The Sydney Opera House was the result of an international competition run in 1956. The term 'Opera House' is a little misleading as the main aspirations of Sydney's music lovers was a good, modern concert hall to serve as a future home for the Sydney Symphony Orchestra. Even in the brief, the 'major hall' was described as dual purpose with an audience capacity of 3000 for concerts and 1800 for opera. The 'minor hall' was described as an auditorium for drama and light opera with a capacity of 1200.

The architect who won the competition was Jorn Utzon. After winning, he faced a number of significant problems resolving the acoustics given the constrains imposed on the internal envelope of each hall by the external shells. Much testing of 1:10 scale models was done in an attempt to solve the acoustics, however, the dual-purpose idea was proving extremely difficult to achieve. As a result of intense financial and political pressure, he resigned from the project after about 10 years.

A new team of architects led by Peter Hall then took over. They began by basically revising the entire design, looking at what exactly could be accommodated beneath the shells. This change of administration almost exactly coincided with the completion of the external shells, the interiors having being left practically empty.

The new team made a submission to the minister for the arts that the major hall should be purely a concert hall, given the city's pressing need for such a facility. It was also proposed that the minor hall be revised so that it became the opera theatre of the complex.

In considering the design of the auditoria, it was decided to reduce as much as possible the distance between performers and the audience, even if this meant having some face the conductor. This resulted in the orchestra platform being located away from the extreme end of the auditorium and occasionally using some of the reversed seating for the choir.

Once again a great deal of model work was done. This meant using an area, on-site, in the basement to construct a 1:10 scale model. The audience was simulated using pieces of neoprene rubber with small cardboard heads.

The points to be noted about the evolution of the basic design are as follows;

• The use of side wall to provide lateral reflections which were almost non-existent in Utzon's designs due to the shape of the shells. In the final designs the need for lateral reflections was recognised. This meant that side walls had to be stepped in a manner approaching the contour of the shell, however, they were required to consist of vertical elements.

• The original ceiling consisted of large 'reversed' arches shaped like catenaries hung longitudinally from the external shell [cf: Figure 1]. However, model testing showed that the orchestra area suffered from a deficiency of early arriving sound meaning that the direct sound dominated over the early reverberant sound and the influence of lateral reflections was negligible. Adding suspended reflectors over the stage did not substantially change this situation. It was therefore decided to raise the ceiling substantially and reduce the distance between the side walls. In addition, soffits were used along the side walls to increase reflections in the side boxes.

• The use of suspended reflectors above the orchestra was studied at all times throughout the design progression. In the final designs, these appear as small circular and later toroidal (slightly convex on the underside) disks suspended from the ceiling by adjustable lines.

After settling on a final design the Sydney Opera House was officially opened in 1973. To assess the final acoustic characteristics a number of test concerts were performed. It was agreed that these would include some acoustical testing involving the firing of a handgun from the stage area and taping the impulse response at a number of positions within the hall. This was repeated with the orchestra playing the first bars of the 'Coriolanus' overture by Beethoven. This was then abruptly interrupted by the conductor to achieve an instantaneous cessation of all instruments. This, it was hoped, would produce a genuine reverberation process with the audience present.

The final figures for the RT are shown in Figure 2.

The internal shape of the halls was not the only problem facing the designers. They also had to contend with train lines running over the Harbour Bridge and Ocean Liners sounding their fog horns when departing.

They solved intrusion problems through the use of a double skin [cf: Figure 3] and tested it using an Army Helicopter hovering around it in different positions.

Conclusion

The development of the design of the Sydney Opera House may probably be regarded as the first major example of a modern concert hall in which the general shape, plus many smaller details, were radically influenced by thorough scale model testing. Such an approach is today championed by many respected acousticians such as Harold Marshall who has recently developed the MIDAS system for automated testing.