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The why and wherefore of designing for sound with textiles.
By Todd Willmert
All building materials are acoustical; all affect how sound is reflected, absorbed or transmitted. Through materials, sound can be sustained through reflection or made to dissipate through absorption. Transmission touches on the how much sound is permitted to pass through the material or materials of a wall or other room boundaries.
As sound strikes a surface it is transmitted, reflected or absorbed, but clearly this is a matter of degree. No material, for instance, is totally absorptive or reflective. And there are variations. Masonry materials like concrete or brick are highly reflective in their most common form, yet aerated, porous concretes can be fairly absorptive. Similarly, fabric stretched directly over a hard substrate will not absorb sound. However, fabric, especially heavy fabric gathered in waves, is highly absorptive.
While all materials enjoy a range of acoustical properties, recent fabric applications suggest an especially rich array. One thinks of stone as reflective-old churches with their polyphonic reverberations. However in a few new fabric projects, fabric is innovatively harnessed as a reflective surface, suggesting a new use that challenges conventional conceptions. In other applications, fabric is utilized in its more traditional sense, a material to absorb and arrest sound, yet the projects provocatively sculpt fabric to trap sound or use a variety of absorptive materials within a sophisticated fabric liner.
A look at a few of these different projects illustrates that fabric, depending on its deployment, enjoys varying acoustical properties. Consider the accompanying materials chart and the range of fabric performance, which is unmatched by other common building materials and finishes. This remarkable flexibility is evident in a survey of recent projects.
Reflecting fabric
Outdoor amphitheaters and concert halls are among the most challenging acoustical problems. That fabric is a key component of recent projects illustrates that the material is a viable alternative, often-lower cost and more aesthetically distinctive to harder, more massive materials and structures.
But not any fabric will do, and the shape of the tensile structure is as critical as the fabric itself. For instance, coated, tightly woven fabrics-vinyl coated polyester and Teflon-coated fabrics-are acoustically reflective. And a double curved surface, which reflects sound in many directions, is ideal to produce a more blended sound. Balancing these factors, as well as others-will the performances be chiefly acoustical, like orchestral, jazz and opera performances, or highly amplified, as in rap or rock-impact the design direction.
Benedict music tent for the Aspen Music Festival
These basic design parameters have been stretched in the renovation of a fabric pavilion in Aspen. Originally built in 1949, the canvas tent housing the Aspen Music Festival never adequately supported the music, despite modifications and replacement over its first 50 years. The canvas was literally heavy, but too lightweight acoustically to reflect sound.
The most recent redesign utilized a Teflon-coated fiberglass. Placed under extreme tension to support the region’s snow loads, the fabric was tested early in the design process to determine its sound reflection properties. Tests found that the fabric was effective into the mid-frequency range of music, but did not support lower frequencies. This deficiency was remedied with heavier surfaces of solid masonry and wood for the stage walls, wood and glass used as well in the roof directly above the stage.
Creating this concert hall space within a fabric structure is novel due to fabric’s basic limitations with regard to sound reflectivity. The use of more massive materials within the complex offset these disadvantages, the result an innovative use of traditional materials and fabric.
Acoustic fabric canopy for Experimental Media and Performing Arts Center (EMPAC) at Rensselear Polytechnic Institute (RPI)
RPI’s new arts center, which houses traditional performance spaces for music, dance and theatre, is high in design and materials. Conceived as an orchestra venue, as well as capable of accommodating electric sound and video projection, the new 18,270m2 structure is traditionally configured. As a long, narrow room, wood and masonry surfaces provide acoustic diffusion.
Designed by Grimshaw Architects, supported to gently reflect high-frequency sound. The ceiling, which much like its wood walls is gently convex, also masks electrical and mechanical equipment above it, and the fabric surface, being backlit, provides a gently glowing surface. In fact, the hall’s most innovative aspect is ceiling – comprised of fabric panels less than one millimeter thick supported on a delicate web of stainless steel cables.
Acoustic consultants Kierkegaard Associates tested 50 different fabrics for the project before settling on Nomex, a canvaslike, flame retardant fabric used more often to make NASCAR driver jumpsuits. Nomex reflects high-frequency sounds but allows lower frequencies to penetrate and reverberate against the ceiling.
