Sound reflections in an Urban Context: The influence of façades on urban noise levels

Sound reflections in an Urban Context: The influence of façades on urban noise levels

Niesten, J.

Knaack, U. (mentor)
Tenpierik, M.J. (mentor)
Krimm, J. (mentor)

Architecture and The Built Environment

Architectural Engineering and Technology

Building Technology

2016-01-27

In a research of the World Health Organization (2011) it is shown that environmental noise is number two on the list of health risks. Environmental noise is linked to certain health conditions ranging from annoyance to heart attacks and is caused by the growing number of noise sources in the urban environment. An important aspect influencing the noise levels in an urban context are the hard, sound reflective, building materials that are mostly used. A recent study from Techen and Krimm (2014a) indicates that surfaces like this can change the noise levels 3 to 8 decibel. To reduce these increased noise levels this research tries to find an answer to the following question: how can an existing façade be adapted in such a way that the impact of the noise reflected by it will be reduced? To answer this question a case study location in Delft is chosen: the Herman Gorterhof. At this location most of the direct sound is blocked, so the sound levels are mostly influenced by reflected sound waves. Based on prior research and on on-site measurements it’s concluded that the most important noise source for this location is traffic noise from the nearby road. This source produces the highest levels for the frequencies of 1000 and 2000 Hertz. In order to define potential approaches to reduce reflections which are influencing the sound levels a literature study is conducted. Sound can be reduced when or before it’s reaching the façade by noise barriers, sound absorbing materials or resonating sound absorbers. For the sound absorbing solutions it’s especially important to consider weather effects which might damage the used materials. When it’s impossible to use these reduction methods the impact of the sound reflection can be reduced by influencing the (direction) of the reflected sound. This can for example be done by introducing an angle or by scattering the sound waves. A computer simulation is done to further investigate the impact of sound reflections at the chosen location and to test the potential solutions. To validate this model the found values are compared to the actually measured sound levels. From the simulations it followed that the sound levels improve up to 17,7 dB(A) when the façade completely consist of a (theoretical) 100% absorbing material. When looking at more realistic façade solutions to prevent reflections, façades which are reflecting the sound waves upward seem to perform the best. A solution like this has an average improvement of 5 dB(A) which is similar to the impact of the theoretical 100% absorbing solution. To investigate the best performing façade solutions in more detail scale model measurements are used. For these measurements a simplified 1:50 model is made from the chosen location and the possible façade solutions. The measurements are conducted in the small semi-anechoic room at the faculty of Applied Physics at the TU Delft. To validate this model the found values are compared to both the values from the on-site measurements as well as the values found using the computer simulations. Besides investigating the sound levels inside the courtyard of the chosen location this scale model is also used to determine the impact of the potential solutions on sound diffraction over nearby buildings, into more quiet areas. From these measurements it followed that the solution which would be reflecting the sound upward did indeed perform the best when just looking at the improvements in the courtyard area. However, when looking at the diffraction it caused over nearby buildings this solution performed the worst. The sound absorbing solution performed the best since this solution only absorbs the sound waves, not (potentially) sending them towards more quiet areas. Based on this results it’s decided to further investigate three potential façade solutions: an absorbing one, a solution which would only reflect the sound upward at the two lowest floors, and the already mentioned sound upward reflecting solution. The reason to add the solution which would only reflect sound upward at the lowest floors was that it performed better than the solution which is just absorbing sound while the impact on the diffracted sound was lower than the more reflecting solution. Based on the conducted measurements and simulations three façade designs are made. The first one is a façade that focusses on applying as much absorption as possible. For this design absorbing materials are selected which are sufficiently impact and weather resistant so they can be applied at the building façade. A porous concrete-like solution is applied at the lowest two floors and the tower which houses the circulation area of the building, while a perforated sound absorber is being used as sound absorbing parapet on the balconies. The second design focusses on reflecting the sound upward and therefore uses mostly hard building materials like glass. At the lowest eight floors a certain angle is introduced in such a way that the sound will be reflected over the nearby buildings, away from the courtyard. To achieve this the balconies need to be extended and closed off with a glass structure. Finally the third design combines the absorbing and the upward reflecting solutions. At the lowest two floors the sound will be reflected upward and at the higher floors the perforated sound absorbing parapet solutions are used to absorb the sound. To determine the impact of the three different façade designs on the sound levels inside the courtyard and on the sound which is being diffracted over the nearby buildings final scale model measurements are conducted. Furthermore the three designs are also rated looking at their architectural impact, complexity, costs and reusability. Using this data the design combining the sound absorption and the sound reflection is selected as the best performing solution, taking the defined context into account. While it improves the sound levels less than the upward reflecting design it only has a minor influence on the diffracted sound. Furthermore this design is still relatively cheap and easily constructed when being compared to the completely reflecting design. It’s expected that this design will on average improve the sound levels inside the investigated courtyard area 5 dB(A) with local improvements up to even 10 dB(A).

acoustics
sound reflections
façades
highrise
environmental noise
urban context

http://resolver.tudelft.nl/uuid:575fc4f9-8fb3-4633-b000-01e332d883dd

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Student theses

master thesis

(c) 2016 Niesten, J.