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Acoustic canopy produces clear sound, clear design

January 1st, 2008 / By: / Feature, Interiors

The Royal Festival Hall (RFH) opened on London’s South Bank in 1951 as a part of the Festival of Britain. On the heels of the devastation brought to the city by World War II, the Festival was a showcase for modern design, a spirit of rebuilding, and progress into the second half of the 20th century. Unfortunately, from the outset RFH suffered from poor acoustics. The original design overestimated the hall’s ability to support sound energy and added multiple sound-absorbing and grounding features — from the lightweight wooden panels which grace the hall’s side walls to the projective geometry of its stage — resulting in an environment in which cross-stage musician communication was difficult and reverberation was less than inspiring.

More than 50 years later, a long awaited renovation was launched with a design team consisting of RFH and SouthBank Centre leadership, Allies and Morrison, architects; Carr and Angier, theater consultants; Max Fordham, mechanical services; and Kirkegaard Associates, architectural acoustics consultants.

The renovation was complete in June 2007 and celebrated with a public opening after more than a two-year shut down. Work to the hall’s interior was extensive, with removal, alteration, and replacement of nearly every surface in the room. Changes to the hall’s visual character were subtle and meant to update, but maintain the vision of the original modern design.

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. With the exception of one narrow strip on each, the blades were angled to direct energy into the sound absorbing audience. This diverted scarce energy from musician ears and closed off the volume above the canopy, stopping it from becoming energized — something important for the reverberation needed for orchestral music. A complete replacement of this canopy was undertaken, and instead of resorting to the acoustics industry standard of arrays of small, hard reflectors, the design team looked for a new approach, ultimately replacing the three wooden reflector blades with three stretched fabric blades.

Monolithic hard canopies, such as in the original hall, tend to stifle concert halls, and arrays with voids can cause interference problems which are perceived as overly bright to discerning listeners. Fabric turns out to be a viable alternative solution to this challenging balancing act, as long as its properties are carefully tuned to the needs of the room. 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. Geometrically speaking, this approach would cut off the upper volume just as the old canopy did, but the carefully-tuned weight and air permeability of the fabric allows middle and low frequency sound to travel straight through to the volume above.

Many acoustic fabrics have been developed to maximize sound absorption, or to maximize transmission, but the world of critical concert hall acoustics has been largely devoid of fabrics which are designed to maintain a balance of absorption, transmission, and reflection. The fabric was developed by Kirkegaard Associates with the help of James Watson and the College of Textiles at North Carolina State University (NCSU), from an idea born primarily of positive experiences at the Benedict music tent in Aspen, Colorado, and even memories of a temporary reflector of canvas duck above stage at New York City’s Carnegie Hall, probably originally meant as a cover for a hole once cut in the ceiling.

The first round of development was, in fact, based on a standard 18 oz. sample of canvas duck, which proved the most promising in rounds of testing of off-the-shelf fabric. Canvas was not a viable final choice, however, because of its lack of flame resistance and its tendency to sag over time.

Flame resistance was of concern from the beginning. Allies and Morrison were drawn to the potential of Nomex fiber—an inherently flame retardant synthetic fiber manufactured by DuPont—and initial fabric prototypes from NCSU confirmed the visual and tactile appeal of the material. Although Nomex tends to discolor when exposed to UV light, the lack of any natural light within Festival Hall allayed any concerns. Getting the right weight and air permeability was a bigger challenge, the solution to which took many iterations of custom fabric woven by NCSU. Each of the iterations was followed by a long string of Kirkegaard-performed acoustic tests for absorption, reflection, and transmission. At two crucial steps in the development, full-scale mockups were conducted, complete with live orchestras, within the pre-shutdown hall.

Using the information learned through the development process, 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.

RFH is used as both an orchestral venue and amplified venue, so the fabric blades are tiltable and retractable (see section above, opposite) to optimize their position for amplified events, as well as to tune the hall for a variety of acoustic performances. The material is beautifully backlit and, with its natural off white color, is well suited for use as a projection screen when needed.

On a cautionary note, fabrics have a tendency to resonate like a drum when stretched too tightly, so installation proceeded with a minimum of tension, just enough to maintain the desired reflector geometry.

Initial reactions to the performance of RFH are very positive, and while it is difficult to completely separate the contributions of individual elements such as the new fabric canopy, every indicator points to its success at its intended goals: improvement of musician communication on stage, pleasant reflections to musicians and audience, and increase of reverberation time. As audiences enjoy the obvious improvements brought by the renovation, RFH managers and musicians are hard at work learning its new character, and honing their skills to take full advantage of what the hall has to offer. As such a prominent part of the hall, the Nomex fabric reflector will be given every chance to prove itself as a viable acoustic element for years to come.

Zackery Belanger, is part of the Room Acoustics Group at Kirkegaard Associates, Chicago, and takes a particular interest in innovative geometry and materials.

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