Improving the Acoustic Absorption of Vertical Greening Systems

Improving the Acoustic Absorption of Vertical Greening Systems

Bakker, Jesse (TU Delft Architecture and the Built Environment; TU Delft Architectural Engineering +Technology)

Lugten, M.C. (mentor)
Ottele, M. (mentor)
Tenpierik, M.J. (mentor)
Pietsch, S. (graduation committee)

Delft University of Technology

Architecture, Urbanism and Building Sciences | Building Technology

2021-06-28

Noise pollution is a growing health concern in cities, especially in busy streets with parallel reflective facades, known as urban street canyons. Here, high flows of vehicular traffic generate a broadband noise, with high noise levels in the low-frequency (LF) spectrum. In the canyons, noise decays slowly due to multiple reflections. To counter this, vertical greening systems (VGS) can be applied as multi-disciplinary solutions. Not only can these systems attenuate noise, they can also e.g. decrease the urban heat island effect and increase biodiversity. Yet, the shortcoming of VGSs is the lack of LF-absorption. Hence, the following research question was asked: “How can the acoustic absorption of vertical greening systems in urban street canyons be improved, especially for low-frequency noise?” Four concept designs were developed consecutively, and evaluated on the basis of design criteria. These criteria included specifications on acoustic absorption, manufacturability, and durability. The final design consisted of a living wall system (LWS) based on hydroponic rock wool, with a parallel array of Helmholtz resonators integrated in its structure. This array was added to increase the LF-absorption of the VGS below 250 Hz. The Helmholtz cavity volumes were not physically subdivided, but rather filled with a locally reactive material with a low flow resistivity, to enforce propagation that is normal to the surface. Additionally, plants can also absorb sound in the visco-thermal boundary layer around their organs, depending on the leaf area density. It was found that sciophytes (shade-loving plants) had most acoustic potential due to their large leave surfaces. Analytical, experimental, and numerical validation was performed. A Matlab script was developed that applied Delany-Bazley-Miki equations in an equivalent fluid model, and a transfer-function approach for the multi-layer complex impedance. The design yields near-unity absorption for f>500 Hz. The resonators extended the LF-absorption (a>0.5) down to 80 Hz. Impedance tube and reverberation room measurements demonstrated that piercing the porous substrate layer with the Helmholtz resonator necks can yield a combined acoustic absorption. Finally, acoustic simulations showed that applying the proposed LWS to the facades in a reference urban street canyon in Rotterdam can substantially decrease sound levels, both inside the street canyon, as well as in the adjacent courtyard canyon. However, the window-to-wall ratio of the facade largely determines to what extent sound levels can be decreased, since vertical greenery cannot be applied in front of windows.

Vertical greening systems
Acoustic absorption
Urban street canyon
Green wall

http://resolver.tudelft.nl/uuid:ebb67d29-4ab9-4c12-873c-c2bfc05d3b36

Student theses

master thesis

© 2021 Jesse Bakker