the design and evaluation of an economically constructed anechoic chamber

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The Design and Evaluation of an EconomicallyConstructed Anechoic ChamberDavid Sun b , Craig Jin a , André van Schaik a & Densil Cabrera ba School of Electrical and Information Engineering, University of Sydney , NSW, Australiab Faculty of Architecture, Design and Planning , University of Sydney , NSW, AustraliaPublished online: 09 Jun 2011.

To cite this article: David Sun , Craig Jin , André van Schaik & Densil Cabrera (2009) The Design and Evaluation of anEconomically Constructed Anechoic Chamber, Architectural Science Review, 52:4, 312-319

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ArchitecturalScienceReviewVolume52.4,pp312-319

The Design and Evaluation of an Economically Constructed Anechoic Chamber

David Sun**†, Craig Jin*, André van Schaik* and Densil Cabrera**

*SchoolofElectricalandInformationEngineering,UniversityofSydney,NSW,Australia**FacultyofArchitecture,DesignandPlanning,UniversityofSydney,NSW,Australia

†Correspondingauthor:ComputingandAudioResearchLaboratory,SchoolofElectricalandInformationEngineering,J03,Level8,UniversityofSydney,NSW,Australia

Tel:612-9351-7257;Fax:612-9351-3847;Email:[email protected]

Abstract: Thispaperreportsonastudyofthedesignofaneconomicanechoicchambersothatacontrolledenvironmentwouldbereadilyavailable for acoustic experiments such as 3-D audio playback through a spherical loudspeaker array. It describes measurements performedto evaluate the room’s acoustical performance against ISO 3745 and some ITU-R BS.1116-1 recommendations. Two custom omni-directional sound sources were developed for the mid and high frequency range to perform some of these measurements. The data fromthese measurements indicate that the room has a noise rating of NR15; the carpeted floor absorbs at least 50% of incident sound energyabove 400 Hz, and the room exhibits free-field conditions in all directions tested up to 2 m from the centre of the room between 400Hz and 20 kHz. Hence, this paper presents a model of how an economic anechoic chamber could be designed for high-resolution spatialaudioapplications (suchas audio reproductionusinghighorder ambisonics), andhowsucha room’s acousticperformancecanbeassessed.

Keywords:Listeningroom,Omni-directionalloudspeaker,Roomcharacterisation,Soundabsorption,Spatialaudio

IntroductionResearch into audio playback techniques such as

multichannel reproduction require a listening environment,which facilitates theproductionof controlled stimuli,whichare used for listening tests. A listening environment tofacilitate these testswasnotavailablesoawayofdevelopingsuch an environment was devised with limited resources.Thedesignwasdeveloped,evaluatedandthisworkledtotheconstructionofanabsorptivelisteningspaceattheUniversityofSydney’sComputingandAudioResearchLaboratory.Thenew listening environment houses a spherical loudspeakerarrayconsistingof32loudspeakersforthecreationofpreciselycontrolled immersive audio. The listening environmentwasconstructedthroughtheuseofabsorptivematerialsbyliningthewalls andceiling,butwithacarpetedfloor. Inorder toachieve an economic solution the absorptive materials usedwere common building materials of varying flow resistivitybasedonanapproachrecentlyrefinedbyXu,Buchholz,andFricke (2005).Thisapproachhasproventobelesscostlythanusingwedge-shapedacousticabsorbers(mostcommonlyusedinbuildinganechoicchambers),despitetheirhighacousticalabsorptionperformance(Delany&Bazley,1977).Theroomhas been evaluated against some recommendations bothwithin ISO 3745, and ITU-R BS.1116-1 standards. Thetwostandardswerechosenoverotheravailable standards forlistening rooms (IEC 1985; NPBC 1992; ITU 1994; AES

