The why and wherefore of designing for sound with textiles
FabricArchitectureMag.com | May 24, 2012
By Todd Wilmert
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 recent fabric projects, fabric is inovatively 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 materials within a sophisticated fabric liner. Still, another application utilizes fabric in a third, or transmissive, mode in addition to reflective and absorptive modes.
A look at a few of these different projects illustrates that fabric, depending on its deployment, enjoys varying acoustical properties. Consider the chart (Fig. 1) 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. (Click on image to enlarge.)
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, with 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 18,270m2 structure is traditionally configured. As a long, narrow room, wood and masonry surfaces provide acoustic diffusion.
Designed by Grimshaw Architects, it is supported to gently reflect high-frequency sound. The ceiling, which much like its wood walls is gently convex, also masks the electrical and mechanical equipment above it, and the fabric surface, being backlit, provides a gently glowing surface. In fact, the hall’most innovative aspect is the ceiling—comprised of fabric panels less than one millimeter thick.
Supported on a delicate web of stainless steel cables, 50 different fabrics were tested for the project by acoustic consultants Kierkegaard Associates before they settled 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.
Fabric traditionally has been utilized to arrest sound, with fabric wrapped acoustical panels a sound control mainstay. This ubiquitous, banal solution is used throughout hospital hallways, nursing homes, and open offices. The level of sound absorption depends on the fabric utilized, with a wool-like material functioning best, especially if it has a fiberglass or sponge texture. In larger spaces, banners or heavy curtains perform similarly.
These common, 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 6 by 60 foot strips of 5/8 inch think 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.
Royal Festival Hall
The Royal Festival Hall (RFH) opened on London’s South Bank in 1951 as a part of the Festival of Britain. The original design added multiple sound-absorbing features—both intentional and unintentional—resulting in an environment in which cross-stage musician communication was difficult and reverberation was low.
A key detriment to the hall’s original acoustics was its above-stage acoustic canopy: a triple monolith of wooden blades spanning nearly the entire width of the space. Historical value was high and the original design was respected in a more recent acoustic and cosmetic renovation, led by a design team consisting of RFH and SouthBank Centre leadership; Allies and Morrison, architects; Carr and Angier, theater consultants; Max Fordham, mechanical services; and Kierkegaard Associates, architectural acoustics consultants. The College of Textiles at North Carolina State University (NCSU) assisted in developing a custom fabric for the renovation. The new RFH opened in June 2007.
Work to the hall’s interior included removal, alteration and replacement of nearly every surface in the room. A complete replacement of the stage canopy was undertaken using three stretched fabric blades. The large, continuous fabric blades are reminiscent of the original wooden blades but are reshaped to provide continuous reflection coverage to musicians as well as audience members.
The weight and air permeability of the fabric allow a balance of middle- and low-frequency sound to travel through it to energize the volume above, while reflecting a pleasing blend of sound to the musicians and audience.
Most acoustic fabrics are developed to maximize sound absorption, transmission or reflection; this fabric was designed for a specific balance of these three properties. At the RFH, Nomex fiber was used for its appealing color and flame resistant properties. Kierkegaard performed measurements of acoustic performance and also conducted full-scale mockups as listening tests, one including a live orchestra.
The final fabric for RFH was woven and finished by British weavers John Heathcoat & Company, who used a calendering (high temperature pressing) process to reduce its off-the-loom air permeability to acceptable levels. A stain treatment process was applied to combat soiling in the coming years. The fabric, installed with a minimum of tension to prevent resonant ringing like a drum head, is translucent when backlit and can be used for projection.
When it comes to architectural fabrics, the important qualities for acoustics are the weight, finish and air permeability, not the fiber.
Can one hear architecture? Perhaps not, but on to 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, expand and stretch fabric’s sound absorbing capabilities. And fabric can also carry multiple use applications, as with the Royal Festival Hall, where it serves absorption, reflection and transmission needs.
Architecture’s visual aspects typically garner the most attention. However, it 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.
Editor’s note: This article, originally published in the Jul/Aug 2006 Fabric Architecture, has been edited and updated by the editors, with new material furnished by Zackery Belanger. ©2012 by the Industrial Fabrics Association International.
Hearing fabric images
The acoustics of the music tent of Colorado’s prestigious Aspen Music Festival must meet high standards for both audience and professional musicians. Minimal supports and the incorporation of heavy materials such as wood and masonry, in Harry Teague Architects’ 1999–2000 makeover, give the audience unobstructed views and strong sound. Acoustics consultants, Kierkegaard Assoc., replaced the original 1949 canvas tent with PTFE-coated fiberglass to reflect sound in the mid-frequency range. To compensate for the low-frequency range of sounds, Kierkegaard added masonry and wood for the stage walls, and heavy wood planks and glass were incorporated into the roof directly above the stage, along with an “acoustic disk” over the tent’s center. These were augmented with sound-absorbing banners (made of the same fabric as the roof, but with only a light PTFE coating) around the perimeter to shorten reverberation times when audience attendance is light or during rehearsals.
The Experimental Media and Performing Arts Center [EMPAC] at Rensselaer Polytechnic Institute, designed by Nicholas Grimshaw & Partners [with Davis Brody Bond; Kierkegaard Assoc., acoustician]. Panels of Nomex, rigged like sails to the ceiling, reflect high-frequency sounds but allow low-frequency sounds to pass through and reverberate against the concrete ceiling above.
Hodgetts + Fung used 16mm thick felt across the ceiling of the auditorium at the Southern California Institute of Architecture (SCI-Arc) in an undulating plane to absorb sound. Suspended from an aluminum truss system and slit in a geometric pattern to create air pockets, the felt is attached to a polypropylene grid with vinyl upholstery buttons.
The Royal Festival Hall in London is used as both an orchestral venue and amplified venue, so the fabric sound blades are tiltable and retractable to optimize their position for amplified events, as well as to tune the hall for a variety of acoustic performances. To avoid sound problems, the fabric is installed with minimum tension, as fabrics have a tendency to resonate like a drum when stretched too tightly.