Alternate focusing fluids for passive acoustic targets H. Boehme and James E. Stockton
AppliedResearch Laboratories,The University of Texasat Austin,P.O. Box8029, Austin, Texas 78713-8029
( Received18June 1990;acceptedfor publication2 August 1990)
Fluid-filled,focusingspheresare oftenusedaspassiveacoustictargetsbecauseof their relativelyhigh targetstrengthand aspect-independent characteristics. A fluid mixture consistingof equalpartsby weightof trichlorotrifluoroethane and carbontetrachloride providesa near-optimumindex of refraction.The incentiveto investigatealternatefilling fluids arisesbecauseof the potentiallyhazardousnatureof carbontetrachloride.Target strength measurements were madeon stainlesssteelsphericalfloatsfilled with pure inert fluorochemicalsas well as other fluid mixtures.Resultsindicatethat passiveacoustictargets canbe successfully designedusingfocusingfluidslesshazardousthan carbontetrachloride. PACS numbers:43.30.Ft, 43.85.Vb, 43.30.Yj INTRODUCTION
In many activesonarapplications,fluid-filled,focusing spheresare used as passiveacoustictargetsthat combine relativelyhigh target strengthand uniform reflectivitywith respectto aspectangle.The acousticbehaviorof suchstructureshasbeeninvestigatedbothanalyticallyandexperimen-
a numberof factors,two of the most importantbeingconvenience and availability. The authors have used thinwalled,stainless steelspheresfilledwith equalpartsof Freon
113© andcarbontetrachloride asreference acoustic targets
for a varietyof sonartestand data acquisitionapplications. The sphericalshellsare availablein a wide varietyof materitally.1-3The acoustic focusing characteristics areachieved als, diameters,and wall thickness;deploymentis facilitated options. 5The by filling thin-walled,metal shellswith fluidshavingsound by an equallywidevarietyof floatconnection filling fluids are readily available from general chemical supspeedslower than that of the mediumin whichthey are to be pliers and mix in all proportions, with no evidence of separaused(usuallyseawater). The optimumindexof refractionof the fillingfluid, relativeto the soundspeedin the surround- tion in time over normal temperature ranges. Target strengthmeasurements havebeenrepeatablewithin + 1 dB ing fluid, appearsto be about 1.8 (Refs. 1,2). for the same sphere diameters and fluid mixtures,evenwhen An idealfillingfluid wouldbeonein whichthe optimum spheres and fluids purchased at different times were comindexof refractionisachievedwith a single-fluidcomponent pared. and maintainedover a broad temperaturerange.In reality, The incentiveto investigatealternatefillingfluidsarises an indexof refractionnearoptimumis achievedat a selected primarily from the potentialcarcinogenichazardof carbon temperature;most fluids have negativetemperaturecoeffitetrachloride;in addition, this chemicalis highly toxic and cients that cause the index of refraction to increase with deirritating by inhalation and ingestion,and actsas a solvent creasingtemperature.Also, a desiredindexof refractionat a selectedtemperatureis commonlyachievedby mixing two
or morechemical components. ToulisI mentions a singlefilling fluid Fluorolube, a chlorotrifluoroethylenepolymer
for manymaterials. 6 The healthhazardof thischemicalis
consideredsufficientlyseriousthat interstateshipmentis restricted.