1996)as theyweremore suitedconsideringourcurrentandfuture needs. Examples of application of listening roomconcepts given by Arato-Borsi, Poth, & Furies, (1998); Bolt(1946);BoreniusandKorhonen(1985);CoxandD’Antonio(2001);IshiiandMizutani(1982);J’awinen,Savioja,Moller,Ikonen, and Ruusuvuor, (1997); Walker (1996) and Walker(1998)andassistedindeterminingwhichprinciplesaremostappropriate.Itisimportanttonotethatthelisteningroomsdescribed in these papers are designed to house relativelysimple audio systems, such as 2-channel stereophony or 5.1channelsurroundsound,andmaynotbedirectlyapplicabletolisteningroomsforhighspatialresolutionaudiosystemswithmanyloudspeakers.

That said, the main criteria for the new listening roomis a floor space greater than 70 m2, a ceiling height greaterthan 2.5 metres, high acoustic absorption, and a maximumnoise rating of NR15. The Noise Rating (NR) is a unit ofmeasure developed by the International Organization forStandardization (ISO), to determine the acceptable indoorenvironmentforhearingpreservation,speechcommunicationandannoyance(seeFigure1).

In this paper we will detail the design and constructionof the listening room,describemethodsused to characterisethe room’s acoustical properties, and give the results of thatcharacterisation.Itishopedthisinvestigationprovidesauseful

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313David Sun, Craig Jin and André van Schaik A New Spatial Audio Listening Room

model for researchers whom may wish to develop a similareconomic facility.

1 Room Design and Construction1.1 Room Geometry

� espace in which the room has been constructed waspreviously an offi cespace with concrete walls and ceiling, andtimber flooring raised 0.2 m above a concrete floor. Beforeadding absorptive lining, the dimensions of the space were12.16 m x 9.48 m x 3.48 m (see Figure 2). Double-glazedwindows are present on the top half of the front wall, andthere are doors on either side of the room. � eroom is on theeighth storey of a building that is not in the direct line of sightto nearby major roads. � eceiling has a maximum height of3.48 m, although the ceiling height varies because there aretwo concrete, structural beams that run along the length of theroom. Given the height limitation of the ceiling, the listeningroom was not designed to be fully anechoic c.f. (Ballagh, 1986; Beranek & Sleeper, 1946; Koidan & Hruska, 1978), sincethere is insuffi cientspace to properly treat the floor. A solidfloor is in many ways easier to use (for example, in positioningloudspeakers) than the wire mesh or metal grid floors that areused in fully anechoic rooms. However, a strong reflectionfrom the floor is not desired in audio reproduction in the mid-high frequency range (in the low frequency range, especiallyfor subwoofers on the floor, the reflection is in phase with thedirect sound, and so usefully reinforces the desired sound).

� erehave been several attempts to determine optimumroom geometry, most recently by Walker (1996) wherelimitations are placed on length, width and height proportionssuch that coloration of low frequency reproduction (due to lowroom mode density) are kept to a minimum. Our room meetsthe criteria 8.2.2.1 – 8.2.2.3 of the BS.1116-1 standard, but

Figure 2: a) Top view of the listening room. b) � ree dimensional view of the room. � e dashed lines represent the traverses used for free-field decay measurements. The mark ‘X’ indicates the centre of the room, which was used for mid and high frequency

measurements. c) The microphone locations used to measure the room’s background noise level.

Figure 1: The Noise Rating curves, developed by the International Standards Organisation. Figure from “http://www.

industrialacoustics.com/uk/reference/bluebook/f_-_nr_noise_rating_curves.html”.

a)

Windowplugs

Doors

Air-conditionvent

Insulation

Ceilingbeams

Air return duct

T8T5

T10 T9

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Left

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ArchitecturalScienceReview Volume52,Number4,2009314

hasalowerheightthanrecommendedforthefloorplan.Thefinished, lined room measures 10.5 m x 8 m x 2.6 m, withareductionofheightto2.4munderneaththeceilingbeams(seeFigure2b)andis the largestroomintheworldthathasusedabsorptionofthistypetoprovideanechoicsurfacesinanenclosedspace.

