JBL 4671 User manual

InstructionManual
MotionPictureLoudspeakerSystems:
AGuideToProperSelectionAndinstallation
A.Introduction: B.SystemsforSmallerHouses:
Inrecentyears,JBLhasemergedasthepredomi-
nantmanufacturerofloudspeakersystemsforthemo-
tionpicturetheater.Ourproductsaretobefoundinthe
mostprestigiousscreeningroomsanddubbingtheaters
inHollywood,includingWarnerHollywoodStudios,Glen
GlennSound,TheBurbankStudios,UniversalStudios,
andTodd-AO.Therecentinstallationofa JBLsystemin
theGoldwynTheateroftheAcademyofMotionPicture
ArtsandSciencesrepresentsa benchmarkforourcom-
pany.Ourcomponentshavealsobeenselectedby
Lucasfilm,Ltd.,fortheirTHXtheatersystem.
ThereareseveralreasonsforJBL'ssuccessinmo-
tionpicturesound.Foremosthereisourconceptofflat
powerresponse,whichresultsinanorderofimprove-
mentinsoundcoverageinthetheater.Second,our
componentsareofprovenreliabilityandhaveextended,
inherentlysmoothresponse.Inaddition,wemanufac-
turecomponentsforallaspectsoftheatersound.JBL's
systemsareverycosteffectivewhencomparedtothe
ventedhorn/multicelldesignsofearlieryears.
Thismanualisintendedforboththeaterdealersand
prospectivepurchasersoftheatersystems.Itwillbeuse-
fulasa guidetotheselectionandinstallationofsystems,
anditwillprovideconsiderabletheoreticalbackground
forthespecifyingengineer.
1. A SummaryofHouseTypes:
Byfar,mostofthetheatersconstructedtodayare
relativelysmallrectangularrooms.Theyareusuallybuilt
ingroupsknownasmultiplextheatersforeconomicrea-
sons. Mostofthesetheatersaremonophonic,witha sin-
gleloudspeakerbehindthescreen,andseatingcapacity
rangesfrom200upto500.
2. GeneralAcousticalCharacteristics:
Motionpicturetheatersaregenerallyacoustically
"dead" rooms,withreverberationtime,eveninthe larg-
esthouses,notexceeding1.25seconds.Inmostmulti-
plextheaters,reverberationtimesareontheorderof0.5
seconds.Theshortreverberationtimeensurescleardia-
logthroughoutthehouse.
Themostcommoninteriortreatmentoftheatersis
double-foldvelour,whichmaybeplacedonthesideand
rearwalls.Thiswillprovidefreedomfromunwantedre-
flections,butitdoesnothingtoisolateonemultiplex
spacefromtheadjacentone.Here,thearchitectmust
relyonsolidblockwallsforgoodisolation.
Anyunusualacousticalproblemsshouldbereferred
toa qualifiedacousticalconsultant.

