B&W electronics 70 User manual

Model70
electronics
B&W
WORTHING. ENGLAND
Tel.Worthing66830
asignificantadvanceon
existingloudspeakersystems

Model70
USERSINSTRUCTIONHANDBOOK
CONTENTS Page
AssemblyInstructionsl
MainsVoltages3
AudioConnections.........4
PositionofSpeakers5
OperationInstructions......6
ServiceandGuarantee7
DesignandDevelopmentStory8
ResponseCurvesandMeasurements...23
SummaryofDetailedSpecification30

-2-
INTRODUCTION.
EachModel70loudspeakersystemcomestoyouintwo
seperatecartons.Thelargercartoncontainsthemain
cabinetandanaccessorybagcontainingplugs,sparefuse
andfixingscrews.Themaincabinetiscompletewithleg
assemblyandelectrostaticpowersupply/filterunit,both
ofwhicharefixedinposition.Thesmallercartoncontains
the701electrostaticunitwhichisreadyforinsertionin
thesocketsprovidedonthetopsurfaceofthemaincabinet.
WARNING.
ITISIMPORTANTTHATTHEASSEMBLYANDINSTALLATION
INSTRUCTIONSBECAREFULLYREADBEFORECONNECTING
THISSYSTEMTOTHEMAINSORAUDIOSUPPLY.
ASSEMBLYINSTRUCTIONS.
Unpackthelargerofthetwocartonsandremovethe
accessorybagfromtheundersideofthemaincabinet.Unpack
theelectrostaticunitfromthesmallercarton.Wesuggest
youretainthespecialPolystyrenepackfromthesmaller
cartonforpossiblefuturetransportationoftheelectrostatic
unit.
Theelectrostaticunitshouldnowbeinsertedinthe
socketsprovidedonthetopsurfaceofthemaincabinet.
Spigotsonbothfeetofthe701electrostaticunitlocatein
thecorrespondingsocketsandtheycanonlybeassembledin
thecorrectmanner.
Havinginsertedtheelectrostaticunitmakesecureby
screwingthroughtheholeprovidedintherearofeachfoot
withthespecialinstrument-headwoodscrewsandcupsincluded
intheaccessorypack.Thesystemisnowready,afterchecking
mainssupplyvoltage,forinstallation.
UNITBEREMOVEDWITHMAINSSUPPLYCONNECTEDORWHILST
THESYSTEMISINOPERATION.

-3-
INSTALLATIONINSTRUCTIONS
MAINS.
BeforeattemptingtoconnectthemainssupplytoyourModel70
pleasecheckyoursupplyvoltageandseethatthisagreeswith
theadjustmentmadetothesystembeforeleavingourfactory.
Asmallredlabelisattachedtothepowersupplypanel
(rearmaincabinet)givingthisindication
AdjustedforEuropean( 200- 240volts§0HzJAC.
AdjustedforAmerican( 110-125volts60Hz.)AC
DuetothegoodregulationoftheEHTpowersupplyadjustment
ofmainsvoltagewillnotnormallybenecessary.Allsystems
aresetbeforeleavingthefactoryforthemarketconcernedas
shownabove.

-4-
(AssemblyInstructions_-_Mains_-__contd.)_ '-
Shouldmainsvoltageadjustmentbenecessarythisiscarried
outbyremovingthepowersupplyandrepositioningonewire
onthemainstransformerterminationboard.Thisworkshould
becarriedoutbyyourdealerwhoisprovidedwiththenecessary
instructions.
Model70requiresconnectiontoanalternatingcurrentmains
supplywiththespecialthreepinplugprovidedwithyour
accessories.Undernormaloperatingconditionsthecurrent
drainisnegligible,theloadbeingonlysome10watts
(onetenththatofa normalreadinglamp)andprovidedthree
corecableisusedformainsconnectionthecorediameteris
innowaycritical.Themainscableconnectionstothethree
pinplugareillustratedinFig.3 below.
Anoutermetalscreenisbuiltintotheelectrostaticunit
forsafetypurposes.Itisimportantthereforetoensure
thatthreecorecableisusedtogetherwitha threepinplug
toyourmainssocket.
AUDIO
Theoutputfromyouramplifierisconnectedviathespecial
2pinnon-reversibleplugprovidedtothesocketmarked
1INPUT' onthepowerunitrearplate.
Thecable(twinortwocore)useafortheaudioleadshould
havelowresistanceandifmorethansaytenfeetofcable
isrequiredforthisleadwewouldrecommendaninnerconductor
of7/029orsimilar.
Whentwoloudspeakersystemsareoperatingforstereo
reproductionitisimportantthattheaudioconnectionsto
the'INPUT'socketarecorrectlyphased.Thered(or
positive)terminalofyouramplifiershouldbeconnectedto
pinoneoftheinputplugonyourloudspeaker.

