A reliable method

The Polish capital Warsaw is getting a new concert hall: Construction on a new venue to house the Sinfonia Varsovia Orchestra is scheduled to start next year. The remarkable design combines the classic shoebox with innovative forms. To ensure the successful planning of the project, acoustic sound tests were carried out on a specially created 1:10 scale model.

Bühnentechnische Rundschau

Back in 2010, the city of Warsaw and the Sinfonia Varsovia Orchestra – the building contractor and the user/operator, respectively – invited tenders for the design of a new concert hall. Construction is due to begin in 2022. The centrepiece of the project will be a large concert hall seating over 1800. In view of the orchestra’s high demands of the acoustics and the hall’s unusual architecture, planning involved room acoustic measurements carried out on a real scale model as well as the use of advanced acoustic computer simulations.

(…)

Why scale model measurements?

Today, it almost goes without saying to use sophisticated computer models when planning complex construction projects such as state-of-the-art concert halls. These supply helpful insights to almost all areas of planning, including the acoustics. For the Warsaw project, too, acoustic tests were carried out on a computer-generated model, using the architect’s BIM (Building Information Modelling) program. This made it possible at an early stage of planning to determine and optimize the key acoustic parameters of the concert hall and its spatial volume, as well as to produce the right geometric basic conditions for ensuring good in-hall sound.

But despite the very helpful findings that can be drawn from computer-generated acoustic models, even the most high-tech simulations are limited in their forecast accuracy, so it can often make sense to carry out additional measurements on a real scale model. This is basically because of the fact sound travels in waves, which cannot be one hundred percent accurately reproduced even by the most state-of-the-art simulation algorithms. (…)

Geometric reflection characteristics can be superbly replicated by means of computer simulations using cone tracing or raytracing. But if the geometric structures and the wavelength of the incident sound are of roughly equal dimensions, there are a lot of effects such as diffraction and refraction, which cannot currently be reproduced with proper accuracy in computer simulations. (…)

Scale model measurements were factored into the planning of the Warsaw concert hall project as well as computer simulations from the start. Although primarily intended for double-checking previous readings, the possibility could not be ignored that the new readings might make it necessary to change and optimize the design of the concert hall. (…)

Precise handiwork – the model construction

In 2019, the Warsaw model designers Pracownia Tryktrak created a 1:10 scale model of the concert hall. They made all the model’s surfaces according to the acoustic specifications and exactly following the architect’s detailed drawings, mostly out of wood and plaster, to a large extent using CNC mills. The concert hall has been designed with a large, structured glass wall at the front. This was reproduced in the model with an accordingly shaped glass surface, which was removable so that modifications to the design could be made even after completion of the first model.

(…) The new Warsaw concert hall is set to be clad in almost entirely sound reflecting surfaces. For the acoustic model, this meant accurately recreating the sound reflecting surfaces. The main materials chosen for the Warsaw concert hall model – wood, plaster, and glass – are excellently suited to this. To give the surfaces the required sound reflecting characteristics, the materials needed to be of an appropriate weight and all the joins and pores in the structure carefully sealed.

The key absorption area in the concert hall is the seating taken up by the audience. To achieve reliable readings from the model measurements, not only these surfaces’ absorption capacity needs to be as precisely reproduced in the model as possible but also the additional reflection and dispersion characteristics, e.g., those formed by the audience’s heads. Prior to taking the model measurements for the Warsaw concert hall, acoustic tests were carried out in the model reverberation room at Müller-BBM with a replica audience. This was optimized so that it could be easily produced in large quantities and to replicate the acoustic characteristics of an audience-occupied area as precisely as possible. The result was a wooden corpus clad in a sound absorbing material.

Model measurements in real conditions

In 2020 Müller-BBM carried out extensive measurements on the model of the Warsaw concert hall, in the same way as in a real hall: An omnidirectional radiating dodecahedron was used for the sound source, positioned in different places on the podium. Miniature measuring microphones were attached to selected audience seats.

The measurements were made using true-to-scale, frequency spectrum sweep-measuring signals. The received signal is used to calculate the ‘room impulse response’. This can be seen as a kind of unique ‘acoustic fingerprint’, containing all the acoustic information on the space’s reverberation microstructure, and from which all the acoustically relevant criteria such as reverberation time and clarity can be ascertained.

The readings confirmed the forecast resulting from the computer simulations, i.e., that the design promised excellent concert hall acoustics, fulfilling all the clients’ requirements. But they also highlighted undesirable acoustic artefacts in some seating areas due to unfavourable sound reflections from curved breastwork surfaces. Consequently, the course and curvature of the breastwork surfaces in question was changed.

Conclusion

As well as the advantages outlined above which real scale models have for analysing the acoustics of complex concert hall geometries, they also have other benefits. Building a model is an excellent way of testing the design. It’s a reliable method for discovering any oversights or weaknesses in the plans and remedying them before construction begins. Models are also a great way to preview and promote projects. The Warsaw concert hall model was split in two once the measurements had been completed. One half is in the care of the Sinfonia Varsovia Orchestra and the other is now being shown at various exhibitions in Poland to publicize and advertise the project.


BTR 5 2021
Rubrik: English texts, Seite 110
von Michael Prüfer

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