BASICGUIDELINES
FOR
THE USE
OF
HORNSAND DRIVERSWITHINASOUNDSYSTEM.
DESIGNING
FOR
INTELLIGIBILITY AND ADEQUATE SPL
The Basic Idea
Manysoundsystemswould havebetter performance
if
thefollowing basic
principlesarekeptinmind Speakerswiththeappropriatecoveragepatterns
shouldbechosen,aimedandpowered
to
achieveauniformdirectfieldinthe
highlyabsorbtiveaudience,with nosoundaimedatthereflectivewallandceil
-
ing surfaces Where multiple speakers are required in order
to
achieve a
uniformdirectfield,theircoveragepatternsshouldbeonlyslightlyoverlapped,
so
thateachsectionof the audienceiscovered bya singlespeaker To the
extentthis ideal is achieved reverberationis minimizedandintelligibilityis
maximized
Thefollowingmaterialexplainstheseconceptsinmoredetailandillustrates
two designapproaches
What isReverberation?
Reverberationisthepersistenceof soundwithinanenclosure,suchasaroom,
afterthe originalsoundhasceased Reverberationmayalsobeconsidered
asaseriesof multipleechoes
so
closelyspacedintime thatthey mergeinto
asinglecontinuoussound Theseechoesdecreaseinlevelwith successive
reflections,andeventuallyarecompletelyabsorbed by the room
Non
-
Reverberant Environments
Anopen,outdoor space isconsidered
to
beanon
-
reverberantenvironment,
as virtually allsound escapestheareawithoutreflection.
Variations inLevelDueto Distancefor Non
-
Reverberant Environments
Innon
-
reverberantenvironments,suchasoutdoors,soundpressurelevelwill
bereducedbyhalf
(6
dB)everytimethedistanceformthespeakerisdoubled
(thisis calledthe inverse
-
squarelaw).Figure
A
shows thedB losses
to
be
expected as distance from the speaker is increased from the one
-
meter
(3 28
-
foot)measuringdistancetypicallyusedinSPL specifications.
Reverberant Environments
Wheresoundisreflectedfromwallsandothersurfaces,thereisapoint beyond
which the
"
reverberantfield
"
dominatesand the sound pressurelevel is
higherand more constantthan predicted by usingthe inverse
-
squarelaw
alone
Variations inLevelDue
to
Distancefor Reverberant Environments
The reverberantfieldwillbegin
todominatetypicallyatdistancesof
10to30
feet Thisdistanceisgreatestfor the least reverberantroomsandspeakers
withnarrowbeamwidthangles Thefrequencyandbeamwidthspeckations
providedbythe datasheetarestillrequired
to
obtainsatisfactorydistribution
of
thedirectsound(ordirectfield)fromthe loudspeaker(s) whichstillfollows
the inversesquare law
It
is the direct signal that contributes
to
speech
intelligibility This iswhy the sound systemdesignershouldseeka uniform
directfield, withaslittlereverberantfieldas possible Forexample,consider
asinglespeakerwithawidebeamwidthangleused
to
coveralong,narrow
reverberantroom The directfieldwill be
so
far belowthe reverberantfieldat
the backofthe roomthat speechwill probably beunintelligible
Calculating Variations inLevelDue
to
ChangesinElectricalPower
Eachtimethe powerdelivered
to
the speakerisreducedbyone
-
half,alevel
dropof3dBoccurs Thenomographof
FigureBshowsthethechangein
dB
to
beexpectedas the powervariesfromtheone
-
wattinputtypicallyusedin
SPL specifications
Power Handling
Thepowerratingof aspeakermustbeknown
to
determinewhetheradesign
iscapable of meetingthe soundpressurelevelrequirementsof the system
Thepowerratingcombinedwiththesensitivitywill enableasystemdesigner
tocalculatethemaximumsoundpressurelevelattainableatagivendistance
DISTANCE
FROM
CHANGE
IN
SPEAKE:IH;
4
(dB)
M
f25
lmm
::$a
M
M
3+m
FIGURE
A
FIGUREB
LevelVariation LevelVariation
WithDistance WithPower
PoweringtoAchieve BothAverage andPeakSPL
Theaveragepower that must bedelivered
to
the speaker@)
to
achievethe
desired average SPL can be determined from the previously presented
materialonspeaker sensitivity, levelvariationwithdistanceand levelvaria
-
tion with power. Enoughadditionalpower must beavailable
to
reproduce
withoutdistortiontheshort
-
termpeaksthatexist invoiceandmusic program.
Thisdifferencebetweenthepeakandaveragecapabilityof asoundsystem,
whenexpressedindB,isoftencalled
"
peak
-
to
-
averageratio,
""
crestfactor
"
or
"
headroom.
"
The peakscanbelarge,as notedearlier:at least
10
times
theaverage
(1
0
dB).
Thebettersoundsystemsare designedfor peaksthat are
10
dBabovethe
average,although
6
dBof headroomissufficientfor most general
-
purpose
voicepagingsystems The
1
0-dBpeaksrequireamplifier powertentimesthat
requiredfortheaveragesound levels The6
-
dBpeaksrequirefourtimesthe
power.
Utilizing Speaker BeamwidthInformationfor Maximum Intelligibility
Knowingthebeamwidthangle
of
aloudspeakercanaidinprovidingaunlorm
directfieldinthe listeningarea After selectingadesiredspeaker location,the
beamwidthangle needed
to
adequatelycover the listenerswithoutspilling
over
to
the walls or ceilings must be determined Once these angles are
known thecorrect speaker can befound by usingcatalogspecifications
UsingEasy
-
VAMP
TM
andFloor
-
PlanIsobars
Insomecircumstances
d
isdesirable
to
useanapproachthat ismoredetailed
thanusingthe basichorizontalandvertical beamwidthangles Environments
which have excessive reverberationor high ambient noise levels make
it
especially difficult
to
achievethe desired
SPL
and intelligibility
In recent years a number of computer based techniques have been
developed
to
help sound system designers Some of the more complex
systems use personal computers with relatively sophisticatedgraphics
Simplersystems,suchas ElectroVoice
s
VAMP
TM
(VeryAccurateMapping
Program) utilize clear overlays and require programmable scientific
calculators However,the hardwareisoftwareandtraininginvestmentrequired
to
utilizeeventhesimpler systemsarenotattractivetosomesoundsystems
designers Because of this University Sound has developed a special
adaptationof VAMP,calledEasy VAMP
TM
whichprovidesasimilardesign
aidwithout the complexityandcostof the VAMP programs
More information
on
boththe Easy VAMP
TM
and floor planisobarscan be
found inthe UniversitySoundGuide
Part Number
531291
-
923
Litho
in
U.S.A
US
42
-
02
-
0261
17