3.SoundLevelRequirements:
Normalpeaklevelsof85dBSPLarecommoninthe
theater.A minimumheadroomlevelabovethisfigure
wouldbe10dB;thatis,thesystemshouldbecapableof
handlingpeaksupto95dB-SPLWhereverpossible,
JBLrecommendsthata headroomfigureof15dBbe
designedintothesystem.
Thefollowingfactorshavebeentakenintoaccount
indeterminingsystemandamplifierchoiceforvarious
housesizes:
a.Roomvolume
b.Roomboundary
c.Reverberationtime
d. Systemsensitivityandpowerrating
e.Systemdirectivityindex
4. RecommendationsforVariousSizeTheaters:
A.Fortheatersseatingupto200patrons;volume
approximately1700cubicmeters(60,000cubicfeet):
*Preferredsystem:Model4673(directradiatorLF)
Powerneededfor100dB:46watts
Recommendedpoweramplifier:JBL6230in
bridgedmode(300wattsinto8 ohms)
Characteristics:Broad-bandsensitivity97dB,one
wattatonemeter;500-HzcrossoverintoBi-Radialhorn;
integralpowerresponsecorrection;smoothresponse
downto45Hz;idealqualityforbothdialogandmusic;
shallowprofile.
*Alternatesystem:Model4671(directradiatorLF)
B.Fortheatersseatingupto350patrons:volume
approximately2800cubicmeters(100,000cubicfeet):
*Preferredsystem:Model4670B
Powerneededfor100dB:30watts
Recommendedpoweramplifier:Singlesectionof
JBL6230(75wattsinto4 ohms,theimpedanceofthe
low-frequencysectionofthe4670Bsystem.)
Characteristics:Sensitivity,100dB,onewattatone
meter;500-Hzcrossoverformostnaturaldialogand
musicquality;integralpowerresponsecorrection;LF
responsesmoothdownto45Hz;shallowprofile
'Alternateventedhornsystems:Models4672Aand
4674A
Powerneededfor100dB:30watts
Characteristics:Mid-bandsensitivity,103dB,one
wattatonemeter;LFresponselimitedduetohorn load-
ing; tobespecifiedwhereavailableamplifierpoweris
limited(typically,50wattsorless)
C.Fortheatersseatingupto500patrons:volume
approximately4500cubicmeters(160,000cubicfeet):
'Preferredsystem:Model4670B
Powerneededfor100dB:50watts
Recommendedpoweramplifier:singlesectionof
JBL6260(300wattscanbedeliveredinto4 ohms,the
impedanceofthelow-frequencysectionofthe4670B
system)
Characteristics:Sensitivity,100dB,onewattatone
meter;500-Hzcrossoverformostnaturaldialogand
musicquality;integralpowerresponsecorrection;
smoothresponsedownto45Hz;shallowprofile
Powerneededfor100dB:46watts
Recommendedpoweramplifier:JBL6230in
bridgedmode(300wattsinto8 ohms)
Characteristics:Broad-bandsensitivity,97dB,one
wattatonemeter;800-Hzcrossoverwithintegralpower
responsecorrection;smoothresponsedownto45Hz;
slightlylessrealisticondialogthanrecommendedsys-
tem; shallowprofile
'Alternateventedhornsystem:Model4674A
Powerneededfor100dB:50watts
Characteristics:Mid-bandsensitivity,103dB,one
wattatonemeter;LFresponselimitedduetohorn load-
ing; shouldbespecifiedonlywhereavailableamplifier
powerislimited(100wattsorless)
*Alternateventedhornsystems:
Models4672Aand4674A
Powerneededfor100dB:23watts
Characteristics:Mid-bandsensitivity,103dB,one
wattatonemeter;LFresponselimitedduetohorn load-
ing; tobespecifiedwhereavailableamplifierpoweris
limited(typically,50wattsorless)
ThesystemsdiscussedaboveareshowninFigure
1 A,andtypicalfree-fieldresponsecurvesareshownat
BandC.
2

FIGURE1.SYSTEMSFORSMALLERHOUSES
FIGURE1A.LINEDRAWINGS
4671
PMAX=
150W
SENSITIVITY:
97dB,
1W,1 M
4673
PMAX=
150W
SENSITIVITY:
97dB,
1W,1 M
4670B
PMAX=
300W
SENSITIVITY:
100dB,
1W,1 M,
4672A
PMAX=
150W
SENSITIVITY:
103dB,
1W,1 M
4674A
PlVlAX=
150W
SENSITIVITY:
103dB
1W,1 M
FIGURE1B.ON-AXISRESPONSEOF4670B (2TT LOADING);
HIGH-FREQUENCYPOWERRESPONSE
CORRECTED.
3
dB
Relative
Response
FrequencyinHz

