Basic of Acoustics

8/22/2019 Basic of Acoustics

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BASICS OF ACOUSTICS


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CONTENTS

1. preface

2. room acoustics versus building acoustics

05

0506060708090910

11

11141616

18

03

04

3. fundamentals of acoustics

4. room acoustic parameters

5. index

3.1 Sound

3.2Soundpressure

3.3Soundpressurelevelanddecibelscale

3.4Soundpressureofseveralsources

3.5Frequenc y

3.6Frequencyrangesrelevantforroomplanning

3.7Wavelengthsofsound

3.8Levelvalues

4.1 Reverberationtime

4.2Soundabsorption

4.3 Soundabsorptioncoefcientandreverberationtime

4.4 Ratingofsoundabsorption


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1. PREFACE

Noiseorunwantedsoundsisperceivedasdisturbingandannoyinginmanyfieldsoflife.Thiscanbeobservedinprivateaswellasinworkingenvironments.Severalstudiesaboutroomacousticconditionsandannoyancethroughnoiseshowtherelevanceofgoodroomacousticconditions.Decreasingsuccessinschoolclassroomsoraffectingefficiencyatworkisoftenrelatedtoinadequateroomacousticconditions.ResearchresultsfromclassroomacousticshavebeenoneofthereasonstoreviseGermanstandardDIN18041onAcousticqualityofsmallandmedium-sizedroomfrom1968anddecreasesuggestedreverberationtimevaluesinclassroomswiththenew2004versionofthestandard.Furthermorethestandardgaveadetailedrangeforthefrequencydependenceofreverberationtimeandalsoextendedtherangeofroomstobeconsideredinroomacousticdesignofabuilding.

Theacousticqualityofaroom,betteritsacousticadequacyforeachusage,isdeterminedbythesumofallequipmentandmaterialsintherooms.Inthesenseofgoodacousticstheroomsshouldcontributetoperceivespeech,musicorothersoundsasnottooloudortooquietandthewecancommunicatewithmucheffortandfeelcomfortable.

ThisbrochurehasbeendevelopedbyCrationBaumanwiththeintentiontogiveanintroductionandprofessionalsupportinthefieldofroomacousticsthatsometimeshastheconnotationofbeingconfusingortoomulti-dimensional.Itilluminatesimportanttermsandexplainsbasicsandinterrelationshipsofroomacoustics.

WithitspaletteofcreativetextilesforroomsCrationBaumanndeliversacousticallyeffectiveaswellasartisticattractivesolutionsforroomacousticquestions.Thebandwidthoftheacousticefficiencyoftextileapplicationsisoftenunderestimated.ForthisCrationBaumannofferswithitslargedocumentationofacousticpropertiesforitsmaterials-thatisavailableseparatelyagreatpotentialinmodernsolutionsforacousticsbytextiledesigninaroom.


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2. ROOM ACOUSTICS VERSUS BUILDING ACOUSTICS

Thedifferencebetweentheeldsofroomacousticsandbuildingacousticsbecomesobviousonlywhenwetakeacloserlookatacousticalquestions.Inbuildingacoustics,thequestionalwaysis:

Whatportionofthesoundreachestheothersideofthecomponentinquestion?Thekeypropertyisthesoundinsulationofthecomponent.Essentially,itisabouttheabilityofcomponentswalls,ceilings,doors,windows,etc.tominimisethesoundtransmissionbetweentworooms.Ahighdegreeofsoundinsulationisusuallyachievedusingsolid,heavycomponentswhichhinderthepropagationofsound.

ThesoundinsulationofpartitionsforairbornesoundisdescribedbythesoundtransmissionlossorratedsoundreductionlossRwthatcanbemeasuredonsiteorinlaboratoryorevencalculated.

Backgroundnoiselevel

Buildingacoustics:

Soundtransmissionbetween

adjacentrooms

Transmitted

Soundlevel

SoundlevelSoundlevel

80dB

60dB

Thequestioninroomacoustics,ontheotherhand,is:

Whatsurfaceshelptocreateoptimumlisteningconditionsinaroom?Thekeypropertyinthiscaseisthesoundabsorptionprovidedbythematerialsusedintheroom.Soundabsorption

describestheabilityofmaterialstoabsorbsoundortoconverttheincidentsoundenergyintootherformsofenergy.Soundabsorptionisachievedbymeansofsoundabsorbers

Roomacoustics:

Acousticqualitywithinaroom

Backgroundnoiselevel

ThesoundabsorptionofasurfaceisdescribedbythefrequencydependentsoundabsorptioncoefcientorsimpliedbyaaveragevaluessuchasworNRC.Thesoundabsorptioncoefcientusuallyismeasuredinspeciallaboratoryroom,so-calledreverberationchambers.

Thetermssoundinsulationandsoundabsorptionarewell-denedandrelatetotheeldsofbuildingacousticsandroomacousticsrespectively.Ifwefeelannoyedbynoisefromanadjacentroom,increasingthesoundinsulationessentiallyhelpstoimprovethissituation.Thesoundabsorptioninaroomcangenerallyonlydecreasethelevelinroombyasmallamount.Decreasingsoundlevelsinaroombyroomacousticmeansisinprinciplemuchsmallerthananyoptimizationofthepartion.


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3. FUNDAMENTALS OF ACOUSTICS

3.1SOUND

Soundcancompriseharmonioustones,music,bangs,noise,crackling,butalsospokenwords.Allofthesesoundeventscauseaslightvariationinairpressurewhichpropagateswithinthesurroundingsofitssource.Wethereforerefertothesoundpressureofatone,ofnoise,speechormusic.Thelouderthesoundevent,theheavieristhispressurevariationandthehigheristhesoundpressure.

