Building From the Sound Up: Acoustic Design at Foster + Partners

Acoustic design is a vital, though often overlooked, stage in architectural and urban planning. Understanding design as an inherently multisensory practice, where buildings are not only seen but heard, Antonella Bevilacqua and Andy Acred discuss how acoustic engineering supports the architectural structures designed at Foster + Partners today.

Acousticians must remain flexible in their thinking, applying expertise at the right moments.

It has long been Foster + Partners’ concern to orchestrate the integration of complex engineering – such as acoustics – with the fine art of architecture, and a fitting example of this was for the studio’s innovative intervention into a building appropriately known as America’s ‘Temple of Invention’ – the Kogod Courtyard at the Smithsonian Institution in Washington, D.C.

The practice proposed enclosing the existing courtyard with a glass roof that would admit sufficient daylight to sustain public events and greenery, yet shut out the weather. Not surprisingly, the simplicity of this brief belied the depth of the challenges. The ability to enjoy music, speeches, and regular conversation in the courtyard was a critical acoustic requirement. That meant countering the long reverberation time that a glass roof and stone facades would normally induce. Innovative acoustic engineering would be essential.

Integrating and balancing the many technical demands became the primary engineering focus of the project. An integrated solution for the roof reduces what would normally have been about a ten-second reverberation time in the space to just three seconds. The surface of each beam in the steel frame is covered by a grille of horizontal 15mm-diameter rods. Behind these rods, a grey fabric lining conceals shredded denim material, which acts as an acoustically absorbent muffler immediately beneath the glass panels. Almost 9,000 pairs worth of used jeans were sustainably recycled into the roof.

As well as an example of inter-disciplinary collaboration, the Kogod Courtyard also neatly illustrates two acoustic concepts that are critical to almost every design. The first is insulation, the control of sound exchange between adjacent spaces, which could be within a building or between outside and inside. High levels of insulation might be required between residential units or, as at the Smithsonian, between a building and its surrounding urban environment (such as a busy street, or airport runway). Conversely, insulation might be reduced across certain spatial divisions so that speech can be clearly understood.

This is cutting-edge trial and improvement design, occurring across disciplines.

The second key concept of acoustic design is absorption, which controls the reverberation in a room – the time it takes for sound to decay away. Large spaces with hard surfaces, such as churches or atria, are more reverberant; they provide a feeling of immersion within the acoustic environment. However, for event spaces, meeting rooms or recording studios, where clarity of speech or music is critical, adding soft, acoustically absorbent materials – like fabric panelling – is needed to reduce reverberation and provide a calm, controlled environment. In addition to insulation and absorption, the function assigned to space is linked to its form and geometry, which also affect sound. It may be desirable to use convex curved features within a performance venue to scatter sound evenly from the stage to the audience, for example. Different priorities may apply elsewhere; enclosed booths that block line of sight (and sound) would be more appropriate in a quiet working space, where privacy is crucial.

Often, different priorities occur within the same building – especially when a mixed-use design ethos is at play. Foster + Partners’ recent Ferring Pharmaceuticals Building in Copenhagen, for example, required sensitive, site-specific acoustic thinking. The central atrium creates open and connected social space, with wooden acoustic panelling strategically placed to absorb excessive noise and prevent a ‘cathedral’ effect. The perimeter offices on each floor continue this project of connection; glass dividers and windows offer external views of the Baltic Sea and internal views of the atrium, connecting users with both the natural world and one another. These perimeter offices, in addition, had to deliver high levels of acoustic privacy. This was achieved by selecting well-insulated glass with precise fitting instructions for the construction team so that sound could not ‘leak’ in or out.

Evidently, the acoustic profile of each Foster + Partners project is unique; during the design phase, the acoustic performance of a given space can be modelled using sophisticated software that reflects the geometry and the properties of a project’s specific materials. If the outcomes of these predictions are not compatible with other parties – such as fire, thermal, plumbing, mechanical and electrical engineers – further adjustments are required. Other metrics, including the sustainability of materials, are also considered. The cycle of acoustic innovation in step with spatial and environmental limitations repeats, each time refining the material properties and geometry of an environment to provide the best possible acoustic experience. The various limitations of a project, paradoxically, allow for an increase in creativity. This is cutting-edge trial and improvement design, occurring across disciplines.

