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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
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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
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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
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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,[email protected],www.akustikbuero-oldenburg.de
CopyrightEGGERHolzwerkstoffeGmbH&Co.OG,St.JohanninTirol,Österreich
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