FIGURE1C.ON-AXISRESPONSEOF4674A (2TT LOADING);
HIGH-FREQUENCYPOWERRESPONSE
CORRECTED.
5.Installation of thesystems: FIGURE2.MOUNTINGTHESYSTEM
A.GeneralComments: A.PositionBehindTheScreen
JBL'stheatersystems are shipped tothe dealer in
componentform.Everyattempt is made to ship the
componentscompletesothatthesystemscan be readily
assembled. All componentsshould be inspectedprior to
beingsent outto thejobsite.Wiringinstructions forall
theatersystems are included inthe dividingnetwork
package.
Manydealerswillwant to takeadvantage of thefact
thatcertainJBLtheaterlow-frequencysystems are
availableloadedfrom the factory.These are:
4647(consisting of 4507loadedwith2225H)
4648(consisting of 4508loadedwith 2-
2225H)
Further, the 4671 OK isa factorypreassembledver-
sion of theutilitysystemwith an oakvinylenclosure and
browngrillecloth.
V3 •SCREEN
B.PhysicalConsiderations:
,ABSORPTIVE
'MATERIAL
STAND
IF
NECESSARY
MOUNTING
OFSCREEN
LOUDSPEAKER
Mounting the systembehind the screen is shown in
Figure2A. The preferredlocation is two-thirdsthedis-
tance upthe screen,andthis may require a platform.
Details of this are shown atB.
4
LF
HornLoading
LF
ReflexLoading
dB,
RelativeResponse
Frequencyin Hz

ELPlatformDetails
Framedimensions
slightlylargerthan
bottomofenclosure.
Topcovered
V2" thickplywood.
Frameandlegs,2x4
lumber
Braces1 x 4 lumber.
Bracingrequiredon
foursides.
AllJBLtheatersystemsmustbeorientedasshown
inFigure 1 A.Inparticular,themodels4670Band4675A
shouldNOTbeassembledwiththelow-frequencyenclo-
surelyingonitsside.
FIGURE3-INSTALLINGTRANSDUCERS
ANDCOMPONENTSINSYSTEMS
Theloudspeakershouldbeplacedona densefoam
rubberpadorsectionofcarpetsothatitcannotpossibly
rattlewhendrivenathighlevels.Whentheloudspeaker
isplacedonanelevatedsupport,itisadvisableto
anchortheloudspeakerwithfourormoreangleironsso
thatitcannotpossiblyshiftposition.
Thebackwallbehindthescreenshouldbecovered
witha soundabsorptivematerial.
1. High-frequencyHornAiming:
Figure3 showsdetailsofmountingthecomponents
inthevarioussystems.Notethatthehigh-frequency
hornsareaimedstraightoutintothetheater.JBLhas
foundthatthismountingwillbebestformostinstalla-
tionsforthereasonsshowninFigure4A.Sincetheoff-
axisresponseofournewhornsisquiteuniformoutto12
kHz,wecantradeofftheinversesquarelossesinthe
roomwiththeoff-axishornlossestoprovidemoreeven
front-to-backcoverageintheseatingarea.
ANGLEIRON
5
A.

FIGURE3,Cont'd
6
HORN/DRIVER
SCREWEDINPLACE
PANELREMOVED
B.
C.4 WOODSCREWS*
INTOFRONTEDGES
OFLFENCLOSURE
"Use#10x2flat-headPhilipswoodscrew;pre-drillholeswithVa"bit.
D.MOUNTINGLFTRANSDUCERSIN4507,4508,
AND4518 E.MOUNTINGLFTRANSDUCERSIN4550BKA
AND4560BKA

FIGURE3,Cont'd
H.(Seenoteintextconcerningpolarity)
INCLUDED
WITH
NETWORK
INSERT
T-NUTS
#6
SCREWS
TERMINALBOARD
PLATEREMOVED
FROMINSIDE
F. G.
For4671,4672A,4673,4674A: For4670B:
3115A
311OA
J.
7

FIGURE4.PROPERAIMINGOFHIGH-FREQUENCYHORNS
A.INVERSESQUARE,OFF-AXISTRADE-OFF
0°(Maximumoutput)
FIGURE4B.CADPDISPLAY;HFHORNAIMEDSTRAIGHTAHEAD
Screen!
HFHorn
10' high
12'
2kHz
8

