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BASICS OF ACOUSTICS

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CONTENTS

1. preface

2. room acoustics versus building acoustics

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0506060708090910

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11141616

18

03

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3. fundamentals of acoustics

4. room acoustic parameters

5. index

3.1 Sound 3.2Soundpressure 3.3Soundpressurelevelanddecibelscale 3.4 Soundpressureofseveralsources 3.5Frequency 3.6 Frequencyrangesrelevantforroomplanning 3.7 Wavelengthsofsound 3.8Levelvalues

4.1 Reverberationtime 4.2Soundabsorption 4.3 Soundabsorptioncoefficientandreverberationtime 4.4 Ratingofsoundabsorption

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1. PREFACE

Noiseorunwantedsoundsisperceivedasdisturbingandannoyinginmanyfieldsoflife.Thiscanbeobservedinprivateaswellasinworkingenvironments.Severalstudiesaboutroomacousticconditionsandannoyancethroughnoiseshowtherelevanceofgoodroomacousticconditions.Decreasingsuccessinschoolclassroomsoraffectingefficiencyatworkisoftenrelatedtoinadequateroomacousticconditions.ResearchresultsfromclassroomacousticshavebeenoneofthereasonstoreviseGermanstandardDIN18041on“Acousticqualityofsmallandmedium-sizedroom”from1968anddecreasesuggestedreverberationtimevaluesinclassroomswiththenew2004versionofthestandard.Furthermorethestandardgaveadetailedrangeforthefrequencydependenceofreverberationtimeandalsoextendedtherangeofroomstobeconsideredinroomacousticdesignofabuilding.

Theacousticqualityofaroom,betteritsacousticadequacyforeachusage,isdeterminedbythesumofallequipmentandmaterialsintherooms.Inthesenseofgoodacousticstheroomsshouldcontributetoperceivespeech,musicorothersoundsasnottooloudortooquietandthewecancommunicatewithmucheffortandfeelcomfortable.

ThisbrochurehasbeendevelopedbyCréationBaumanwiththeintentiontogiveanintroductionandprofessionalsupportinthefieldofroomacousticsthatsometimeshastheconnotationofbeingconfusingortoomulti-dimensional.Itilluminatesimportanttermsandexplainsbasicsandinterrelationshipsofroomacoustics.

WithitspaletteofcreativetextilesforroomsCréationBaumanndeliversacousticallyeffectiveaswellasartisticattractivesolutionsforroomacousticquestions.Thebandwidthoftheacousticefficiencyoftextileapplicationsisoftenunderestimated.ForthisCréationBaumannofferswithitslargedocumentationofacousticpropertiesforitsmaterials-thatisavailableseparately–agreatpotentialinmodernsolutionsforacousticsbytextiledesigninaroom.

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2. ROOM ACOUSTICS VERSUS BUILDING ACOUSTICS

Thedifferencebetweenthefieldsofroomacousticsandbuildingacousticsbecomesobviousonlywhenwetakeacloserlookatacousticalquestions.Inbuildingacoustics,thequestionalwaysis:

Whatportionofthesoundreachestheothersideofthecomponentinquestion?Thekeypropertyisthesoundinsulationofthecomponent.Essentially,itisabouttheabilityofcomponents–walls,ceilings,doors,windows,etc.–tominimisethesoundtransmissionbetweentworooms.Ahighdegreeofsoundinsulationisusuallyachievedusingsolid,heavycomponentswhichhinderthepropagationofsound.

ThesoundinsulationofpartitionsforairbornesoundisdescribedbythesoundtransmissionlossorratedsoundreductionlossR’wthatcanbemeasuredonsiteorinlaboratoryorevencalculated.

