JAS000999 Riverbank New Lab 1958

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THEACOUSTICAL

Volume 30

JOURNALOF THE

SOCIETY OF AMERICANumber 11

NOVEMBER. 1958

New Airborne Sound Transmission Loss Measuring Facility at RiverbankR. L. RmIaXr)s

Armour Research oundation,RiverbankAcoustical aboratories, eneva,llinois(ReceivedJuly 28, 1958)

A new sound ransmissionossmeasuring acility has been in use at Riverbank AcousticalLaboratoriesfor more than a year now. This paper describeshe two reverberation ooms, nstallationof the test panel,instrumentation,and measurement echnique.The several checksmade for measurementaccuracyandrepeatability are discussed. est resultsare given comparing he old method of test with the new, andresultsare alsogiven for a folding door measured t four testing aboratories n the United Statesand Canada.

I. INTRODUCTION

HE oundransmissionossTL) fawallrartition is definedas the ratio of incident totransmitted sound energy, with a reverberantsoundfield in both source and receiving rooms. Becauseofthe necessityor reverberant ound ields, he measure-ment of sound ransmissionoss equires pecial uildingfacilities which are both large and costly. RiverbankAcousticalLaboratoriescompleted n December,1956,a second reverberation room to be used for soundtransmissionoss esting. This new facility now meetsexplicitly he standards f both the AmericanSocietyfor Testing Materials and the American StandardsAssociation 2 for airborne sound transmission lossmeasurements.

II. TEST ROOMS AND TECHNIQUE1. Test Rooms

Figure 1 shows plan view of the two reverberationrooms.The new room, shownon the left, contains he ASTM Designation 90-55,"Standard ecommendedracticefor laboratorymeasurement f airbornesound ransmissionossof building floorsand walls," American StandardsAssociation,New York. This standard s presentlyunder revision.2 ASA DesignationZ24.19-1957, "American standard recom-mendedpractice or laboratorymeasurement f Airbornesound

sound sourceand the larger room receives he trans-mitted sound.The new room s rectangular,20 ft longby 14 ft wideby "11.5t high,and hasa volumeofabout 3200 cu ft. The insidesurfaces re hard plasterand concrete, nd there are severalprotuberancesntothe room which help to increasesound diffusion. Theroom absorptionbelow 1000 cps is 30 to 50 sabins.The larger reverberation oom is the one regularlyused for soundabsorptionmeasurements nd is 27 ftlong by 19.7 ft wide by 19.1 ft high with a volume ofabout 10 100 cu ft. To increasesound diffusion, thisroom contains two 6-by-12-ft steel reflector vaneswhich rotate at a rate of one rpm during the test, anda number of 12-in.-diam pillars, 4, 6, and 8 ft high,standingon the floornear the roomwalls.The absorp-tion of this room below 1000 cps s 100 to 110 sabins.This room has 18-in.-thick walls and a 22-in.-thickceiling of solid masonry, and is structurally isolatedfrom the source oom (and the rest of the building)with a separate oundationand surrounding 0-in. airspace.The test panel is positioned n the receiving oomwall, and is installedas shown n Fig. 2. The panel istransmissionossof building floorsand walls," American Stand-ards Association,New York.

999Copyright 1958 by the AcousticalSociety of America.

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usually constructednside a wood frame with insidedimensions 6.5 in. wide by 81 in. high, and is trans-ported on a specialcart to the openingwhere it isrolled up againsta permanentwood rame with insidedimensions he sameas thoseof the test panel. Sealingstrips aced on two sideswith sponge ubber are thenplacedaround he other sideof the panel rame, and athird removable frame is clamped tight against thepanel frame. This installation echnique llowsrapidchangeof panelsand provides solidcouplingbetweenthe test panel and the surrounding all, yet is airtightbecauseof the sponge ubber seals.Also, particularcare s taken to seal around he test panel nsideof itsown frame. Caulking compounds very effective orsealing small cracks and is generally used for thispurpose. eavierwalls,suchas brickor concrete lock,must be built directly in the test opening:A specialadapterpanel s used or the installationof doorsandpanels smaller than the standardsize. This adapterpanel s discussedn detail n a later section.

