Hitting the right timbre
With the plentiful use of timber, Pringle Richards Sharratt, architect of the new music school and auditorium at Shrewsbury School, has been able to overcome the problem of acoustics, and at the same time design a building appropriate to its country surroundings Acoustics is a tricky subject. Look at most well-designed buildings and you can immediately grasp the main points of the architect's thinking and, unless any tricks have been played deliberately, of the structure. Glance at a section, at the orientation and at any shading and vents, and you can get a pretty good idea of the environmental strategy as well. But acoustics is a subject with far more pitfalls, affected in a complex manner by shape, by choices of material and even by the routes by which air enters and leaves a building.
John Pringle of Pringle Richards Sharratt, architect of the new music school and auditorium at Shrewsbury School, says:
'Acoustics has far more hidden traps than other aspects of the building'.At Shrewsbury, these traps had to be avoided, and at the same time the school had to be built on a restricted budget (at £1,350,000 for 1,315m 2only £1,025/m 2) and to a tight schedule.
Pringle worked with Richard Cowell of Arup Acoustics to produce a solution that kept the architect's vision intact while providing an excellent acoustic and adopting solutions that were new to the UK. It makes plentiful use of timber, partly for its acoustic properties and partly because it seemed appropriate to a building in the grounds of a country school.
The auditorium sits at the centre of the building, with the practice rooms around it.
One of the first changes that Cowell and Pringle came up with was to set the auditorium more deeply into the ground to give it more height, and hence volume. The sloping tiled roof, with a clerestory near the top, rises above the practice rooms but manages not to dwarf them. Facing outwards, these practice rooms have splayed outer walls that prevent standing sound waves developing, and they benefit from daylight and views.
But the elliptical form that gave the practice rooms their splayed walls posed potential problems for the auditorium itself.
Any concave curved surface can focus sound in an undesirable manner. Architect and acoustician therefore devised an octagonal shape for the auditorium to fit within the ellipse. For the ceiling, explains Pringle: 'We went out to tender on a studwork panel system of joists with plywood screwed on either side to create a stressed skin. But this didn't give enough mass for acoustic attenuation.' Looking for alternatives, the architect, which has worked in Germany, came across a system from Merk Holzbau. Its Dickholz (literally 'thick wood' or laminated timber) panels are made of spruce and were supported on an elliptical spruce ring beam. They were strong enough not to need additional structural support and in fact could support directly a high-level cupola containing lighting, ventilation and sound baffles.
Cowell explained that 'in order to stop absorption at very low frequencies, we needed to have a stiff roof structure. On the long sides of the roof, we wanted to put more material. In the end, for economy, the panels were framed out and stiffened.'At the ends it was necessary to have convex panels. These panels do curve into the room but, due to the optical effects of the roof shape, this is scarcely visually perceptible.
Because of this and other requirements, the Dickholz panels in the auditorium had to be lined. However, they are visible in their original form in the practice rooms. Pringle was full of admiration for Merk's technology and craftsmanship: 'They took our 3D CAD files, then programmed directly the CNC cutting machines, to a very precise geometry This meant there were few carpenters needed on site but that they, subcontracted to Merk were excellent.' In fact Pringle also persuaded them to construct the building's larch-framed windows which they did 'beautifully'.
The building stands on a slope so tha part of it can be set into the ground. Practice rooms for noisy activities are kept on this lower level, well insulated acoustically from their surroundings. A corridor running around the auditorium isolates it from the practice rooms, which are of a shape tha means they throw their sound outwards Timber fins between the rooms prevent the transmission of sound from one room to another around the perimeter.
The building stands on a brick plinth constructed in traditional 'English garden wall' bond to echo its surroundings. Above this level, it is built using timber-frame 'breathing wall' techniques, with the wal panels in Western Red Cedar.
With engineer Buro Happold, it was decided to use natural ventilation, not just in the practice rooms but also in the main auditorium. Air is drawn in from the undercroft mainly from under the seating, and by a stack effect rises to the roof from where it is extracted. Sound attenuators at low and high level have all been sized similarly, presenting an economy of scale.
Describing the collaboration in genera terms, Cowell says: 'I think it was very productive. I felt that John [Pringle] managed to pull it all together quite nicely.'
Reaction from the users has been positive For a modest sum, the school now has a handsome building with excellent acoustics plenty of daylight, low running costs and good environmental credentials.
START DATE April 2000 FINISH DATE February 2001 ARCHITECT Pringle Richards Sharratt STRUCTURAL AND SERVICES ENGINEER Buro Happold CREDITS ACOUSTIC CONSULTANT Arup Acoustics QUANTITY SURVEYOR Davis Langdon & Everest CONTRACTOR & PLANNING SUPERVISOR Bowmer & Kirkland