1.2 Cooling and VentilationToprovidetheroomwithventilation,fourmetalductswere

installedontherightwall(seeFigure2a).Themetalductsleadto anair compressor, situatedon the roofof thebuilding tominimisenoise.Anairreturnductwasinstalledontherightsidewhichalsoservesasachannelforfeedingcablesbetweenthecontrolroomandthelisteningroom(seeFigure2a).

1.3 Sound AbsorptionAnalgorithmbyXuetal., (2005)wasused todesign the

multi-layeredabsorptivelining.Thealgorithmprovidedacostanalysistoabsorbsoundinaroomgivenacombinationoffibrousabsorbers, total lining thickness, and theabsorptionrequiredforagivenfrequencyrange.Fibrousabsorbersheetswerelinedagainst thewalls andheld inplacebyplastic stakesglued tothewalls.Thealgorithmdeterminedthatonemetrethickofabsorberwithincreasingflowresistivitytowardsthehardwallandceilingwasneededforanechoicsoundabsorptiondownto100Hz.Giventhelowceilingheight,theceilingbeamswerelinedwithfewerlayers,achievinganechoicabsorptiondownto200Hz(althoughthisprobablyhaslittleeffectonperformanceconsideringthatthewidthofthebeamsismuchlessthanthe100Hzwavelength).Toabsorbsoundonthefrontwall,whichcontainedlargewindows,fiveremovablealuminiumplugswere

constructed(seeFigure3c).Otherplugs(seeFigures3a,b)wereconstructedtocoverthedoors,andair-conditioningducts.Allplugsemployedthesameabsorptivematerialasthewalls.Thewooden floor was covered with carpet and two layers of feltunderlay.Theuseoftwolayersofunderlaywasdecideduponafterperformingmeasurementswithcarpet samplesusinganimpedancetube(Brüel&Kjær,2005).

2 Measurements2.1 Sound Sources

Totesttheroom’sacousticresponse,threesoundsourceswereused tocover the low,midandhigh frequencies respectively.Together,thethreesoundsourcescoveredfrequenciesbetween10 Hz and 20 kHz. The mid- and high- frequency soundsourceswere custombuilt to optimise omni-directionality atthosefrequencies.Forthelowfrequencies,aWhiseProfunder319Asub-woofer(seeFigure3f )wasused.Themidfrequencysound source consisted of an Aurasound NSW2-326-8AmountedononeendofaPVCpipe,50mmindiameterandonemetreinlength,andfilledwithporousabsorbingmaterial.The multi-layered absorption used to cover the walls of thelisteningroomwasappliedtofillthePVCpipetoachieveananechoicterminationdownto100Hzbehindthedriver(seeFigure3g).Forthehighfrequencysoundsource,aTOATU-50compressiondriverwasencapsulatedinawoodenboxandfilledwithdense soundabsorbingmaterial. A1.8mcoppertubeof10mmdiameterwasmountedontheopeningofthecompressiondriverandsurroundedbya1.8mPVCpipe(seeFigure3h).ThePVCpipewaslaggedwithinsulationmaterialtopreventsoundfromleakingalongthelengthofthetube.Oncethesoundsourcesweremade,theirdirectionalitywasmeasured

Figure 3: a) A plug for an air conditioning duct. b) Door plugs to the left and the right. c) Bottom half of the plugs to cover the front wall. d) The listening room, front wall showing the windows covered by the plugs. e) The listening room, back wall. f ) Low frequency sound source.

g) Mid frequency sound source. h) High frequency sound source.