Asarule,soundalwayspropagatesintoallthreedirectionsofspace.Withmanysoundsourcesthesoundradiationde-pendsontheorientationofthesource;inmostcasesitissufcient,however,toassumeroughlyauniform,omnidirec-tionalsoundradiation.Soundsourcesofthistypearereferredtoasomnidirectionalsoundsources.Todayitisalsopos-sibletoselectverytightlyrestrictedsoundradiationdirectionsbymeansofspecialloudspeakerssothattheradiatedsoundcanbedirectedspecicallytoaparticularposition.Thismethodisused,forexample,whenttinglectureroomswithelectroacousticequipment.Here,ithastobetakenintoaccountthatthesoundenergydecreasesconsiderablywithincreasingdistancefromthesoundsource.Intheareasoccupiedbytheaudience,however,thesounddistributionshouldbeasuniformaspossible.Toachievethiseffect,alargernumberofloudspeakersmayhavetobeused.

Asarule,soundalwayspropagatesintoallthreedirectionsofspace.Withmanysoundsourcesthesoundradiationdependsontheorientationofthesource;inmostcasesitissufcient,however,toassumeroughlyauniform,omnidi-

rectionalsoundradiation.Soundsourcesofthistypearereferredtoasomnidirectionalsoundsources.Inprincipalonehastodifferentiatebetweenairbornesound,soundinliquidsandsoundinsolidbodies.Generallysoundisapropagationofpressureanddensityvariationinanelasticmedium.Ifsoundtravelsthroughawalloranotherpartitiontheairbornsoundisconvertedtovibrationofthewallandthenradiatedfromthevibratingwallasairbornsoundtotheroom.

Unwantedsoundeventscanbenamedasnoise.Thisdenitionshowsthattheperceptionofsoundshasstrongsubjectiveaspects.Psychoacousticsasabranchofacoustics,oralsonoiseeffectresearch,dealswiththerelationshipbetweenoursubjectiveperceptionandthesoundsignalswhichareobjectivelypresent.Oftenadifferencebetweenwantedsoundsuchasmusicinaconcertoravoiceofaspeakerundunwantedsoundliketrafcnoiseormusicoftheneighbourismade.


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3.2SOUNDPRESSURE

Soundcancompriseharmonioustones,music,bangs,noise,crackling,butalsospokenwords.Allofthesesoundeventscauseaslightvariationinairpressurewhichpropagateswithinthesurroundingsofitssource.Wethereforerefertothesoundpressureofatone,ofnoise,speechormusic.Thelouderthesoundevent,theheavieristhispressurevariationandthehigheristhesoundpressure.Theminimumsoundpressurethatahumanbeingcanperceiveisaround20Pa=0.00002Pascal,averylowvalueshowinghighsensitivityofthehumanauditorysystem.Soundpressurevaluesof20Pascalwilldamagethehearingsystemforveryshortexposuretimes.

Time(sec)

SoundpressureinPascal

3.3SOUNDPRESSURELEVELANDDECIBELSCALE

Thestrengthofasound,thesoundpressure,usuallyisgivenassoundpressurelevelorsoundlevel.Asoundpressurelevelof0decibelrefersbydenitiontothesoundpressurelevelwherehumanperceptionbegins.Thisdenitionprovidesascalebetween0decibel(abbr.:dB)andabout140dB.Constantsoundlevelsofmorethan80dBorveryshortnoisesofmorethan120dBcanirreversiblydamagetheauditorysystem.

Decibel

intolerable

veryloud

loud

quiet

veryquiet

inaudiable

aircraftengine

discotheque,jackhammer

tickingwatch

breathing

whispering

140dB(A)

120dB(A)

100dB(A)

80dB(A)

60dB(A)

40dB(A)

20dB(A)

0dB(A)

loudcommunication,

busyofce

quietcommunicationquietlibrary

absolutesilence

heavytrafc

*DefinitionseeChapter5

*


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3.4SOUNDPRESSUREOFSEVERALSOURCES

Anincreaseinthenumberofsoundsourcesbyafactoroftwoalwaysresultsinanincreaseofthelevelby3dB,afactorofteninanincreaseby10dB,andafactorofonehundredinanincreaseby20dB.

SOUNDPRESSUREINCREASEFORIDENTICALSOUNDSOURCES

Thefollowingtablegivesasimpleruleofthumbfortheadditionoftwosoundlevels.Firstofallthedifferencebetweenthetwolevelsshouldbecalculated.

Example:Fortwosourcesof45dBand52dB,respectively,thedifferenceof7dBmeansanincreaseby1dB,whichisaddedto52dBandthusresultsinatotallevelof53dB.

Numberofidenticalsoundsources Soundpower Soundpressure Soundpressurelevel

100 10+20dB

10 3,2+10dB

4 2+6dB

2 1,4+3dB

1 10dB

ExampleAlarmclock IncreaseofdBvalue

1 62dB

2 62+3=65dB

3 62+5=67dB

4 62+6=68dB

5 62+7=69dB

10 62+10=72dB

15 62+12=74dB

20 62+13=75dB

50 62+17=79dB

100 62+20=82dB

Soundpressureleveldifference 0to1 2to3 4to9 morethan10

Levelincrease(tobeaddedtothehighervalue) +3dB +2dB +1dB +0dB


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3.5FREUENCY

Thefrequencyofasoundwavedescribesthenumberifpressurechangesoroscillationspersecond.Itisoftenabbrevi-atedbytheletterfandhastheunit1Hertz(short:Hz).Afrequencyof1000Hzmeans1000oscillationspersecond.Thesoundpressureorsoundlevelisperceivedasloudnessandisoneimportantdimensionfortheperceptionofsound.Equallyimportantisthefrequencycontentofthesoundorspectrum.Puretonesaresoundwithonlyonefrequency.

