Robotic Methods in AcousticsAnalysis and Fabrication Processes of Sound Scattering Acoustic Panels

James Walker1, Isak Worre Foged21CITA/Royal Danish Academy of Fine Arts 2CITA/Royal Danish Academy of FineArts & Institute of Architecture & Design, Aalborg University, Denmark1jamesbrucewalker@gmail.com 2iwfo@create.aau.dk

This research explores the design, fabrication and testing of acoustic panels in thecontext of architectural acoustics. A method of fabricating curved acoustic panelswith locally diffusive geometry will be presented. A novel method of testing thesound distribution of a panel in 3-dimensions will also be presented, which uses arobotic arm to position a microphone along a hemispherical toolpath. My aim isto present a theoretical process by which one could start to correlate geometryand sound scattering in a multi-dimensional design space, and how this might fitinto the context of architectural project, as shown in Figure 1.

Keywords: Architectural Acoustics, Digital Fabrication, Robotics, EmpiricalTesting

INTRODUCTIONContemporary concert halls employ custom acoustictreatment which are mass-customised to respond todiffering acoustic conditions across the auditorium,whereas in the past standardised products wouldhave been used. These custom acoustic treatmentscandisplaydifferentiated acoustic qualities includingabsorption, scattering and diffusion; also known as‘hybrid surfaces’ due to their ability to performwithinmultiple acoustic criteria (Cox and D’Antonio 2016,p. 313). The ability to create differentiated buildingelements using digital fabrication makes it possibleto vary acoustic qualities throughout a space (Bon-wetsch et al. 2008, p. 365). These treatments areoften integrated into the wall or ceiling, as opposedto being distinct features, which allows the treat-ment to blend in with the architectural style. How-ever, because the treatments are custom made for

the project they do not come with the testing datathat would comewith a standardised product, whichmeans we cannot accurately simulate the acousticsof a space in a digital model with these treatmentspresent.

These treatments are oftenmade from a gypsumcomposite and previous solutions to their fabricationoften involves the use of CNC milling to either millan EPS foam mould for GRG (glass reinforced gyp-sum), as in the case of Guangzhou Opera House (Ex-ton 2011, p. 121) or to mill the gypsum fibreboardpanel directly, as in the case of the ElbphilharmonieConcert Hall (Koren and Müller 2017, p.127). As partof this investigation, custom robot arm end-effectorshave been created, including a hot wire cutter andhot knife cutter. This paper will propose a method offabricating panels using amulti-axis robotic armwiththese tools to create EPS foammoulds for GRG.

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Figure 1Theoretical processof designingcustom acoustictreatment in thecontext ofarchitecturalproject.

Current solutions for testing custom acoustictreatment involves empirically testing prototypeswith specialised equipment or using software to dig-itally predict the performance of the panel prior tofabrication. When testing these treatments empir-ically a reverberation or anechoic chamber with aspecific apparatus is required, depending on the as-pect of performance being tested. However, accessto this type of space and equipment is not alwaysreadily available. This paper proposes that if multi-axis robotic fabrication is to beused in the fabricationprocess then why not take further advantage of thistool and use it to position a microphone when test-

ing the panel. This provides amore accessible way totest panels post-fabrication and could help to createa closer loop between the fabrication and testing ofacoustic panels. This test could be used to analyse ifa panel has an equal distribution of sound in all direc-tions or that a panelmight intentionally reflect soundin one direction more than others. The data fromtesting these panels can then be used correlate ge-ometry with sound scattering properties (Reinhardtet al. 2017, p. 156).

The investigation will present the combinationof a design, analysis and fabrication method, the re-sults and discuss the potential of these methods.

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METHODSTools for fabricating EPS moulds have been createdincluding a large hot wire cutter and small hot knifeto be used as robot arm end-effectors, shown in Fig-ure 2. A two-step method has been employed whichfirstly uses a large hot wire cutter mounted to therobot arm to create the curvature of the global sur-face, shown in Figure 3, which is intended todirect re-flections and scatter sound due to convex curvature;then using a hot knife to carve out the local surfacecondition, which is intended to scatter sound further.This fabricationmethod could allowus to create free-formmoulds faster than CNCmilling, due to the abil-ity of these tools to cut a finished surface in one pass,whereas a CNC may need multiple passes to achievea similar result (Clifford et al. 2014, p. 8). The foamused to create the moulds can even be fully recycledif a thin latex sheet is used to protect themouldwhilecasting (Clifford et al. 2014, p. 13).