Backgroundnoiselevel

Buildingacoustics:

Soundtransmissionbetween

adjacentrooms

Transmitted

Soundlevel

SoundlevelSoundlevel

80dB

60dB

Thequestioninroomacoustics,ontheotherhand,is:

Whatsurfaceshelptocreateoptimumlisteningconditionsinaroom?Thekeypropertyinthiscaseisthesoundabsorptionprovidedbythematerialsusedintheroom.Soundabsorptiondescribestheabilityofmaterialstoabsorbsoundortoconverttheincidentsoundenergyintootherformsofenergy.Soundabsorptionisachievedbymeansofsoundabsorbers

Roomacoustics:

Acousticqualitywithinaroom

Backgroundnoiselevel

ThesoundabsorptionofasurfaceisdescribedbythefrequencydependentsoundabsorptioncoefficientorsimplifiedbyaaveragevaluessuchasαworNRC.Thesoundabsorptioncoefficientusuallyismeasuredinspeciallaboratoryroom,so-calledreverberationchambers.

Theterms“soundinsulation”and“soundabsorption”arewell-definedandrelatetothefieldsofbuildingacousticsandroomacousticsrespectively.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;inmostcasesitissufficient,however,toassumeroughlyauniform,omnidirec-tionalsoundradiation.Soundsourcesofthistypearereferredtoasomnidirectionalsoundsources.Todayitisalsopos-sibletoselectverytightlyrestrictedsoundradiationdirectionsbymeansofspecialloudspeakerssothattheradiatedsoundcanbedirectedspecificallytoaparticularposition.Thismethodisused,forexample,whenfittinglectureroomswithelectroacousticequipment.Here,ithastobetakenintoaccountthatthesoundenergydecreasesconsiderablywithincreasingdistancefromthesoundsource.Intheareasoccupiedbytheaudience,however,thesounddistributionshouldbeasuniformaspossible.Toachievethiseffect,alargernumberofloudspeakersmayhavetobeused.

Asarule,soundalwayspropagatesintoallthreedirectionsofspace.Withmanysoundsourcesthesoundradiationdependsontheorientationofthesource;inmostcasesitissufficient,however,toassumeroughlyauniform,omnidi-rectionalsoundradiation.Soundsourcesofthistypearereferredtoasomnidirectionalsoundsources.Inprincipalonehastodifferentiatebetweenairbornesound,soundinliquidsandsoundinsolidbodies.Generallysoundisapropagationofpressureanddensityvariationinanelasticmedium.Ifsoundtravelsthroughawalloranotherpartitiontheairbornsoundisconvertedtovibrationofthewallandthenradiatedfromthevibratingwallasairbornsoundtotheroom.

Unwantedsoundeventscanbenamedasnoise.Thisdefinitionshowsthattheperceptionofsoundshasstrongsubjectiveaspects.Psychoacousticsasabranchofacoustics,oralsonoiseeffectresearch,dealswiththerelationshipbetweenoursubjectiveperceptionandthesoundsignalswhichareobjectivelypresent.Oftenadifferencebetweenwantedsoundsuchasmusicinaconcertoravoiceofaspeakerundunwantedsoundliketrafficnoiseormusicoftheneighbourismade.

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3.2SOUNDPRESSURE

Soundcancompriseharmonioustones,music,bangs,noise,crackling,butalsospokenwords.Allofthesesoundeventscauseaslightvariationinairpressurewhichpropagateswithinthesurroundingsofitssource.Wethereforerefertothesoundpressureofatone,ofnoise,speechormusic.Thelouderthesoundevent,theheavieristhispressurevariationandthehigheristhesoundpressure.Theminimumsoundpressurethatahumanbeingcanperceiveisaround20µPa=0.00002Pascal,averylowvalueshowinghighsensitivityofthehumanauditorysystem.Soundpressurevaluesof20Pascalwilldamagethehearingsystemforveryshortexposuretimes.

Time(sec)

SoundpressureinPascal

3.3SOUNDPRESSURELEVELANDDECIBELSCALE

Thestrengthofasound,thesoundpressure,usuallyisgivenassoundpressurelevelorsoundlevel.Asoundpressurelevelof0decibelrefersbydefinitiontothesoundpressurelevelwherehumanperceptionbegins.Thisdefinitionprovidesascalebetween0decibel(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,busyoffice

quietcommunicationquietlibrary

absolutesilence

heavytraffic

*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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3.5FREqUENCY

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[ 20–20.000Hz] Ultrasound

Frequencies–measuredinHertz(Hz)