Thesensitivityofthehumanauditorysystemishighlydependentonfrequency.Itisparticularlypronouncedinthefrequencyrangeofhumanspeechbetween250Hzand2000Hz.Thisisveryusefulwhenwelistentosomeonespeak,butdisruptionsinthisfrequencyrangeareperceivedasparticularlyannoyingandcanstronglyaffectcommunication.Withtoohighorlowfrequencies,ourhearingabilitydecreases.

Anoiseloudnessratingwhichistomeetthedemandsofthehumanauditorysystemneedstotakeintoaccountthefrequencycharacteristicofthehumanauditorysystem.Themediumfrequencies,atwhichthehumanauditorysystemisparticularlysensitive,areweightedmoreheavilythanthehighandlowfrequencies.ThisweightingresultsinthetermdB(A)forsoundpressurelevels,i.e.theso-calledA-weightedsoundpressurelevel.Nearlyallregulations,guidelines,standardvalues,limitvalues,recommendationsandreferencestosoundpressurelevelsusevaluesexpressedindB(A).

Infrasound Audiblerange[ 2020.000Hz] Ultrasound

FrequenciesmeasuredinHertz(Hz)

10Hz 100Hz 1.000Hz 10.000Hz 100.000Hz

Bat

Triangle

Organ

Violin

Contrabass

Grandpiano

Malevoice

Femalevoice

Phone


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3.6FREUENCYRANGESRELEVANTFORROOMPLANNING

Thefrequencyrangetobetakenintoaccountwhenplanningaroomisbasedonthehumanauditorysystemontheonehandandwhatistechnicallysensibleandfeasibleontheother.Frequenciesabove5000Hzareattenuatedbytheairtosuchadegreethatitisnotsensibletotakethemintoaccountwhenplanningtheacousticsofaroom.Below100Hz,otherphysicalimplicationsofsoundpropagationneedtobetakenintoaccount.

Theinternationallystandardisedtestmethodsfordeterminingthesoundabsorptionbyparticularmaterialsarebasedonthefrequencyrangefrom100Hzto5000Hz.Correspondinglyithasbeendecidedtofocusroomacousticplanningonthefrequencyrangebetween100Hzand5000Hz,asarule.

Relevantfrequencyranges

10Hz 100Hz 1.000Hz 10.000Hz 100.000Hz

Relevantfrequencyrangesfrom100upto5.000Hzforroomplanning.

3.7WAVELENGTHSOFSOUND

Eachfrequencyofsoundisassociatedwithasoundwaveofaparticularwavelength.Inair,a100Hzwavehasanextensionof3.40meters,whereasa5000Hzwavehasanextensionofonlyabout7centimeters.Accordingly,thesoundwavesrelevantforroomacousticshavealengthofbetween0.07mand3.40m.Aswecansee,thedimensionsofsoundwavesarewellwithintherangeofthedimensionsofroomsandfurnishings.Thefollowinggureshowstherangeofallsoundwavelengthsrelevantforroomacoustics.

Wavelengthsl

l

l

Time(sec)

SoundpressureinPascal


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3.8LEVELVALUES

Therelevantparameterforanobjectiveassessmentofthenoiseimpactataworkstationistheso-calledratinglevel,whichconsists,ontheonehand,ofthemeasured,time-averagedsoundpressurelevelinaroomand,ontheotherhand,ofadjustmentsinaccordancewiththecharacteristicofthenoiseaswellasitsdurationofimpact.

Theratinglevelisusuallybasedonaratingperiodof8hours.Highbackgroundnoiselevelsinofceroomswilllikelyaffecttheintellectualefciency.Forthisreason,severalregula-tionsandstandardscontainrecommendationsregardingthemaximumpermissiblebackgroundsoundpressurelevel.

ThefollowingtableshowsthevaluesoftherecommendedbackgroundnoiselevelinaccordancewithDINEN11690:

Conferenceroom Officeroom Openplanoffice

dB(A)

100

50

30-35dB(A)30-40dB(A)

35-45dB(A)

65-70dB(A)

Industrialworkplace


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4. ROOM ACOUSTIC PARAMETERS

4.1REVERBERATIONTIME

Thereverberationtimeisthebasisforratingsofroomacousticquality.Putsimply,thereverberationtimeindicatestheperiodoftimeittakesforasoundeventtobecomeinaudible.Technically,thereverberationtimeThasbeendenedasthetimerequiredforthesoundpressurelevelinspacetodecayby60dB.Thismeansthat,ifaroomisexcitedwithabangof95dB,thereverberationtimeindicatestheperiodoftimewithinwhichthenoiseleveldropsto35dB.Thiscanbeafewtenthsofaseconduptoseveralseconds.Thereverberationtimecanbedeterminedforeachenclosedspace.

Thisobjectivelymeasurablequantityallowsdifferentroomstobecomparedwitheachotherandtheirroomacoustic

qualitytobeassessed.Whileareverberationof4to8secondsisquitenormalforachurch,thevaluesaimedatforthereverberationtimeinconferenceorofceroomsarequitedifferent.Thefollowingtableprovidesanoverviewofthetypicalreverberationtimesofdifferentroomtypes.