Figure 2Hot wire cutter andhot knife robot armend-effectors.

It is important to distinguish between diffusion andscattering, diffusion is how uniformly distributedsound is reflected; whereas scattering is the ratioof sound reflected in a non-specular way. The dif-fusion coefficient is used to define the quality of apanel; however, the scattering coefficient tends to

be used for acoustic simulation to define the quan-tity of sound that is scattered (Cox and D’Antonio2016, p. 87). The diffusion coefficient is calculatedusing a goniometer which measures the direction ofreflections and it requires up to 37 microphones, ar-ranged in either a semicircle or hemisphere depend-ing on whether the test is 2D or 3D. If a panel scat-ters sound in multiple planes then a 3D setup mustbe used (Ibid., p. 89).

A tool for attaching a microphone to the robotarmhas been fabricated. It is designed to have amin-imal surface area so that it will not cause unwantedreflections that disturb the measurement (Ibid., p.93). This testing method is based on the 3D go-niometerwhich outlined in theAES-4id-2001 and ISO17497-2:2012 standards. The setup, shown in Fig-ure 4, uses the guidelines for performing the test in anon-anechoic room (Ibid., p. 91). The tool is used totake measurements along a hemispherical toolpathat 61 planes of measurement. Multiple microphonesare not required as a single microphone can be posi-tioned by the robot andmeasurements can be takensequentially. A loudspeaker plays test tones and theSPL (sound pressure level) of the reflection is mea-sured at eachplane. Using this data, a polar balloon isconstructed to represent a multi-dimensional under-standing of sound reflections. A polar balloon is a 3Dversion of the polar patternwhich is amode of repre-sentation to describe the radiation pattern of micro-phones, loudspeakers and acoustic panels.

Figure 3Hot wire cutterrobot arm tool inthe CITA robot lab.

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Figure 4Testing processbased on guidelinesfor using thegoniometer in anon-anechoicroom.

DESIGN EXPERIMENTATIONThe design of the panels has been explored basedon the limitations of the tools. The hot wire cuttercan only create ruled surfaces and therefore cannotoffer free-form surface creation comparable to CNCmilling. In order to create more complex geome-tries, a hot knife tool was created which features arigid piece of metal with high resistivity which canbe shaped with different profiles. The method isdefined by a directional sweep of a profile throughfoam and is distinct from CNC milling which is non-directional due to its spinning drill-bit (Clifford et al.2014, p. 8). The large hot wire cutter tool is firstused to ‘rough’ the foam block, removing the bulkof the unused material and creating a negative ofthe global surface which can be used in the mould-ing process subsequently. A hot knife tool is thenattached in order carve out locally diffusive surfacegeometry. A curved profile has been utilised at dif-ferent sizes to create a grid of convex nodes, thesize of these nodes was varied on different panels

to see how this affected scattering. After the EPSfoam moulds have been created they are cast witha gypsum and glass fiber mixture. At scale the glassfibers give the panel a greater shear strength and al-lowmuch thinner depths of material.

The panels are measured using sine wave testtones at 7 frequency bands from 125 Hz to 8 kHz.The panel is irradiated with sound using a station-ary loudspeaker which is designed with a flat fre-quency response andhas a frequency range of 54Hz -30kHz. When the sound stops the reflection from thepanel is recorded using an omni-directional acous-tic testing microphone which is also designed witha flat frequency response and has a frequency rangeof 15 Hz to 20 kHz. The planes of measurementare constructed along a hemisphere centred on thepanel and the microphone is positioned normal tothe hemisphere to measure the directionality of thereflection most accurately.

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Figure 5Convex nodepanels, 20mmradius (right),50mm radius (left).

Figure 6Results shown in 3Dpolar cloud and 2Dpolar pattern forSPL level ofreflection of panelsat 1kHz.

RESULTSThree panels have been fabricated using the pro-cess outlined: a panel which is globally curved withno local surface carving, one which has been carved

with the hot knife tool using a 20mm curved pro-file and another with that has been carved with a50mm profile, the latter two are shown in Figure 5.These samples were designed to represent a varietyof surface conditions thatmight be used as an acous-tic treatment. The measurements shown in Figure6 were taken while playing a 1kHz test tone, whichrepresents a key frequency for both vocal and musi-cal acoustical conditions. The data is converted froman impulse response to frequency response using aFourier transform, and the SPL is used in the repre-sentation of a polar cloud, as shown in Figure 7.