10Hz 100Hz 1.000Hz 10.000Hz 100.000Hz

Bat

Triangle

Organ

Violin

Contrabass

Grandpiano

Malevoice

Femalevoice

Phone

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3.6FREqUENCYRANGESRELEVANTFORROOMPLANNING

Thefrequencyrangetobetakenintoaccountwhenplanningaroomisbasedonthehumanauditorysystemontheonehandandwhatistechnicallysensibleandfeasibleontheother.Frequenciesabove5000Hzareattenuatedbytheairtosuchadegreethatitisnotsensibletotakethemintoaccountwhenplanningtheacousticsofaroom.Below100Hz,otherphysicalimplicationsofsoundpropagationneedtobetakenintoaccount.

Theinternationallystandardisedtestmethodsfordeterminingthesoundabsorptionbyparticularmaterialsarebasedonthefrequencyrangefrom100Hzto5000Hz.Correspondinglyithasbeendecidedtofocusroomacousticplanningonthefrequencyrangebetween100Hzand5000Hz,asarule.

Infrasound Audiblerange[ 20–20.000Hz] Ultrasound

Relevantfrequencyranges

10Hz 100Hz 1.000Hz 10.000Hz 100.000Hz

Bat

Triangle

Organ

Violin

Contrabass

Grand piano

Male voice

Female voice

Phone

Relevantfrequencyrangesfrom100upto5.000Hzforroomplanning.

3.7WAVELENGTHSOFSOUND

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

Wavelengthsl

l

l

Time(sec)

SoundpressureinPascal

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3.8LEVELVALUES

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

Theratinglevelisusuallybasedonaratingperiodof8hours.Highbackgroundnoiselevelsinofficeroomswilllikelyaffecttheintellectualefficiency.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,thereverberationtimeThasbeendefinedasthetimerequiredforthesoundpressurelevelinspacetodecayby60dB.Thismeansthat,ifaroomisexcitedwithabangof95dB,thereverberationtimeindicatestheperiodoftimewithinwhichthenoiseleveldropsto35dB.Thiscanbeafewtenthsofaseconduptoseveralseconds.Thereverberationtimecanbedeterminedforeachenclosedspace.

Thisobjectivelymeasurablequantityallowsdifferentroomstobecomparedwitheachotherandtheirroomacousticqualitytobeassessed.Whileareverberationof4to8secondsisquitenormalforachurch,thevaluesaimedatforthereverberationtimeinconferenceorofficeroomsarequitedifferent.Thefollowingtableprovidesanoverviewofthetypicalreverberationtimesofdifferentroomtypes.

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

Thesubjectiveimpressionofthesoundqualityofaroomallowseventhenon-experttodrawconclusionsastohowthereverberationtimeprogresseswithinthedifferentfrequencyranges.If,forexample,speechinaroomsoundsblurred,andifitisverydifficulttounderstandeachother,itcanbeassumedthatthereverberationtimeistoolong.Acoustically“dry”inthiscontextmeansthatthesoundisabsorbedunnaturallyfast.Ifthishappensonlyathighfrequencies,theroomsounds“hollow”or“booming”,whereasatlowfrequenciesitsounds“piercing”and“sharp”.

Reverberationtime

0 1,0 2,0Time(sec)

typicalreverberationtimeforofficerooms:0,5–0,8sec

Reverberationtime:1,8sec

60dB(A)

100

50

in(dB)Soundpressurelevel

Typeofroom Reverberationtime(exemplary)

Church approx.4–8seconds

Classroom–mediumsized 0,6seconds

Officeroom–dependingonsize 0,5–0,8seconds

Concerthallforclassicalmusic approx.1,5seconds

Performance Reverberationtimeatlowfrequencies

Reverberationtimeathighfrequencies

Subjectiveimpression

speech toolongtoolongtooshorttooshort

toolongtooshorttoolongtooshort

blurred,difficulttounderstandhollow,buteasytounderstandpiercing,clanking,sharp,difficulttounderstanddry,buteasytounderstand

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

Aroomusuallybecomesmorereverberantwithincreasingheight.Absorbingsurfaces–suchascarpets,curtainsandsoundabsorbingceilings,butalsofurnitureorpeoplepresentintheroom–reducethereverberationtime.