Ithasadirecteffectonspeechintelligibilityinaroom.Ingeneral,speechintelligibilityinaroomdecreaseswithin-creasingreverberationtime.Thisdoesnotmean,however,thattheshortestpossiblereverberationtimeisalwaysthebestsolution!Verypoorspeechintelligibilityusuallydoessuggest,though,thatthereverberationtimeistoolong.

Thesubjectiveimpressionofthesoundqualityofaroomallowseventhenon-experttodrawconclusionsastohowthereverberationtimeprogresseswithinthedifferentfrequencyranges.If,forexample,speechinaroomsoundsblurred,andifitisverydifculttounderstandeachother,itcanbeassumedthatthereverberationtimeistoolong.Acousticallydryinthiscontextmeansthatthesoundisabsorbedunnaturallyfast.Ifthishappensonlyathighfrequencies,theroomsoundsholloworbooming,whereasatlowfrequenciesitsoundspiercingandsharp.

Reverberationtime

0 1,0 2,0Time(sec)

typicalreverberationtimeforofcerooms:0,50,8sec

Reverberationtime:1,8sec

60dB(A)

100

50

in(dB)Soundpressurelevel

Typeofroom Reverberationtime(exemplary)

Church approx.48seconds

Classroommediumsized 0,6seconds

Ofceroomdependingonsize 0,50,8seconds

Concerthallforclassicalmusic approx.1,5seconds

Performance Reverberationtimeatlowfrequencies

Reverberationtimeathighfrequencies

Subjectiveimpression

speech toolongtoolongtooshorttooshort

toolongtooshorttoolongtooshort

blurred,difculttounderstandhollow,buteasytounderstandpiercing,clanking,sharp,difculttounderstanddry,buteasytounderstand


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Onwhichfactorsdoesthereverberationtimedepend?Thereverberationtimedependsmainlyonthreefactors:-thevolumeoftheroom,-thesurfacesoftheroomand-thefurnitureintheroom.

Aroomusuallybecomesmorereverberantwithincreasingheight.Absorbingsurfacessuchascarpets,curtainsandsoundabsorbingceilings,butalsofurnitureorpeoplepresentintheroomreducethereverberationtime.

0 0,5 1,0 1,2 Time(sec)

Reverberationtime:0,5secWITHproductsofCrationBaumann

Reverberationtime:1,2secWITHOUTproductsofCrationBaumann

100

50

SoundlevelindB


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Theshapeofaroomisusuallyofminorimportanceforthereverberationtime.Onlyiftheroomacousticrequire-mentsareveryhigh(e.g.inconcerthalls)oriftheshapeisveryunusual,e.g.vaultedsurfacesorheavilyvaryingroomheights,doesshapebecomeanessentialfactor.TherecommendationsgiveninDIN18041shouldalwaysformthebasisforanyroomacousticplanning.DIN18041Acousticqualityinsmalltomedium-sizedroomsformsthebasisfortherecommendationsregardingtheacousticdesignofsmalltomedium-sizedrooms.Withregardtotheoptimumreverberationtime,DIN18041distinguishesbetweenthreedifferentroomcategories:music,speechandcommunicationandteaching.Roomsoftheusagetypemusicaremusicclassroomsandhallsformusicpresentations.Speechinthebroadestsensecomprisesallroomswhereaspeakerspeaksinfrontofanaudience.Communicationandteachingcomprisesalltypeswhereseveralpeoplespeakatthesametime,i.e.teachingroomsaswellasconferencerooms,multipleoccupancyofces,servicepoints,callcentersandroomswithaudiovisualpresentationsorelectroacousticuses.

Twoexamples:

Example1:

Aconferenceroom(usagetype:communicationandteaching)withavolumeof250m3shouldhaveareverberationtimeof0.60s.

Example2:Achambermusichall(usagetype:music)withavolumeo550m3shouldhaveareverberationtimeof1.30s.

2,6

2,4

2,2

2,0

1,8

1,6

1,4

1,2

1,0

0,8

0,6

0,4

0,2

30 100 1.000 5.000 10.000 30.000

ReverberationtimeTSOLL

ins

Roomvolume V in m

Music

Speech

Teaching, Communication


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4.2SOUNDABSORPTION

Thesoundabsorptioncoefcientdescribesthepropertyofamaterialtoconvertincidentsoundintootherformsofenergye.g.thermalorkineticenergyandthustoabsorbit.

Case1:Soundcompletelyabsorbed(soundabsorptioncoefcient=1)noreection

Theotherextremeisfullsoundreection.Alltheincidentsoundisreected.

Case2:Soundcompletelyreected(soundabsorptioncoefcient=0)

Case3:Soundpartlyabsorbed(soundabsorptioncoefcient=between0and1)

Soundcompletelyabsorbed

Soundcompletelyreected

Soundpartiallyabsorbed

Thefrequency-dependentsoundabsorptioncoefcientofamaterialisdeterminedbymeansofaspecialacousticmaterialtestmethodtheso-calledreverberationroommethod.Forthistest,amaterialsampleisplacedintothereverberationroom,whosereverberationtimehasbeendeterminedpreviouslywithoutthesample.Fromthechangeinthereverberationtimewiththesamplepresentintheroom,thesoundabsorptioncoefcientScanbedeterminedforeachone-thirdoctavebetween100Hzand5000Hz.Thisyields18one-thirdoctavevalueswhichuniquelydescribetheabsorptionbehaviorofthematerial,i.e.towhatextentandatwhatfrequenciesthematerialabsorbsthesound.

Solvingroomacousticproblemswithmeasurementsshouldalwaysuseon-thirdoctavebandresolutioninfrequencyasmanyproblemsoccurinsmallfrequencybandsandrequireadequatesolutions.


