Engineering agency LBP|Sight has won the Vernufteling award, organised last week by NLIngenieurs and the engineers’ association KiviNiria. LBP|Sight’s acoustical floor in the Dutch Maritime Museum saves visitors from drowning in sound in the courtyard.
The courtyard is a place of pride for the fully restored and recently reopened Maritime Museum in Amsterdam. A huge glass roof has been constructed over the courtyard, with numerous fine steel beams in the form of a compass rose supporting it. The only trouble is that sound would bounce endlessly between the stone floor, the glass roof and the walls of the courtyard. Reverberation times of 8 seconds were calculated, which would result in a disastrous cacophony for guides and visitors.
Engineering Agency LBP|Sight developed a sound-absorbing floor, covered with travertine, which reduced the reverberation time from 8 to 3 seconds. The jury was charmed by the novelty of the solution in combination with its simplicity. So how did they do it?
“It’s clever, smart and elegant,” says Professor Diemer de Vries, emeritus professor of acoustics at TU Delft. “The idea is much the same as the better known perforated ceilings, which absorb sound according to the principle of the Helmholtz resonator.” Like a bottle, the resonator has a small opening and a large volume behind it. The air in the mouth acts as a piston against the compressible air inside. Together they form a mass-spring system with a tuneable resonance frequency. Normally the resonator will amplify sounds with frequencies near its resonance frequency. But stuff the volume with mineral wool, and it will specifically absorb sounds close to its resonance frequency – which is a few hundred Hertz for absorbing speech.
“The new thing is that they have applied the same principle to the floor instead of the ceiling,” explains Prof. De Vries. Crossing grooves between the tiles act as the opening of the resonator, while mineral-wool-filled cavities beneath the tiles absorb the sound. “An additional benefit of a sound-absorbing floor is that it’s usually closer to the source – talking humans – than a perforated ceiling. About 50 percent of the produced sound gets absorbed right away.”
A problem could be the maintenance of the tiles, as suggested by the professor’s wife. De Vries: “Women think of these things: how to keep it clean?” If the 4-millimetre grooves slowly silt up, the damping will go away and the courtyard will drown in sound. “I’d like to know what they’ve come up with to counter that effect,” the professor says. “They seem smart enough.”
Luc Schaap (MSc), who works as a consultant for LBP|Sight on the project, says it’s implausible that the grooves will silt up. “We’ve long discussed the width of the grooves. If too small, things will stick in it, and if the width is too large, ladies on heels will suffer. Four millimetres is actually quite wide, but still safe for the ladies.” Besides, people will enter the museum elsewhere and will cross several fibre mats before reaching the courtyard. Much has been done to limit dirt coming in. If ever a major cleaning would be needed, all tiles can be removed from the supporting frames with special pincers.
Both the jury and Prof. De Vries expect the sound-absorbing floor to be applied more widely in future.