The relatively inert fluorochemicalshave some of the desiredcharacteristicsof filling fluidsfor focusingspheres, namely,low soundspeed,low vaporpressure,characteristic impedancenearthat of water, and high chemicalinertness. marketedby Pennwalt Corporation,to achievevariousvalHowever, they typically do not mix with other low-soundues of refractive index. Marks and Mikeska 3 used a fluid speed chemicals,including other fluorochemicals,which mixture consistingof equal parts by weight of trichlorotripreventsadjustmentof soundspeedfor optimum index of fluoroethane (DuPont-Freon113©;AlliedCorp.-Genesolv refraction.Two suchfluorochemicals, FC-43 © andFC-70 © D © ) andcarbontetrachloride(CC14) to achievean indexof purchasedfrom Pennwalt Corporation,were usedin pure refractionnear 1.8. A studyof fluidsfor acousticlenseswas form as filling fluids in separate10-cm (4-in.)-diam, stainperformedfor the U.S. Navy Ship Researchand Developlesssteelspherical floats. 7Two otheridenticalspheres were ment Laboratory, Panama City, FL (now Naval Coastal
(CF2CFC1)x, alsocalledbythetradenameFS-5© (Hooker ElectrochemicalCo.). Folds2 useda mixtureof Fluorolube andanotherfluorochemical FC-75,a "Fluorinert ©" liquid
Systems Center),by Leaderet al.,4 of PennwaltCorpora-
filledwith pureFreon113©,anda 50/50 mixtureof Freon 113© and carbontetrachloride.The stainless steelsphere
tion, in an attempt to identify liquid mixturesthat would haveappropriatesoundspeedsand temperaturecoefficients filledwith the 50/50 mixture representeda referenceacoustic target for which considerablemeasurementdata have near that of seawater. beenaccumulated.Table I providessomeof thepropertiesof I. FLUID-FILLED
PASSIVE
ACOUSTIC
TARGETS
The selectionof a particularfillingfluidor fluidmixture for focusingsphereacoustictargetsgenerallydependsupon 2484
J. Acoust. Soc. Am. 88(5), Nov. 1990;
the fluids used (additional fluids listed will be discussed shortly).
Acoustic target strengthmeasurementswere made at Lake Travis Test Station,an acousticcalibrationfacility op-
0001-4966/90/112484-03500.80;
@ 1990 Acoust. Soc. Am.; Letters to the Editor
2484
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TABLE I. Properties of fluidsusedfor focusing sphereacoustictargets. Soundspeed
Density
Temperature
Av/A T
( 105cm/s)
(g/cm3)
(øC)
(m/s øC)
0.655 0.687 0.719
1.85 1.94
25 25
1.57
20
NA NA -- 3.34
0.935
1.59
20
-- 2.7
1.170
0.79
20
-- 4.0
Fluid FC-43 © FC-70 © Freon 113 ©
-10
Carbon
tetrachloride
(CCl4) Ethyl alcohol (C2H5OH) Methylene
ß
•
50/50F113/CCl •
67/33F113/EA
................... •;!• .............. 65/35F 113/MC
chloride
(CH•_CI2)
1.092
1.34
20
NA
-30 10
100
NA-not
FREQUENCY
1000 - kHz
available
FIG. 2. A comparison of measured acoustic targetstrengths for 10-cmdiam spheresfilled with three differentfluid mixtures.
eratedby Applied ResearchLaboratories,The Universityof Texasat Austin, and locatedon a fresh-waterlake approximately 15milesfrom Austin, TX. Measurementsweremade by suspending the sphericaltargetsunder teston monofilament line at a depth of about6 m. An acousticprojectorand hydrophonewere suspendedat the samedepth. A transmit pulse length of 0.2 ms and a receivergate length of 0.1 ms wereusedfor all measurements. The targetecholevelswere suchthat reflectionsfrom the front surfaceof the spheres,as well as the focusedechoes,were clearly discernible.The receive gate was positionedwithin the steady-stateregion of the incidentpulsesand focusedechoes;the amplituderatios were usedin computing target strength. Figure 1 shows a comparison of measured target strength(in dB re: 2-m radius,perfectlyreflectingsphere) valuesasa functionof frequencyoverthe frequencyrangeof 40-280 kHz. The water temperatureduring thesemeasurementswas 14 øCand the fluid-filledsphereswere allowed to stabilize at this temperature before measurementswere made.As seenin this figure,the targetstrengthvaluesfor the pure liquids are considerablybelow that of the 50/50 mixture exceptat the lower frequencypart of the curves.However, the curvesare relatively constantwith frequencybetween 40 and 100 kHz and might representuseful target strength values for some applicationsin this frequency
Since the chemical constituent Freon 113 © is less hazardous than carbon tetrachloride
in the fluid mixture
con-
ventionally used,the two additional chemicalslisted in Table I were selectedasreplacementsfor carbontetrachloride. Linear proportionsof the chemicalconstituentsof the fluid mixtureswere usedto estimatepercentagesby volume that would yield an index of refraction of about 1.8 for the mixtures.No problemswereencounteredin mixingvariouspro-
portionsof Freon113© andeitherethylalcoholor methylene chloride.A 67/33 mixture of Freon 113© and ethyl alcoholanda 65/35 mixtureof Freon 113© andmethylene chloride were prepared. These mixtures were used to fill identical 10-cm-diam, stainlesssteel spherical floats, and target strengthmeasurementswere made. A third identical
spherefilledwitha 50/50 mixtureofFreon113© andcarbon tetrachloridewas again included in the acousticmeasurements.