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Octaveaveragefrequency

one-thirdoctavebandstep octavebandstep

400 500 630 800 1.000 1.250 1.600 2.000 2.500 3.150 4.000 5.000200 250 315100 125 160

Itisnotonlythechoiceofmaterial,however,whichisresponsibleforthesoundabsorptioninaroom.Whatismostimportantisthetotalareaofthismaterialpresentintheroom.Theequivalentsoundabsorptionareahasbeenintro-ducedtoprovideameasureforthesoundabsorbingperformanceofasoundabsorberactuallypresentintheroom.ItisdenedastheproductofthesoundabsorptioncoefcientSofamaterialandthesurfaceofthismaterial.

Calculationoftheequivalentsoundabsorptionofsurfacesinaroom:

A=s11+s22+s33++sn+n+A1+A2++An

Atotalequivalentsoundabsorptionareainarooms1surfacesizeofmaterial1,e.g.acousticceiling1soundabsorptioncoefcientofmaterial1s2surfacesizeofmaterial2,e.g.carpet2soundabsorptioncoefcientofmaterial2Snsurfacesizeofmaterialnnsoundabsorptioncoefcientofmaterialn

10Hz 100Hz 1.000Hz 10.000Hz 100.000Hz

Relevantfrequencyrangesfrom100upto5.000Hzforroomplanning.


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4.3SOUNDABSORPTIONCOEFFICIENTANDREVERBERATIONTIME

Inafullyfurnishedroomwithdifferentsurfaces,forexample,eachmaterial(e.g.carpets,plaster,acousticceiling,cur-tains,windows,shelves,etc.)canbeallocatedasoundabsorptioncoefcient,andbymultiplyingthiscoefcientbythesurfaceofthismaterial,theequivalentsoundabsorptionareacanbecalculated.Theequivalentsoundabsorptionareasofallmaterialsarethenaddedtodeterminethetotalequivalentsoundabsorptionareaoftheroom.ThereverberationtimeofaroomcanbederivedfromthecalculatedtotalequivalentsoundabsorptionareausingtheSabineformula.

Sabineformula:

TReverberationtimeVVolumeoftheroomATotalequivalentsoundabsorptionarea

Asoundabsorberof10m 2withasoundabsorptioncoefcientof0.50hasanequivalentsoundabsorptionareaof5m2andthushasthesameeffectasasoundabsorberof20m2withasoundabsorptioncoefcientof0.25orasoundabsorberof5m2withasoundabsorptioncoefcientof1.00.Inafullyfurnishedroomwithdifferentsurfaces,forexample,eachmaterial(e.g.carpets,plaster,acousticceiling,curtains,windows,shelves,etc.)canbeallocatedasoundabsorptioncoefcient,andbymultiplyingthiscoefcientbythesurfaceofthismaterial,theequivalentsoundabsorptionareacanbecalculated.Theequivalentsoundabsorptionareasofallmaterialsarethenaddedtodeterminethetotalequivalentsoundabsorptionareaoftheroom.

4.4RATINGOFSOUNDABSORPTION

Intheprevioussectionstheadvantagesoflookingatthesound,thereverberationtimeandthesoundabsorptioncoef-cientinafrequency-dependentcontexthavebeenexplainedingreatdetail.Severalinterestedpartieshave,however,expressedtheirdesireforsimpliedvalues,whichmightnotpermitdifferentiatedplanning,butwouldallowroughcomparisonstobemadebetweendifferentsoundabsorbersorpreliminarystatementsregardingthebasicsuitabilityofproductsforparticularapplications.Suchvaluesshouldalsoenableasimpliedplanningofroomswithlowrequire-mentsregardingtheiracousticquality.Againstthisbackdrop,singlevaluesofsoundabsorptionhavebeendenedinEuropeandtheUSwhichdifferslightly.ThemostcommonsinglevalueofsoundabsorptioninEuropeistheso-calledweightedsoundabsorptioncoefcientw,whereasintheEnglish-speakingworlditistheNoiseReductionCoefcient(NRC)ortheSoundAbsorptionAver-age(SAA).

Allprocedurestodetermineofsinglenumberratingsrelyontestsinthereverberationchamberwithon-thirdoctave

bandresolution.

Weightedsoundabsorptioncoefcientw(DINENISO11654):Inordertodeterminetheweightedsoundabsorptioncoefcientw,themeanvaluefortheoctavecentrefrequencybetween125Hzand4000Hzisdeterminedfromthreeone-thirdoctavevalues.18one-thirdoctavevaluesarethusconvertedinto6octavevalues.Themeanvalueoftherespectiveoctaveisthenroundedtothenearest0.05;itisreferredtoasthepracticalsoundabsorptioncoefcientp.Thepracticalsoundabsorptioncoefcientpbetween250Hzand4000HziscomparedtothereferencecurvegiveninDINEN11654.Thiscomparisongivesasinglevalueoftheweightedsoundabsorptioncoefcientw.Deviationsbymorethan0.25betweenthecurveandthereferencecurveareindicatedbymeansoftheshapeindicatorsL,MorH,dependingonwhethertheyoccurat250Hz(L),at500Hzor1000Hz(M),orat2000Hzor4000Hz(H).Theresultingvaluesare,forexample,w=0.65(H),w=0.20orw=0.80(LM).

T=0,163 VA


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Deviationsbymorethan0.25betweenthecurveandthereferencecurveareindicatedbymeansoftheshapeindicatorsL,MorH,dependingonwhethertheyoccurat250Hz(L),at500Hzor1000Hz(M),orat2000Hzor4000Hz(H).Theresultingvaluesare,forexample,w=0.65(H),w=0.20orw=0.80(LM).