Figure 2: a) A plug for an air-conditioning duct. b) Door plugs to the left and the right. c)

Bottom half of the plugs to cover the front wall. d) The listening room, front wall showing the

windows covered by the plugs. e) The listening room, back wall. f) Low frequency sound

source. g) Mid frequency sound source. h) High frequency sound source.

c)

h)

e)d)

b)

g)

a)

f)

a) b) c)

f )e)d)

g) h)

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315DavidSun,CraigJinandAndrévanSchaik ANewSpatialAudioListeningRoom

byplacing them ina largeanechoic chamberandmeasuringthe pressure at various locations around it (InternationalOrganizationforStandardization,2003).Thedatafromthesemeasurementsshowedthatthemidfrequencysoundsourceisomni-directionalbetween200Hzand1.6kHz,whilethehighfrequencysoundsource isomni-directionalbetween1.6kHzand 20 kHz. For reference,Table I describes the tolerancesfor sourceomni-directionality, andmore information canbefoundinISO3745.

2.2 Noise FloorAfter the sound absorbing lining was completed, a Brüel

&Kjær2250soundanalyzerwasusedtomeasuretheroom’sbackgroundnoiselevel.Theanalyzerwasplacedatninedifferentlocations(seeFigure2c),ataheightof1.3maboveground,andthesoundpressurelevel(un-weighted)wasmeasuredover40secondintervals.

2.3 Free Field DecayTodeterminethelisteningroom’sfreefieldequivalencefor

mid and high frequencies, a sound source was placed in themiddleoftheroomataheightof1.3m.Fourtraversesweresetuporiginatingfromtheacousticcentreofthesoundsourceandextendingtotheuppercornersatthefrontwall,andlowercornersatthebackwall,asspecifiedinISO3745:2003.Afifthtraversewassetuporiginatingfromtheacousticcentreofthesoundsourceandextendingnormaltotheleftwall.Becauseofitslargesizeandweight,forlowfrequencies,thesub-woofer

wasonthefloorcentre,andtraversesweresetuporiginatingfrom the base of the sub-woofer and extending to the topcornersof theroom. Fishingwirewith0.1mmarkingswasusedtomarkthemeasurementpositions. AdiscretetraversesystemwasusedinsteadofonesusedbyCunefareetal.,(2003)andBieselandCunefare(2003),duetotimeconstraint.Themeasurements were performed by playing sound out of asourceusingacomputerconnectedtoanApogeeDA-16X,D/Aconverter,while simultaneously recording the soundstimuliusing aBrüel&KjærmicrophonewithType2669body andType 4192 capsule. The microphone was connected to aneight-channel preamplifier, Digidesign PRE, while the outputofthepreamplifierwasconnectedtoanApogeeAD-16XA/Dconverter.Theconverterwasconnectedtothesamecomputerused to play the sound stimuli. The sound stimuli consisted

Figure 4: The background noise level measured over 40 seconds at different locations within the room, at a constant height of 1.5 m.

Frequency (Hz) Anechoic Hemi-anechoic

≤630 ± 1.5dB ± 2.0dB

800-5000 2.0dB ± 2.5dB

6300-10000 ± 2.5dB 3.0dB

>10000 ± 5.0dB 5.0dB

Table I: Theallowabledeviationsfromdirectionalityforomni-directionaltestingofsources.

Sound pressureleveldB

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of eight second, logarithmic sine sweeps, at32-bitdepth, andsampledat48kHz.Thefrequencyrangeofthesweepsvarieddepending on the type of sound source. For the sub-woofer,midfrequency,andhighfrequencysoundsources,thefrequencyrangeofthesweepvariedfrom10Hzto2kHz,20Hzto5kHz,and500Hzto22.5kHz,respectively.Thesoundpressurewasmeasuredateachmarkingforninesecondsalongthetraverses,starting0.5mawayfromthesoundsource. Fifteeniterationswere performed at each position, and these were coherentlyaveragedtoimprovethesignal-to-noiseratio(SNR).