Absorbing fabric
Fabric traditionally has been utilized to arrest sound, with fabric wrapped acoustical panels a sound control mainstay. This banal solution often is used throughout hospital hallways, nursing homes, and open offices. The level of sound absorption depends on the fabric utilized, with a woollike material functioning best, especially if it has a fiberglass or sponge texture. In larger spaces, banners or heavy curtains perform similarly.
These pedestrian solutions are being innovatively challenged. Different types of fabric and their articulation-as in the SCI-Arc’s ceiling, or assembly, as in London’s Skyscape-illustrate that fabric is also an excellent, interesting sound control remedy.
SCI-Arc acoustical ceiling
Felt is not commonly thought of as a building material, but in the refurbishment of an auditorium at Los Angles’ Southern California Institute of Architecture (SCI-Arc), it holds center stage. As an overhead baffle for the reverberant space, the felt canopy, designed by Hodgetts + Fung, undulates across the ceiling plane. Slits arranged in a geometric pattern create air pockets to dampen sound.
Reverberations were always a problem in this utilitarian space, a problem addressed with the felt in a project that took SCI-Arc students a mere three days to fabricate and install. Suspended by an aluminum truss, the felt field is composed of eight 1.8m by 18.3m strips of 16mm thick felt. The fabric is attached to a polypropylene skeleton with vinyl upholstery buttons. With its gentle contours and punctuating strips, the canopy has a provocative elastic quality that visually complements its acoustical performance.
London’s Skyscape
Situated next to the Millennium Dome (now called the O2 Dome) during the UK’s Millennium Celebrations in 2000, Skyscape was a temporary auditorium containing two 2500-seat cinemas, and support spaces like a ticket office and refreshment area. A fabric roof was critical for the 21m by 119m structure’s rapid, nine month construction.
A prime project challenge was the acoustic requirements, which required the fabric roof to arrest sound so that both local residents and Dome occupants would not be unduly disturbed. The roof ultimately was to achieve a performance sound reduction of 30dB, which is about the same level that a 50mm solid timber roof would yield. It also had to function acoustically for the Skyscape occupants themselves, primarily by reducing reverberation time, which can be high in fabric building envelopes.
An inner lining that mirrored the contoured external roof shape, a PTFE-coated glass cloth, was developed to meet these specific requirements. No suitable market-ready product existed to produce such a liner, so Architen Landrell Ltd. worked with a supplier specializing in noise control solutions, one that produced sound absorbent panels for cars, tractors and other vehicles. Long reverberation times can render sounds unintelligible. If a sound lingers excessively, noise levels unduly build up and can ruin performances or screenings. The first goal of the liner was to address this issue. Fire retardency and other requirements ultimately lead to “rockwool” or glass wool-the same insulation typically used in residential and commercial construction.
The main sound isolation element of the liner is the central layer, with its high damping properties. As with the glass wool, several materials were tested, including lead sheeting. In the end, a “polymeric mass layer” was utilised-a rubbery substance made from waste materials and chalk. The sheet had sufficient tensile strength, was fire retardant, had some elasticity and was lightweight, all crucial traits given the design parameters.
The diagram illustrates the assembly of elements, all which had to mirror the shape of the outer tensile fabric. A laminating process cleaved the glass wool to the polymeric mass, as well as the outer fabric. The assembly was then sewn into a quilted finish, with FTL Design Engineering Studio of New York consulting on the actual patterning. The final result is an inner liner with the distinctive look of a giant “puffa jacket,” one that, as testing proved, envelops the cinemas to create ideal acoustic chambers.
Conclusion
Can one hear architecture? Perhaps not, but on the other hand architecture does not produce light either, and it is readily seen. An architectural project has acoustical properties just as it has visual qualities. Of course fabric has always been used to warm a space and dampen sound – think tapestries on stonewalls. But the projects shown here illustrate fabric stepping beyond this more traditional conception. Fabric can be part of a building material palette where sound reflection is a key concern, as represented by the Benedict Music Tent and EMPAC. Novel applications, as at SCI-Arc and London’s Skyscape, expand and stretch fabric’s sound absorbing capabilities.
Architecture’s visual aspects garner the most attention, with glossy magazine photos required to publish any project. However, architecture also engages the aural sense. Fabric, with its wide range of acoustic performance, as well as other advantages, is increasingly utilized in the material palette of designs where acoustics are a key concern.