-5-
AssemblyInstructions- Audio- contd.:-
Ifphasingiscorrectthisisevidentbya relativelysmall
centralimagewhenthelistenerisseatedona centreline
betweentheloudspeakersandbothloudspeakersarefedfrom
amonophonicsignal.Ifphasingisincorrectthesound
imagewillbemuchbroaderandlesswelldefined.A reversal
oftheamplifierorloudspeakerconnectionsfromonechannel
willconfirmthecorrectnessofphasingandillustratethis
point.
POSITIONINGOFLOUDSPEAKERSINLISTENINGROOM.
DuetothewidehorizontaldispersionoftheModel70system
loudspeakerplacingtoobtainanevenstereoimageisnotas
criticalaswithmanyconventionalsystems.Itwillalso
befoundthatthestereoimageispreservedovera wide
listeningangle.Thesuggestedplacingfora pairof
Model70reproducersisillustratedinFig.4 below.
Fig.4.
Itisimportanttorealisethatthebassresponseiscontrolled
underlisteningconditionsbytheroomdimensionsandthe
position(relativetowallsorcornerplacing)"whichthe
loudspeakeroccupies.Thecloserthesystemisplacedto
awallorcornerthegreaterwillbethecouplingtothe
room.Asa generalrulethoseroomswithsimilardimensions
(i.e.a squareroom) arethemostdifficultfromthe
viewpointofobtainingextendedresonantfreebassresponse.
Ifyourlisteningconditionsaresuchthatthebassresponse
isexcessivewithina narrowbandoffrequenciesitmaybe

-6-
itmaybefounddesirabletowithdrawtheloudspeaker
fromthecornerorwallbysometwelvetotwenty-four
inches.
OPERATION,
Attherearoftheelectrostaticunitwillbefounda
removableabsorbentpad,thefunctionofwhichistoreduce
rearradiationinlocationswheretheloudspeakeris
placedclosetoa wallorcorner.Thispadprovidesa
usefulmethodofvaryingthedistributionpatternofthe
electrostaticunitabove400Hz.(lowermid.freguencies)
tosuitbothambientconditionsandtheuser'spersonal
preferanceintermsoftheratioofdirecttoreflectedsound.
Thisabsorbentpadcanbereadilywithdrawnwiththefingers
fromitsrecessintherearofthe701electrostaticmount.
Withtherearpadwithdrawngreaterambiencewillbeexperienced
and,dependingonthereflectivenatureofthewallsand
furnishingsbehindtheloudspeakertherewillbea slight
increaseinsoundlevelabove400Hzwiththeelectrostatic
operatinginthiscondition.Ifitisdesiredtooperate
Model70atveryhighvolumelevelsorwherehighpowered
continoussinewaveinputsareappliedtherearabsorbentpad
shouldbeplacedinposition.
ItwillbeobservedthatontherearpanelofthePowerUnit
thereisa neonindicator.Thisshowsthatmainssupplyis
functioning.Iftheneondoesnotlightupwhenmainsis
connectedthefuseshouldbechecked.A sparefuseisprovided
butrepeatedreplacementshouldnotbemadeandifthis
failsyoushouldconsultyourdealer.
ANCILLARYEQUIPMENT.
Model70probablyaddslessintermsofcolourationand
distortionthananyotherloudspeakersysteminexistance.
Itwillbeanalyticalofthesignalwhichitreceivesand
thereforebenefitsfromusewithancillaryequipmentof
thehighestavailablequality.

