Page 16 - Winter 2011

Fig. 5. Fabric panels in place over the lower “sawtooth” walls in the Concert Hall at the Sydney Opera House.
Fig. 6. One of the adjustable medium density fiberboard (MDF) panels covering the lower sawtooth walls during the 2009 acoustics trials in the Concert Hall at the Sydney Opera House.
the room might play in this phenomenon. One common thread we have noted is that rooms with large amounts of fine and medium-scale sound diffusing shaping on the walls and ceiling tend to have this undesirable quality of harshness at high dynamic levels.
12 Acoustics Today, January 2011
One illustrative example of this is from the Concert Hall at the Sydney Opera House. Harshness in the stalls level seating and on stage was acknowledged as a serious acoustical defect in this hall in a number of inde- pendent studies conducted between
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ing the stage and stalls seating.
This wall shaping was presum- ably intended to provide diffused lat- eral reflections to improve listening conditions, but the sound reflected by the sawtooth paneling was harsh and abrasive for audiences listening in the stalls seating as well as to musicians on stage. Musicians felt they could not make a beautiful sound in the hall, and audiences concurred. Careful listening pointed to the sawtooth walls as the source of the harshness, and as a simple means of demonstrating this to listeners and musicians, we suggested a mock-up with fabric hung in front of the saw tooth walls, pictured in Fig. 5. The fabric made a dramatical- ly positive contribution to the quality of the musical sound even though it diminished the loudness and enveloping qual-
ity one would desire from these side-arriving reflections. The temporary fabric was installed in 2007 and has remained in place for three years awaiting funding to replace it with timber paneling, which would provide sidewall reflec-
tions rather than absorbing them.
As proof of the timber paneling concept, and for public
as well as stakeholder encouragement in this iconic space, we prepared a second demonstration in 2009 that replaced the fabric in front of the sawtooth walls with sealed medi- um density fiberboard (MDF) panels. The MDF panels were designed to be adjustable to allow experimentation and optimization of wall angles as can be seen in Fig. 6. Over the course of the trials, the orientation of the panels was optimized in plan and section. Improvements were clearly noticeable to musicians onstage, but the improve- ments were most significant in the stalls seating, borne out by consistently positive comments from both audience and
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us that a better understanding of the positive and negative influences of sound diffusion is essential for progress in con- cert hall design. In the case of the Sydney Opera House Concert Hall, less diffusion from the essentially flat MDF panels was clearly preferred. Unfortunately, this acoustics trial did not reveal the precise mechanisms at work here. Is it low level edge-diffracted sound from the points of the saw- tooth shaping? An imbalance between high and mid-fre- quency sound levels? “Smearing” of the reflection structure in the time domain? Perhaps some combination of all these effects?
Our own studies of this issue concentrated on the acoustics impact of the “sawtooth” shaping of the lower sidewalls flank-
1996 and 2003.
musician surveys.
This experience was one of several that have convinced