2.4 Floor AbsorptionTheabsorptioncoefficientofthecarpetedfloorwasestimated

usingtheimpulseresponsesmeasuredalongthetraversesoftheroom.Theabsorptioncoefficient,α ,isestimatedas:

Hz to 2 kHz, 20 Hz to 5 kHz, and 500 Hz to 22.5 kHz, respectively. The sound pressure was

measured at each marking for nine seconds along the traverses, starting 0.5 m away from the

sound source. Fifteen iterations were performed at each position, and these were coherently

averaged to improve the signal-to-noise ratio (SNR).

2.4 Floor Absorption

The absorption coefficient of the carpeted floor was estimated using the impulse responses

measured along the traverses of the room. The absorption coefficient, ! , is estimated as:

2

1 r

d

E r

E d!

" #= $ % &

' ([1]

where r is the distance travelled to the receiver by the first reflection, d is the distance to the

receiver for the direct sound,r

E is the energy received from the first reflection, andd

E is the

energy of the direct sound. Using the measured impulse responses, the direct sound and first

reflection are identified and their corresponding energy calculated. The factor ( )2

r d is used to

compensate for the distance travelled under the assumption that the sound intensity decreases

inversely with distance squared. A value of 1! = indicates that a material absorbs all incoming

sound and a value of 0! = indicates that a material is fully reflective. In order to estimate the

start of the first reflection within the impulse response, a model was derived to estimate the first

reflection path length. The various values for the absorption coefficient that were obtained along

one traverse were then averaged to provide a value for! .

(1)

whereristhedistancetravelledtothereceiverbythefirstreflection,disthedistancetothereceiverforthedirectsound,Er is the energy received from the first reflection, and Ed istheenergyofthedirectsound.Usingthemeasuredimpulseresponses,thedirectsoundandfirstreflectionareidentifiedandtheircorrespondingenergycalculated.Thefactor(r/d)2 is used to compensate for the distance travelled under theassumptionthatthesoundintensitydecreasesinverselywithdistancesquared.Avalueof








2

1 r

d

E r

E d!

" #= $ % &

' ([1]




E is the



r d is used to







=1indicatesthatamaterialabsorbs all incoming sound and a valueof








2

1 r

d

E r

E d!

" #= $ % &

' ([1]




E is the



r d is used to







=0 indicatesthat a material is fully reflective. In order to estimate thestart of the first reflection within the impulse response, amodelwasderivedtoestimatethefirstreflectionpathlength.The various values for the absorption coefficient that wereobtainedalongonetraversewerethenaveragedtoprovideavaluefor








2

1 r

d

E r

E d!

" #= $ % &

' ([1]




E is the



r d is used to







.

3 ResultsInthefollowing,wepresentdataregardingthebackground

noise level of the room, data from the measurements along

traverses set upwithin the room, anddata onfloor acousticabsorption.

3.1 Background Noise LevelConsider now Figure 4, which shows the measured

backgroundnoise level in1/3-octavebands for our listeningroomatmidday.ThedatapresentedhereshowsthattheroomhasamaximumnoiseratingofNR15,satisfyingtheBS.1116-1recommendationforbackgroundnoise.Itisimportanttonotethatthedatashownincludesthesoundlevelmeter’sinherentsystem noise, which dominates at and above 400 Hz. It isalsoevidentfromthisfigurethatthelisteningroomprovidesa lownoise soundenvironment in thehigh frequencyrange.Forexample,thereislessthan15dBSPLofbackgroundnoisepresentintheroomabove200Hz,whichatsuchlevelsdoesnotprovidesignificantmaskingeffectswhencomparedtothehumanhearingthreshold(i.e.20

3 RESULTS

In the following, we present data regarding the background noise level of the room, data from the

measurements along traverses set up within the room, and data on floor acoustic absorption.