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

Reverberationtime:0,5secWITHproductsofCréationBaumann

Reverberationtime:1,2secWITHOUTproductsofCréationBaumann

100

50

SoundlevelindB

13

Theshapeofaroomisusuallyofminorimportanceforthereverberationtime.Onlyiftheroomacousticrequire-mentsareveryhigh(e.g.inconcerthalls)oriftheshapeisveryunusual,e.g.vaultedsurfacesorheavilyvaryingroomheights,doesshapebecomeanessentialfactor.TherecommendationsgiveninDIN18041shouldalwaysformthebasisforanyroomacousticplanning.DIN18041“Acousticqualityinsmalltomedium-sizedrooms”formsthebasisfortherecommendationsregardingtheacousticdesignofsmalltomedium-sizedrooms.Withregardtotheoptimumreverberationtime,DIN18041distinguishesbetweenthreedifferentroomcategories:“music”,“speech”and“communicationandteaching”.Roomsoftheusagetype“music”aremusicclassroomsandhallsformusicpresentations.“Speech”inthebroadestsensecomprisesallroomswhereaspeakerspeaksinfrontofanaudience.“Communicationandteaching”comprisesalltypeswhereseveralpeoplespeakatthesametime,i.e.teachingroomsaswellasconferencerooms,multipleoccupancyoffices,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

Rev

erbe

ratio

n tim

e TS

OLL in

s

Roomvolume V in m3

MusicSpeechTeaching, Communication

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4.2SOUNDABSORPTION

Thesoundabsorptioncoefficientαdescribesthepropertyofamaterialtoconvertincidentsoundintootherformsofenergy–e.g.thermalorkineticenergy–andthustoabsorbit.

Case1:Soundcompletelyabsorbed(soundabsorptioncoefficientα=1)noreflection

Theotherextremeisfullsoundreflection.Alltheincidentsoundisreflected.

Case2:Soundcompletelyreflected(soundabsorptioncoefficientα=0)

Case3:Soundpartlyabsorbed(soundabsorptioncoefficientα=between0and1)

Soundcompletelyabsorbed

Soundcompletelyreflected

Soundpartiallyabsorbed

Thefrequency-dependentsoundabsorptioncoefficientofamaterialisdeterminedbymeansofaspecialacousticmaterialtestmethod–theso-calledreverberationroommethod.Forthistest,amaterialsampleisplacedintothereverberationroom,whosereverberationtimehasbeendeterminedpreviouslywithoutthesample.Fromthechangeinthereverberationtimewiththesamplepresentintheroom,thesoundabsorptioncoefficientαScanbedeterminedforeachone-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.ItisdefinedastheproductofthesoundabsorptioncoefficientαSofamaterialandthesurfaceofthismaterial.

Calculationoftheequivalentsoundabsorptionofsurfacesinaroom:

A=s1α1+s2α2+s3α3+…+sn+αn+A1+A2+…+An

A–totalequivalentsoundabsorptionareainarooms1–surfacesizeofmaterial1,e.g.acousticceilingα1–soundabsorptioncoefficientofmaterial1s2–surfacesizeofmaterial2,e.g.carpetα2–soundabsorptioncoefficientofmaterial2…Sn–surfacesizeofmaterialnαn–soundabsorptioncoefficientofmaterialn

10Hz 100Hz 1.000Hz 10.000Hz 100.000Hz

Organ

Infrasound Audiblerange[ 20–20.000Hz] Ultrasound

Bat

Triangle

Violin

Contrabass

Grand piano

Male voice

Female voice

Phone

Relevantfrequencyrangesfrom100upto5.000Hzforroomplanning.

