New Controllable Sound Absorbers Made of Vacuum Packed Particles - New Contributions in Information Systems and Technologies - page 283

3.2 Test Sample
Theexperimentwasconductedinaccordancewiththeappropriatestandard[9].
The properties of the structure during research were controlled by a vacuum
pump.Twogaugesprovidedreliablecontrolofpressureinsidethesample.The
elastic container was constructed in aspecificwaysoastoonlyfilltheexact
shape of test tube with granular material. It was constructed from a delicate
elasticmaterialformed in a cylindricalshape (scheme in fig. 1).The influence
of the container wall material was considered negligible. To keep the required
cylindricalshapeduringexperiments,tworeinforcementsweremade.Thefront
ofthesamplecontainedaverythinteflonring,andthebackwasmadeofasteel
diskofproperthicknesswithasmallairvalveforpressurecontrol.Thediskalso
actedasa'rigidwall'anditspurposewastoreflect100%oftheacousticwaves.
Fig. 1. Schemeoftestsample.1-reardisk,2-granularmaterial,3-frontmembrane,
4-pressurevalve.
Additionalmeasurementsweremadetoverifythecorrectnessoftheproposed
sampleconstruction.Theresultsforseveralsamples(including reference)were
compared to those performed with a standard piston/wall that came with a
selected test stand (the pressure was then controlled with a small 1mm tube
connectedtothefrontofsample).Comparisonofthequantitiesoftheabsorp-
tion coecient delivered in these two different ways has shown no measurable
differences.
Sealingofthefrontareaofthesamplewasaccomplishedbyaverythinand
delicate plastic material. It is obvious that it had some negative influence on
the overall absorption, but at the same time it seems to be the only solution
toconstructahermeticsamplethusenablingstructuralcontroloverthetested
materials.
Thegrainsofthetestedplasticswerebarrel-rollershaped(fig.2).Theirdi-
mensionswereabout2-3mmindiameterand3-5mminlength.Intable1there
isacomparisonofthemassesofthevariousgrainmaterialsfillingthesample.
3.3 Test Stand
Theteststandthatwasusedinthepresentedresearchhadtwofrequencyranges.
Forthepurposeofthis work,arangefrom500Hz to6400Hz waschosen.As
aresultofthat,thesampleshadadiameterof28.8mm,whiletheirlengthwas
200mm.