3.1 Background Noise Level

Consider now Figure 3, which shows the measured background noise level in 1/3-octave bands

for our listening room at midday. The data presented here shows that the room has a maximum

noise rating of NR15, satisfying the BS.1116-1 recommendation for background noise. It is

important to note that the data shown includes the sound level meter’s inherent system noise,

which dominates at and above 400 Hz. It is also evident from this figure that the listening room

provides a low noise sound environment in the high frequency range. For example, there is less

than 15 dB SPL of background noise present in the room above 200 Hz, which at such levels

does not provide significant masking effects when compared to the human hearing threshold (i.e.

20 Paµ , at 1 kHz). There is a noticeable peak at 100 Hz, which can be attributed to the ceiling

fans that are operating in the room directly below. Finally, it is important that access to areas

adjacent to the room is restricted during measurements because footsteps can be heard when a

person walks heavily in the adjacent corridor because no special provision has been made for

insulation against structure-borne sound.

[Insert Figure 3]

3.2 Measurements Along Traverses

Figures 4, 5, and 6 show the inverse of pressure as a function of distance for selected one-third

octave frequency bands. The measured data and the free-field ideal are shown, which is a

,at1kHz).Thereisanoticeablepeakat100Hz,whichcanbeattributedtotheceilingfansthatareoperatingintheroomdirectlybelow.Finally,itisimportantthataccesstoareasadjacenttotheroomisrestrictedduringmeasurementsbecause footstepscanbeheardwhenapersonwalksheavilyintheadjacentcorridorbecausenospecialprovisionhasbeenmadeforinsulationagainststructure-bornesound.

3.2 Measurements Along TraversesFigure5,6,and7showstheinverseofpressureasafunction

of distance for selected one-third octave frequency bands.Themeasureddata and the free-field ideal are shown,whichis a straight line extrapolated from the origin and the firstmeasurementobtained at 0.5maway from the source. If aroomisperfectlyanechoic,thentheinverseofsoundpressuredoubleswitheverydoublingofdistance. Ineachfigure, thesoundsource(whereLFistheWhiseProfunder319A,MFistheAurasoundNSW2-326-8A,andHFistheTOATU-50),thecentrefrequencyofthe1/3-octaveband,andthenameofthetraverseareindicated.

Figure 5: The inverse of pressure as a function of distance within selected one-third octave frequency bands for traverse T8, which extends from the centre of the room to the top left corner of the front wall.

The theoretical ideal is shown as a line.Figure 5. The inverse of pressure as a function of distance within selected one-third octave

frequency bands for traverse T8, which extends from the centre of the room to the top left corner

of the front wall. The theoretical ideal is shown as a line.

a) b) c) d)

e) f) g) h)

Distance (m)

Inve

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ofP

ress

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ConsidernowFigure5,whichshowstheinverseofpressureasafunctionofdistancefortraverseT8,whichextendsfromthesourcetothetopleftcorneronthefrontwall.Thisfigureshowsthatforlowfrequencies,suchas200Hz(seeFigure5a),thelessthan ideal initial increase in inversepressureasa functionofdistancesuggestspossibleroommodaleffectsorfloorreflectioneffectsinthisfrequencyband.Thisalsosuggeststhattheroomhaspoorabsorptionatlowfrequencies.However,above200Hz(seeFigures5bto5h),thedatashowthattheroomexhibits1/rcharacteristicsacrossmostfrequenciesupto4mawayfromthesource.Beyond4m,thereisadecreaseinpressureduetotheleftceilingbeamblockingthesound(i.e.introducingsomerefraction towards the ceiling for sound passing though theabsorptive lining,andperhapsdiffraction towards theceilingafterthebeamobstruction).Thedatafor1.6kHz(seeFigure5d)and3kHz(seeFigure5e)showoscillatingpatterns,whichsuggestperiodsofconstructiveanddestructiveinterferenceduetothealuminiumdoorframes.