FIGURE4C.CADPDISPLAY;HFHORNAIMED10°DOWN
Screen]
HFHorn
10'high
12'
2kHz
Furtherevidenceofthisbenefitisshowninthe
CADP(seereference)coverageplotshownatB.Here,
thefront-to-backvariationincoverageisonly5 dB,and
thismeansthatthemajorityofpatronswillreceiveexcel-
lentcoverage.Comparethiswiththecoverageshownat
C.Here,wehavetiltedthehigh-frequencyhorndownby
only10degrees!Theresultingcoveragevariationhas
increasedto8 dB.
Onlywherethereisa pronouncedrake(8degrees
ormore)tothefloorshouldthesystembeangledslightly
upwardtowardtheback wall. Thiscanbeaccomplished
byplacinga stripofwoodbeneaththefrontedgeofthe
loudspeaker.
C.WiringConsiderations:
AllJBLnetworkandtransducerterminalsarecolor
codedsothatconsistentpolarity(phase)ofallsystems
isassured.
Inthefollowingsystems,allconnectionsbetween
transducersandnetworksshouldbebetweenlikecoded
terminals.Inotherwords,redtored,andblacktoblack:
high-frequencysectionshouldbewiredinreverse
polarity:
4670B
4673
Thereasonforthishastodowiththeprecise posi-
tioningofthehigh-frequencydriverwithrespecttothe
low-frequencyacousticcenter.Thereisone-halfwave-
lengthshiftatthecrossoverfrequencyinthesesystems
whichrequiresthepolaritychangetomaintainproper
summationwiththelow-frequencysection.
Wiringfromtheprojectionboothtothescreen loud-
speakersshouldconformtolocalcodes.Thewiregauge
selectedshouldbesuchthatelectricallossatthe loud-
speakerdoesnotexceed0.5dB.Formostapplications
insmallerhouses,AWG#12(2.053mm)copperwire
willbesufficient.ThetableshowninFigure5 showsthe
wireresistanceforvariousdoublerunsofcopperatvar-
iousgauges.For8-ohmloudspeakers,thetotalwire
resistanceshouldnotexceed0.552ohms.
FIGURE5.TABLEOFWIRELOSSES
4671
4672A
4674A
Inthefollowingsystems,thelow-frequencysection
shouldbewiredredtoredandblacktoblack,butthe
WIRERUN(DOUBLE;COPPER)
NO. 10
NO .12 NO .14
NO .16
NO. 18
10M (33')
.02
.032
.05 .08
.125
20M (66') .04
.064
.10
.16
.250
30M (100')
.06
.096
.15 .24
.375
40M (132')
.08
.128 .20
.32 .500
50M (165') .1
.160 .25
.40
.625
60M (200') .12 .192 .30
.48
.750
FOR 8HLOAD, LINE RESISTANCESHOULDBE0.552ORLESS.
9
Nornalizeddirectfield.

