103002_P24_26_Microphones:FEATURE/TECHNICAL PAGE 22/5/09 08:45 Page 24
microphones
KeepingMicrophonesQuiet
Microphonesarewonderfulat
UnavoidablePhysics things.Everyspringsystemhasanatural
pickingupsound–butalsoall
Aswithsomanyaudiomatters,the resonantfrequency,andatthepointof
behaviourofsuspensionsisextremely resonanceastimuluswillbeaccentuated
sortsofnoisesthatwewould
dependentonfrequency.Handlingnoise, ratherthandamped.Thisfundamental
rathernothear.ChrisWoolf
thegeneraltermwhichcanusefullyinclude frequencywillalsohaveharmonicswhich
MIBSdescribeshowexisting
actual‘handling’aswellasnoise willrepeatthesamebehaviourathigher
transmittedthroughpolesandstands, frequencies,thoughincreasinglymore
andnewmicrophone tendstohaveaspectrumthatistilted damped.Sinceresonanceisrarelyidentical
suspensionsworkand,as
heavilytowardsLF.Asthefrequency oneachaxis,microphonescanfrequently
buckandyawrathervigorouslynearthese
usual,givesustheevidence.
increasessoitbecomesprogressively
hardertomovethephysicalmassofa particularfrequencies.However,above
microphonebody;thushandlingnoise roughlythreetimestheresonantfrequency
M
icrophonesneedtobeheard–it rarelyhasmuchcontentaboveafew
thefairground-ridebehaviourcalmsdown
istheirraison d’etre –butthey hundredHertzunlessthemicrophoneis
anditispossibletomakeasuspension
shouldbetranslatorsofsound verysmallandlight,suchasasub-miniature
isolatewithreasonableefficiency(see
ratherthanoriginators.Theiroutputshould personaltype.Howeverthelow
Figure1).
onlybethesoundthatentersfromoutside frequenciesarealsothehardestonesto
throughthegrille.Someself-noiseis eradicate.
inherentineverymicrophone;thejostling Anisolatingsuspensioncanbeanalysed
ofairmoleculesagainstthediaphragm,or asatypicalspringsystem–mass,
thethermalnoiseintheelectronicsofan complianceanddamping.Don’tworry,this
impedanceconverter.Howeverthese isn’tgoingtobecomeaphysicslesson,but
contributions–usuallyjustacontinuous thepointisthattherulesarefixedand
blandhiss–willbeverylowcomparedto defineexactlywhatwillhappeninany
wantedsound.Whatirritateslistenersfar particularset-up.Oneofthemostawkward
moreisthemuchhigherlevelsoferratic, oftheseisthat,forafixedmassof
impulsivenoisethatareoftenmechanically microphone,thelowerthefrequencyof
transmittedtothemicrophone. thehandlingnoisethefurtherasuspension
Microphones,aseveryoneknows, hastobeabletowobbleinordertogivea Figure 1: Typical isolation mount
detectsoundbysensingthemovementofa particulardegreeofisolation.
behaviour
lightdiaphragmrelativetothefixed, Thismeansthatforlowfrequenciesthe
massivereferenceofabackplateora idealsuspensionshouldallowthe
Thustheresonancepointisimportant
magnet.Howeverifthediaphragmisheld microphonetomovemanycentimetresand
indefininghowasuspensionfunctions,and
stillbyinertialforces(mostlythoseofthe beabletobringitbackgentlytoitsoriginal
needstobeaslowaspossible.Ideallyit
massofairrestingagainstit)andthe positionwithminimalovershoot(perfectly
shouldbethreetimeslowerthan20Hzto
referencemovesinstead...wehavean damped).Soundsgreat,ineverysense,
allowfulluseoftheaudioband.Withlarge,
accelerometer.Thisbehaviouris exceptthatpracticalhigh-compliance
heavymicrophonesthisisfeasible(though
unavoidable–andnomicrophonecan suspensionscanalsobearecipeforthe
notalwayseasy).Withlightermicrophones
distinguishbetweenthetwofunctions. microphonetowobbleup,downand
itgetsprogressivelymoredifficult–the
Obviouslytheaimshouldbetokeep sidewaysratherwildly.Thatmakesthem
springelementhastobemuchmore
themicrophonebodycompletelyfreefrom verydifficulttocontrol,say,onaboom
complianttomatch–andwiththesmallest,
external,mechanicallytransmittednoise, pole.Fortunately,microphonesarenotas
lightestdesignsbelovedoflocation
butthatisnotsosimple.Themicrophone sensitivetohandlingnoiseoneveryaxis.
recordists,gettingthefundamentaldown
hastobephysicallysupportedinspaceand Theoneaxialtothediaphragm(Z)isfar
below20Hzcantakesomeingenuity.
probablyconnectedtotheoutsideworld moreimportantthantheothertwo–X
Thebiggestproblemiscombiningthis
viaacable–bothrouteswhichinevitably (sideways),andY(upanddown)–soa
highcompliancewithsufficientcontrolto
provideapathwayformechanicalnoise. sophisticatedsuspensioncanexploitthisin
preventthemicrophonesloppingaround
Thecableisfrequentlyignoredalthoughit ordertogivebothgoodisolationandgood
sovigorouslythatitstartstogenerateextra
isahighlysignificantconduit,butthiscan control.
handlingnoise,becomesuncomfortableor
besolvedrelativelysimplybyusingathin,
distractingforartistesunderneath,oreven
flexibletypeofcableandaproperly StrangeThings
crashesintothebasketofawindshield.
anchoreddecouplingloop.Thesupportis Attheverylowestoffrequencies
Beingabletotailorverydifferentdegreesof
rathermoretricky. suspensionsstarttodosomeverystrange
complianceintoeachaxis,andtodampany
24 LINEUPSummer2009
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