Equation 2 Objective source broadening
4 The development of the concert hall
4.10 The lateral directed reflection sequence hall
When asked in the mid-1960s to assess the competi-tion for and subsequently to advise on the design of a new hall for Christchurch, New Zealand, Marshall (1979a, 1979b) was dismayed to discover the lack of existing recommendations regarding the appro-priate shape for a concert hall enclosure. In con-sidering this problem, he developed a new theory proposing that ‘lateral reflections were the most important single component of the early reflection sequence’ (section 3.2). This proposal provided a further explanation for the subjective superiority of classical concert halls. The Christchurch Town Hall of 1972 was the first product of this hypothesis (Marshall and Barron, 2001). It was not an example of timid acoustic design.
The architects, Warren and Mahoney, had pro-posed a near-elliptical plan with a cantilevered
gallery. The brief specified a flat central stalls floor.
The elliptical plan has the particular visual virtue that from opposite the stage one tends to ‘convert’
the form into a circular one, providing the desirable illusion of proximity to the stage. In spite of the large audience of 2338 plus 324 in choir seating, none are more than 28 m from the stage front (Figure 4.33).
With a ceiling as much as 21 m above the floor there is a sensation of vastness, while the arena form imparts a vivid sense of occasion. The reflectors also contribute markedly to the visual impression (Figure 4.34).
Fourteen vertical wall elements define the ellip-tical plan and generate adjacent seating areas.
The primary aim of the acoustical design was ‘the provision of unmasked lateral reflections’. The architects and consultants, A.H. Marshall and W.A.
Allen, carefully developed a design with individual seating groups in the gallery for each vertical wall element. Figure 4.35 shows how each seating group
The development of the concert hall 111
Figure 4.33 (a) Plan and (b) cross-section of the Town Hall, Christchurch, New Zealand
Figure 4.34 Town Hall, Christchurch, New Zealand
112 The development of the concert hall
engenders three reflecting surfaces: an inclined balcony front to reflect sound into the central stalls, a balcony soffit to enhance reflections to the over-hung seats and, most significant, a large suspended reflector providing lateral reflections both into adja-cent gallery seating areas and into the stalls. Two of these elements, the balcony soffit and the sus-pended reflector, also serve an essential masking function to avoid focusing by the elliptical plan. At stalls level the seat rake and soffit design sufficiently limit the exposed wall height. Immediately above balcony seating but below the reflectors the sur-faces are treated with absorbent. Orientation of the gallery reflectors was refined with both an acoustic scale model and a computer model. Opposite the stage it was found necessary to use dihedral reflec-tors in order to achieve reflections from the side.
The volume above the reflectors is substantial; the total volume of 20700 m3 generates a reverberation time of 2.3 seconds when the hall is occupied with audience.
Subjective response to the Christchurch Town Hall has, in the main, been highly favourable, though for some its marked character is not optimal. As a
personal view, the most remarkable feature is not the spatial character of the sound but the sense of intimacy and identification with the performance.
With a high degree of clarity, it is possible to listen with ease to individual musical lines. Yet there is also a rich sense of reverberation; the long reverberation time has seldom led to criticisms of excess. In the flat stalls seating, there can be problems hearing the woodwind due to obscured line-of-sight. Those who criticize the acoustics are probably unenthu-siastic about reflections from large plane surfaces.
The successor to this design in Wellington, New Zealand, aimed to overcome these shortcomings.
The objective behaviour of Christchurch Town Hall is interesting, showing sizable deviations from average behaviour. The results given in Appen-dix C were measured in 1983 (and are thought to be more reliable than those quoted by Marshall, 1979a, owing to a superior sound source). Partic-ularly marked is the difference between the early decay time (EDT) and reverberation time, with the EDT being only 82 per cent of the latter at mid-frequencies. In subjective terms this explains why there is no sense of excessive reverberation.