Themodels4670Band4675Ahave4-ohmlow-
frequencysections,andthetotalwireresistanceshould
notexceed0.276ohms.
C.SystemsforLargerHouses:
1. GeneralComments:
Theclassofhousesconsideredherewillallhave
multichannelsystems.Seatingcapacitymaybeaslittle
as500andaslargeassomeoftheoldmoviepalacesof
yesterday.Therearefewofthelatterleft,exceptinlarge
metropolitanareas.Forthemostpart,thelargertheaters
havebeenconvertedintomultiplextheaters.
Thenumberofloudspeakersbehindthescreenwill
beeitherthreeorfive,andtherewillalwaysbea sur-
roundchannelconsistingofsixormoresmaller loud-
speakersarrayedonthesideandrearwalls.
ThesesystemswillbecapableofplayingDolby
films, andthird-octaveequalizationwillbeemployed,as
partoftheDolbyequipment,toadjustthesystemtothe
roomaccordingtoa standardresponsecurve.
Ingeneral,theclassofaudioequipmentusedin
theseinstallationswillbetheequalofthefinestequip-
mentspecifiedforhigh-qualitysoundreinforcementsys-
temsinauditoriumsandconcerthalls.
DONOTmixdifferentsystemsinthemultichannel
screenloudspeakerarray.Notonlywilltherebe dif-
ferencesinpowerhandling,theremaybepolarity dif-
ferences,andthesewilladverselyaffectstereophonic
performance.
Wewillnowcoverindetailtherequirementsfor
thesesystems.
2. SystemRecommendationsforLargeHouses;
500to2000seats;volume14,000cubicmeters
(500,000cubicfeet):
*Preferredsystem:Model4675A(directradiator
LF)
Powerneededfor100dB:90watts
Recommendedpoweramplifier:JBL6260,one
loudspeakersystemperchannel(300wattsinto4 ohms)
Characteristics:Broad-bandsensitivity100dB-
SPL, onewattatonemeter;JBL'smostpower-flatand
accuratesystem;useslargeBi-Radialhornforprecise
90-by-40degreepatterncontroldownto500Hz;integral
powerresponsecorrection;extendedresponsedownto
45Hz
Forverylargehousesandforthehighestdegreeof
performanceinprofessionaldubbingtheatersand
screeningrooms,the4675A-2maybespecified.Werec-
ommendthatbiamplificationbeseriouslyconsidered
withthissystemforoptimumperformance.Thebroad-
bandsensitivityofthe4675A-2is103dB,onewattat
onemeter.
*Alternateventedhornsystems:
Models4676B-1and.4676B-2
Powerneededfor100dB:80and53watts,
respectively
Recommendedamplifier:JBLmodel6260,one
channelperloudspeakersystem(300watts)
Characteristics:Mid-bandsensitivity,106and109
dB-SPL,respectively;recommendedforretrofitinlarge,
olderhouseswherea completeconversiontoall-new
hardwareisnotfeasible.LFresponserollsoffbelow80
Hz
Thesystemsdiscussedinthissectionareshownin
Figure6A.ResponsecurvesareshownatB andC.
3.Systemimplementation:
A.Electricalconsiderations:
Thesystemsintendedforlargerhousesmaybe
implementedwithJBL's3160high-levelnetwork,with
wiringasshowninFigure7AthroughB.
However,foroptimumperformance,werecommend
biamplificationusingtheJBLmodel5234Aelectronic
dividingnetwork.The5234Aisavailablewitha 500-Hz
cardwhichcontainshigh-frequencypowerresponse
correction. Implementationofbiamplificationisshownin
Figure7C.
Thechiefvalueofbiamplificationisthereductionof
intermodulationdistortiononpeakprogramsignals.Fur-
thermore,largerpoweramplifiersmaybespecifiedfor
theLFsectionsthanthoseindicatedforusewiththe
high-leveldividingnetwork.
10

FIGURE6.SYSTEMSFORLARGETHEATERS
A.
LINE
DRAWINGS
4675A 4675A-2 4676B-1 4676B-2
PMAX=
300W
SENSITIVITY:
100dB,
1W,1 M
PMAX=
300W
SENSITIVITY:
100dB,
1W,1 M
PMAX=
300W
SENSITIVITY:
106dB,
1W,1 M
PMAX=
300W
SENSITIVITY:
109dB,
1W,1 M
FIGURE6B.ON-AXISRESPONSE,4675A(2ir LOADING);
HIGH-FREQUENCYPOWERRESPONSECORRECTIONADDED.
dB,
Relative
Response
11
FrequencyinHz

FIGURE6C.0N-AX1SRESPONSEOF4676B-1 (2ir LOADING);
HIGH-FREQUENCYPOWERRESPONSECORRECTIONADDED.
The LF sections of thesesystems are fourohms in
impedance,andthatconsiderationshould, asinthe pre-
vioussection,dictate the choice of wiregauge for hook-
up.
It is important inany biamplifiedinstallation to
include DC blockingcapacitors in output of thehigh-fre-
quencyamplifier, as shown in Figure7C. The capacitor
protects the high-frequencydriverfrom any inadvertent
DC or low-frequencytransientswhichcoulddevelopdur-
ingpower-up or power-downphases of amplifieropera-
tion. Figure 7D presents a table of capacitorsizes for
variousapplications.
FIGURE7.WIRINGDIAGRAMSFOR
LARGERSYSTEMS
FIGURE 7A.
For4675 and
4676B-1(Seenote in
textconcerning
polarity)
12
LF
HornLoading
LF
ReflexLoading
dB,RelativeResponse
Frequencyin Hz
3160