from
electronics
manufacturersof
electronic
andacoustical
equipment
THEDESIGN& DEVELOPMENTOF
THEMODEL70
MODEL70,thefirstofourloudspeakersystemswhereall
unitsaredesignedandbuiltwithinourownfactory,represents
whatwebelievetobea successfulattempttoproducea
loudspeakersystemfortheperfectionistwhichisa worthwhile
advanceonexistingsystems.
INTRODUCTION
IncontemplatingthedesignandmanufactureofModel70
certainguidelineswerelaiddownsometwoyearsagoatthe
commencementofthedesignprogramme:-
1)Newthinkingontheacousticaldistributionpattern,
especiallywithinthevitallyimportantmidfrequency
regionof400Hzto5 kHz,aimingtowardsexploring
theadvantagesofa distributionpatternapproximating
tospherical.
2)A midfrequencyunitcapableofhandlinginputpowers
intheregionof25wattsr.m.s.withdistortion
factorstotheorderof0.5%orlower.
3)A reallylowdistortionbassradiatorcapableoftruly
complementingaspects1)and2)abovewithoutoccupying
anundulylargeenclosurevolume.
4)Explorationofthelisteningadvantagesbyincreasing
reverberantto directsound.
5)Allunitsbedesignedandbuiltwithinourownfactory
toensuremaximumqualitycontrolandretentionofthe
exclusiveoriginaldesign.
6)Stylingandgeneralappearancetobeacceptableina
widerangeofdomesticfurnishings.

-9-
ItwasfurtherdecidedthattheconceptofModel70should
beoneofaninternationalproductand,whilstparagraphtwo
detailinga highpowerhandlingcapacitywasconsistentwith
Worldmarkets,certainotherdesignandconstructionalfeatures
werelaiddownasbeingessential:-
a)Thatthesystembeina modularformallowing
easyassemblybyoverseasagents.
b)Thatthebasschamberloading,consistentalways
withtheultimateperformance,berelatively
simpleandcapableofbeingaccuratelycopiedin
distantmarketswhereitisobviouslyuneconomical
toshipcompletesystems.
c)Asanextensionof2)thatallunitsbeeasily
removabletofacilitatespeedyservicinginoverseas
marketswheretechnicalservicefacilitiesare
limited.
DESIGNPERAMETERS
Ithasbeengenerallyacceptedforreasonstoowellknown
toberepeatedthat,ifa loudspeakersystemisaimingat
optimumperformance,thefrequencyspectrumshouldbesplit.
Theprecisedivisionoffrequenciestobehandledbythevarious
unitsandreasonsforthecrossoverfrequencychosenwillbe
discussedlaterinthisrelease,butinitiallyletussurveythe
alternativesdealingfirstwithfrequenciesbelow400Hz.
Thealternativesareelectrostaticormovingcoilunitsand
itwasdecidedatanearlystageinthedesignthatthelatter
methodshowedobviousadvantages.Forexample,onanexisting
fullrangeelectrostaticloudspeakerverycarefulmeasurements
weremadeandtotalharmonicdistortionat60Hzwith15watt
r.m.s.inputwasintheregionof50%.Onanalternativefull
rangeelectrostaticAmericandesignwhichwassampled,the
dimensionsofthisunitweresuchastoexcludeitsuseinthe
majorityofhomeinstallations.Whilsta veryselectedfewmay
acceptpanelsintheirloungesomesevenfeethighandthree
feetWideitwasfeltthatonappearancealonethepoints
discussedintheintroductionwereinnowaycateredfor.
Asa resultoflongdiscussionsbetweenthewriterand
Mr.DennisWarditwasfeltthatthemovingcoilsystemshouldbe