It probably occurs because a high proportion of sound leaving the source is reflected onto absorb-ent seating. This suggests that in designs of this nature a long reverberation time is not a luxury but is essential. In line with the shorter relative EDT, there is a high value for the objective clarity. Analy-sis shows that this is mainly caused by a low level of late sound relative to theory. Late sound energy may well be screened by the reflectors. The meas-ured total sound level is less than theory, but with such a wide discrepancy between reverberation time and early decay time, the validity of the theory is somewhat compromised. Total sound values are all however above the 0 db criterion. The measured objective source broadening is typical rather than exceptional. Also of interest is the observation that all gallery seats receive a reflection within 20 ms of the direct sound.
The new hall for Wellington of 1983, known as the Michael Fowler Centre, had to be designed in a mere six weeks, so it was natural to use the Figure 4.35 Individual seating group engendering
reflecting surfaces in the Town Hall, Christchurch, New Zealand
The development of the concert hall 113
Christchurch design as its starting point (Marshall and Hyde, 1979). Warren and Mahoney were again the architects, while Marshall was joined by Hyde as acoustic consultants. The Wellington plan form is virtually identical but in this case the stalls flooring is raked (Figure 4.36). The total seat capacity is 2566 including choir. As a design criterion, research since the Christchurch design had indicated less stringent requirements for the creation of source broadening.
Strong lateral reflections on paths remote from the audience seating planes were now the aim, with less emphasis on lateral reflection arrival time. Experi-ence with the Christchurch hall had indicated two risks associated with specular reflections from plane surfaces: false localization and tone colouration.
With reflection from a highly scattering surface, these subjective risks disappear. Traditionally the effectiveness of scattering surfaces had been a hit-and-miss affair until Schroeder proposed a series of slot diffuser designs with predictable behaviour (section 2.6.4). At the time the quadratic residue dif-fuser (QRD) was the most promising to give deter-ministic scattering over a specific frequency range.
The Michael Fowler Centre was the first concert hall to use such diffusers.
Below some frequency, scattering surfaces are no longer effective. To maintain ‘warmth’, Marshall wished the reflections to be specular over the range of the audience attenuation dip, and subjective experiments suggested that the scattering action should be effective above about 350 Hz (QRD design frequency 500 Hz). At high frequencies there was concern that the diffuser fins would reflect sound back towards the source. This backscatter-ing behaviour had been predicted by H.W. Strube of Göttingen and was indeed experienced in subjec-tive recordings in a 1:10 scale model of the hall. But as well as the change from specular to scattering surfaces, the reflectors became independent of the room boundaries. While in Christchurch, reflectors direct sound to the same side of the auditorium, in Wellington the QRD surfaces reflect to the opposite side. In addition to the diffusing reflectors in Wel-lington, lower convex surfaces direct sound to the neighbouring side wall before reaching the listener.
Model tests also confirmed the need for some addi-tional plane reflecting surfaces located behind the major reflectors to serve adjacent seating areas. A further difference with Christchurch is that all but the rear gallery seating blocks have an opening Figure 4.36 Michael Fowler Centre, Wellington, New Zealand
114 The development of the concert hall
behind them to allow late reflected sound to reach overhung seats. This last feature works most effectively.
In subjective character the Michael Fowler Centre shares many characteristics with its Christch-urch forebear. Again there is intimacy, envelopment and a remarkable degree of transparency, enabling individual musical lines to be followed. The objec-tive measurements (Appendix C) point to the minor failing of a slight lack of sense of reverberation for the Romantic repertoire. The early decay time (EDT) is 83 per cent of the reverberation time at mid-frequencies, but with a conventional reverberation time of 2 seconds (occupied) the EDT becomes less than optimal. Presumably the reason for the shorter reverberation time than in Christchurch is inciden-tal absorption by the greater area of exposed reflec-tor (the volume is in fact larger at 22700m3).
A third design based on the Christchurch model opened in the Hong Kong Cultural Centre in 1989 (Marshall, Nielsen and Halstead, 1998). This uses yet another reflector scheme, but again with QRD sur-faces. Marshall’s design with Hyde and Paoletti of the Segerstrom Hall in Orange County, California, is a further radical departure (section 10.6).