FIGURE7,Cont'd.
FIGURE7C.
Biamplification
withthe5234A,
Useplug-in500-Hz
card52-5222
withhigh-frequency
powerresponse
equalization
JBL5234A
LFOUT'
DCBLOCKING
CAPACITOR
FIGURE7D.BLOCKINGCAPACITORVALUESFORDRIVERPROTECTION
CAPACITOR JBL FOROPTIMUMRESULTS
VALUE PART ATTHESECROSSOVER
NUMBER FREQUENCIES*
36JXF 62772-0360 500Hz
24jxF 62772-0240 800Hz
*160 OPERATIONASSUMED
HFAMPLIFIER
HFOUT
LFAMPLIFIER
BIAMPLIFICATION
FIGURE7B.
For4676B-2
(Note:SplayeachHF
horn 221/2° offcenter;
totalsplayis45°)

B.Physicalconsiderations:
High-frequencycomponentsshouldbemountedas
showninFigure8.
FIGURE8.MOUNTINGTHE 2360HIGH-FREQUENCYHORN
BI-RADIALHORN
LFENCLOSURE
ALIGNREAROF2506
WITHREAROF
LFENCLOSURE
Asindicatedintheprevioussection,theloudspeak-
ersshouldbepositionedaboutone-thirdthewayupthe
screen. Ifthescreenisflat,asshowninFigure9A,the
flankingloudspeakersshouldbetoedinslightly.Where
thescreeniscurved,asshownatB,theloudspeakers
maybepositionednormaltothescreenateachposition.
Thehigh-frequencyhornsshouldbeplacedwithin75-
100mm(3to4 in)fromthescreen.
FIGURE9.PLACINGTHELOUDSPEAKERSBEHINDTHE SCREEN
A.FLATSCREEN
14
2506ADAPTER
HFDRIVER
VIEWFROMABOVE

B.CURVEDSCREEN
Thebestpossibleacousticalloadingofthesystem
totheroomisobtainedwhentheloudspeakersare
mountedina largeflatbaffle,asshowninFigure10.
Here, the4675A-2systemhasbeenspecifiedforextra
acousticalpoweroutputinthe40Hzto500Hzrange.
Thebaffleshouldnotbeinterrupted,anditshould
extendtheentirewidthoftheloudspeakerarray.
MechanicaldetailsofthisbafflearegiveninFigure11.
Ideallythebaffleshouldbecontinuousina multi-loud-
speakerscreenarray.
FIGURE10-PHOTOOFACADEMYOFMOTIONPICTUREARTSANDSCIENCESSYSTEM
VIEWFROMABOVE
15

FIGURE 11. DETAILSOFLARGEFLATBAFFLE
4' BY8'1"THICKPARTICLEBOARD
2' by4'STIFFEN-
ERSMOUNTEDAS
SHOWNONBACK-
SIDEOFBAFFLES
PANELSBOLTEDTO
EACHOTHERAND
TO4508ENCLO-
SURES, ASSHOWN.
16
FRONTVIEW
TOPVIEW
ADDED2 BY4
BRACES(AT3
PLACESONBACK
OFEACHBAFFLE)