2. InstrumentationA diagram of the instrumentationused for themeasurementss shown n Fig. 3. Two WesternElectric

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type 633A microphones re used, one in each room,and are connectedo two identicalsystems achcon-sisting f a voltageamplifier, ttenuator, ariablebandpass filter (Allison type 2-B), and one channelof atwin-channel raphic evelrecorder SoundApparatusCompanymodelR-Z). A specialevelequalizing ircuitestablishes nd maintainsa constantsoundpressurelevel n the receivingoomby adjusting he level n thesource oom. This insures known, constantsignal-to-noise ratio on the low level side and facilitates reductionof the data.

3. TechniqueFor the test, the frequency f the sources swept n7.5 min from 100 o 5000cps,and s warbled -20 cpsat a rate of 5 timesper sec.The sweep s slowenoughso that the soundpressure evel in each room is thesameas it wouldbe for steadystate at all frequencies.During the sweep he sound pressure evels in eachroom are simultaneouslyecorded.A typical chart

Fro. 3. Block diagram of instrumentation or soundtransmission loss measurements.from the twin-channelgraphic evel recorder s shownin Fig. 4. The top half is a graph of the soundpressurelevel in the sourceroom, and the bottom half that ofthe receiving room. The heavy line is a smoothedaveragedrawn by eye and, in the caseof the receivingroom side, is straight becauseof the level equalizer.The rapid fluctuations about the average line aretypical of measurementsmade in a diffusesound ield.If the warbler and reflector vanes were not used thefluctuationswould be lessrapid and much greater namplitude making a smoothedaveragemore difficultto obtain.After two runsare made, he pickupsystemsare completely nterchanged nd two more runs made.The sound eduction,or differencen soundpressurelevel between he two rooms, s obtainedby averagingthe differences between the smoothed lines of each ofthe four charts.By interchanging ickup systems nydifferences n response etween them are eliminated.The sound eductioncurve s used n the preparationof the sound ransmissionoss curve as reported,



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RIVERBANK TRANSMISSION LOSS FACILITY 1001

Fro. 4. Typical chartfrom the twin-channelgraphic evel recorder.

accordingo Buckingham'sequationTL= Lx--L.q-10 logS-- 10 logA, (1)

where L=sound pressure evel in source room, L.=sound pressure evel in receiving oom, S=area oftest panel, fff', and A =absorption of receiving oom,sabins. Since the absorption of the receiving roomremains virtually constant the year around and ischecked epeatedly or sound bsorptionmeasurements,this measurement is not made for each TL test unlessthe test panel itself adds significantly to the roomabsorption.The test report includesa descriptionofthe test panel, the measurement echnique, and agraph of the TL vs frequency.Also, the TL valuesaretabulatedat eachof the nine standard requencies'andthe arithmeticaveragegiven.

III. CHECKS ON MEASUREMENT ACCURACYAND REPEATABILITY

Because walls with a known sound transmission lossare not available, t is impossibleo checkdirectly theaccuracyof the system. We do know, however, ofcertain conditions which must be fulfilled if the measure-ment is to be correct. Checks on how well these condi-tions are met are discussed below.

1. Flanking PathsSpecialcare s taken to see hat no significantpathsof flanking sound transmissionexist. How the testpanel is sealed nto the test openinghas already beendiscussed. ne suggestedmethod .' o test the effective-a E. Buckingham, "Theory and interpretation of experimentson the transmissionof sound through partition walls," Sci.

PapersNatl. Bur. Standards 0, 193 (1925), S506.

nessof the room walls in retarding the transmission fsoundbetweenrooms s to build in the test openingthe same structure as that of the walls. This was notdone becauseof the immensity of the walls, but ameasurement as made of the TL through wo plasterpartitions of identical construction 2-by-4-in. woodstudsspaced 6 in. on-centers,-in. gypsumboard ath,and -in. plaster each face), one installed in thesource room wall and the second n the receivingroom wall (standardposition),which providedstruc-tural isolation between the two partitions. The parti-tions were separatedby a 12-in. air space.The resultsof this measurement re shown n Fig. 5. The uppercurve is the TL of both partitions; the lower curve isthe TL of one of the partitions tested alone, and is

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Fro. 5. Sound transmissionoss of single and double plasterpartitions. For the double partition test, the partitions werestructurally solatedwith a 12-in. air spacebetween.TL valuesgreater han thoseof the upper curve can be measured.