-10-
capableofmeetingtheinitialdesignrequirementssetout
inparagraphthreeabovewithanenclosuresizeofsome2-3
cubicfeet.
Insofarasthemidandupperfrequencieswereconcerned
thechoiceofunitswasthreefold- Movingcoil,Electrostatic
orIonic.ItwillbenotedthattheIonicunitiscurrently
usedinourP2monitor.
AtanearlystageindevelopmentnewalternativeIonic
unitsweremanufacturedinprototypeformwitha viewto
increasingpowerhandlingcapacityandextendingthefrequency
responseat leasttwooctavesbelowthatprovidedby the
existingFaneunit.Whilstcertainprogresswasmade,both
thephysicaldimensionsofthehornloadingandtheR.F.
oscillatorpowerrequirementsshowedthat,althoughimprovement
intheIonicunitwaspossible,thedisadvantageswerethoseof
physicalsizeandadequateR.F.screening.Forinstance,one
prototypehada hornlengthofsometwofeetandanR.F.
oscillatorof70watts,a totallyunmanageablequalitybothin
termsofsizeandadequatescreeningtoprovidefreedomfrom
T.V.I.ForthesereasonstheIonicmethodwasdismissedand
attentionfocusedonElectrostaticandMovingcoilunits.
Althoughconsiderableadvancehasbeenmadeinthedesign
anddevelopmentofMovingcoilunitstheydonot,inthewriter's
opinion,approachtheElectrostatictransducerinsofaraslow
distortion,transientbehaviourorflexibilityofbroadpolar
distribution.
EvaluationscarriedoutbyMr.Harwoodona somewhatsimilar
'stateoftheart'projectand,forthoseinterested,reference
maybemadetothreearticlespublishedin"WirelessWorld"in
1968whereitwasshownthatevenofferinganidealisedMoving
coilunittoa slotradiator,thepolarcharacteristicwasnot
wellmaintainedabove3 kHz.Itwasdecided,therefore,to
explorethepossibilityofmakinganelectrostaticunit,working
fromfirstprincipleswitha viewtocomparativemeasurements
betweentheElectrostaticandMovingcoilunitswhichcouldbe
developed.
Somesixmonthsafterthedesignprogrammewasoriginated
aseriesofprototypeunitswereproducedwhichenableda prime

-11-
decisiontobetakenthattheElectrostatictransducer,subject
toperformancebeingthefirstconsiderationandpricesecondary,
wasthebestmethodtoapproachthefinaldesign.The
followingconditionsonthebasicdesignwere,therefore,
decidedupon:-
1)Thatthebottomfouroctavesshouldbedealtwith
byMovingcoilunitandthata development
programmebelaunchedtomeetthestringent
requirementslaiddown.
2)ThatanElectrostatictransducerbedeveloped
forfrequenciesabove400Hzwiththepossible
additionofa "supertweeter",shouldthisbe
foundtobenecessary.Thatwithinthisbasic
designconsiderableworkwouldbeneededin
termsofimproveddistributionpatternand
increasedpowerhandlingcapacityonElectrostatic
units.Existingsampleswhichweremeasured
showedthistobetwooftheirweakerfeatures.
DETAILEDDISCUSSIONONBASICUNITSFORMINGTHE
COMPLETEDMODEL70SYSTEM
MidHighFrequencyElectrostaticUnit.
Itisnottheintentionofthisreleasetoprovide
productiontechniquesorinformationwhichhavebeenexpensive
bothintermsoftimeandeffortandwhichhavemadeit
possibleforustosuccessfullymanufactureanElectrostatic
unit.Sufficienttosaythatatanearlystagefullreference
wasmadetoallexistingpatentsonElectrostaticunitsasit
wasinnowayourintentiontoinfringetheseandtoproduce
newmaterialonwhichweourselvescouldlaypatentclaims.
Theproblemswhichweovercameconcernedsuchmattersas
diaphragmstability,methodsofdiaphragmcoating,A.C.
protectiveinsulationasbetweenfixedplates,mechanical
stabilityandmanyotherfactorswhich,totheuninitiated,may
seemsimplebutformeda majorpartoftheconsiderable
developmentexpenseinwhichwehavebeeninvolved.

