#de bijloke

[rockwise]

De Bijloke Zomerbar

Bruno De Groote en Ben Faes

August 27th 2017

[fromthesoundup]

Mysterious missing reflections at De Bijloke

I arrived at De Bijloke, a concert hall housed within a medieval hospital in Ghent, with high hopes for the acoustics. The hall is particularly narrow, and narrowness is good, because it means strong side wall reflections will arrive at listeners' ears soon after the direct sound. Strong, early-arriving side reflections make the sound image louder, clearer and broader, and can make the musicians sound closer than they look, contributing to a perception of intimacy. To be precise, De Bijloke is 3 meters narrower than the Grosser Musikvereinsaal, the benchmark by which all rectangular halls are measured, so I imagined the sound might be even louder, clearer, broader and more intimate than in that most acoustically-acclaimed of halls.

Before I deliver my verdict, a bit of history. De Bijloke was constructed in the 13th century, when Ghent thrived as a major cloth manufacturing center, to replace the older (and likely overcrowded) Maria Hospital of the Cistercian order of nuns. Erecting its massive timber truss ceiling apparently required that a "whole forest" of oak trees in southern Belgium be felled. After the French Revolution, De Bijloke became a civilian hospital, and it remained in operation until the early 20th century. After lying vacant for a number of decades, the building was in 1999 repurposed as a concert hall.

Taking my seat about two thirds of the way back just to the right of center, I strained my ears to hear the woodwinds warming up over the din of the audience. "Sound is a little distant," I noted, but hoped that it was simply because of the chatter. But the sound didn't improve once the concert began. The program began with Schumann's piano concerto in a minor. The sound was reasonably clear and warm, but it lacked intimacy. I felt removed from the music, disengaged, on the outside. At intermission, I moved up several rows to an empty seat about halfway back. The program continued with Bruckner's 7th symphony and despite the increased orchestral forces and my new position closer to the stage, the sound remained far away, over there. I could hear some sound energy coming from above but almost none coming from the sides. As a result, the sound image was compressed, constrained, unable to grow or swell or bloom. If I had walked into the concert hall blindfolded, I would have guessed I was sitting in a much wider room with far more than its 975 seats (Geffen Hall, formerly known as Avery Fisher Hall, came to mind).

I felt disappointed not because the acoustics were poor but because I had failed to predict as much. How could I have been so far off the mark?

According to a 2003 article in Sound & Communications Magazine, De Bijloke's weak, distant sound impression can be partly blamed on the room's timber rafters, shown below, which "act as a huge baffle and soak up any potential reverberation." 

The article profiles a survey done by Artec Consultants soon after the hall's opening in which it was found that the room had a reverberation time of only 1.3 seconds, far below the recommended 2.0 seconds for Romantic orchestral music. Because historic preservation laws precluded any invasive changes to the building, Artec recommended enlivening the room electronically, and the management installed a LARES elecro-acoustic enhancement system made up of 90 loudspeakers on the ceiling beams (look closely and you will see a few in the photo) and under the floor.

During the concert I attended, the sound did not strike me as being unusually dry, so the system must have been turned on. But what about the weak lateral sound? The smooth, massive stone walls were clearly not absorbing sound like the ceiling rafters. Scanning the room for clues, I noticed something odd up at the front. The portable reflector panels standing upright at the edge of the stage weren't parallel to the adjacent side wall.

The side walls, I could now see, were not vertical! They leaned outwards by a few degrees, pushed over hundreds of years by the lateral force of the dense, massive ceiling trusses. A walk around the outside of the building confirmed the finding. Here I am, standing upright in the modern corridor that surrounds the historic structure. Observe the angle of the buttresses compared to that of the new mullions to the right.

A little angle goes a long way acoustically. A vertical wall will send a strong side reflection to everyone in the audience if there is a reasonably high stage and/or a reasonably steep rake.

A slight angle, however, will reflect energy upwards, over the heads of the audience.

For sound to reflect off an angled side wall and reach a good portion of the audience, the stage must be very high or the rake must be very steep. At De Bijloke, the stage is only about a meter high and the rake is very shallow. A steeper rake might have facilitated strong, early lateral reflections audible but it would have also reduced the room's acoustical volume, further driving down the reverberation time—a damned if you do, damned if you don't scenario.

It appears that the consultants at Artec and LARES were aware of the lack of lateral energy in the room but were unable to do much about it. The article in Sound & Communications explains:

The hall's management was concerned about the location, number and visibility of the speakers, preferring that they not be seen at all... The overhead speakers were not so much the issue - they could be painted and tucked on the stage side of the ceiling beams to hide them from the audience. But, because no speakers could be placed on the walls, this meant finding a different solution for the lower lateral components.

If De Bijloke’s management had opted for speakers on the walls, the angled side walls might have been acoustically straightened, providing strong lateral reflections to the audience and giving the sound the presence and intimacy it so badly lacks. Instead, the “different solution” was to locate speakers in the air plenum underneath the raked floor, which in my experience did not make up for the missing side wall reflections. 

I suppose there are two design lessons to be learned from the case of De Bijloke. First, make sure your side walls are straight. Second, make sure that your side walls will be straight for the next 800 years, because who knows, people may still be listening to Bruckner in 2816.

Program Notes

De Bijloke March 12, 2016

DeFilharmonie Edo de Waart, conductor Jan Lisiecki, piano

Robert Schumann (1810-1856) Piano Concerto in a minor, op. 54

Anton Bruckner (1824-1896) Symphony no. 7 in E Major (version Nowak, 1954/2003)

[froy]

Proclamatie

This slideshow requires JavaScript. Na 6 jaar studeren, hard werken, zwoegen en zweten is het vanavond eindelijk zover: de proclamatie. Nadat ik in september afstudeerde was het vanavond het officiële moment. Ik kreeg een echt diploma (en een kopie in het Engels) en mocht met alle andere afgestudeerden het podium van het adembenemende muziekcentrum “De Bijloke” op voor de bijhorende foto’s en het…

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