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4.3SOUNDABSORPTIONCOEFFICIENTANDREVERBERATIONTIME

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

Sabineformula:

T–ReverberationtimeV–VolumeoftheroomA–Totalequivalentsoundabsorptionarea

Asoundabsorberof10m2withasoundabsorptioncoefficientof0.50hasanequivalentsoundabsorptionareaof5m2andthushasthesameeffectasasoundabsorberof20m2withasoundabsorptioncoefficientof0.25orasoundabsorberof5m2withasoundabsorptioncoefficientof1.00.Inafullyfurnishedroomwithdifferentsurfaces,forexample,eachmaterial(e.g.carpets,plaster,acousticceiling,curtains,windows,shelves,etc.)canbeallocatedasoundabsorptioncoefficient,andbymultiplyingthiscoefficientbythesurfaceofthismaterial,theequivalentsoundabsorptionareacanbecalculated.Theequivalentsoundabsorptionareasofallmaterialsarethenaddedtodeterminethetotalequivalentsoundabsorptionareaoftheroom.

4.4RATINGOFSOUNDABSORPTION

Intheprevioussectionstheadvantagesoflookingatthesound,thereverberationtimeandthesoundabsorptioncoef-ficientinafrequency-dependentcontexthavebeenexplainedingreatdetail.Severalinterestedpartieshave,however,expressedtheirdesireforsimplifiedvalues,whichmightnotpermitdifferentiatedplanning,butwouldallowroughcomparisonstobemadebetweendifferentsoundabsorbersorpreliminarystatementsregardingthebasicsuitabilityofproductsforparticularapplications.Suchvaluesshouldalsoenableasimplifiedplanningofroomswithlowrequire-mentsregardingtheiracousticquality.Againstthisbackdrop,singlevaluesofsoundabsorptionhavebeendefinedinEuropeandtheUSwhichdifferslightly.ThemostcommonsinglevalueofsoundabsorptioninEuropeistheso-calledweightedsoundabsorptioncoefficientαw,whereasintheEnglish-speakingworlditistheNoiseReductionCoefficient(NRC)ortheSoundAbsorptionAver-age(SAA).

Allprocedurestodetermineofsinglenumberratingsrelyontestsinthereverberationchamberwithon-thirdoctavebandresolution.

Weightedsoundabsorptioncoefficientαw(DINENISO11654):Inordertodeterminetheweightedsoundabsorptioncoefficientαw,themeanvaluefortheoctavecentrefrequencybetween125Hzand4000Hzisdeterminedfromthreeone-thirdoctavevalues.18one-thirdoctavevaluesarethusconvertedinto6octavevalues.Themeanvalueoftherespectiveoctaveisthenroundedtothenearest0.05;itisreferredtoasthepracticalsoundabsorptioncoefficientαp.Thepracticalsoundabsorptioncoefficientαpbetween250Hzand4000HziscomparedtothereferencecurvegiveninDINEN11654.Thiscomparisongivesasinglevalueoftheweightedsoundabsorptioncoefficientαw.Deviationsbymorethan0.25betweenthecurveandthereferencecurveareindicatedbymeansoftheshapeindicatorsL,MorH,dependingonwhethertheyoccurat250Hz(L),at500Hzor1000Hz(M),orat2000Hzor4000Hz(H).Theresultingvaluesare,forexample,αw=0.65(H),αw=0.20orαw=0.80(LM).

T=0,163× VA

17

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

Basedontheαwvalue,soundabsorberscanbeclassifiedintodifferentsoundabsorberclasses.αwvaluesofmorethan0.90,forexample,belongtosoundabsorberclassA,valuesofbetween0.15and0.25belongtoclassE.

Single-numbervaluescommonlyusedintheUS

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

SAA(ASTM423):AnothervalueusedintheUSistheSAA(SoundAbsorptionAverage).Itisdeterminedbycalculatingthemeanvaluefromtwelveone-thirdoctavevaluesofthesoundabsorptioncoefficientbetween200Hzand2500Hzandthenroundingtheresulttothenearest0.01.

ADVANTAGEOFSINGLE-NUMBERVALUES:Soundabsorberscanberoughlyclassifiedandthuscomparedwithoneanother.

DISADVANTAGEOFSINGLE-NUMBERVALUES:Asingle-numbersoundabsorptionvalueisalwaysanextremelysimplifiedvalue.Soundabsorberswithverydifferentabsorptionspectracanhaveidenticalsingle-numbervalues.Thismaysometimesresultintheuseofasoundabsorberwhichisnotsuitablefortheexistingconditions.Frequenciesbelow200Hzarenottakenintoaccount.