• The panel with no local surface carving wasfound to have the strongest reflections, espe-cially on the incidence angle from the loud-speaker, which could be due to the lack oflocally diffusive geometry and could also becaused by a focusing effect from it’s globalcurvature.

• The panel with 20mm radius convex nodesfeatured weaker reflections than the previouspanel with a similar overall distribution, po-tentially due to their similar global curvature.

• The panel with 50mm radius convex nodeswas found to have the most evenly diffuseddistribution, showing the least high SPL re-flections comparedwith theother panels. Themoreequal distribution couldbeexplainedbythe 50mm convex node radius, because it isacknowledged that the scattering of higherfrequencies is influenced by the width of ge-ometry (Koren and Müller 2017, p.126).

DISCUSSIONA method of fabricating acoustic panels and a novelmethod of testing them has been shown. We arecurrently in the process of identifying problems withthe testingmethod, such as the environmental noisecaused by performing the test in a robotic lab, specif-ically ventilation and mechanical sounds, and at-tempting to mitigate these issues.

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Figure 7Data processing ofmeasurements topolar clouds andpolar balloons.

The room is reverberant, as opposed to anechoic, thisis not a problem in a 2D goniometer setup which useboundary microphones to remove the floor bound-ary, however, with a multiple plane method this so-lution does not apply. In this setup there are po-tential unwanted reflections from the floor and wallswhich can invalidate the measurement if not prop-erly acoustically treated. In terms of evaluating theresults, polar clouds were produced in this investiga-tion, however, to truly evaluate the performance ofthe panels then diffusion, scattering and absorptioncoefficients must be calculated. Methods of under-standing these areas of performance are being stud-ied in more recent investigations. Another consid-eration is the design of the panels, as the sampleswere designed to present a variety of surface con-

ditions, however, if they were optimized first in thedigital model then their acoustic performance couldbe much greater. This would also reduce the costsassociated with a trial and error approach. We hopethat further studieswill be able to create stronger cor-relations between geometry and sound. By study-ing global surface direction and local surface geome-try, more acoustic tendencies can be identified, thenlarger scale prototypes can start to test differentiatedqualities across a surface. This study is the first steptoward an empirical and localised understanding ofmulti-dimensional sound scattering using a roboticarm, with further development this could start to in-form how large scale acoustic treatment is created ina hybrid digital and analog design space enabled byrobotics.

REFERENCESBonwetsch, T., Baertschi, R. and Oesterle, S. 2008

’Adding Performance Criteria to Digital FabricationRoom-Acoustical Information of Diffuse Respon-dent Panels’, Acadia Conference Proceedings, NonStandard Production Techniques Tools, TechniquesandTechnologies-AddingPerformanceCriteria toDig-ital Fabrication, Minneapolis, p. 364–369

Brander, D., Bærentzen, A., Fisker, A., Gravesen, J., Nørb-jerg, T.B. and Steenstrup, K. 2016 ’Designing for Hot-Blade Cutting’, Advances in Architectural Geometry,vdf Hochschulverlag AG an der ETH Zürich, pp. 306-327

Clifford, B., Ekmekjian, N., Little, P. and Manto, A. 2014,’Variable Carving Volume Casting’, Robotic Fabrica-tion in Architecture, Art and Design, 1, pp. 3-15

Cox, T.J. and D’Antonio, P. 2016, Acoustic Absorbers andDiffusers: Theory, Design, and Application, Taylor &Francis, London

Exton, P. 2011 ’The Room Acoustic Design of theGuangzhou Opera House’, Proceedings of the Insti-tute of Acoustics, Dublin, pp. 117-124

Koren, B. and Müller, T. 2017 ’Digital Fabrication of Non-Standard Sound-Diffusing Panels in the LargeHall ofthe Elbphilharmonie’, Fabricate, Stuttgart, pp. 122-129

Reinhardt, D., Cabrera, D. and Hunter, M. 2017 ’A Mathe-matical Model Linking Form and Material for SoundScattering’, CAAD Futures: Future Trajectories of Com-putation in Design, Istanbul, pp. 150-163

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