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1002 R. L. RICHARDS

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Fxo. 6. Crosssectionshowingconstruction f one end of adapterpanel. This adapter s used or installingdoorssmaller n size hanthe regular est opening.typical of the other. Thesecurves, ogetherwith soundreduction measurements made between each room andthe adjacent air space, ndicate that the approximatelimit of TL values which can be measured are at least55 db in the low-frequency ange and 80 db in thehigh range.

A special dapterpanel s used or the installationof36-by-80-in.doorsand panels,which are smaller hanthe standard size opening.This adapter panel consistsof a frame of 2-by-12-in. umber divided vertically into3 sections.The middle section s 36 in. wide for receivingthe test door, and the two end sections,each 13 in.wide, are filled with gypsumboard sheetsas shown nFig. 6. If a doorwith a largeTL is tested n the opening,the sound energy transmitted through the adapterpanel itself becomes mportant. The TL through theadapter panel was determinedby filling the openingsolidlywith brick (12 in. thick). The TL of the brickwas krown from previous tests and was' negligiblecompared o the adapter at most frequencies. he TLof the adapter panel, measured n this manner, isshown n Fig. 7. If the measuredTL of a door beingtested n the adapterpanelcomes o within about 12 dbof the TL of the adapterpanel, hen the true TL of thedoor is found from the relationship hat the total soundenergy transmitted betweenrooms s the sum of eachcontributing element. n this case,Sr= Slrl-Jr-S,.r,., (2)

whereS= total area, Sq-S,.; r= measured ound rans-mission coefficientof door and panel; S=area ofadapter panel; S2= area of test door; r = sound rans-

missioncoefficientof adapter panel; and r,.=soundtransmission oefficient f test door. After solving orthe sound transmission coefficient of the doorr,.= (S/S,)(r-r)+r, (3)

the true TL of the door s obtainedby multiplying helogarithim of 1/r by 10. This adjustmentbecomeslarge and uncertain if the measuredTL of the door iswithin about 3 db of that of the adapter.Adapterwallsof masonryhave beenused or measuring oorswith avery high TL.

2. DiffusionThe useof Buckingham'squation or the determina-tion of sound transmission loss assumes that the sourceand receiving oomsound ieldsare reasonably iffuse.The special means used to increasesound diffusionhave been discussed in Sec. II. One check which in-

dicates sound diffusion is the effect of microphoneposition on test results. A seriesof sound reduction

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curves were run on a test panel at several differentmicrophone ositionsn both rooms,with the reflectorvanes in the receiving oom stationary. These curveswere all drawn on one graph and the mean ine drawnthrough he enclosed rea. Deviations rom the meanwereas muchas 2.5 db at a few of the lower requencies,but usuallywere ess han 1.5 db. In fact, deviations fabout the same size are noted when comparingrunsmade with the microphone ositionunchanged as inthe normal test procedure)and the reflectorvanesstarted at differentpositions t the beginningof eachrun. All microphone ositionswere chosen t least 4 ftfrom any room surfaceand at a distance rom thesoundsourcewhere the direct soundpressureevel wasat least 10 db below that of the reverberant level.

3. Reversal of Sound FlowMeasurements were made on three different testpanels, irst in the normalway, then by moving hesoundsource nto the large room and using the small

room to receive the transmitted sound. The results of



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RIVERBANK TRANSMISSION LOSS FACILITY 1003these checksare shown n Fig. 8. The top graph is theTL of a 2-in.-thickpanel with -}-in. plywood acesandglasswool packed n the core space.The middle graphrepresents wood stud panel with plywood faces andthe lower graph a wood stud, gypsumboard lath, andplaster partition. These curvesare smoothedaveragesof the original TL curves,so minor variations are notshown.Except for valuesat a few isolated requencies,the curvesare within the systemmeasurementerror.An additionalmeasurement f the wood-stud,plywood-faced panel was made after the absorption of thereceiving room was increased o about 200 sabins,almost double the normal amount. The results of this

