WHAT IS QWord文档下载推荐.docx
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WHAT IS QWord文档下载推荐.docx
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WhatisQ
byDickPiercefromapostingofhisonthenewsgrouprec.audio.tech,14Nov1995
compiledbysanford1@
"
Q"
isoneofthosedimensionlessnumbersthatcausesnosmallamountofconsternationamongstthosewhodon'
t
understandwhatitmeans,aswellasamongthosethatTHINKtheydo!
However,theprinciplebehindQ,whenusedinthecontextofloudspeakers,isVERYsimple.
1)Itissimplytheratiobetweenenergystoringandenergydissipativemechanismsatresonance.
2)Inelectricalterms,itistheratioofthereactancetotheresistance.
AhighQindicatesthatfortheamountofenergystoredinaresonantsystem,themechanismsthatdissipatethatenergy
aresmall.Soahigh-Qsystemwilltendtohavearesonancethatdecaysslowly,becausetheamountofresistance
availabletodissipatetheenergyissmallcomparedtotheamountofenergystored.
Alow-Qsystemwilltendtodampentheresonantmotionquickly,becausetheenergyisdissipatedquicklyandremovedfromtheresonantsystem.
Thereareprimarily2energydissipatingmechanismsavailableinaloudspeakerdriver:
mechanicalandelectrical(thereis
another,acoustical,butitisVERYsmallwhencomparedtotheothermechanisms).
Themechanicaldissipativemechanismsareprimarilythefrictionallossesinthedriver'
ssuspension,and,toalesserextent,acousticabsorption.
Thereare,essentially,twoelectricalmechanismsforenergydissipation:
theDCresistancetothevoicecoilandtheoutputresistanceoftheamplifier.Inalmostallcases,theDCresistanceofthevoicecoilcompletelydominates.
Thesetwomechanisms,mechanicalandelectrical,determine,respectively,themechanicalQ(Qms)andtheelectricalQ
(Qes)oftheloudspeakerdriver.TheirparallelcombinationdeterminesthetotalQ(Qts)oftheloudspeakerdriver."
Amen.
WhenwemountaloudspeakerdriverontoabafflesystemwealsohavetotakeintoaccounttheQofthebafflesystem
toarriveatthetotalsystemQ.ToworkoutthetotalQofthedriverandbafflesystemyousimplymultiplythebaffle
systemQwiththetotalQoftheloudspeakerdriver.
Closedboxesstoreenergythatinteractswiththeloudspeakerdriverincomplexways,especiallyinventedenclosures.Boxesthemselvesalsohaveresonances.
Normallyahigh-Qclosedboxiscombinedwithlow-QloudspeakerdrivertogiveadesirabletotalsystemQ.
Butwhenwemountaloudspeakerdriveronanopenbafflethissituationisreversed.Anopenbafflestoresnoenergyandhasalow-Qof0.2andCarverchosetouseahigh-QwooferwithatotalQof3+toarriveatadesirabletotalsystemQ.
Carver'
shigh-Qwooferwasalsochosenforanothergoodreasontodowithmountingawooferonanopenbaffle.
Aswedecreaseinfrequencyorincreaseinwavelength,thesysteminitiallybehavesasaninfinitebaffle.Whenthewavelengths
arelongenoughtobeaquarterofthebaffledimensions,thewavesbegintocanceleachotheraroundtheedgesofthe
dipolebaffle.
Thewavetravelsouttotheedge(1/4)andbacktotheoppositesideofthevibratingspeakercone,whereitisexactlyoutofphaseandcancelsout.
Quarterwavecancellationonanopenbaffleisafirstorderphenomenon-theroll-offoccursat6dBperoctave.Whenwereachthefree-airresonancepointofthehigh-Qwooferweaddtothisthesecond-ordersub-resonancefall-offofthehigh-Qwoofertoendupwithathird-orderor18dbperoctavefall-offbelowthefree-airresonancepointofthehigh-Qwoofer(thismakesagoodrumblefilterintheCarvercase).
Inloudspeakerliteraturewecanlookatthefamilyofcurvesforthefrequencyresponseofaloudspeakerdriveronan
infinitebaffleaswedecreasethefrequency.
Startingwiththerolled-offcurve
whenQ=0.5(criticallydamped),thenthe
ButterworthgraphwithQ=0.71(maximallyflat),thena
littlerippleatQ=1,thenclearlya
bumped-upgraphatQ=1.4.
