D.SurroundChannels:
1=Genera!requirements:
Whilethereislittleconsistentpracticein
specifyingandimplementingsurroundchannels,
wecanlaydownthefollowingrequirements:
a.
Thetotalacousticalpowerdelivered
bytheensembleofsurround
loud-
speakersshouldbeequaltooneof
thescreenchannels.Therearepro-
bablynotmanysurroundchannels
whichmeetthisrequirement,but
itisessentialifthefullimpact
ofsurroundinformationistobe
appreciated.Whilea 15-dBheadroom
factorisa partofthepowercalcu-
lationsforthescreenchannels,it
maybeunreasonabletodemandthat
ofthesurroundchannel.Thus,a
requirementthatthesurroundchannel
beabletoproducea levelinthe
houseof95dB-SPLwillsuffice.In
thecaseofthedistributed200mm
(8")speakersystem,a levelof
92dBisdeemedsufficient.
b.Thequantityofsurroundloudspeakers
shouldbesufficientsothatlisteners
arenotawareofonlyoneofthem.
Thisusuallymeansa minimumof
eight:threeeachonthesidewalls
andtwoontheback
wall.
c.Thesurroundloudspeakersshould
exhibitwidedispersionsothatthe
entireaudienceareacanappreciate
evencoverageacrossthefrequency
range.
d.
Surroundloudspeakersshouldbe
unobtrusiveandnotinterferewith
theaterdecor.Thisrequirementmay
runcountertosomeoftheacoustical
requirementsinthatsizeisrelated
tosystemefficiency.
TheJBL4401monitorloudspeakerworks
extremelywellsincea smallnumberofthem,
usuallylesstham12,willprovidethedesired
levelinmanyhouses.TheMC4401mounting
cradleprovidesa convenientmethodof
wall-
mountingandanglingthe4401system.TableIV
presentsdetailsofsurroundloudspeaker
selection.
Theuseofmultiple200mm(8")loudspeakers
hasbeenfavoredbyDolbyLaboratories.Whilethe
hardwarecostsmaybereasonable,thelaborcosts
forthisapproachareaccordinglyhigher.Forthis
typeofsystem,theJBL8140co-motionaltrans-
ducerwillbea logicalchoice.Thebafflesused
insuchaninstallationshouldideallybe
con-
structedaccordingtothetransducermanufac-
turer'splansorrecommendationssothatlow-
frequencyresponsewillbeadequate.A lowercost
approachcallsforusingoneofthemanystandard
tiltedfrontenclosures.Asa rule,thesearetoo
smallinvolumeforproperlow-frequencyresponse,
andtheclientshouldunderstandthisattheoutset.
The4671theatersystemisrecommendedfor
high-powersurrounduse.Withanefficiencyof
3.5%,relativelyfewoftheseunitsarerequired.
Theywouldnormallybespecifiedwhentheentire
theaterreproductionsystemwascalleduponto
producelevelsinthe105-to-110-dBrange.
2.
Poweringthesurroundchannel:
Figure6 presentsdetailsoftheimplementa-
tionofthesurroundchannel.AtC throughE,we
showdetailsoftheelectricaldistribution.Depen-
dingonthenumberofsurroundloudspeakers,the
hook-upwillbeseries-parallel,andtheresulting
impedancecanusuallybeheldtonolessthan4
ohms.
Anamplifiershouldbechosenwhose4-ohm
powerratingwillnotbeexceeded.Insomecases,
theJBLmodel9375linematchingautotransformer
willbeusefulinmaintainingthedesiredim-
pedance.Forthe200mm(8")loudspeakers,a
70-voltdistributionsystemshouldbeused.See
JBLTechnicalNote,Volume1,Number2,for
detailsofthismethod.
E.SubwooferSystems:
1.
Background:
Universal'sintroductionof"Sensurround"
duringtheseventiesbroughtspecialeffectsvery-
low-frequencychannelsintothemotionpicture
theater.Typically,thesechannelscovertherange
from20-25Hzupto40-50Hz.Quitehighacoustical
poweroutputisrequired,sincetheearisrelatively
insensitivetoextremelylowfrequencies.
2.
Implementation:
Mostinstallationsofsubwoofersaredictated
bytherequirementsofparticularscreenings,and
instructionswillgenerallycomefromthestudios
orthroughthefilmdistributors.TheDolbymodel
CP200processorprovidesa low-frequencyoutput
thatmaybeusedtodrivea subwooferchannel
directly.
TableV presentsdataonselectingthe
numberofsubwoofermodulesneededforparti-
cularsoundlevelsintheatersofdifferent
volumes.Theenclosurerecommendedhereisthe
4518.
Itsspecificationswiththe2245HLFdriver
aredetailedincolumn4 ofJBLTechnicalNote,
Volume1,Number1.Itisimportanttoprovide
adequateandstablepowerforpropersubwoofer
operation.
Wheneverpossible,theamplifiers
shouldbelocatedattheloudspeakersthemselves.
Notethatasthenumberofmodulesis
doubled,
thesoundpressureincreases6 dB.
ThreedBareduetothedoublingofpower