D.SurroundSystems:
1. Generalcomments:
Whilethereislittleconsistentpracticeinspecifying
andimplementingsurroundchannels,wecansetdown
thefollowinggeneralrequirements:
a.Thetotalacousticalpowerdeliveredbythe
ensembleofsurroundloudspeakersshouldbe
equaltooneofthescreenchannels.Thereare
probablynotmanysurroundsystemswhich
meetthisrequirement,butitisessentialifthefull
impactofsurroundinformationistobeappreci-
ated. Whilea 15-dBheadroomfactorisa partof
thepowercalculationsforthescreenchannels,it
maybeunreasonabletodemandthatofthesur-
roundchannel.Thus,a requirementthatthesur-
roundchannelbeabletoproducea levelinthe
houseof95dBwillsuffice.Inthecaseofdistrib-
uted200mm(8in)loudspeakers,a levelof92dB
isdeemedsufficient.
b.Thequantityofsurroundloudspeakersshouldbe
sufficientsothelistenersarenotawareofany
oneofthem.Thisusuallymeansa minimumof
eight:threeeachonthesidewallsandtwoonthe
back wall.
c.Thesurroundloudspeakersshouldexhibitwide
dispersionsothattheentireaudienceareacan
appreciatesmoothcoverageacrossthe fre-
quencyrange.
d. Surroundloudspeakersshouldbeunobtrusive
andnotinterferewiththeaterdecor.Thisrequire-
mentmayruncountertosomeoftheacoustical
requirementsinthatsizeisrelatedtosystem
efficiency.
2. Choosingtherightsurroundloudspeakers:
JBLhasrecentlyintroducedthemodel8325Athree-
waysystemforsurroundapplications.Withinputpower
capacityof80wattsanda sensitivityof91dB,onewatt
atonemeter,tentotwelveofthesesystemsarea rea-
sonablematchfora screenarrayof4675Asystems.
Morethantenmayberequiredforbroadercoveragein
largerhouses.
Inlargertheaters,the46120Khasbeensuccess-
fullyusedasa surroundloudspeaker.Thissystemhasa
sensitivityof97dB,onewattatonemeter,andpower
inputis400watts.Obviously,fewerofthesesystems
wouldberequiredthanwiththe8325A.
Ingeneral,wewouldrecommendthefollowing
quantitiesofthevarioussurroundloudspeakersforrea-
sonablematchingwitha screenloudspeakerarray
consistingof4675A's:
8325A10to12
4671or 4671 OK6 to8
46120K6 to8
DolbyLaboratorieshassuggestedtheuseof mul-
tiple200mm(8in)loudspeakersforthesurroundchan-
nel. Upwardsof60suchloudspeakersmaybeusedin
thisconfiguration,mountedingroupsofsixoreightand
arrayedhorizontallyalongthewalls.TheJBLmodel
8140Co-Motionaldriverisidealforthisapplication
becauseofitsextremelysmoothandwidehigh-fre-
quencydispersion.
3.Implementationofthesurroundsystem:
Figure12showsthegeneralmountingforsurround
loudspeakers.Notethattheycoverthebacktwo-thirds
ofthehouseandthattheyarelocatedone-thirdthedis-
tanceupthewalls.
A.SIDEVIEW
D
17
FIGURE12.LOCATINGTHESURROUND
LOUDSPEAKERS
B.TOPVIEW

FIGURE13.WIRINGTHESURROUNDLOUDSPEAKERS
Figure13showsdetailsofelectricalhook-up.
Series-parallelwiringwillbenecessarytoensurethat
eachamplifierseesa proper load. Mostamplifierstoday
havea 4-ohmrating,andloadimpedancesanywherein
the4 to8-ohmrangeareacceptable.
JBLrecommendsthatsurroundloudspeakersbe
individuallywiredbacktotheprojectionboothforeasein
isolatingproblemsandreconfiguringtheloudspeakers
forsplitsurroundchannels.AWG#12(2.053mm)cop-
perwirewillgenerallybesufficientforwiringthe
surrounds.
ESubwooferSystems:
1. Background:
Speciallow-frequencyeffectsinthemotionpicture
theaterdatebacktothethirties,butthepresentinterest
datesfromUniversal's"Sensurround"intheseventies.
Sincethattime,speciallow-frequencychannelsforcov-
eringtherangefrom20to40Hzhavebecomecommon
inlargetheaters,andtheDolbyCP200processingunit
hasa summedoutputbelow100Hzthatcanbeuseddi-
rectlyfordrivingsubwooferamplifiers.
2. Implementation:
JBLmanufacturesthe4645subwoofersystemfor
motionpictureapplications.Thissystem,shownin Fig-
ure14A,usesthe2245H460mm(18in)transducer,
whichisnotedforitslowdistortionandforitstotalexcur-
sioncapabilityof19mm(0.75in).
FIGURE14A
4645SUBWOOFER
PMAX=
300W
SENSITIVITY:
95dB,
1W,1 M
18
8UNITS= 12UNITS=
Ail LEFT
SIDE
RIGHT
SIDE
LEFT
SIDE
RIGHT
SIDE
5.3ft