Basedonthewvalue,soundabsorberscanbeclassiedintodifferentsoundabsorberclasses.wvaluesofmorethan0.90,forexample,belongtosoundabsorberclassA,valuesofbetween0.15and0.25belongtoclassE.

Single-numbervaluescommonlyusedintheUS

NRC(ASTM423):TheNRC(NoiseReductionCoefcient),whichiswidelyusedintheUS,isdeterminedbycalculatingthemeanvaluefromfourone-thirdoctavevaluesofthesoundabsorptioncoefcient(250Hz,500Hz,1000Hzand2000Hz)androundingtheresulttothenearest0.05.Ifthenumberisattheexactmid-pointofthenumbersdivisibleby0.05,thevalueisalwaysroundedup(example:0.625=>0.65;0.675=>0.70).

SAA(ASTM423):AnothervalueusedintheUSistheSAA(SoundAbsorptionAverage).Itisdeterminedbycalculatingthemeanvaluefromtwelveone-thirdoctavevaluesofthesoundabsorptioncoefcientbetween200Hzand2500Hzandthenroundingtheresulttothenearest0.01.

ADVANTAGEOFSINGLE-NUMBERVALUES:Soundabsorberscanberoughlyclassiedandthuscomparedwithoneanother.

DISADVANTAGEOFSINGLE-NUMBERVALUES:Asingle-numbersoundabsorptionvalueisalwaysanextremelysimpliedvalue.Soundabsorberswithverydifferentabsorptionspectracanhaveidenticalsingle-numbervalues.Thismaysometimesresultintheuseofasoundabsorberwhichisnotsuitablefortheexistingconditions.Frequenciesbelow200Hzarenottakeninto

account.

Soundabsorberclass w-value

A 0,901,00

B 0,800,85

C 0,600,75

D 0,300,55

E 0,150,25

notclassied 0,000,10


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5. INDEx

A-WEIGHTEDSOUNDPRESSURELEVELdB(A)TheA-weightedsoundpressurelevelistheweightedaveragevalueofthesoundpressurelevel(dB)asafunctionofthefrequencyofasound.Theweightingtakesintoac-counttheabilityofthehumanauditorysystemtoperceivesoundpressurelevelsortonesofdifferentfrequenciestoadifferentdegree.Thissensitivityisparticularlypronouncedinthemediumfrequencyrange,i.e.therangeofhumanspeech.NearlyallregulationsandguidelinesindicatevaluesexpressedindB(A).

EUIVALENTSOUNDABSORPTIONAREATheequivalentsoundabsorptionareaAisdenedastheproductofthesoundabsorptioncoefcient ofamaterialandthesurfaceSofthismaterial.

AURALISATIONAuralisationisamethodforsimulatingtheacousticproper-tiesofaroom.Withthismethod,theeffectsofcertainacoustictreatmentscanbeauralisedasearlyasthedesignstage.

BUILDINGACOUSTICSBuildingacousticsisabranchofbuildingphysics,oracous-tics,whichdealswiththeeffectofthestructuralconditionsonthepropagationofsoundbetweentheroomsofabuild-

ingorbetweentheinteriorofaroomandtheoutsideofthebuilding.

RATINGLEVEL(L r)

TheratinglevelLr(Lforlevel,rforrating)istherelevantparameterforobjectivelyassessingthenoiseimpactataworkplace.Apartfromweightingthesoundpressurelevelasafunctionofthefrequency(seeA-weightedsoundpressurelevel),adeterminationofthesoundpressureleveltakesintoaccountcertainadjustmentswhichdependonthecharacteristicofthesound(e.g.impulsivenessorclearprominenceofindividualtones)anditsdurationofimpact.

TheratinglevelisalsoexpressedindB(A).

DECIBEL(dB)Logarithmicallydenedunitofmeasurementwhichex-pressesthesoundpressurelevel.Therelevantscaleforhumanbeingsis0dBto140dB.0dBreferstoasoundpressureof20Pa.

SINGLENUMBERVALUESOFSOUNDABSORPTIONSo-calledsinglenumbervaluesareusedforasimpliedrepresentationofthefrequency-dependentparameterofthesoundabsorptioncoefcientaswellasforaroughcomparisonofdifferentsoundabsorbers.InEurope,theweightedsoundabsorptioncoefcient winaccordancewithDINENISO11654iscommonlyused.IntheUS,theNRCandSAAvaluesarewidelyused.Alloftheabovevaluesarebasedonmeasurementsofthesoundabsorptioninone-thirdoctaveandoctaveincrements.Foradetailedacousticplanningofaroomitisnecessarytoknowthesesoundabsorptionvaluespreciselyinone-thirdoctaveoratleastinoctaveincrements(seeoctaves).

FREUENCYFrequencyindicatesthenumberofsoundpressurechanges

persecond.Soundeventswithahighfrequencyareperceivedbythehumanearashigh-pitchedtones,soundeventswithalowfrequencyaslow-pitchedtones.Soundssuchasnoise,roadtrafc,etc.,normallycompriseagreatnumberoffrequencies.Themeasurementunitoffrequencyishertz(Hz),1Hz=1/s.Humanspeechisintherangebetween250Hzand2000Hz.Theaudiblerangeofhumanbeingsisbetween20Hzand20000Hz.

REVERBERATIONROOMReverberationroomsarespeciallaboratoryroomswithwalls

whichreecttheincidentsoundwavestoaveryhighdegree.Reverberationroomshaveparticularlylongreverberationtimesacrosstheentirefrequencyrange.

