FUNDAMENTALS OF ACOUSTICS(1)

FUNDAMENTALSOFACOUSTICS(1)XuXiaomeiEmail:[e-mail]•Prerequisites:Anunderstandingofbasicmathematics,thefundamentalprinciplesofmechanics,electricity,andaccesstoascientificcalculatorObjective:•Throughoutthecourseemphasisisplacedonfundamentals,withdiscussionsandproblemsextendingintosomephasesandapplicationsofacousticsThegeneration,transmissionandreceptionofacousticwaves•Oncompletionofthiscoursestudentswillbeabletounderstandthetermsusedtodescribesoundwaves,thegeneration,transmission,andreceptionofacousticwavesandtheirapplicationtonumerousfields;therelationshipbetweensoundpressure,soundintensityandsoundpower;andthetermsusedtodescribesoundfields.contentsIntroductionWhatsoundiscausedby…whatasoundwaveis…whatacycleis….Whatissound?Soundismechanicalwaveofenergythatchangesthepressureofitsmedium(airorwater)asitmoves.Thesechangesofpressurearedetectedbyoursenseofhearingandtransmittedtoourbrainsforinterpretation.Soundwavesaredescribedbytheirwavelength,amplitudeandfrequencyandintensity(indecibels).Anysound,whateveritmightbe,iscausedbysomethingvibrating.Withoutvibrationtherecanbenosound.Thevibratingbodycausestheairparticlesnexttoittovibrate;thoseairparticles,inturn,causetheparticlesnexttothemtovibrate.Inthiswayadisturbanceoftheairmovesoutfromthesourceofthesoundandmayeventuallyreachtheearsofalistener.Whenwehearasound,airvibratesagainstoureardrumscausingthemtovibratealso.Thesevibrationsaredetectedandanalyzedbyourbrains.Althoughitisusuallyairthatactsasthetransmittingmedium,soundcanbetransmittedbyothermedia,e.g.water,buildingstructuresThetransmittingmediumSounddoesnotconsistofairmovingtowardsusinbulk;ittravelsthroughtheairasasoundwave.Asoundwaveconsistsofadisturbancemovingoutfromasourcetosurroundingplaceswiththeresultthatenergyistransferredfromoneplacetoanother.SoundwaveAsthewavepasses,thedisturbanceofparticlesisinthedirectionofthewavetravel.Thedisplacementofparticlesofthemediumresultsinalternateregionsofhighparticledensityandlowparticledensity.Regionsofhighparticledensityarecalledcompressions.Regionsoflowparticledensityarecalledrarefactions.CompressionsandrarefactionsRarefactionsandcompressionsbothmoveinthedirectionofthewavetravel.Theparticlesofthemediumdonotmovebodilyinthedirectionofthewavemovement;theyvibrateabouttheirnormalpositions.Eachcompletevibrationofaparticleiscalledacycle(i.e.fromitsstartingposition,toamaximumdistanceinonedirection,backthroughthestartingposition,thentoamaximumdisplacementintheoppositedirectionandbacktothestartingplace).acycleAssoundspreadsoutinanopen-planoffice,itmeetsobstacles:floor,ceiling,partitions(partial-heightscreens),lightfixtures,furniture,etc.Theseobstacleschangethepathofthesound.Whendesigninganopen-planofficetoblocksound,designersmustconsiderallofthesepaths.transmissionAttenuation:Assoundwavespropagatethroughanymedium,thesoundenergydiminishesduetospreading,scattering,absorption,andsoundtransmissionloss.•Spreadingoccursasthesoundexpandsinsphericalwaves.Thesoundleveldecreasesasthedistancefromthesourcebecomeslarger•Scatteringoccursasthewavedirectionchangesthroughdiffractionorreflection;•Absorptionoccursasthesoundentersaporousmaterialandgetstrappedintheairpockets.ThetrappedsoundenergyisconvertedtootherformsofenergyHowdoessoundtravelthroughwater?Waterisanidealmediumforthetransmissionofsound.Thespeedofsoundinwateris5timesfasterthanitisinair.Soundtravels334meterspersecond(m/s)inairandapproximately1500m/sinwater.Thisspeedcanbealtereddependingonthesalinity,temperatureanddepthofthewater.NoiseNoiseisdefinedasunwantedordamagingsound,thatis,soundwhichinterfereswithwhatpeoplearetryingtodo,orsoundwhichhasanadverseeffectonhealthorsafety.Tobeabletodealwiththeproblemsofnoisewemustfirsthaveanunderstandingofbasicacoustics-thenatureofsoundanditsphysicalproperties.Examplesnoiseatworks