Figure2 showsa comparisonof the targetstrengthmeasurementsfor the three differentfilling fluid mixturesmade overthe frequencyrangeof 40-280 kHz and at a water temperatureof 20 øC.In general,the measuredtarget strength valueswerecomparablefor all threefluid-filledspheresover the entire frequencyinterval. However, the mixtures containingethyl alcoholand methylenechlorideexhibitedreso-
range.Attemptsto mix FC-43© and FC-70© with other
nance characteristics, while the conventional 50/50 mixture
fluorochemicalsand with glycerin to modify the sound speedwere unsuccessful.
did not. The apparentresonances are the subjectof further studiesand may representmanifestationsof the resonances
referredto by Flax et al.,8andby Murphyet al.9 -10
II. CONCLUSION
Lesshazardouschemicalcomponentshavebeenidentified to replace carbon tetrachloridein mixtures to use as focusingfluidsin passiveacoustictargetswithout reduction in measuredtarget strength over a significantfrequency range.Single-component fluidsthat are relativelyinert were also identifiedthat might be usedover a more limited fre-
-2O
-25 -•0 ß --
100FC70
quency
-35
range.
......... • 100FC43 • •
-4olo
100Fl13
I
I
I
I I
lOO FREQUENCY
lOOO
- kHz
W. J. Toulis,"AcousticFocusingwith SphericalStructures," J. Acoust. Soc. Am. 35, 286-292 (1963).
FIG. 1. A comparisonof measuredacoustictargetstrengthsfor several10cm-diamfluid-filledsphericalfloats. 2485
J. Acoust. Soc. Am., Vol. 88, No. 5, November 1990
D. L. Folds,"TargetStrengthof FocusedLiquid-FilledSpherical Reflectors," J. Acoust. Soc. Am. 49, 1596-1599 ( 1971 ). Letters to the Editor
2485
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B.M. MarksandE. E. Mikeska,"Reflections fromFocused Liquid-Filled SphericalReflectors,"J. Acoust.Soc.Am. 59, 813-817 (1976).
G. R. Leader,A. H. Fainberg,R. L. Atwell,andJ. R. Soulen,"Fluidsfor Acoustic Lenses," Pennwalt Corporation, Research and Development Department,King of Prussia,PA, August1969.
7The4-in.-diam,20-gauge, type304stainless steelfloatswerefittedwitha single(•)-in. stainless steelplug with eye.The plug unscrewed to allow filling and the eye was usedfor support (with monofilamentline) of the fluid-filledsphereduring acousticmeasurements.
8L. Flax,L. R. Dragonette, andH. Uberall,"TheoryofElasticResonance
Metalfloatsareavailable froma varietyofmanufacturers including Nich-
Excitationby SoundScattering,"J. Acoust.Soc.Am. 63, 723-731 (1978).
olson Division, Datron Systems,Inc., P.O. Box 113, Wilkes-Barre, PA; and ChicagoFloat Works, 230 ScottStreet,Elk Grove Village,IL.
9j. D. Murphy, J.George, A. Nagl,andH. •berall,"Isolation oftheReso-
6MaterialSafetyData SheetNo. 410, revisionA (Genium,Schenectady,
nant Component in Acoustic Scattering from Fluid-Loaded Elastic SphericalShells,"J. Acoust.Soc.Am. 65, 368-373 (1979).
NY, 1980).
ERRATUM
Erratum: "Stimulus selection in adaptive psychophysical procedures" [J. Acoust. Soc. Am. 87, 2662-2674 (1990)] David M. Green
Psychoacoustics Laboratory,Psychology Department,University ofFlorida,Gainesville, Florida32611
(Received5 July 1990;acceptedfor publication7 July 1990) ß
PACS numbers:43.66.Yw, 43.10.Vx [WAY]
The firstequationof a recentpaperhasan exponentin the wrong position. The equationshouldread
instead of
probability ofA-- .•IAc(x,)[1--A•(xi)] . i=l
probability ofA= • A(xi)c[1--A(x,)]•, i=1
2486
J. Acaust.Sac. Am. 88 (5), November1990
(1)
I regret the error.
@ 1990 Acaust.Sac. Am.; Erratum
2486
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