BASICS OF ACOUSTICS
2
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 3.4 Soundpressureofseveralsources 3.5Frequency 3.6 Frequencyrangesrelevantforroomplanning 3.7 Wavelengthsofsound 3.8Levelvalues
4.1 Reverberationtime 4.2Soundabsorption 4.3 Soundabsorptioncoefficientandreverberationtime 4.4 Ratingofsoundabsorption
3
1. PREFACE
Noiseorunwantedsoundsisperceivedasdisturbingandannoyinginmanyfieldsoflife.Thiscanbeobservedinprivateaswellasinworkingenvironments.Severalstudiesaboutroomacousticconditionsandannoyancethroughnoiseshowtherelevanceofgoodroomacousticconditions.Decreasingsuccessinschoolclassroomsoraffectingefficiencyatworkisoftenrelatedtoinadequateroomacousticconditions.ResearchresultsfromclassroomacousticshavebeenoneofthereasonstoreviseGermanstandardDIN18041on“Acousticqualityofsmallandmedium-sizedroom”from1968anddecreasesuggestedreverberationtimevaluesinclassroomswiththenew2004versionofthestandard.Furthermorethestandardgaveadetailedrangeforthefrequencydependenceofreverberationtimeandalsoextendedtherangeofroomstobeconsideredinroomacousticdesignofabuilding.
Theacousticqualityofaroom,betteritsacousticadequacyforeachusage,isdeterminedbythesumofallequipmentandmaterialsintherooms.Inthesenseofgoodacousticstheroomsshouldcontributetoperceivespeech,musicorothersoundsasnottooloudortooquietandthewecancommunicatewithmucheffortandfeelcomfortable.
ThisbrochurehasbeendevelopedbyCréationBaumanwiththeintentiontogiveanintroductionandprofessionalsupportinthefieldofroomacousticsthatsometimeshastheconnotationofbeingconfusingortoomulti-dimensional.Itilluminatesimportanttermsandexplainsbasicsandinterrelationshipsofroomacoustics.
WithitspaletteofcreativetextilesforroomsCréationBaumanndeliversacousticallyeffectiveaswellasartisticattractivesolutionsforroomacousticquestions.Thebandwidthoftheacousticefficiencyoftextileapplicationsisoftenunderestimated.ForthisCréationBaumannofferswithitslargedocumentationofacousticpropertiesforitsmaterials-thatisavailableseparately–agreatpotentialinmodernsolutionsforacousticsbytextiledesigninaroom.
4
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.
5
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.
6
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
*
7
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
8
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
9
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
10
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
11
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
12
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
14
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.
15
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.
16
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
18
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.
19
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”).
20
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.
21
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
22
CréationBaumannisrenownedforhighqualitytextilesforinteriordesign.Thankstoourin-housedesignstudioandourownproductionfacilities,unconventionalcreationsarepossible.Ouroffercomprisescustomizedsolutionsandinteriorshadingsystemsaswellaslightcontrol,dimming,andsoundabsorptionsolutions.CréationBaumannisalsosynonymouswiththeextravagant:600differentdesignsin6,000differentcolours.
Foracompletelistingofourshowroomsworldwide,pleasevisit:www.creationbaumann.com
CréationBaumannAGBern-Zürich-Strasse23|CH-4901LangenthalTelefon+41(0)629196262|Fax+41(0)[email protected]|www.creationbaumann.com
Top Related