Thefollowingtableindicatestheacousticalpower
outputwhichcanbeexpectedintwosizesoftheaters.
NUMBEROFMODULES EFFICIENCY MAXIMUM
ELECTRICAL
POWERINPUT
ACOUSTIC
POWER
SOUNDLEVELS
500- 1000-
1000SEATS2000SEATS
1
2%
300WATTS 6WATTS 101dB 100dB
2
4%
600WATTS 24WATTS 107dB 106dB
4 8% 1200WATTS 96WATTS 113dB 112dB
8
16%
2400WATTS 384WATTS 119dB 118dB
Itisessentialthatonlythemoststableamplifiersbe
usedforsubwooferapplication.Thetransducerscanbe
paralleledfor4-ohmoperation.
inputcapability,andanotherthreedBareduetothe
increaseinefficiencyresultingfrommutualacoustical
couplingbetweentransducers.
Substantialsoundpressurelevelsarenecessaryat
verylowfrequenciestomatchthe85dBreferencelevel
inthetheater,astheequalloudnesscontoursshownin
Figure14Bindicate.Notethatwitheachdoublingof
transducers,themaximumsoundpressurelevel
increases6 dB.ThreedBareduetodoublingthepower
Subwooferloudspeakersshouldbelocatedfairly
closetogether,andtheyshouldbeplacedattheinter-
sectionofa wallandfloor.Carefulbracingandmounting
onrubberorNeoprenepadsmaybenecessaryto mini-
mizerattling.
FIGURE14B.ROBINSON-DADSONEQUALLOUDNESSCONTOURS
F.AcousticalResponse:TheConceptof
FlatPowerResponse
Whileloudspeakersystemsarefairlyflatonaxis,
theytendtonarrowconsiderablyintheircoverage
anglesathighfrequencies.Theirpowerresponserolls
offnaturally.Ifa loudspeakermaintainsfairlyconstant
horizontalandverticalcoverageanglesovermostofits
frequencyrange,thenflaton-axisresponsewillalsoim-
plyflatpowerresponse.Sinceabouthalfofthesound
heardinthetheaterhasbeenreflectedatleastonce,itis
importantthatloudspeakersexhibitfairlyflatpower
responseifthereproducedsoundistobenatural.
19
LOUDNESS
LEVEL(PHON)
SOUND-PRESSURE
LEVEL
INdBRE20N/m2
FREQUENCYINHz
MAF
CURVE

Anotherconsequence of usingloudspeakerswith
flatpowerresponse is thatlittlesystemequalizationwill
berequired.Figure 15 illustratesthis. AtA,weseethe
boundaryabsorption ina typicalwell-designedtheater.
Notethat itis quiteflatover the frequencyrange.High-
frequencylosses duetoair absorption are shown atB,
andtypicalhigh-frequencyscreenlosses are shown at
C.When weaddallof theselosses, wegetthe response
shown atD,and this is exactly the acousticalresponse
wewouldobserve inthe room ifwe wereusing a loud-
speakerthat had flatpowerresponse.
FIGURE15.RESPONSE IN THE HOUSE
ROOMBOUNDARYLOSSES
EXPRESSED INdB
(TYPICAL)
AIRLOSSES AT15 METERS,
50% RELATIVEHUMIDITY
SUM OF CURVES
A, B,ANDC
20
dB,RELATIVELEVEL
dB,RELATIVELEVEL
ON-AXIS
SCREENLOSSES
(15 MIL THICKVINYL, 8% OPENING)
dB,RELATIVELEVEL
dB,RELATIVELEVEL