Big Windows in Porto
Acoustic specialist Renz van Luxemburg was closely involved in the design of the Casa da Música by OMA in Porto. Two spectacular big windows connect the concert hall with the city outside. What did it all involve? Jeroen Mensink interviewed Van Luxemburg.
Both the front and back walls of the big concert hall are entirely glazed. The double expanse of glass behind the orchestra is no less than 15 by 22 metres in area. It is made of curved and laminated strips of glass 1200 mm wide and 400 mm deep. The opposite wall is made of sheets of glass 900 mm wide and 300 mm deep; the differences between the corrugated forms prevent disturbing interference.
''Most concert halls are introverted bastions entered only by the elite. The good thing about this concept, and the strength of this hall, is the clear message it sends out: everyone is welcome to enter. That's entirely new.''
''When it comes to sound, such vast expanses of glass in a concert hall raise a number of issues. The acoustics in the hall, for example, and the need to prevent sound travelling from inside to outside. And also from outside to inside, a vital consideration in this building, since we had to cater for very quiet passages and recording sessions. The limit in such cases is 20 dB, and even 15 dB, an extremely low level, is desirable. If you measure the sound level outside in a quiet area, the sound of the meter alone gives a reading of around 20 dB. And here we had to deal with a busy traffic intersection, which generates around 75 dB, not to mention vibrations. On top of that there are police cars and ambulances racing past.
Good sound insulation can be achieved by placing two sheets of glass at some distance from each other, preferably over an expansion joint. A reasonable distance is a cavity a half metre in width. The hall and façade in Porto are separated by the foyer, which forms a buffer for sound, light and heat. The foyer was programmed there from the outset.''
''The sound in the hall has to be distributed evenly to create the right acoustics. That explains why there are never any perfectly vertical or horizontal planes in a concert hall. Smooth vertical planes in particular are disastrous. In a theatre the main aim is to enable the audience to understand the speaker. What you often do there is design from the ceiling down, and the walls are usually acoustically inert. It's the very opposite in a concert hall. The ceiling is less important; it's the side walls that ensure an even distribution of sound to all the seats. The front and back walls can cause bothersome sound reflections. The back wall in particular can cause annoying reflections when the orchestra is playing. The result is a harsh-sounding tap like an echo – a disaster during concerts. The first idea for that big glass façade was a transparent plane that would break up the sound and send it in all directions so that it would be really diffuse.''
''The idea for the curved glass emerged fairly quickly. The glass couldn't take the form of a flat plane, for the sound had to be broken up. At a certain moment the corrugated form arose. At first I was hesitant about a uniform corrugated surface. Sound would bounce off it and travel in parallel directions instead of being broken up. But in the end we went ahead with it, because the podium is at a lower level than the window. That means that the sound hits the glass at an oblique angle and that's when totally different things happen. After that it was a matter of making models, studying the different components, and finding out where things go acoustically wrong in the model. When there seemed to be no difficulties I thought that the model wasn't reliable. So we examined the whole thing from other perspectives and consulted colleagues. Theory would seem to suggest that a regular corrugated form doesn't work, but that can't be proven. Instead of examining how well something worked, I tried to find out why certain options were not possible and what could go wrong. And so in the end we concluded there was nothing wrong with such an acute angle.''
''Another influence on the design was the available size of curved-glass sheets and the sizes suitable for transport. Acoustically, it's interesting to have a single 22-by-15-metre sheet of glass. But that's wasn't on, so we had to design joints. We made an assumption about the effect that the sealant joints between the sheets of glass would have the sound. Measurements now show that the sealant has no negative effect on the sound insulation. In fact, it has a positive effect. If a sheet of glass is too big and too rigid, it starts to vibrate. The separate sheets here transmit energy to one another and that only benefits the sound insulation. Lucky breaks? We weren't depending on them, but we were open to the possibility.''