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A FEW BASIC RULES
As with any project, an opera project starts with a program: What does the end user (and the payer too!) actually want (e.g., capacity, shape of the hall, type of performance)? This means the various requirements must be identified and the relevant acoustic objectives stated. In today’s spirit of sustainable development, one must be conscious that a team effort (i.e., architects, structural engineers, HVAC, and acoustics, just to name a few) occurs and make sure that the various solutions that are considered at the design stage are compatible with each other. Acoustics is often part of a global problem that can only be solved by a complete design team fed the relevant data by the end user.
Assuming that the emphasis on the facility really is opera, the first point will be to define its capacity. This is no academic question, as an opera is staff-consuming and one has to pay their salaries: In addition to the musician and the singers, one needs the electricians and machinists to work the various devices and handle the back stage and gratings. In order to balance the finances, this means that a large audience must be accommodated. In order to achieve such a feat, balconies will be needed. More to the point, according to the size and shape of the orchestra pit (which typically occupies what would be the four front rows), the capacity of the audience may be limited.
In order to increase capacity, it might be tempting to increase both the height (to add one more balcony) and the length (to add more rows). However, the former move will increase the RT lower limit, while the latter will boost the chances to have an echo appear, while decreasing the stage visibility of the audience. More to the point, the local urban planning rules may not take too kindly to a higher-than-usual building being erected, so it will be necessary to dig, and apart from the usual construction problems, this will also double the needs for safety exits according to most safety rules.
Then there will be a few choices to make regarding the stage size and equipment: Does one want a swiveling double stage so as to be able to change the scene in a hurry? Does one need some stage machinery to elevate or lower part of the stage floor? How much side space will be needed? The answer to those questions will have an impact on both the required surface and the acoustic noise control measures to be implemented.
Now, what acoustics are looked for? The answer will be complex, as it depends on the wishes of the conductor (who might prefer an open pit as a better showcase to his orchestra), the physical constraints of the facility due to the available volume and shape, and eventually the doctrine of the local cultural authorities: Does one favor the speech intelligibility or the music fullness? Usually, one will end up with absorptive boxes (in order to avoid unwanted coupled echoes) and diffusive treatment over the front of the balconies, while the walls will be treated with either diffusive or absorptive coverings in order to suit the reverberation time targets. The ceiling will be partly reflective, and its shape will be designed so as to help propagate the sound energy from the stage and pit areas to the audience. The stage tower will be treated to a shock-resistant absorptive material.
Eventually, one will usually end up with a volume in the 6000 to 10,000 m3 range .
There is no question about the air handling in the facility. The best way to cool the audience is to insufflate air at low speed under the seats. But care must also be taken for a proper cooling and ventilation of the orchestra pit due to the amount of active people and the lighting fixtures. Last but not least, the stage must be ventilated and cooled too. The problem is to achieve this without having the back scenes moving due to the air speed, which calls for sizable louvers to achieve the required low speed and large flow rate.
Are we done now? Not yet, as the performance hall is only a small part of the whole facility. In order to operate smoothly, one needs rehearsal rooms, at least one for the choirs and one for the orchestra, not to mention the various groups of instruments. While there probably will not be a performance held during the rehearsals, all those spaces must be able to operate simultaneously. This usually requires highly insulated rooms built as a box in box. One must also take into account the need for consequent height and volume (e.g., both the choir’s and the orchestra’s rooms will be double height compared to other spaces).
Last but not least, there will probably be ballet included in the activities of the opera house. This means that a dance rehearsal and training room must be provided. Keeping in mind that a minimum 4.5 m clearance is required for such a room and that 20 people jumping together provides quite a lot of excitation for the structure, strong impact noise control measures will be needed. In addition, the dancers are—understandably—particular on the kind of flooring they want, so this must be cleared well in advance during the design process.
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