Acoustics.

In developing an acoustic design strategy for Glyndebourne, the acousticians were faced with the usual dilemmas presented by an opera venue, compounded by the added challenge of a circular space. While undoubtedly bringing the audience close to the performer, the auditorium configuration of a multi-tiered horseshoe set within a circular plan was potentially susceptible to acoustic focusing problems. The auditorium can act like a curved mirror, concentrating sound in isolated pockets.

The aim throughout was to reconcile the conflicting requirements of resonance with clarity. In practice this meant achieving a room resonance long enough at the right frequencies to provide the warmth and richness for music, but not too long so that words would be masked or become unintelligible. The geometry is the key. As Derek Sugden of Arup Acoustics remarked 'With the right geometry it is possible to produce a rich sound similar to Bayreuth or Teatro Colon without any sacrifice of intelligibility'.

For the room resonance, the general aim was to achieve a reverberation time of 1.4 seconds, and this effectively determined the immense volume of the auditorium. Initial design strategies sought to achieve 8 cubic metres of air per audience seat; the final scheme has 6.5 cubic metres per seat. This design decision was courageous. The Royal Opera House has a reverberation time of 1.1 seconds, and that of Glyndebourne's previous auditorium was 0.8 seconds.

A secondary strategy was based on the need to achieve a quiet background. A background noise level of PNC 15 (Preferred Noise Criterion) was adopted -- effectively less than the noise made by an average opera house audience. This has been achieved by the use of massive walls and ceiling elements, acoustic doors to the rear stage, and intense attention to such details as the form and operation of the door seals. Background noise from the services installations has also been minimised. The displacement ventilation system helps to limit flow-generated noise due to the low velocities of the supply points located under most of the audience seats. In addition, the main plant room was located in a separate building.

Extensive model testing was a critical element of the acoustic design process. An acoustic 1:50 scale model was built and tested at Arup Acoustics' Cambridge laboratory. This assisted in pinpointing various locations where intuition suggested focusing might have arisen. When such adverse effects were noticed, treatment could be effected by simple modification at 1:50 scale. Several possible ceiling designs were also tested to establish which produced adequate diffusion of sound. The height of the ceiling was such that ceiling reflections back to the stalls area could have been disturbing. Similarly, each wall surface was considered, and the need for either diffusion, reflection or acoustic absorption determined.

The results of the acoustic modelling were subsequently integrated into the architectural scheme. Visual features of the acoustic design include the shape and form of balcony fronts, the shape and convex form of box raised walls, the slatted treatment of parts of the side walls, convex panels on walls at certain levels and ceiling design details. The combined effect of such features was to remove adverse focusing effects, and yet distribute useful reflected sound to the audience in order to achieve the necessary clarity. Further acoustic testing work was carried out during the design to check that the wall panelling introduced to alleviate focusing did not introduce excessive low frequency absorption. Tests were also made to determine absorption coefficient of the seating. This was a critical element of the overall acoustic absorbency, as most other finishes were acoustically hard (there is no carpet, no acoustic tiles and little in the way of acoustically absorbent panels in the auditorium).

Most of the measures discussed so far relate to achieving excellent conditions for the audience, but the requirements of the pit musicians and the singers were also considered. Shawe Taylor i n 1948 advised, 'Opera is a composite art, and one of its principal attractions has always been the thrill of a great voice greatly used. This is a truth which no opera house in the world can safely neglect'. For the singers, the acoustic team has sought to use the balcony fronts taken to the stage edge to overcome the defect of many modern opera house proscenium designs which, while meeting recent theatrical requirements, remove the beneficial reflecting surfaces of the proscenium splay. (The surface at the side of the stage is useful to assist the singer's voice transmission to the distant audience on the far side of the orchestral pit.) For the musicians, the depth and flexibility of the pit has been designed to achieve tolerable conditions. The orchestra pit rail has been made semi-perforate with a slatted treatment, with extra material provided if an unperforated pit rail is subsequently felt necessary.

This appraisal necessarily concentrates on the development of the acoustic design and the resultant auditorium form and finishes. Initial results indicate that the design aims have been met, and the first trial performances have been notable for the gloriously rich quality of the orchestral sound. However, the ultimate acoustic appraisal will not begin until 28 May, when the 1994 Glyndebourne Festival opens with Le nozze di Figaro.

Derek Sugden describes the historical origins and evolution of Glyndebourne's horseshoe form:

The traditions of European opera grew out of the intellectual theories of the Florentine Camerata overlaid with influences of music and drama from the church and commedia dell'arte. Just as the Camerata grew out of an intellectual movement with the aim of discovering the essentials of Greek drama, so the opera house developed from the Classical amphitheatre. The audience was soon enclosed and the walls brought round to frame the action of the drama in a U-shaped plan, ensuring visual and aural intimacy. From these beginnings, the Italian horseshoe developed and has maintained it pre-eminence for over 300 years. There have been various modifications to the basic horseshoe geometry; the truncated ellipse and the hull-shaped plan developed by the Galli da Bibienas, of which the most outstanding example is the Markgrafliches Opernhaus in Bayreuth (p62). There have been many smaller opera houses based on rectangular plans such as the Drottningholm Court Theatre of 1766. In 1876 Wagner returned to the amphitheatre for his 'democratic space' (p65) to house a totally new ethic and aesthetic of opera performance. Yet the qeometry of the horseshoe has always ensured visual and acoustic intimacy, with strong side reflections from balconies to ensure clarity. Despite their generous volumes, old theatres tended to be full of soft finishes and curtains giving rise to reverberation times of one second or less. This favoured the clarity of the voice, but was detrimental to the music, particularly the orchestral sound. The French opera houses were often particularly dry, as was Bordeaux before its recent restoration. This followed the theories of the great trio of Corneille, Racine and Moliere who campaigned for the primacy of words, whereas the Italians always felt that melody, harmony and timbre were more important to the opera. This is a basic difference of opinion that has evolved throughout the history of opera. The calculated resolution of this apparent contradiction was central to acoustic design of the new Glyndebourne.