未命名1.jpg
WhentheQishigherthananyyoucanusuallyfindinloudspeakerdrivercataloguesyoustarttogetboostingabovethe
resonancepointoftheloudspeakerdriver,andasufficientlyhigh-Qwillresultinaslopeofabout6dBperoctaveabove
thefree-airresonancepointoftheloudspeakerdriver.
Thisincreaseof6dBperoctaveofthehigh-Qloudspeakerdrivercanbeusedtocounteractthe6dBperoctavequarterwavecancellationtogiveaflatfrequencyresponserightdowntothefree-airresonantfrequencyoftheloudspeakerdriver.ThisisamuchmoreelegantsolutiontotheproblemofquarterwavecancellationonanopenbaffletothatusedbyCelestion,etcofusingaconventionallow-Qwooferwithelectronic
equalizationsinceitdoesnotinvolveadditionalamplifierpowerandthenecessityofelectronicequalizationequipment.
High-Qwoofersarerelativelyeasytodesign/make.Bothwaysofovercomingquarterwavecancellationonabaffleentail
theuseoflongthrowwoofersforthesafeoperationofthewoofer.
TDA-7294
2010-8-0120:
21:
57
再来一段:
二阶低通的
SecondOrderPassiveResonantFilters
lcr_circuits_2nd_order_low_pass_filter.gif
Figure2-BasicLCRsecondorderlowpassresonantfilter.
NowAVcontainstwofrequencydependantimpedanceswhichmeansthatitisinverselyproportionaltoω2
AV=ZC/(R+ZL+ZC)
Equation3LCRSecondOrderLowPass
Givinganabsolutevalueof:
|AV|=1/√((1-ω2LC)2+(ωRC)2)
Equation4LCRSecondOrderLowPass
I'
llletyoudothesubstitutionshere,rememberaswiththefirstorderfiltersyouneedtotreateachphaseindividuallytogetatrueabsolutevalue,drawaphasediagramifyouthinkitwillhelpyou.YoushouldalsoconsiderwhatAVwouldlooklikeifVoutweretakenacrosstheresistorinductorcombinationinstead.
Sincethegainisnowinverselyproportionaltothesquareofthefrequencytheroll-offofthefilterwillbetwiceassteepasthefirstorderfilter(rememberwe'
reworkinginthelogdomainwhenmeasuringthegainvsfrequency)makingitmoreprecise,thisisshowninfigure3.Sowhereasthefirstorderfilterdropsby6dBeverytimethefrequencydoublesthesecondorderfilterdropsby12dB.Infactforannthorderfiltertherolloffwillben*6dBperoctave.
lcr_circuits_2nd_order_low_pass_respopnse.gif
Figure3-Frequencyresponseofasecondorderlowpassfilterwithf0=5kHz.(clicktoenlarge)
ResonantFrequency
Theresonantfrequency(ω0orf0)isthepointatwhichtheimpedanceofthecapacitorandinductorareequal,itisatthispointthatthebreakhappensinthegainofthefilter(seefigure3).ForasimpleRLCcirucititisdefinedas:
ω0=2πf0=1/√(LC)
Equation5ResonantfrequencyofanLCRcircuit
YoucanverifythisyourselfbymakingZC=ZLandre-arrangingtosolveforω.
QFactor
TheQfactorisdeterminedbytheresistorinserieswiththeinductorandcapacitor,itdescribesthesharpnessoftheresonancepeakorkneethatthefilterwillhave.Theeffectthatthishasisshowninfigure4,notehowasQincreasesthepeakbecomesgreater.Thiscouldcauseproblemsifyoudon'
twanttoamplifyyoursignal.
Qiscalculatedusingthefollowingformula:
Q=ω0L/R=1/ω0CR
Equation6QfactorforanLCRcircuit.
lcr_circuits_effect_of_q.gif
Figure4-EffectsofQonthe'
knee'
ofthefilterfrequencyresponse.
Phase
Thephaseoftheoutputdiffersfromthatoftheinputby:
&
angAV=tan-1(ωRC/ω2LC-1)
Equation7PhasedifferencebetweeninputandoutputofanLCRcircuit.
37:
56
再来两张图:
贝塞尔滤波器、巴特沃思滤波器、切比雪夫滤波器的特性。
可以看出他们的Q值是不一样的。
F_3_11.gif
F_3_13.gif
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