-12-
FIG2
Earlyexperimental
electrostaticunit
comprising6 separate
modules
Fig2 illustratedaboveshowsoneoftheearly
experimentalElectrostaticunitswhichwemade.This
assembly,consistingofsomesixmodules,wastheprime
factorindecidingtoproceedwiththeElectrostatic
transducer.Toneburstoscillogramsandfreedomfrom
harmonicdistortionwerequitesuperlative.Itsprincipal
limitationswere,however,threefold:-
1)Highdiaphragmresonance
2)Poorpolardistribution
3)Lowacousticaloutput
EarlyprototypesoftheElectrostaticunitconsistedof
variousassembliescomprisingfromfourtosixteenofthe
experimentalplatesillustratedwerearrangedinevery
imaginablecombination.Asthemajorityoftheseunitswere
individuallyproducedthispartofthedevelopmenttooksome
twomonths.Whilstyieldingmuchusefulinformationit
pointedtotheinescapabletruththattheElectrostatic
transducer,behavingasitdoesina truepistonfashion,must
offera curvedsurfaceiffreedomfromseriousphasingproblems
withinthemidfrequency.region-istobeavoided.
Theoutcomeofalltheforegoingworkwasthedecisionto
developanElectrostaticunit
a)ofadequateareatoprovidethe
acousticaloutputrequired
b)witha continuouslycurvedsurfaceto
providethesmoothhorizontaldistribution
required,makingtheverticaldimensions
relativetolengthsufficientlysmallto
approximatetoa livesource.

-13-
Handmadeprototypeswereconstructedofvarious
dimensionsandtheinitialconceptwasthatthisunitshould
bemountedvertically.Itwasfoundhowever,thatwhilst
excellentresultscouldbeobtained,thepracticaldimensions
ofsucha unitmadeittotallyimpossibletoaccommodate.
Forinstance,a verticalcurvedunitoneinchinwidthwould
becomeverydirectionalat10kHzandwouldrequirea length
ofsomeeightfeettoprovidea similarradiatingareatothat
ofthefinal701assembly.
Parallelwiththeworkwhichwasbeingcarriedouton
shape,sizeandconfigurationoftheElectrostaticunitthe
possibilityofprovidinga curvedfree-standinghorizontal
radiatorbecameanincreasinglyobvioussolution.Figs.3 and
4belowillustratethefinalElectrostaticassembly.
FIG3
Model701
Electrostaticunit
frontview
FIG4
Model701
Electrostaticunit
withWtM cover
removed
Thepolardiagrams,illustratedinFigs.5 and6 indicate
thewideandextremelyevenpolarresponseachievedfromthe
final701assembly.Evenat15kHz,overan80deg.arc- 2 db.
ismaintained.Allmeasurementstakeninouranechoicchamber
withallB & K equipment,usingtype4133halfinchcalibration
microphone

•14-
FIG5
Generalpolar
diagramofModel70
illustratingwide
andevenhorizontal
distribution
FIG6
Horizontal
distribution
Model70at15kHz