Soundabsorberclass αw-value

A 0,90–1,00

B 0,80–0,85

C 0,60–0,75

D 0,30–0,55

E 0,15–0,25

notclassified 0,00–0,10

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5. INDEx

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

EqUIVALENTSOUNDABSORPTIONAREATheequivalentsoundabsorptionareaAisdefinedastheproductofthesoundabsorptioncoefficientαofamaterialandthesurfaceSofthismaterial.

AURALISATIONAuralisationisamethodforsimulatingtheacousticproper-tiesofaroom.Withthismethod,theeffectsofcertainacoustictreatmentscanbe“auralised”asearlyasthedesignstage.

BUILDINGACOUSTICSBuildingacousticsisabranchofbuildingphysics,oracous-tics,whichdealswiththeeffectofthestructuralconditionsonthepropagationofsoundbetweentheroomsofabuild-ingorbetweentheinteriorofaroomandtheoutsideofthebuilding.

RATINGLEVEL(Lr)TheratinglevelLr(Lfor“level”,rfor“rating”)istherelevantparameterforobjectivelyassessingthenoiseimpactataworkplace.Apartfromweightingthesoundpressurelevelasafunctionofthefrequency(seeA-weightedsoundpressurelevel),adeterminationofthesoundpressureleveltakesintoaccountcertainadjustmentswhichdependonthecharacteristicofthesound(e.g.impulsivenessorclearprominenceofindividualtones)anditsdurationofimpact.TheratinglevelisalsoexpressedindB(A).

DECIBEL(dB)Logarithmicallydefinedunitofmeasurementwhichex-pressesthesoundpressurelevel.Therelevantscaleforhumanbeingsis0dBto140dB.0dBreferstoasoundpressureof20µPa.

SINGLENUMBERVALUESOFSOUNDABSORPTIONSo-called“singlenumbervalues”areusedforasimplifiedrepresentationofthefrequency-dependentparameterofthesoundabsorptioncoefficientaswellasforaroughcomparisonofdifferentsoundabsorbers.InEurope,the“weightedsoundabsorptioncoefficient”αwinaccordancewithDINENISO11654iscommonlyused.IntheUS,theNRCandSAAvaluesarewidelyused.Alloftheabovevaluesarebasedonmeasurementsofthesoundabsorptioninone-thirdoctaveandoctaveincrements.Foradetailedacousticplanningofaroomitisnecessarytoknowthesesoundabsorptionvaluespreciselyinone-thirdoctaveoratleastinoctaveincrements(see“octaves”).

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

REVERBERATIONROOMReverberationroomsarespeciallaboratoryroomswithwallswhichreflecttheincidentsoundwavestoaveryhighdegree.Reverberationroomshaveparticularlylongreverberationtimesacrosstheentirefrequencyrange.

REVERBERATIONROOMMETHODThereverberationroommethodisusedfordeterminingthefrequency-dependentsoundabsorptioncoefficient.Asampleofthematerialtobetestedisplacedintothereverberationroom.Thesoundabsorptionofamaterialcanthenbecalculatedfromthechangeinthereverberationtimeoftheroom.

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

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ACOUSTICqUALITYTheacousticqualityofaroomreferstoitssuitabilityforaparticularuse.Itisinfluencedbythepropertiesoftheboundarysurfaces(walls,ceiling,floor)andthefurnishingsandbypersonspresentintheroom.

