Noise levels can be reduced by absorbing the sound in tubes with cleverly designed shapes. A tried-and-tested technique used in machines, Professor Arjan van Timmeren (Urbanism) is going to use STW funding to investigate whether it can be applied in the built environment as well.
The function of Herschel-Quincke tubes is based solely on their shape. Sound is absorbed and directed through two passageways of different lengths. This passive destructive interference (PDI) results in the (partial) elimination of sound. "As the distance travelled differs between the two tube inlets, the waves meet in antiphase`, explains project leader Professor Arjan van Timmeren (Environmental Technology & Design). "That is what reduces the sound." The principle is comparable to antisound without the need for an active source of sound or energy.
Until recently, the Herschel-Quincke principle could not be used in buildings because of the complex design involved. The long tubes needed for the low frequencies in particular are difficult to achieve in compact volumes. This has changed with the emergence of 3D printers. Thanks to 'additive manufacturing', tubes or a series of tubes can in principle be produced in any shape or composition required. Only a few millimetres in diameter, the tubes can be designed specifically for selected disturbing frequencies. This would allow specific noise to be reduced at a railway station, transformer station, airport or motorway. There are particularly good opportunities for PDI sound absorption in places where problems are caused by just one or a few specific frequencies.
The advantage of using a 3D printer is that any shape can be created and no repetitions are needed for an exclusive shape to be affordable. The tube elements can be produced as wall panels but abstract designs are also possible. The Delft researchers also envision opportunities for sound absorption in the form of free-standing elements or elements suspended from the ceiling. Noise-resistant objects or works of art in urban outdoor areas are other options currently being explored.
Preliminary laboratory tests have meanwhile been conducted. The results were promising and confirmed the calculations. The follow-up research, funded by the STW, for the ADAM project (Acoustics by parametric Design and Additive Manufacturing) is expected to result in working prototypes. Further performance improvement is still needed, however. For example, technological advances should prevent burrs in the tubes caused by inaccuracies of the 3D printer. Burrs can have an adverse effect on sound proofing. For the ADAM project, TU Delft (an interdisciplinary team consisting of Dr. Martin Tenpierik, Dr. Michela Turrin and PhD candidate Foteini Setaki) is working together with acoustic specialists from the faculties of Architecture and Applied Sciences. The Dutch acoustic consultancy agency Peutz has made their test facilities available for research purposes. The Belgian company Materialise is involved in the development of the 3D print technology.