Figure 6 shows the inverse of pressure as a function ofdistancefortraverseT9,whichextendsfromthesourcetothebottomrightcorneronthebackwall.Thedatainthefigureshowthattheroomdoesnotabsorblowerfrequenciesaswellashigherfrequenciesnearthefloor.Thisisevidentatfrequencies200Hz,400Hz,and800Hz(seeFigures6a,b,andc)wheretheoscillationsdeviatefromtheidealmorethanthoseshownatthehigherfrequencies.Acrossallfrequenciesthedatacloselymatch the ideal for a limited distance. The peaks at 2.5 mfor200Hz,and4mfor400Hzcanbeexplainedasfollows.At approximately 2.5 m along the traverse, the destructiveinterferenceisatamaximumbecausethepathlengthdifferencebetweenthedirectandfirstreflectionfromthefloorat2.5mcorrespondstothehalf-wavelengthfor200Hz.Asfrequencyincreases, themaximumwill shift to the right, such that therelationship between the path length difference and the halfwavelengthofthefrequencyunderinspectionholds.

Figure 7 shows data for traversesT5 andT10, whereT5extendsfromthefrontofthesub-woofertomidwaybetweenthefrontandbackceilingcornerstotheleft,andT10extendsfrom the sourcenormal to the leftwall. Thedata from thisfigureindicatesthattheroomdoesnotabsorblowfrequenciesasefficientlyashighfrequencies.Atlowfrequencies,thereisevidence of constructive interference, such as seen in roommodesinthe25Hz,31.5Hz,100Hz,and200Hzbands(seeFigures7a,b,d,ande).Meanwhile,at50Hz(seeFigure7c),therearepatternsofconstructiveanddestructiveinterference.For high frequencies, the data closely match the ideal,exhibiting1/rcharacteristicsandthereforeefficientabsorptionofhigh frequency contentwhich is evidentbetween800Hzand20kHz(seeFigures7gtol). At3kHz,thepresenceofoscillations in the data suggests constructive and destructiveinterference.ThisisalsotrueinFigures5eand6efortraversesT8andT9.Finally,at20kHz,thedecreaseinpressurebelowthe theoretical ideal past 2 m indicates the possibility of airabsorptionbecomingmoresignificant.

3.3 Floor AbsorptionFigure8 shows theabsorptioncoefficient as a functionof

frequencybasedonmeasurementsfromtraversesT8,T9,andT10 (seeFigure2b). Thedata indicate that at 200Hz, thefloor has an absorption coefficient in the order of 0.37. Asfrequency increases, thefloor’s absorptionefficiency increasesto0.83at1.6kHz,and0.92at16kHz.ConsidernowTableI,whichshowsthenormalincidenceabsorptioncoefficientsofvariouscarpetsamplesfrommeasurementsobtainedwithinanimpedancetube.Althoughthetableshowsdataforfrequenciesupto4kHz,thedatainFigure8closelymatchesthatofTableII. Note that between 1.6 kHz and 4 kHz the absorptioncoefficientreducesto0.55,whichislessthanthe0.73obtainedviatheimpedancetubemethod.Furthermore,thedipseenat3kHzcanalsoexplainthepresenceofoscillations(asmentioned

Figure 6: The inverse of pressure as a function of distance within selected one-third octave frequency bands for traverse T9, which extends from the centre of the room to the bottom right corner of the back wall. The

theoretical ideal is shown as a line.Figure 6. The inverse of pressure as a function of distance within selected one-third octave

frequency bands for traverse T9, which extends from the centre of the room to the bottom right

corner of the back wall. The theoretical ideal is shown as a line.

a) b) c) d)

e) f) g) h)

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Figure 7: The inverse of pressure as a function of distance within selected one-third octave frequency bands for traverses T5 and T10, where T5 extends from the front of the sub-woofer to the ceiling edge at the left wall, and T10

which extends from the centre of the room, perpendicular to the left wall. The theoretical ideal is shown as a line.

Figure 8: The absorption coefficient of the floor calculated from traverses T8, T9, and T10. Also shown is the mean across the three traverses and the data from measurements performed using an impedance tube.