NOISENoisecomprisesallsoundswhich,duetotheirloudnessandstructure,areconsideredasharmfulorannoyingorstress-fulforhumanbeingsandtheenvironment.Itdependsonthecondition,preferencesandmoodofapersonwhethersoundsareperceivedasnoiseornot.Theperceptionofsoundsasnoiseandthewayinwhichpeopleareaffectedbyitdepend,ontheonehand,onphysicallymeasurablequantitiessuchasthesoundpressurelevel,pitchofatone,tonalityandimpulsiveness.Ontheotherhand,certainsubjectivefactorsalsoplayarole:atbedtimenoiseisper-ceivedasextremelyannoying.Thesameistrueforactivitieswhichrequireahighlevelofconcentration.Ifwelikecertainsounds,wewillnotperceivethemasannoyingevenathighvolumes;soundswhichwedonotlikeareannoyingtousevenatlowvolumes(e.g.certaintypesofmusic).Further-more,howwefeelataparticulartimealsoinfluencesoursensitivitytonoise.Ifanactivityisdisruptedordisturbedbyoneormoresounds,thisisreferredtoasnoisepollution.Weareparticularlysensitivetonoiseifverbalcommunica-tionisaffected,e.g.ifaloudconversationattheneighbor-ingtablemakesitdifficultforustolisten,andifwehavetoconcentrateorwanttosleep.

REVERBERATIONTIMEPutsimply,thereverberationtimeindicatestheperiodoftimeittakesforasoundeventtobecomeinaudible.Technically,thereverberationtimeThasbeendefinedasthetimerequiredforthesoundpressurelevelinspacetodecayby60dB.

OCTAVEBANDSAcousticparameterssuchasthesoundpressurelevelorthesoundabsorptioncoefficientareusuallyexpressedinincrementsofoctavesandone-thirdoctaves.Thepreciseknowledgeofacousticpropertiesinthesmallestpossiblefrequencystepsofsoundisaprerequisiteforadetailedacousticdesign.Forroomacousticstherelevantoctavefrequenciesare125Hz,250Hz,500Hz,1000Hz,2000Hzand4000Hz.Theoctaveincrementsareobtainedbydoublingthepreviousfrequency.Eachoctavecomprisesthreeone-thirdoctavevalues(seealso“singlevalues”).

POROUSABSORBERSPorousabsorberscomprise,forexample,mineralfibres,foams,carpets,fabrics,etc.Theeffectoftheporousabsorbersisduetothefactthatsoundisabletoentertheopenstructuresofthematerialwhere,bythefrictionofairparticles,thesoundenergyisconvertedintothermalenergyatthesurfaceofthepores.Porousabsorbersachievetheirbesteffectatmediumandhighfrequencies.

PSYCHOACOUSTICSBranchofacousticsornoiseeffectresearchwhichdealswiththesubjectiveperceptionofobjectivelypresentsoundsignals.Furthermore,psychoacousticsstudiestheinfluenceofalistener’spersonalattitudesandexpectationsontheperceptionofsoundevents.

RESONANCEABSORBERThistermcomprisesalltypesofabsorbersusingaresonancemechanismsuchasanenclosedairvolumeoravibratingsurface.Resonanceabsorbersaremainlysuitableforab-sorbingsoundofmediumtolowfrequencies.Themaximumeffectofresonanceabsorbersisusuallyrestrictedtoacer-tainfrequencyrange(seealso“porousabsorbers”).

SOUNDABSORBERSoundabsorbersarematerialswhichattenuateincidentsoundorconvertitintootherformsofenergy.Adistinctionhastobemadebetweenporousabsorbersandresonanceabsorbersorcombinationsoftheseabsorbertypes.

SABINEFORMULAIfthevolumeandthetotalequivalentsoundabsorptionareaofaroomareknown,thereverberationtimecanbeestimatedusingtheSabineformula,where“T”istherever-berationtime,“V”isthevolumeoftheroomand“A”isthetotalequivalentsoundabsorptionarea.Thecloserelationshipbetweenthevolumeofaroom,thesoundabsorptionofthesurfacesofthisroom,andthereverberationtimewasdiscoveredthephysicistWallaceClementSabine(1868-1919).Hefoundoutthattherever-berationtimeTisproportionaltotheroomvolumeVandinverselyproportionaltotheequivalentsoundabsorptionareaA:T=0,163xV/ATheequivalentsoundabsorptionareaAisthesumofallsurfacesSpresentintheroom,eachmultipliedbyitscor-respondingsoundabsorptioncoefficientα:A=α1S1+α2S2+α3S3+…+αnSn

SOUNDABSORPTIONCOEFFICIENTαThesoundabsorptioncoefficientαofamaterialindicatestheamountoftheabsorbedportionofthetotalincidentsound.α=0meansthatnoabsorptionoccurs;theentireincidentsoundisreflected.Ifα=0,5,50%ofthesoundenergyisabsorbedand50%isreflected.Ifα=1,theentireincidentsoundisabsorbed,thereisnolongeranyreflection.