-15-
Withthe701andanexperimentalbassradiatoritwas
immediatelyobviousthatthestabilityofthestereoimagewas
improvedbeyondanyothersystemwehadtested.Therewasan
excellentabsenceof'source*insofarastheloudspeakers
wereconcerned.IfreferenceismadetotheHasseffectit
willbenotedthatoneoftheprimecriterialaiddownfor
stabilityofstereoimageisthatanin-phaseequalamplitude
signalbereceivedfroma widefrontalarcina horizontal
planewithparticularreferencetothemidfrequencyregion.
Considerableworkwascarriedoutonvariousdegreesof
curvaturebeforedesignwasfinalised,butasmuchofthis
workconcernsproductiontechniquesitwillnotbediscussed
indetail.
Hunt("Electroacoustics"HarvardUniversityPress)has
beenthestartingpointformostworkersontheElectrostatic
transducersandthisisanopportunepointinthedevelopment
storytomentiontheacousticalload.Attheendofthis
detailedtreatmentHuntprovedmathmaticallythattheambient
airprovided85%oftheloadonthediaphragmandthisfact
wasconfirmedemphericallythroughoutthedevelopment.We
instancetheoriginalXPprototypewhichshoweda dropin
sensitivityof3 db.whenmountedona smallJigtestingbaffle.
Onthecurrent701assemblyanappreciablechangeinwhite
noisecanbedetectedassoonasitisloweredtotopofthe
cabinet.Thiseffectissomewhatfrequencydiscriminating
due,presumably,toanotherfactorcontainedintheoriginal
workbyHunt.Thefactthatatfrequenciesabovesay10kHz
diaphragmmasstendstobecomea dominantfactor.
Toprovethelatterpointoneonlyhastoadd
supplementarydiaphragms.Inthecaseofthe701E.L.S.one
supplementarycoverreducedtheoutputabove9 kHzappreciably.
Acolleaguewhowasworkingwiththewriteratthisstage
ofthedevelopmentremarked,"Itwouldseemthattheideal
mountingforanelectrostaticunitissuspensioninspace."
Thiswouldappeartobea fairsubmissionoffactbutonewhich
inpracticehastobesomewhatmodified.Itwillbenoted,
however,thatthemountingforthe701assemblyisfree-standing
withaslittlesurroundingenclosureaspossible.

-16-
Thewriterhasanobjectiontothereproductionofsoloists
appearingsometwelvetoeighteeninchesfromthefloorandfor
thisreasontheE.L.S.unitonModel70islocatedapproximately
atearlevelwhenseated.
Thewholeprojectofdevelopingtheelectrostaticunithas
provedsoextensivethatitwouldrequirea workofmuchgreater
scopethanthisshortarticletodealwithevena reasonable
proportionofthesubjectmatter.Thewriterdoes,however,
feelsomesatisfactioninsettingoutinprintcertainbasic
principleswhichfewothermanufacturershaveattemptedto
discuss.
CROSSOVER& FILTERNETWORK
ThecrossoverandfilternetworkoftheModel70issplit
intotwosectors.TheseareillustratedbyFigs.7-10below.
FIG7
Model70power
assemblyunit
E.L.S.matching
and.filter
network,removed
fromcabinet
and'mated
with701
FIG8
Model70power
supplyunitas
above,topview

-17-
FIG9
Model70Bass
Radiatorlowpass
section
FIG10
Undersideof
Model70power
supplyshowing
E.L.S.matching
transformerand
variouscomponent
parts.
Beforediscussingalldesignandelectricalcharacteristics
oftheseunitsitmaybeofinteresttonotefromFigs.7-10
howstraycapacitieshavebeenreducedtoa minimumbycareful
designandpositioningofunits.Thisfactorplaysan
importantpartinsensitivityand"topoctave"performanceof
electrostaticunits.
Apoorcrossoverunitorincorrectlychosencrossover
frequenciescanmaranotherwisegoodloudspeakersystem.The
writerhasalwaysfeltthatsomewhereinthe400Hztothe600Hz

-18-
regionwasthecorrectoneintermsofminimalintermodulation
distortionwithintheallimportantmidfrequencyband.Due
tothesuccessfulcompletionofthe701design,a crossover
frequencyof500Hzwaspossibleandprovedsuccessfulinall
respects.
Thehighpassfilterofanelectrostaticunitrequires
specialconsiderationifthefullpotentialoftheunitisto
berealised.Attentionmustbepaidtotailoringthestop
bandresponsetotherequiredsloperate.Thenaturalresponse
ofanelectrostaticassemblyistoriseverysteeplyimmediately
atdiaphragmresonanceandproduceanalmostimpossiblematching
characteristicforthebassradiator.Thisriseiscontrolled
partiallybyaccuratediaphragmtensionbutonemustalso
restrictinputwithinthefirstoctaveaboveresonanceand,
coupledwiththefallingimpedancecharacteristicofthe
capacityload,theE.L.S.presentsdifferentandsomewhat
exactingrequirementsintermsoffilternetwork.
TheremainderofthecrossoverunitonModel70is
conventionalbutitisworthmentioningthatnolessthan74
mfd.ofpaperisusedinthiscircuit- ata costofone
componentthatwouldalmostbuya cheaploudspeakersystem.
ReversibleelectrolyticcapacitorswhilstsatisfactoryatA.C.
powersfora limitedperiodofusebecomeconductiveandrender
thefilternetworkuselessunderthestringentconditionslaid
downfora systemsuchasModel70.
Onefinalcomponentworthyofmentionistheseriesfeed
inductoronthehighpasssection.Thiscomponentcan
seriouslymodifythecontrollingfeedbackappliedtothevoice
coiland,becauseithandlesconsiderablecurrent,wedesigned
aspecialferriteinductorwhichreducedtheD tot.at25watts
inputtowellbelow0.5%atallfrequenciesfrom30Hz.
Conventionalcomponentssampledshowedfiguresintheregionof
2.5%