REVERBERATIONROOMMETHODThereverberationroommethodisusedfordeterminingthefrequency-dependentsoundabsorptioncoefcient.Asampleofthematerialtobetestedisplacedintothereverberationroom.Thesoundabsorptionofamaterialcanthenbecalculatedfromthechangeinthereverberationtimeoftheroom.

BACGROUNDNOISELEVELUsually,soundswhichdonotcontainanymeaningfulinformationarereferredtoasbackgroundnoise(e.g.noisefromairconditioningortrafc).ThebackgroundnoiselevelismeasuredindBor,byweightingitsfrequenciesinaccor-dancewiththehumanauditorysystem,indB(A).Thebackgroundnoiselevelindicatesthesoundpressurelevelwhichhasbeenexceededduring95%ofthemeasurementperiod.Ithasadirecteffectonspeechintelligibility.


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ACOUSTICUALITYTheacousticqualityofaroomreferstoitssuitabilityforaparticularuse.Itisinuencedbythepropertiesoftheboundarysurfaces(walls,ceiling,oor)andthefurnishingsandbypersonspresentintheroom.

NOISENoisecomprisesallsoundswhich,duetotheirloudnessandstructure,areconsideredasharmfulorannoyingorstress-fulforhumanbeingsandtheenvironment.Itdependsonthecondition,preferencesandmoodofapersonwhethersoundsareperceivedasnoiseornot.Theperceptionofsoundsasnoiseandthewayinwhichpeopleareaffectedbyitdepend,ontheonehand,onphysicallymeasurablequantitiessuchasthesoundpressurelevel,pitchofatone,tonalityandimpulsiveness.Ontheotherhand,certainsubjectivefactorsalsoplayarole:atbedtimenoiseisper-ceivedasextremelyannoying.Thesameistrueforactivitieswhichrequireahighlevelofconcentration.Ifwelikecertain

sounds,wewillnotperceivethemasannoyingevenathighvolumes;soundswhichwedonotlikeareannoyingtousevenatlowvolumes(e.g.certaintypesofmusic).Further-more,howwefeelataparticulartimealsoinuencesoursensitivitytonoise.Ifanactivityisdisruptedordisturbedbyoneormoresounds,thisisreferredtoasnoisepollution.Weareparticularlysensitivetonoiseifverbalcommunica-tionisaffected,e.g.ifaloudconversationattheneighbor-ingtablemakesitdifcultforustolisten,andifwehavetoconcentrateorwanttosleep.

REVERBERATIONTIMEPutsimply,thereverberationtimeindicatestheperiodoftimeittakesforasoundeventtobecomeinaudible.Technically,thereverberationtimeThasbeendenedasthetimerequiredforthesoundpressurelevelinspacetodecayby60dB.

OCTAVEBANDSAcousticparameterssuchasthesoundpressurelevelorthesoundabsorptioncoefcientareusuallyexpressedinincrementsofoctavesandone-thirdoctaves.Thepreciseknowledgeofacousticpropertiesinthesmallestpossible

frequencystepsofsoundisaprerequisiteforadetailedacousticdesign.Forroomacousticstherelevantoctavefrequenciesare125Hz,250Hz,500Hz,1000Hz,2000Hzand4000Hz.Theoctaveincrementsareobtainedbydoublingthepreviousfrequency.Eachoctavecomprisesthreeone-thirdoctavevalues(seealsosinglevalues).

POROUSABSORBERSPorousabsorberscomprise,forexample,mineralbres,foams,carpets,fabrics,etc.Theeffectoftheporousabsorbersisduetothefactthatsoundisabletoenterthe

openstructuresofthematerialwhere,bythefrictionofairparticles,thesoundenergyisconvertedintothermalenergyatthesurfaceofthepores.Porousabsorbersachievetheirbesteffectatmediumandhighfrequencies.

PSYCHOACOUSTICSBranchofacousticsornoiseeffectresearchwhichdealswiththesubjectiveperceptionofobjectivelypresentsoundsignals.Furthermore,psychoacousticsstudiestheinuenceofalistenerspersonalattitudesandexpectationsontheperceptionofsoundevents.

RESONANCEABSORBERThistermcomprisesalltypesofabsorbersusingaresonancemechanismsuchasanenclosedairvolumeoravibratingsurface.Resonanceabsorbersaremainlysuitableforab-sorbingsoundofmediumtolowfrequencies.Themaximumeffectofresonanceabsorbersisusuallyrestrictedtoacer-tainfrequencyrange(seealsoporousabsorbers).

SOUNDABSORBERSoundabsorbersarematerialswhichattenuateincidentsoundorconvertitintootherformsofenergy.Adistinctionhastobemadebetweenporousabsorbersandresonance

absorbersorcombinationsoftheseabsorbertypes.

SABINEFORMULAIfthevolumeandthetotalequivalentsoundabsorptionareaofaroomareknown,thereverberationtimecanbeestimatedusingtheSabineformula,whereTistherever-berationtime,VisthevolumeoftheroomandAisthetotalequivalentsoundabsorptionarea.Thecloserelationshipbetweenthevolumeofaroom,thesoundabsorptionofthesurfacesofthisroom,andthereverberationtimewasdiscoveredthephysicistWallace

ClementSabine(1868-1919).Hefoundoutthattherever-berationtimeTisproportionaltotheroomvolumeVandinverselyproportionaltotheequivalentsoundabsorptionareaA:T=0,163xV/ATheequivalentsoundabsorptionareaAisthesumofallsurfacesSpresentintheroom,eachmultipliedbyitscor-respondingsoundabsorptioncoefcient :A=1S1+2S2+3S3++nSn

SOUNDABSORPTIONCOEFFICIENTThesoundabsorptioncoefcient ofamaterialindicates

theamountoftheabsorbedportionofthetotalincidentsound.=0meansthatnoabsorptionoccurs;theentireincidentsoundisreected.If=0,5,50%ofthesoundenergyisabsorbedand50%isreected.If =1,theentireincidentsoundisabsorbed,thereisnolongeranyreection.

