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1、CHAPTER1BasicAcousticalQuantities:LevelsandDecibelsLEOL.BERANEKConsultantCambridge,Massachusetts1.1BASICQUANTITIESOFSOUNDWAVESSoundWavesandNoiseInthebroadestsense,asoundwaveisanydisturbancethatispropagatedinanelasticmedium,whichmaybeagas,aliquid,orasolid.Ultrasonic,sonic,andinfrasonicwavesa
2、reincludedinthisdefinition.Mostofthistextdealswithsonicwaves,thosesoundwavesthatcanbeperceivedbythehearingsenseofahumanbeing.Noiseisdefinedasanyperceivedsoundthatisobjectionabletoahumanbeing.Theconceptsbasictothischaptercanbefoundinreferences1–7.PortionsarefurtherexpandedinChapter2.SoundPress
3、ureApersonwhoisnotdeafperceivesassoundanyvibrationoftheeardrumintheaudiblefrequencyrangethatresultsfromanincrementalvariationinairpressureattheear.Avariationinpressureaboveandbelowatmosphericpressureiscalledsoundpressure,inunitsofpascals(Pa).∗Ayoungpersonwithnormalhearingcanperceivesoundint
4、hefrequencyrangeofroughly15Hz(hertz)to16,000Hz,definedasthenormalaudiblefrequencyrange.Becausethehearingmechanismrespondstosoundpressure,itisoneoftwoquantitiesthatisusuallymeasuredinengineeringacoustics.Thenormalearismostsensitiveatfrequenciesbetween3000and6000Hz,andayoungpersoncandetectpres
5、suresaslowasabout20µPa,which,whencomparedtothenormalatmosphericpressure(101.3×103Pa)aroundwhichitvaries,isafractionalvariationof2×10−10.∗Onepascal(Pa)=1newton/metersquared(N/m2)=10dynes/cm212BASICACOUSTICALQUANTITIES:LEVELSANDDECIBELSPureToneApuretoneisasoundwavethatcanberepresentedbytheequ
6、ation,p(t)=p0sin(2πf)t(1.1)wherep(t)istheinstantaneous,incremental,soundpressure(aboveandbelowatmosphericpressure),p0isthemaximumamplitudeoftheinstantaneoussoundpressure,andfisthefrequency,thatis,thenumberofcyclespersecond,expressedinhertz.Thetimetisinseconds.PeriodAfullcycleoccurswhentvari
7、esfromzeroto1/f.The1/fquantityisknownastheperiodT.Forexample,theperiodTofa500-Hzwaveis0.002sec.Root-Mean-SquareAmplitudeIfwewishtodeterminethemeanvalueofafullcycleofthesinewaveofEq.(1.1.)(oranynumberoffullcycles),itwillbezerobecausethepositivepartequalst