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NEW TEST FACILITY..... OLD TEST FACILITY

FIo. 9. Comparisonof test resultsbetweennew andold measuring acility.of a reverberantsource oomand a special onreverber-ant receiving oom with certain known characteristics.Inherent in this measurement echniquewere severalassumptions bout the receivingroom sound ield notsusceptible o experimentalproof, and the difficultiesin obtaining an accuratemeasurement f TL by thismethodwerenever fully resolved.It was necessaryo destroy he old measuringacilityin building the new one, and unfortunatelyonly a fewtest panels could be kept for correlation purposes.Enough nformationhas been gatheredon similar con-structions now, however, to give a fairly reliableindication of the difference in results between the oldand the new. In Fig. 9 are shown he resultson threepanels which were measuredwith the old facility andthen remeasured fter the change. t can be seen hatthe difference s a function of frequency and is notconsistentor differentpanels,so correction f the oldmeasurementscan be applied only to the averagesound transmission oss value. Average differences fall the panelswhichwereretested, nd thosenew panelswhich were similar in construction o older panels,ranged from 4 db to nearly 10 db, but most werebetween 5 and 6 db. This means that the old results,

while too high on an absolutebasis,did put panels ntheir approximate rank order and can be used withreasonable ccuracy with a 5 or 6 db subtraction)nsituationswherea precise nowledge f TL vs requencyis not required.2. Correlation with Other Laboratories

At this time there s very little informationavailablecorrelatingRiverbank test results with those of otherlaboratories. n fact, the only material which hasrecentlybeenmeasured t eachof the principal testinglaboratories s an accordian-type olding door. Theresults of these measurements are listed in Table III.Txm:. III. Sound transmission loss values measured at fourtest laboratories.A different folding door was tested at eachlaboratory, although all were standard No. 228 Hufcor doors,described s follows: full accordiandoors,with 55 oz per sq yd5-ply, laminated overs achsideof enclosederticalpantographframe; sweepstrips top and bottom of one side to provide con-

tinuous contact with the floor and ceiling. These tests weresponsored y the Hough ManufacturingCorporation, anesville,Wisconsin.Sound Transmission Loss, dbFrequencycps Lab. As Lab. Bb Lab. Co Lab. D d

125 11.0 11.0 14.0 18.0175 9.0 9.0 18.0 16.0250 9.5 10.0 13.0 15.0350 11.0 10.0 14.0 15.0500 13.5 10.0 16.0 16.0700 17.5 17.0 20.0 20.01000 21.0 23.0 23.0 25.02000 25.0 26.0 29.0 27.04000 28.5 28.0 35.0 32.0av 16 16 20 20 Lab. A: Riverbank Acoustical Laboratories, August, 1957.b Lab. B: National Research Council, Ottawa 2, Canada, February, 1958.* Lab. C: Geiger and Hamme Laboratories, 3250 East Morgan Road,Ann Arbor, Michigan, June 1957. (Reported as sound reduction since noadjustment is made for 10 logs and 10 logA.)d Lab. D: National Bureau of Standards, Washington 25, D.C., October,1957.

It is recognizedhat this type of constructions not anideal choice o make sucha comparison ecause f thedifficulty in obtaining a uniform sealingcondition.Also, a differentdoorwas supplied o each aboratory,although achdoorwas a regularproductionmodelofidenticaldesign,and each nstallationwas supervisedby a representative f the doormanufacturer.It is plannedo conduct seriesof round-robintests on several different materials'iat each of theselaboratories. The results of these correlation tests willbe embodiedn a report to the Societyat a later date.

ACKNOWLEDGMENTSThe author is deeply ndebted o his colleagues tArmour Research Foundation, Mr. Ralph Huntley,Supervisor f RiverbankAcoustical aboratories,ndMr. Franklin Tyzzer, SeniorPhysicist, or their aid inpreparing he material or this paper.

JAS000999 Riverbank New Lab 1958

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