SOUNDATTENUATIONSoundattenuationdescribestheabilityofmaterialstoabsorbsoundortoconvertthesoundenergypresentintootherformsofenergy,i.e.ultimatelyintothermalenergy(seealsosoundinsulation).


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SOUNDINSULATIONSoundinsulationreferstotherestrictionofthepropagationofsoundthroughtheboundariesofaroom.Soundinsula-tionis,therefore,ameasuretoseparateroomsacousticallyfromunwantedsoundfromadjacentroomsortheoutside.Thishasnothingtodo,however,withtherequiredacousticsoundattenuationwithinaroom(seealsosoundabsorp-tion).Soundinsulationisafundamentalparameterofbuildingacoustics.Adistinctionhastobemadebetweenairbornesoundinsulationandimpactsoundinsulation.Airbornesoundiscreatedbysoundsourcespresentintheroomwhicharenotimmediatelyconnectedtotheboundarysurfaces,e.g.peoplewhoaretalking.Impactsound,ontheotherhand,resultsfromstructure-bornesound(footfalls,knocking),whichinturnexcitesthewallsorceilingstoradiateairbornesound.Airbornesoundinsulationandimpactsoundinsulationbothhavetofulltherequirementsestablishedinrelevantbuildinglaws.

SOUNDPRESSURE

Allsoundeventshaveincommonthefactthattheycauseslightvariationsinairpressurewhichcanpropagateinelasticmediasuchasairorwater.Wethereforerefertothesoundpressureofatone.Theheavierthepressurevariationsare,thelouderisthesoundevent.Thefasterthevariationsoccur,thehigheristhefrequency.

SOUNDEVENTSGeneraltermfortones,music,bangs,noise,crackling,etc.

SOUNDSHIELDINGAsoundshieldisbasicallyanobstaclewhichinterruptsthe

directpropagationofsoundfromasourcetoareceiver.Itcanconsistinamovablepartitionoranattachmenttobeplacedontopofadesk.Cabinetsandotherlarge-surfacepiecesoffurniturecanalsofunctionassoundshields.Soundshieldscanbeprovidedwithasoundabsorbingsurfacewhichadditionallyreducesthepropagationofsound.

SOUNDSPECTRUMThesoundspectrumdescribesthefrequencycompositionofthesound.Puretonesaresoundeventsofasinglefre-quency.Asuperpositionoftonesofdifferentfrequenciesis

referredtoasnoiseorsound.

SOUNDWAVESVariationsinairpressurewhicharecausedbysoundeventsarereferredtoassoundwaves.Thelengthofthesoundwavesdenesthefrequencyandtheirheightdenesthelevel.Longsoundwaveshavealowfrequencyandareper-ceivedaslow-pitchedtones.Shortsoundwaveshaveahighfrequencyandareperceivedashigh-pitchedtones.Inair,a100Hzwavehasanextensionof3.40meters,whereasa5000Hzwavehasanextensionofapproximately7centimeters.

SOUNDMASINGSoundmaskingspecicallyusesnatural(e.g.birdstwitter-ing)orarticial(e.g.noise)soundsinordertoblanketothersounds.Thismethodcanbeused,forexample,todrownoutinformation-containingsoundsiftheotherbackgroundnoiseistooweaktomaskthem.

SOUNDPRESSURELEVEL(LP)Thesoundpressurelevel(Lforlevelandpforpressure)isalogarithmicquantityfordescribingtheintensityofasoundevent.Thesoundpressurelevelisoftenalsoreferredto

assoundlevel,whichisactuallynotquitecorrect.Thesoundpressurelevelisexpressedindecibels(abbreviatedasdB).Soundpressuresaremeasuredusingmicrophones.Themeasurablelevelrangestartsatjustbelow0dBandendsatapproximately150to160dB.


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MANYTHANSFORTHESUPPORTTOTHEAUSTIBROOLDENBURG

ThephysicistsDr.CatjaHilgeandDr.ChristianNockefoundedanacousticconsultingcompanyinOldenburg(Germany)in2001.Theyworkasspecializedengineersforarchitects,expertwitnessesforcourtsandconsultantsintheeldofacoustics.Architecturalacousticsforclassrooms,ofcesandotherfacilitieshasbecomeonemajorfocusofthecompany.

ContactdataAkustikbroOldenburg,atharinenstr.10,26121Oldenburg,Germanyt+494417779041,f+494417779042,info @akustikbuero-oldenburg.de,www.akustikbuero-oldenburg.de

CopyrightEGGERHolzwerkstoffeGmbH&Co.OG,St.JohanninTirol,sterreich


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CrationBaumannisrenownedforhighqualitytextilesforinteriordesign.Thankstoourin-housedesignstudioandourownproductionfacilities,unconventionalcreationsarepossible.Ouroffercomprisescustomizedsolutionsandinteriorshadingsystemsaswellaslightcontrol,dimming,andsoundabsorptionsolutions.CrationBaumannisalsosynonymouswiththeextravagant:600differentdesignsin6,000differentcolours.

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Basic of Acoustics