SOUNDATTENUATIONSoundattenuationdescribestheabilityofmaterialstoabsorbsoundortoconvertthesoundenergypresentintootherformsofenergy,i.e.ultimatelyintothermalenergy(seealso“soundinsulation”).

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SOUNDINSULATIONSoundinsulationreferstotherestrictionofthepropagationofsoundthroughtheboundariesofaroom.Soundinsula-tionis,therefore,ameasuretoseparateroomsacousticallyfromunwantedsoundfromadjacentroomsortheoutside.Thishasnothingtodo,however,withtherequiredacousticsoundattenuationwithinaroom(seealso“soundabsorp-tion”).Soundinsulationisafundamentalparameterofbuildingacoustics.Adistinctionhastobemadebetweenairbornesoundinsulationandimpactsoundinsulation.Airbornesoundiscreatedbysoundsourcespresentintheroomwhicharenotimmediatelyconnectedtotheboundarysurfaces,e.g.peoplewhoaretalking.Impactsound,ontheotherhand,resultsfromstructure-bornesound(footfalls,knocking),whichinturnexcitesthewallsorceilingstoradiateairbornesound.Airbornesoundinsulationandimpactsoundinsulationbothhavetofulfiltherequirementsestablishedinrelevantbuildinglaws.

SOUNDPRESSUREAllsoundeventshaveincommonthefactthattheycauseslightvariationsinairpressurewhichcanpropagateinelasticmediasuchasairorwater.Wethereforerefertothesoundpressureofatone.Theheavierthepressurevariationsare,thelouderisthesoundevent.Thefasterthevariationsoccur,thehigheristhefrequency.

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

SOUNDSHIELDINGAsoundshieldisbasicallyanobstaclewhichinterruptsthedirectpropagationofsoundfromasourcetoareceiver.Itcanconsistinamovablepartitionoranattachmenttobeplacedontopofadesk.Cabinetsandotherlarge-surfacepiecesoffurniturecanalsofunctionassoundshields.Soundshieldscanbeprovidedwithasoundabsorbingsurfacewhichadditionallyreducesthepropagationofsound.

SOUNDSPECTRUMThesoundspectrumdescribesthefrequencycompositionofthesound.Puretonesaresoundeventsofasinglefre-quency.Asuperpositionoftonesofdifferentfrequenciesisreferredtoasnoiseorsound.

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

SOUNDMASkINGSoundmaskingspecificallyusesnatural(e.g.birds’twitter-ing)orartificial(e.g.noise)soundsinordertoblanketothersounds.Thismethodcanbeused,forexample,todrownoutinformation-containingsoundsiftheotherbackgroundnoiseistooweaktomaskthem.

SOUNDPRESSURELEVEL(LP)Thesoundpressurelevel(Lforlevelandpforpressure)isalogarithmicquantityfordescribingtheintensityofasoundevent.Thesoundpressurelevelisoftenalsoreferredtoas“soundlevel”,whichisactuallynotquitecorrect.Thesoundpressurelevelisexpressedindecibels(abbreviatedasdB).Soundpressuresaremeasuredusingmicrophones.Themeasurablelevelrangestartsatjustbelow0dBandendsatapproximately150to160dB.

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MANYTHANkSFORTHESUPPORTTOTHEAkUSTIkBüROOLDENBURG

ThephysicistsDr.CatjaHilgeandDr.ChristianNockefoundedanacousticconsultingcompanyinOldenburg(Germany)in2001.Theyworkasspecializedengineersforarchitects,expertwitnessesforcourtsandconsultantsinthefieldofacoustics.Architecturalacousticsforclassrooms,officesandotherfacilitieshasbecomeonemajorfocusofthecompany.

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

CopyrightEGGERHolzwerkstoffeGmbH&Co.OG,St.JohanninTirol,Österreich

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