Speakers_ Construction and Power

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  • 10/7/2015 Speakers:Constructionandpower

    http://education.lenardaudio.com/en/05_speakers_2.html 1/6

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    ConstructionandpowerTheconstructionofspeakersisapproachedinthesamewayasmusicalinstrumentmaking.Finetolerancesandattentiontodetailmakelargedifferencestoperformance.Largemusicalinstrumentsandspeakerssuitlowfrequenciesandviceversa.Eachspeakerandinstrumentcanonlyfunctionefficientlywithlinearitywithin3octaves(octaveisratio1:2).Theoreticallyasinglespeakerwouldhavetochangediameterfrom(1in24ft)(20mm8m)tomaintainsimilarlevelanddispersionoverthefrequencyspectrum.

    Themajorityconsistofpaperorplasticmouldedintoaconeshape,looselysuspendedinaframesoastoeasilymovebackandforthtovibratetheair.Gluedtothebackoftheconeisacoilofwire(voicecoil)withinastrongmagnetfield.Passingelectricitythroughwirecausesamagneticfieldaroundthewire,whichattractsorrepels,causingtheconetomovebackandforth.Thelargerthemagnetandvoicecoil(BL)thegreaterthepowerandefficiencyifwellmade.Externallyvibratingtheconewillcausethevoicecoiltogenerateelectricity.Aspeakercanworkwellasamicrophoneespeciallyforbassdrums.

    Theenergyofthemagnetisconductedthroughthemildsteelpoleplatesandpolepieceandconcentrated(northsouth)acrossthegap.Hopefullythevoicecoilhasbeenperfectlycentredinthegap.Theclearancesareveryverysmall,lessthanhalfabeesdick.Thesmallerthegapthemoreintensethemagneticfieldthegreatertheefficiency.Theslightestvariationsinalignmentduringmanufacture,causelargevariationsinperformance.Notwospeakersormusical

    Topic2of4

    SpeakersPrinciples Power Impedance(Z) Cabinets

  • 10/7/2015 Speakers:Constructionandpower

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    instrumentscanbeidentical.

    VoiceCoils.Passingelectricitythroughwirecausesamagneticfieldaroundthewire.Changingpolarityoftheelectriccurrentthroughthewirealsochangesthepolarityofthemagneticfieldcreatedaroundthewire.Theinteractionofthetwomagneticfields,causesthevoicecoiltobepushedoutofthegapforwardorbackward,dependingonthepolarityoftheelectricitythroughthevoicecoil.

    Speakerpolarity.Thestandardtestforaspeaker,istoputabatteryacrossthespeakerterminals.Whenthe+ofabatteryisputontothe+markedterminalofthespeakertheconeshouldmoveout.Thisisalsothecorrecttestforcheckingthepolarityofspeakersinstereosystems.1.5Voltbatteryissafetouse.Warning.Neverusethistestonacompressiondriver.

    Mostvoicecoilsaredoublelayered,woundwithenamelcoatedcopperwirearoundaformerthenbondedwithanepoxycompoundandbakedinanoven.Voicecoilscanalsobewoundwithrectangularaluminiumwiretoachievelessweightandgreateraccuracy.Aconespeakerisapprox2%efficientthereforeapprox98%oftheelectricalenergyiswastedasheat.

  • 10/7/2015 Speakers:Constructionandpower

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    VoicecoilLength.Atbassfrequenciesthevoicecoilhastomovebackandforthalongdistance,especiallyathighpower,comparedtothehighfrequencies.Duringmovement,the%ofvoicecoilinthegapmustremainconstant.Thevoicecoilcanbelongandthepoleplatethinorthevoicecoilshortandthepoleplatethicktoachievethesameoutcome.Therearearguedadvantagesanddisadvantagesbothways.Midandhighfrequencyspeakersconesonlymoveasmalldistance,comparedtothelargemovementofbassspeakers.Thevoicecoillengthandpoleplatethicknessaresimilar.

    Midandhighfrequencyspeakersareapprox6dBto10dBmoreefficientthanbassspeakers.Insmallpassivesystemsthemidandhighfrequencyspeakersareattenuatedtomatchthelessefficientwoofer.Inactivesystemsthebassspeakerisdrivenwithhigherpower.Anexampleistwo15inwoofersdrivenwith400Wattstomatcha12inmidspeakerdrivenwith200Watts.

    VoiceCoilDiameter.Onthesamediameterspeakerasmallvoicecoilhaslesscontrolovertheconecomparedtoalargevoicecoil.Withasmallvoicecoiltheconeisabletobemoreresonantcomparedtothesamesizeconewithalargevoicecoil.Somesmallvoicecoilspeakersmayappeartobemoreefficientbutthisextraefficiencymaybeonlyattheoneresonantbassnote.Atfrequenciesabovethisresonantbassnotethespeakermaybelessefficientcomparedtothesameconewithalargervoicecoil.Costandperformanceofspeakersincreasewithvoicecoilsize.

    Thelargerthevoicecoilthebetterthecontroloverthecone,andthereforeimproveddampingandlinearity.Largevoicecoilsareexpensivetomake(limitapprox4in100mm).Assemblingthespeakerisalsomoredifficult,tolerancestakentogreateraccuracy.Thelargerthevoicecoildiameterthelargertheareaofthemagneticgap.Tokeepthefluxdensityofthemagneticenergy(perunitarea)inthegapthesame(forthespeakertoretainthesameefficiency)magnetsizeandthereforemassofthespeakermustbeincreased.Themajoradvantageoflargervoicecoilsisgreaterpowerhandling.

