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The typical outcome of continuous technologicalprogress,development and
popularization of transport in industrial areas is an increase of the number of
soundsources.Thisleadstoanoverallincreaseofacousticalenergythathumans
are in contact with. This occurrence is accumulating despite new regulations
(standards,EUdirectives,laws)thatdefinepermissiblenoiselevelsforhumans
andtheenvironment.
2 Absorbing Materials
Thespecificapplicationofapassivebarrierinacousticsshouldbeginfirstlywith
understandingtheneeds.Itshouldbeprecededwithcomplexresearchregarding
the problematic noise (frequency, level, overall analysis). If for instance there
is a need to decrease the sound level of machines in a production hall (with
operators working nearby), the best solution would probably be to construct
barriersorhousingsforallmachinesortheirelementsresponsibleforthenoise.
This solution enables good access and provides a good working environment.
In the case of a noisy machine working in a separate area, the best idea is to
isolateitfromtherestoftheproductionarea.Everysinglenoiseissueshouldbe
treatedseparatelythen.Therearenouniversalsolutionsthatarebestinevery
situation.Sometimeswhenmakingnoisemeasurementsonanobject,avibration
studyshouldbeincluded(NVH-noiseandvibrationharshness).
Tomaximizetheacousticeffectivenessfunctionalityofasound-proofstructure
(e.g. in resistance to environmental conditions), often multi-layered structures
are used. In these constructions several materials are combined and shielded
withstiffplastictoprotectthemfromchemicalormechanicalfactors[17].That
sortofconstructionisreasonablebecauseitproducesanonlineardistributionof
absorptioncoe cientinthefrequencydomain.Moreover,thecoe cientvalueis
rarelylinearlydependentonthethicknessoftheabsorbingmaterial,soaproper
combinationofseveralindividualthinlayerswithvariouscharacteristicsmight
givebetterresultsthanusingathickermaterialofonetype[13].
Absorbing materials most often have a porous, fibrous or similar structure
[1].Acousticabsorptionisaccomplishedwithacomplexvisco-thermaldiffusion
mechanism [13]. In literature there are a number of papers concerning these
mattersveryclosely.Acousticabsorptionisafunctionofdensity,porosity,tor-
tuosity etc. [2],[4]. Most tests show that the performance of these materials is
satisfactory above 1 kHz [5]. In these tests reliable results most often can be
foundbetween100and7000Hzsincethesearethelimitationsofthecheapand
fastKundtpipemethod.Itisrarelyfoundinthepublishedresultsthatthetest
frequenciesequalaman'shearingrange,becausethatkindofresearchrequires
acompletelydifferentsetupandaspecializedfacility.
Nowadaysthereisageneralsearchforgreensolutionsinengineering.Thiscon-
cernsalsoabsorbingstructures.Itturnsoutthatinnovativematerialscanbefound
amongindustrial(production)wasteandnaturalorbiodegradablematerials[6],
[7],[12].Theirconsiderableadvantageisalowerpriceandoftencommonavailabil-
ity(comparedtoconventionalmaterials).Theresearchregarding'green'absorbing
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