De Spiegel opera theatre in Zwolle, opened in 2006, can be considered as the theatre which makes the most optimal use of variable acoustics. Not only did the auditorium have to be suitable for opera and symphonic concerts (1,000 seats), but also for intimate plays (850 seats), in all situations with natural acoustics. This theatre can be considered as a sublimation of the development in variable acoustics as applied by Peutz in earlier halls. The starting point was a much more intimate and compact (round/circular-shaped) theatre with smaller volume (3,500 m3) compared with earlier multi-purpose halls, which is very beneficial for natural speech. Based on Peutz’ previous experience in the Royal Albert Hall in London a 7 m high gallery was constructed as well as movable ceiling elements to obtain the additional 4,000 m3 of acoustical volume required. Combined with a large orchestra shell on stage, a large volume increase of more than 300% up to 11,000 m3 (see figures 1, 2 and 3) was achieved as well as a variable reverberation time ranging from 0.9 s. to 2.0 s, as is required for symphonic music.
To make sure this new concept would work in every respect, the acoustical behaviour was studied using a 1:12 scale model and subjective evaluations were carried out through auralisation. A great variety of room acoustical parameters were measured in this study and attention was given to the variable acoustics and the reduction of round-shape induced echoes. The results of the scale model study were also used to design the final diffusive wall and ceiling elements. Also the seats were tested in the acoustic laboratory of Peutz for their acoustic properties, and provided with a perforation in the bottom of the seat to minimize the difference in absorption with and without occupation as much as possible (see figure 4). Initial measurements and listening tests in the hall confirmed all previous assumptions.
After the opening of the hall in 2006 the acoustic result appears to meet all expectations. The planned variation is achieved with acoustic reverberation times which vary between about 1.4 seconds for opera, 0.9 seconds for theatre and 2.0 seconds for symphonic concerts. Pre-eminently opera performances come to their full advantage. In addition the room is suitable for intimate theatrical and musical performances and also for symphonic concerts. The stage acoustics and resonance from the room was qualified by musicians as excellent. In all respects it proved to be a magnificent hall.