  • 10/7/2015 Speakers:Constructionandpower

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    SquareWave.Itisnotpossibleforaspeakerormusicalinstrumenttoproduceanacousticsquarewave.Asquarewavecanonlyexistinelectronicswhere'physical'massandforcedonotexist.Aspeakerwillaveragetheelectricalenergyofasquarewaveasthefundamentalperiodofasinewave,withdiscordantthirdharmonicresonances.

    Aspeakerrespondsonlytotheresultantenergyofanelectricalsignal,butnottotheseparatecharacteristicsofphasebetweenvoltageandcurrentwithinthatsignal.Alsotounderstandthisdemonstrateshowmuchoftheaudiophileandprofessionalaudiomythsabout'electronictimealignment'aresimplymarketingfabrications.Howeveracousticalwavelengthtimealignmentbetweenthephysicaldistancesofspeakercomponents(wooferandtweeter)isimportantandshouldbetakenintoconsideration.

    Whatgovernsthe3octaverule?

    Soundisawavemotionofairhavinglengthandcorespondingheight,describedasfrequency.Atfrequencieswherewavelengthsareequaltoandbecomelongerthanthespeakerdiameter,decreasinginfrequency,thefrequencyresponseissmooth,anddispersionwidens(directivity).

    Theoreticallytomaintainthesamepower,theconemustmove4timesthedistance,foreachoctavedecrease.Conemass,andreducingairresistanceasthefrequencydecreases,causethis.Thesameasamusicalstringedinstrument(constantvelocity).

    Theupperlimitforhighfrequenciesiswherewavelengthsbecomesmallerthanthespeakerdiameter.Thiscausesnodeandlobedistortionwithinthecone(chaoticresonances).Dispersionnarrowstoabeamandthefrequencyresponsebecomeschaotic,soundingharshandscreechy.

    Thelowerlimitiswherewavelengthsbecome10timesthespeakerdiameter.Conemovementissogreatitbecomesinefficient,nonlinearandgeneratesintermodulation(affectingothernotes).Thefashionforbassspeakerstobesmallcompromisesperformanceregardlessofmarketingclaims.

  • 10/7/2015 Speakers:Constructionandpower

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    Anidealsystemwillhave4speakers,eachcovering23octaves.Eg.(1infortheharmonics).(45inforuppervoice).(810inforlowervoice).(1215inforbass).professionalsystemsusehornsandcompressiondriversforextrapowerandefficiency.

    Speakerpower

    ConeSpeakersareapprox2%efficient.powerandefficiencyisdependantonBL,whichismagnetandvoicecoilsize.Bisthetotalmagneticenergyinthegap.Lislengthofvoicecoilwire.Alsomassandareaofcone,suspensioncomplianceandfrequency.

    Basspower.Themaximumdistancetheconecanmovelimitsbasspower.Tomaintainthesameacousticoutputtheconemustmove4timesthedistanceforeachoctavedecreaseandviceversa.1/4distance,foreachoctaveincrease.Twotimesduetoconemasschangingdirection.Twotimesbecauseairisslippery.Asfrequencydecreasesairslipssidewaysdecreasingresistance.Asfrequencyincreasesairdoesn'thavetimetoslipsideways,increasingresistanceagainstthecone(radiationresistance).

    Atwavelengthsgreaterthan10timesthespeakerdiameter,conemovementissogreatitbecomesnonlinearandinefficient.Somemanufacturersmakethesuspensiontighttolimitconemovement,compromisingbassperformanceforahigherpowerrating.

    Highfrequencypowerislimitedbyexcessiveheatthatcandestroythevoicecoil.

    Speakermanufacturershavedifficultyspecifyingpowerhandling.Inmarketingtherehasbeenamoderntrendtosellspeakersasbeingcapableof1,000Wattsplus,similartosellingamotorvehicleasbeingcapableofgoingatthespeedoflight.Atbassfrequenciestheconeexcursionislimitedbythetightnessofthesuspensionandmaybeatmaximumbetween20100Watts.Abovemaximumexcursionthebassnotesbecomecompressed.

  • 10/7/2015 Speakers:Constructionandpower

    http://education.lenardaudio.com/en/05_speakers_2.html 6/6

    EndofTopic2 WrittenbyJohnBurnett SeguirCreated:17Dec2008

    Conemovementcanbesogreatthatatthepointwherethevoicecoilandconearejoineditcanhitthepoleplate,makingaloudcrackingsoundandthereforewarning.Thevoicecoilmaybecapableof50200Wattsbeforeitburnsout(dependingontime).Atmidandhighfrequencies,coneexcursiondoesnotlimitpowerhandling.Thevoicecoilwillkeepgettinghotteruntilitburnsup,andstopabruptlywithoutwarning.Sometimesexcessiveheatwillcausethevoicecoiltobecomeloosefromitsformer.Thiswillbeheardasanannoyingmechanicalrattlingsound.

    Othertypesofspeakers

    Domethemajorityoftweetersaredome,similartotheconespeaker.

    Ribbontweetersareexpensive,lessefficient,notcommon,haveexcellentperformance.

    Electrostaticspeakersareexpensiveveryfragileandhaveanichefollowing,excellenttransients.

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    SpeakersPrinciples Power Impedance(Z) Cabinets