Recent Trends in Porous

www.SandV.com12SOUND&VIBRATION/JULY2010Sound-absorbingmaterialsabsorbmostofthesoundenergystrikingthem,makingthemveryusefulforthecontrolofnoise.Theyareusedinavarietyoflocations–closetosourcesofnoise,invariouspaths,andsometimesclosetoreceivers.Althoughallmaterialsabsorbsomeincidentsound,theterm“acousticalma-terial”hasbeenprimarilyappliedtothosematerialsthathavebeenproducedforthespecificpurposeofprovidinghighvaluesofabsorption.Themajorusesofabsorbingmaterialsarealmostinvariablyfoundtoincludethereductionofreverberantsoundpressurelevelsand,consequently,thereductionofthereverbera-tiontimeinenclosures,orrooms.Awiderangeofsound-absorbingmaterialsexist.Inthe1970s,publichealthconcernshelpedchangethemainconstituentsofsound-absorbingmaterialsfromasbestos-basedmaterialstonewsyntheticfibers.Although,thesenewfibersaremuchsaferforhumanhealth,morerecently,issuesrelatedtoglobalwarmingmayincreasetheuseofnaturalfibersinsteadofsyntheticones.Noiseandvibrationareofconcernwithmanymechanicalsys-tems,includingindustrialmachines,homeappliances,vehicles,andbuildings.1-3Noiseandvibrationcanalsobeusedasthesourceofsignalsformachinerydiagnosticsandhealthmonitoring,althoughthistopicwillnotbediscussedhere.4,5Oncenoiseandvibrationsourceshavebeenidentified,thenoiseandvibrationofmachinerycanbereducedbytheuseofvibrationisolation,barriers,sound-absorbingmaterials,machineenclosuresorbycabinenclo-suresusedtoprotectpassengersinthecaseofaircraftorvehicles.6,7Sound-absorbingmaterialsshouldalwaysbeusedinconjunction