlyawellcontrolledwaveform,asshowninFig.2.Thetransducersusedinthisworkhaveafarmoreextendedbandwidth(spectrumshowninFig.3)comparedwithtypicaltransducersusedinaudiometry(e.g.,TDH-39headphones).Thusbyliftingsomeoftheequipmentbottlenecks,itwaspossibletodemonstrateamuchshorterthresholdfordiscerningtemporalerrors,thanhasbeenachievedbefore(asnotedintheIntro-ductionsection,thede“nedinreference[31]corre-spondstoaninterpulsedelayof=10+thatismuchlongerthanthethresholdobtainedhere).Thisnewlowerthresholdshouldbetakenintoaccountinthedesignandsetupofaudiocomponentsifthehighesttransparencyistobeachieved.Whilethepresentdemonstrationofdiscriminabilityatthemicrosecondtimescaleusedsimple(square-waveform)high-bandwidthsignals,realisticmusicalsoundsalsocarrycontentinthistemporal-spectralrange.Measurementsofspectraofvariousmusicalinstrumentsshowthattheseextendintotheultrasonicrange[38]andevenbeyond100kHz[39].Inthetimedomain,ithasbeendemonstratedthatseveralinstruments(xylophone,trumpet,snaredrum,andcymbals)haveextremelysteeponsetssuchthattheirfullsignallevels,exceeding120dBSPL,areattainedinunder10s[2,38].Besidesultrasonicspectralcontentandmicrosecond-rangeonsetdurations,athirdaspectofmusicalsoundthatdemandsfasttemporalresolutionisthereverberation.Atransientsoundproducesacascadeofre”ectionswhosefrequencyofincidenceuponalistenergrowswiththesquareoftime;therateofarrivalofthesere”ectionsdN/dtistheroomvolume)approachesonceeverysafteronesecondfora2500mroom[2].Henceanaccuracyofreproductioninthemicrosecondrangeisnecessarytopreservetheoriginalacousticenvironmentsreverberation.Thepresentexperimentalresultthuspro-videsaconcretebasisfortheanecdotalclaimsbyau-diophilesofsensitivitytoveryshorttime-domainerrors(suchasaninsuciencyofsomecommonlyuseddigitalsamplingrates)asdiscussedintheIntroductionsection.Whiletheneurophysiologicalbasisthatunderliestheob-servedfasttemporaldiscriminabilityisnotofprimaryinterestforsoundreproduction,thepresentresultnever-thelessdoesshedsomelightonthisissue.Thestartingpointofanyhearingsensationinvolvesexcitationoftheinnerhaircellsinthecochlea.Threedeterminantscanchangetheperceptofasound.Oneisachangeinthestimulusfrequency/ies,whichchangestheCFsandloca-tionsalongthebasilarmembranewhereIHCsaremax-imallyexcited.Anotherisachangeinloudness,whichchangesthedegreeofexcitationandthewidthofthe 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