x
-19-
BASSRADIATOR
FIG11.
DW13/3,final
productionversion
asusedinthe
Model70
Itwasmentionedearlierintheintroductorydiscussion
thereasonsforchoosingthemovingcoilasopposedtothe
electrostaticprincipleforthelowestfouroctavesofthe
system.Therequirementssetoutforthefinalperformance
ofthisunitwerethatitshouldmatchascloselyaspossible
inallrespectsthatofthe701electrostaticunit.The
writer'scolleague,Mr.DennisWard,despitesometwentyyears
experienceinthedesignandmanufactureofmovingcoilunits
feltthata distortionfactorintheorder1%withinputsof
25wattswasa somewhatimpossibletask.However,workwas
commencedonprototypeswitha viewtoexploringthepossibility
ofa verylongthrowlinearsuspensionunitworkingasa true
piston.
Apistonarea(nominal12")waschosenasbeingreasonably
consistentwiththeenclosurevolumetobuilda finalsystemwith
aresonancebelow40Hzanda freeairresonanceintheregion,
of18- 22Hz.

-20-
Expandedpolystyreneseemedtheobviouschoiceforcone
materialasmediumandhighfreguencieswerenotinvolvedand
indeeda numberofearlyprototypesweremadewiththismaterial.
Extendedlisteningtestsproved,however,thatthepolystyrene
conematerialshowed'readout'colourationwithinthe200- 400
Hzregion.Experimentswerethenmadewithanexactlysimilar
unitminusthepolystyreneslabandgirderassemblyanditwas
determinedbyrelatingfrequencyresponsecurvestothetwo
unitsthatenergygeneratedprimarilyattheapexofthecone
waslargelybeinglost,presumablyduetokineticconversion
withinthepolystyrene,andinturnthepolystyrenewas
partiallybreakingupandprovidingthissmallbutobjectionable
colouration.
Thefinalconeassemblydoes,infact,consistofa
laminatedstructureinordertoprovidetherequiredstiffness
andisfurtherbracedandmassadjustedbysmallrectangular
sectionsofsyntheticrubberaffixedatcriticalpositionson
thefrontfaceofthecone.
Oneoftherelativelyuntouchedaspectsofbassradiator
designhasbeentheeffective'Q'atresonance.A simple
bridgemethodwasevolvedforthismeasurementandconsiderable
workcarriedoutintermsofsuspensioncomplianceanddiaphragm
masstoreducethisfigurewellbelowunity.Untilambient
roommeasurementsweretakenitwasnotfullyrealisedhow'room
gain'couldplayanimportantpartinmagnifyinga relatively
smallriseintheresponseasshownonananechoicplottoan
objectionableonenotethump.
LOUDSPEAKERMEASUREMENTS
Thewriterisa firmbelieverinmeasurementatallstages
ofdesignandproduction.Althoughdesignersinthepasthave
suggestedthatthemeasurementofloudspeakerresponseisof
secondaryconsideration,wefeelthatthisviewistakenbecause
theartofinterpretatingloudspeakermeasurementsisa subject
onwhichthereisconsiderablescopefordevelopment.For
instancemanymanufacturersanddesignersproudlypublisha
beautifulaxialplotoftheirsystemandwhilsta straightline
responseisa goodstartingpointthismeasurementisalmost