G.R.A.S. Sound & Vibration
Sound-intensity Probe Type 50AI - Page 13
4. Handling and Assembling the Probe
4.1 The Microphones
TheMicrophonesType40AI(Fig.1.3)areapairofspecialfree-eldmicrophoneswith
extremelywell-controlledphasecharacteristics.Theyaredeliveredasamatchedpaireachwith
individualcalibrationdataaswellasdataondifferencesbetweentheirphaseresponses.
Thesemicrophoneshaveauniquepressureequalisationsystemwhichensuresawelldened
lower-limitingfrequencyandanextremelylowsensitivitytosoundpressuresatthepressure
equalisationchannels.Therefore,theycanbecalibratedinsingle-portphasecalibratorssuchas
theG.R.A.S.IntensityCalibratorType51AB.
4.2 Probe Design
ThedesignoftheSound-intensityProbeminimisesacousticreectionsandtheinuenceof
diffraction.Thishasbeenachievedbyremovinganyphysical,reectivecomponentsfromthe
soundpathat0°incidence.Sincesoundwavesat0°incidencearethemaincontributorstothe
totalsoundintensitylevel,itisimportantthatdisturbancesinthisdirectionbeminimised.
Thethin,2mmdiameterpreampliercableswillhavenoinuenceonthesoundeldsincethey
constituteahighlyirregularsurfacewithnegligiblereections.Theeffectsofacousticdiffraction
andreectionfromthephysicalpartsoftheSound-intensityProbearebelow0.15dB.
Microphonesaresuppliedaspairs(Type40AI)orassets(Type40AK)whichincludespacers
andthreepreamplieradapters(tworight-angledandonestraight).
Thedistancesbetweenmicrophonesandpreampliershavebeenkepttoaminimuminorder
toavoidproblemswithanystraycapacitanceandsensitivitytovibration.Whileamplitude
characteristicsarelittleinuenced,thephasecharacteristicsofaSound-intensityProbecan
becriticallyaffectedbyevenverysmallvibrationsintheconductorscarryingtherawsignals
fromthemicrophones.Therefore,the¼″preampliersaremountedinrigidcontactwiththe½″
microphonesviashortadapters(right-angledand/orstraight).Thisalsoeliminatesproblems
withnon-matchingcapacitancesbetweenmicrophonesandpreampliers,whichcouldgiverise
to phase problems.
4.3 Physical Strength
Fromaphysicalpointofview,aSound-intensityProbeshouldberobustandeasytoassemble
anddismantle.Typically,therearetwopointsinaSound-intensityProbewhichcanbeidenti-
edascriticalforphysicalstrengthandarethemostlikelytosufferdamageandarethemost
difculttorepair.Thesepointsarethethreadsonmicrophonesandpreampliersaswellason
themicrophones’protectiongrids.Theconnectionsbetweenmicrophonesandpreampliersare
verydelicateandcarrybothmicrophonesignalsandmicrophonepolarisationvoltages.There-
fore,thepreamplierthreadsoftheType50AIaresupportedbystainlesssteel½″to¼″adapt-
ers.Thereisalsoaprotectiveguardwithinthe¼″housingofeachpreamplier.Inaddition,the
microphones’protectiongridsaremadeofstainlesssteeltowithstandroughphysicaltreatment
sinceabuckledordamagedprotectiongridwillalmostinvariablydamageamicrophone’sdia-
phragmbeyondrepair.
4.4 Assembling the Probe
TheSound-intensityProbecanbeassembledineitherastraightcongurationorasymmetri-
calcongurationThestraightconguration(Fig.4.1),isforintensitymeasurementscloseto
surfacesandgeneralsourcelocationmeasurements.Thesymmetricalconguration(Fig.4.2),
isidealforsoundpowermeasurements,forexampleaccordingtointernationalstandardISO
9614-2“Acoustics-Determinationofsoundpowerlevelsofnoisesourcesusingsoundinten-
sity”wherearotationtestisrequired.
TheprobeontheType50AIconsistsofthepartsshowninFig.4.4.Assembleasfollows.