The fan-shaped hall

A major misconception of many of the designs in the first half of the twentieth century was the (tacit?) assumption that there was one ‘perfect acoustics’, that there was no distinction between the appropriate conditions for the spoken voice and instrumental music. In view of the totally different forms developed historically for drama theatres and concert halls, this was a blinkered view. In the 1920s a new art form developed with its own auditorium type: the cinema. The acoustic requirements for cinema are not particularly stringent and in order to maximize the audience size the obvious plan form is the fan shape, originally developed so successfully by the ancient Greeks for outdoor theatre. Inevita-bly the fan-shape plan was also adopted for concert halls, however during the last thirty years or so this plan form has become anathema to many acousti-cians. There is more than one reason why the fan shape is unflattering to concert performance.

The most obvious problem with the fan shape is that the rear auditorium wall is automatically generated as a concave curved surface, which pro-duces a focused echo back to the stage. There is a simple remedy for this in tilting the rear wall to reflect sound down onto the audience. The alterna-tives of fragmenting the rear wall surface to make it scattering or placing absorbent on it have often

been used but if the degree of focusing is too great, echoes may still be audible. Of these remedies, only by rendering the surface scattering is the rear wall retained as reflecting. As a major bounding surface it is desirable that it should not absorb acoustic energy.

Other acoustic problems of the fan-shape plan include the following. The extreme width at the rear of the hall tends to leave seats in the centre rear of the stalls with few early reflections. This is particularly the case for reflections from the side.

As seen in Figure 3.13, the angle of arrival for side wall reflections is small in the fan shape. One of the acoustic characteristics therefore of this plan form is a limited degree of source broadening. The acoustic imperfections of the fan shape also extend to the later part of the received sound. Whereas in the rectangular plan there is vigorous interreflec-tion of sound between the parallel side walls, the potential for multiple reflection is much reduced in the fan shape (Figure 4.19). This compromises the sense of feeling surrounded by sound. It also leaves a low level of late sound towards the rear of the hall, where the sense of reverberation is lower. Kroks-tad, Strøm and Sørsdal (1968) report from compu-ter studies a lack of mid-period reflections (40–180 ms) in fan-shaped designs. This may contribute to an early decay time shorter than the reverberation time, again contributing to a less reverberant sensa-tion. The degree to which a fan-shape plan suffers these various deficiencies is predictably influenced by the angle of fan; larger angles of fan have more extreme acoustics.

Subjectively therefore the fan-shaped hall tends towards, though is not as extreme as, the directed sound hall discussed in the previous section. The sound is more frontal than lateral and the sense of reverberation is diminished by the limited degree of diffusion. If the ceiling reflections are strong, the sound quality can become harsh due to tone colouration effects. Within these halls pronounced variations in quality can often be observed, with a tendency for a dull sound towards the rear of the stalls seating and a lesser sense of reverberation at seats distant from the stage. Two extreme cases

96 The development of the concert hall

with a semicircular plan are discussed in sections 11.1 and 11.2.

As an example of a more realistic fan shape, the 2700-seat Alberta Jubilee Halls, Canada, are of interest (Figure 4.20). These virtually identical halls in Calgary and Edmonton bear some resem-blance to the earlier Kleinhans Music Hall of 1940 in Buffalo. Opening in 1957, they were designed by the Alberta Department of Public Works, not in fact as pure concert halls but rather as multi-purpose spaces with stage flying facilities, though concerts were a major use employing an orchestral shell on stage. Among design details which deserve note is the elaborate diffusing structure on the rear wall which successfully suppresses an echo (Northwood and Stevens, 1958). The front sections of the ceiling were made convex to render less intense the ceiling reflections to the front stalls with delays of 40–50 ms.

The auditorium volume (21500m³) was select-ed as intermselect-ediate between the requirements for speech and music, giving an occupied reverberation time of 1.4 seconds. That this was short for music

use was noted by many of the listeners at a test concert. Objectively the seating area with minimum sound level in the hall was found to occur in the middle of the ground floor, testifying to the hole-in-the-middle reflection problem with this plan form.

On the other hand at ‘the rear of the top balcony ...

the sound had brilliance and an incisive quality of attack that would please the “hi-fi” enthusiast, but [with] much less large-hall effect’. This proves to be due to the facetted ceiling design which con-centrates reflections onto the top balcony. O’Keefe has provided an incisive analysis of the acoustics of these Jubilee and other post-war halls, which sug-gests that a major issue for concert acoustics is the height–width ratio (O’Keefe, 2002). He found that wide, low halls suffered not only from poor spatial sound but also poor reverberance and sound level.

As a modern comment on these designs, both cities have subsequently commissioned new concert halls following classical rectangular lines:

the Jack Singer Concert Hall in Calgary of 1985 (Forsyth, 1987) and the Winspear Centre in Edmon-ton of 1997. The Alberta halls themselves under-went major refurbishment in 2005 to improve their acoustics; a major modification was to raise seating sections next to the side walls, thereby creating reverse-splay seating areas rather than fan-shaped (Jordan and Rindel, 2006).

In Europe, the Konserthus in Oslo of 1977 with 1700 seats is a further example of a fan-shape plan that has yet to be seriously modified (Jordan, 1980).

The architect best known for using almost exclu-sively the fan-shape plan for auditorium designs was the Finnish architect, Alvar Aalto (1898–1976).

Most notable are two halls in Helsinki: the Kultuuri-talo of 1958 (1500 seats) and the Finlandia Concert Hall of 1971 with 1750 seats (Forsyth, 1985). For the Finlandia Hall, Aalto acted as his own acoustic con-sultant, yet subsequently the hall’s acoustics have been rated as disappointing. At the time of writing there is some electronic enhancement in the hall and the city of Helsinki is building a new concert hall. Even in the hands of a famous architect, the fan-shape plan can offer inferior acoustics for sym-phony concerts.

Source

Source

Figure 4.19 Multiple reflections from the walls of a rectangular plan and a fan-shape plan

The development of the concert hall 97

Figure 4.20 (a) Plan and (b) long section of the Alberta Jubilee Halls, Canada

98 The development of the concert hall

4.6 The synthesis in British