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Sound resolution

BFLS and Arup skilfully tackled tricky questions of acoustic design in their extension to the Royal Welsh College of Music and Drama

The orthodox holistic notion of pulling together every aspect of a design in one stroke is a valid methodology and goes hand in hand with the pursuit of a unifying concept, although it often follows a series of iterations. But the design of some projects is more sequential.

BFLS’ concept for an extension to the Royal Welsh College of Music and Drama (RWCMD) in Cardiff, which opened last November, tackled the trickiest elements – the performance spaces and the urban context – before addressing other requirements. ‘The starting point was the interior of the concert hall and theatre,’ says BFLS director Jason Flannigan.

The new construction comprises performance and rehearsal spaces, teaching studios and exhibition facilities, added to the college’s existing accommodation in a converted stables block and drab 1970s building, forming a new public face for RWCMD.

Prior to this, the college orchestra rehearsed and performed in a local church and occasionally in Cardiff’s St David’s Hall.

It was vital to resolve the knotty cluster of variables controlling the acoustics

Acoustic design
The principal challenge in the design of the performance space was the acoustics of the 450-seat Dora Stoutzker Hall, Wales’ first purpose-built chamber recital hall. It was essential to minimise noise from the adjacent A470 trunk road and building services, as well as breakout to the training theatre and drama studios. ‘A low “noise floor” enhances the drama of music performance, with increased awareness of reverberant sound,’ explains Arup acoustics team associate director Ian Knowles.

The hall’s 400mm-thick reinforced concrete drum and perimeter buffer space, together with its 300mm-thick concrete lid, which is independent of the weathering roof above, achieved the specified NR-15, and its air displacement system maximises ventilation efficiency and minimises potentially noisy air velocity. ‘When it’s completely silent, you can hear the blood in your ears,’ says Flannigan.

Beyond this, it was vital to resolve the knotty cluster of variables controlling the acoustics quality of the room. This involved using geometry and absorption to balance direct and reverberant sound and loudness, taking into account the different uses of the hall. ‘It’s an unusual mix,’ says Knowles. ‘You start with a precedent study and we looked at overlays of the auditorium of Dixon Jones’ Kings Place and the Wigmore Hall in London.’ The Dora Stoutzker Hall’s modulated geometry, with many different reflective surfaces and finishes, provides a richly populated impulse response, which is more rewarding for the audience.

The damped, suspended timber platform is resonant for cellos and bases, and the upper room surface, which is concrete with vertical timber fins, is the hall’s main source of accumulated reverberations. Small-scale diffusion here is minimised to avoid dulling reverberation and reduce duration.

The ceiling’s three layers of plasterboard, weighing 40kg/m2, minimise excessive low frequency absorption, and its glazed apertures for production lighting define the boundary volume and insulate the hall from the attic, where the walkways and gantries are situated. Diffusion is quite shallow in the stalls (25-50mm) and bay ‘columns’ create large-scale diffusion. The balcony fronts are angled upwards to reflect lateral energy into the upper part of the hall in order to enhance the sound development of reverberations. As part of the variable acoustic system, banners that drop out of the ‘top-hat’, modify the reverberations in the upper hall.

The ceiling is formed of a series of overlapping profiles, with a maximum recess depth of 300mm, optimising lighting angles to the stage from the lighting bridges, and providing acoustic diffusion without obstructing energy transmission from platform to audience. Arup modelled the hall using Odeon proprietary acoustic software, drawing from scratch rather than importing elements from architectural drawings, since its model needed to be precise and include only key elements. Superfluous detail would have caused problems. ‘It was the acoustic equivalent of a CGI,’ says Flannigan.


Urban design and layout
Situated on the site of a former car park, the BFLS extension adds to a formal arrangement of civic buildings grouped around the Grade I-listed rectangle of Cathays Park that lies to the north of the College’s original home in Cardiff Castle. ‘It’s the first building you see after the civic centre,’ says Flannigan. Taking the form of the performance spaces, which had already been established, and the height restriction set by the planners as a starting point, BFLS used the maximum height as a horizontal datum, which set the roof’s level and shuffled around the volumes below.

It floats above the elliptical volume of the concert hall, separated from the adjacent blocks by a triple-height entrance foyer, fully glazed on its east and west sides, visually linking Cathays Park and Bute Park to the west. The roof overhang provides shading and its GRP soffit panels reflect light and colour into the college, tapering in section to 50mm at its edge. The roof, which is syphonically drained, also tapers on plan towards its south end, where it cantilevers 10m and picks up an axis from the castle.

The addition’s scale builds up as ground level falls from south to north, and the largest space, the concert hall, is situated at the north end, enabling its height to be maximised. The concert hall’s inclined Western Red Cedar slatted cladding follows its stepped section profile. ‘The fins have concealed, spigoted connections, and they twist on plan to confuse the
aging process,’ says Flannigan.

From certain angles, these twists create the effect of deep bands, which are alternately solid and perforated. Internally, the 160-seat Richard Burton Theatre has a full fly tower with a tension wire grid, and an auditorium, which works with a proscenium arrangement. The space known as the Linbury Gallery, between the extension and the 1970s building, was glazed over and inspired by Cardiff’s city centre arcades, forming RWCMD’s principal circulation space, which connects to the Bute Theatre. Music from the open windows of practice rooms on the upper floors filters into this gallery, which, like all the other spaces, apart from the concert hall and the theatre, is naturally ventilated, helping RWCMD to achieve a BREEAM Excellent rating.

BIM software
While BFLS worked in MicroStation, Generative Components parametric software and Rhino, multi-disciplinary engineering consultant Mott MacDonald used Revit BIM software to model the structural steelwork. Given RWCMD’s complexity, with large structural steelwork spans that needed to be coordinated with elaborate ventilation services, it helped to model the project in 3D.

Mott imported the BIM model into Navisworks to detect and resolve clashes with services, eliminating the need for any redesign. Mott’s BIM model was also exported to the 3D structural analysis package STADD for concrete frame and steelwork design, and used in conjunction with in-house steelwork connection design software to collate member end forces for subcontractor connection design. Mott also exported the BIM model to AutoCAD documents, issued as a tender supplement. The BIM model was issued to steelworks and roofing and piling contractors in geometric form. Pile schedules, including toe levels, cut-off levels and location coordinates, were produced automatically from the BIM model.

Practical completion
At £3,300 per square metre, BFLS’ extension is good value, considering the height of some of its volumes. There are places where the budget wears thin, however, especially on the west elevation where the addition adjoins the 1970s building. This is very much the rear elevation. It’s also disappointing to pass through the new entrance foyer to the terrace, which overhangs the canal, and not to be able to proceed further. Nevertheless, the project is well worked through and practical, with some clever details, such as concealed lights in balcony fronts and in the shadow gaps below the overhanging roof. As Flannigan notes, ‘the form changed quite a lot’. BIM played an essential role in the structural design and procurement, helping the project team to meet its time and cost constraints on a tight site and allowing the college to continue to operate during the construction phase. Tackling the most challenging problems first and starting as it intended to go on also helped.

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