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Step 1: Choosing the characteristics of our concert hall
Shape, dimensions, materials...
In addition to the course book “Architectural Acoustics” by Marshall Long we used the following books as sources of inspiration when deciding on the different parameters and general shape and layout of our hall:
Beranek, L: Concert and opera halls – How they sound
Hammond, M. : Performing Architecture
Lord, P. : The Architecture of Sound
Mehta, M .: Architectural Acoustics, Principles and Design
We started by deciding the volume and dimensions of our hall. We used the Boston Symphony Hall and the Vienna Grosser Musikvereinssaal (photos below) as a reference in our design (shoebox shape). Indeed, this kind of shape is considered as being acoustically good for music purpose because of side reflections. Furthermore, it is very simple to draw. More complex shapes can give better results, but they would be too complicated to model.
Figure 1: Boston Symphony Hall
Figure 2: Vienna Grosser Musikvereinssaal
The basic shape of our hall can be seen in the following picture.
Figure 3: basic shoebox
Dimension: (38.5m; 22.5m; 18m) - Volume: 15592 cubic meters
Our first goal was to create a hall in which 1500 people (+100 musicians) would fit. However we wanted the hall to be as small as possible in order to get the best acoustic quality possible (low initial time delay gaps). Finally we get the following sketches. We choose deliberately two different shapes as we would like to compare them in the end (with regard to the important metrics). The only difference between the two halls is the shape of the walls around the stage. In the first one we keep the shoebox form, in the second the walls follow the shape of the stage and the leaning ceiling replaces two reflectors:
Figure 4: Final version of the Concert hall (1)
Figure 5: Final version of the Concert hall (2)
We had to take into account several parameters when designing the hall and choosing the materials.
- Early reflections
The sound transmitted from an orchestra is radiated in all directions and travels through the air at about 344 m per second and within 1 or 2 seconds is reflected many times over from the different surfaces of the space. To understand the effect of the acoustical attributes of a hall on the music, we must consider the reflections as divided into two time intervals. First, the “early sound”, defined as the direct sound and those reflections that take place within 80 ms after the arrival of the direct sound. Second, the reverberant that is created by the many reflections that occur subsequently.
These reflections come from the side walls, the ceiling overhead and the walls of the stage enclosure. If they arrive at the listener’s position from the side walls, i.e. from lateral directions, they appear to broaden the source and this increase the apparent source width (ASW). Increasing ASW lends quality to the music heard in a concert hall.
In order to get a better ASW we must try to reduce as much as possible the initial time delay gap. Measuring the distance between stage and the receiver point, once directly, once including a reflecting surface, we can deduce this time delay. We quickly found out that the first lateral reflections coming from the sidewall came much too late after the direct sound (> 28ms). In order to reduce the delay we lifted up the side seats, which creates a new, shorter path for the reflected sound. This is called a hall-within-a-hall method. The side seats happen to appear as terraces overlooking into the hall. They are just lifted by 1 meter but that should be enough for the audience in the middle, lower part of the ground floor to get earlier lateral reflections.
Another short path is to the upper side balconies . It depends on the point where we measure whether the first reflections come from the side walls or the upper balconies.
- Sight lines
Since is it important that every spectator sees the orchestra in addition to hearing it, we have to check that all sight lines are free. This is especially important for the balconies. For the ground floor, it is enough to have a slight inclination, i.e. in our case around 7 degrees for the middle and for the side parts. For the rear balcony we need an inclination of 25 degrees so that the persons sitting in the very last row still see the stage. For the side balconies, we choose an inclination of 14 degrees against the center of the hall. So the first look of the audience sitting in the side balconies goes towards the center of the hall and not the stage. That's why everyone has free view on the music players.
- Diffusion / Scattering / Reflection
In order to get a good sound quality at every point in the concert hall, the sound should be directed into every direction. This can best be achieved by making the surfaces on which the sound waves are reflected, diffusive. An important surface is the external part of the side balconies. That is where the early reflections come from, which are that important for the audience to feel "surrounded" by the music. In order to get the reflected sound waves directed into the middle of the hall and not the ceiling, we smooth the rough edges of the lower part of the side balconies and make them round. That way the sound waves are diffused into the hall where the audience is sitting. Furthermore we shape them with semi-circular panels in plywood in order to increase the scattering:
Table 1: scattering and absorption coefficients of the upper balconies
Furthermore the side walls have also to be scattering in order to get the sound field in the hall as diffuse as possible. What's important for the walls is not to absorb too much the low frequencies produced by the orchestra. However room surfaces made of lightweight panels with air cavity backup increase low frequency absorption, thereby reducing the warmth. Thus, only hard and heavy surfaces, such as concrete and masonry, are recommended for music hall interiors. If any lightweight paneling is used, it has to be thin and mounted directly on heavy surfaces without an air cavity backup.
We avoid large reflective surfaces since they produce "harsh" reflections, referred to as acoustic glare. In order to get the sound from the orchestra directed to the audience, we tried first to use double curved reflectors fixed above the stage, but as the calculation time was really huge, we chose to simplify the panels to pyramid shaped ones. They are not as diffusive as the curved ones but the results stay reasonable. Thus we use 2 or 4 pyramid shaped panels made out of a material of 20kg/m2 of 11m x 9m each, which are attached at the ceiling in a manner that it is possible, at any moment, to change their height and angle, depending on the music which is played on the stage.
What should absolutely be avoided are bunched reflections coming from behind the spectator since it distorts the sound. Therefore we use Schröder diffusers on the back wall to get a more diffuse field. The back wall consists of many small rectangular holes with different depths, which reflect different frequencies in different ways. We don't imitate the diffusers in Sketchup but just apply a scattering coefficient on the back wall surface which corresponds approximately to this kind of diffuser. For the side walls approximately the same problem appears. We need a lot of scattering but with an absorption as low as possible. So we choose the same kind of diffusers but another material so that the absorption is reduced.
Table 2: absorbtion and scattering coefficients of the rear wall
Table 3: absorption and scattering coefficients of the side walls
- Audience
The audience is simply simulated as a surface which is at an approximate height of 1.15m (average height of a seated person, male and female). The bodies have a non negligible absorption coefficient and the heads have a certain scattering coefficient too. However the audience is not taken into account when calculating the metrics like brilliance, warmth etc. Instead the absorption coefficients of the seats have to be integrated into the model. Further information on the seats can be found in Step 2: Choosing the seats for the audience.
Table 5: absorption and scattering of the seats
Some other details on the used materials:
- The floor is made absorbing by putting heavy carpets directly on the concrete.
Table 6: Scattering coefficients for the floor carpet
- The stage is made out of some kind of wood.
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