Acoustic calculations were informed by the dimensions and materials chosen by the design team for the Canopy. After a series of models, and various design iterations, a convex canopy (derived from a double hyperbolic curve) was proposed. With this shape, as opposed to an initial concave cantilevered shape, the sound is more uniformly distributed in the square. The structure was also considered more stable from a seismic standpoint.

This digital realisation of sound – rather than taking us further from the material world – brings us closer to the spatial realities of acoustics and, by extension, of architecture. People hear in three dimensions, all the time; even when we are looking ahead, we are constantly receiving sounds from areas we can’t see – above, below, behind. Our brains instantaneously process this information and build a 3D image of the space around us. In other words: we can use the ear to see. Auralisation harnesses this immersive fact of acoustic design, and, in doing so, brings to the fore the spatial and material aspects of architecture. The visual representation of a building, usually expressed in sketches, plans, photographs, or films, risks flattening and muting it. Auralisation resists this flattening – or rather, perceives a space in ways that bypass or build beyond vision.

A biophilic approach to design is also closely related to the mood and emotions of a space. The natural world is not only a source of acoustic modelling but generates sounds (rain, the surf of the sea, birdsong in a tree canopy) that have been proven to reduce stress and anxiety and increase concentration and relaxation. Sometimes desirable sounds are carefully introduced into a space to mask unpleasant noises (such as airplanes, traffic, or mechanical ventilation). James Carpenter’s water feature, Icefall, in Foster + Partners’ Hearst Tower and the integration of a tropical canopy in the South Beach project in Singapore are two examples of this site-specific sonic sensitivity. Acoustic design, then, must adopt a symbiotic relationship with the surrounding environment, working with both passive and active, natural and industrial strategies.

One of Foster + Partners’ most notable projects in terms of both acoustics and ventilation is the Bloomberg headquarters in London. The project had to overcome the challenge of naturally ventilating a large, deep-plan building in a busy and noisy central London location. To do this, custom facade openings – visible from the outside as distinctive vertical bronze fins – were designed to both freely allow fresh air into the building and to block outside noise. Ventilation openings at the top of a central atrium were introduced to avoid noise disturbance from rooftop equipment.

Inside, between the office floors, is the multifunctional integrated panel ceiling, with its unique aluminium ‘petal’ design. These unique petals were shaped to maximise the cooling potential of the radiant ceiling, while allowing sound to pass through to the acoustic absorption installed behind, on the soffit, thus allowing for control of reverberation within the space. The petals also scatter sound, thus improving privacy – always a challenge in an open-plan environment.

Bloomberg is a key demonstration of integrating acoustic engineering into a broader design process. Its BREEAM Outstanding rating (the highest green rating possible under this assessment) attests that there is a place for acoustic design within the complex network of aesthetic and mechanical practices that comprise a sustainable building. Acoustics should not be an ornamental post-script since their early consideration in the design process can vastly improve a user’s experience of a space. Rather, the key is early and sustained collaboration.

Acoustic design must adopt a symbiotic relationship with the surrounding environment, working with both passive and active, natural and industrial strategies.

Designer Bruce Mau argues that in a field ‘caged by vision,’ architecture’s aim should be one of ‘orchestration.’ Mau’s ‘orchestration’ is a wonderfully aural metaphor that reminds us that buildings are not only seen but heard. Acoustic design, like architectural practice in Foster + Partners in general, lends itself to a model of ‘orchestration’. Each participant offers a distinct set of skills which, when conducted together, create unique harmonies and rhythms, perhaps even productive dissonances. By building from the sound up, the interdependent design decisions that influence sound can be newly considered and appreciated. Only through this attunement to architectural space can new possibilities be heard.