Figure 7. The inverse of pressure as a function of distance within selected one-third octave

frequency bands for traverses T5 and T10, where T5 extends from the front of the sub-woofer to

the ceiling edge at the left wall, and T10 which extends from the centre of the room,

perpendicular to the left wall. The theoretical ideal is shown as a line.

a) b) c) d)

e) f) g) h)

i) j) k) l)

Distance (m)

Inve

rse

ofP

ress

ure

(m2 /N

)

Figure 8. The absorption coefficient of the floor calculated from traverses T8, T9, and T10. Also

shown is the mean across the three traverses and the data from measurements performed using an

impedance tube.

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above)inFigure7i.Theseresultsarenotsurprisinggiventhethicknessof carpet, asonewouldexpect that apaddedfloorwoulddolittletoabsorblowfrequencyenergy.

DiscussionWe have successfully built a listening environment using

unconventionalmethodsandachievedourgoaltoprovidefree-fieldequivalencebetween400Hzand20kHzinalldirectionsupto2metresfromthecentreoftheroom,asprovenviaperformanceevaluation.Betweenthesefrequencies,theroomhasanintrusivenoiseratingofNR15,whileandthefloorabsorbsatleast50%ofincidentsoundenergyabove400Hz.Thefloorprovidesasolidsurface for working without introducing significant reflectiveenergy for precise audio reproduction. We will construct aspherical32-loudspeakerarrayintheroom,usingitsabsorptivepropertiestominimisereflectionsduringplayback.

ReferencesArato-Borsi,E.,Poth,T.,&Furies,A.(1998).“NewReferenceListening

RoomforTwo-ChannelandMultichannelStereophonic,”in104thAESConvention(AES,Amsterdam).

Audio Engineering Society, (1996). “AES20-1996: recommendedpractice for professional audio – Subjective evaluation ofloudspeakers,”(NewYork).

Ballagh,K.O.(1986).“Calibrationofananechoicroom,”JournalofSoundandVibration105,233-241.

Beranek,L.L.,&Sleeper,J.H.P.(1946).“TheDesignandConstructionof Anechoic Sound Chambers,” J. Acoust. Soc. Am. 18, 140-150.

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Borenius,J.,&Korhonen,S.U.(1985).“NewAspectsOnListeningRoomDesign,”in77thAESConvention(AES,Hamburg).

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Cox, T.J., & D’Antonio, P. (2001). “Determining optimumroom dimensions for critical listening environments: A newmethodology,”in110thAESConvention(AES,Amsterdam).

Cunefare,K.A.,Biesel,V.B.,Tran,J.,Rye,R.,Graf,A.,Holdhusen,M.,&Albanese,A.M.(2003).“Anechoicchamberqualification:Traversemethod,inversesquarelawanalysismethodandnatureoftestsignal,”J.Acoust.Soc.Am.113,881-892.

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Koidan,W.,&Hruska,G.R. (1978). “Acousticalpropertiesof theNationalBureauofStandardsanechoicchamber,”J.Acoust.Soc.Am.64,508-516.

Walker,R.(1996).“OptimumDimensionRatiosForSmallRooms,”in100thAESConvention(AES,Copenhagen).

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Xu, J.F., Buchholz, J.M., & Fricke, F.R. (2005). “Flat-walledmultilayeredanechoiclinings:Optimizationandapplication,”J.Acoust.Soc.Am.118,3104-3109.

Table II: Thenormal incidenceabsorptioncoefficientsofvariouscarpet-underlaycombinations.

Frequency (Hz) 125 250 500 1000 2000 4000

Carpet 0.10 0.10 0.12 0.11 0.22 0.50

Carpet + 1 underlay 0.15 0.12 0.22 0.57 0.73 0.85

Carpet + 2 underlay 0.10 0.41 0.50 0.82 0.90 0.73

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the design and evaluation of an economically constructed anechoic chamber

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