Acoustically effective façades

Abstract

The silhouettes of the great European metropolises are characterised by a high density of high-rise facades made of glass, metal or stone. On one hand this density stands for economic power and good employment values. On the other hand, the large sound-reflecting surfaces in the cities are responsible for an increase in sound pollution in their vicinity. They cause the most frequent inner-city sound source, traffic noise, to double in perceived intensity. Direct sound and reflected sound add up in the urban environment. This effect of sound level increase through reflection has been subject of acoustic research for some time. In architecture, however, the topic of noise reflection has been completely neglected. But it is exactly these concentration processes in the continuously growing metropolises that make architects face problems that they cannot solve with their own tools alone. For example, placing bedrooms in the quiet part of a building is only possible if there are quiet areas. But highly concentrated areas as a result of creating additional living space are often surrounded by 4 or more sound sources. Hereby, the noise originating from air traffic further aggravates the situation because this type of noise generally impacts the urban space from above. These developments require us to discuss the topic of the reflective properties of facades within the disciplines acoustics and architecture. The fundamental question was: Is it possible to develop a design strategy that can be employed by architecture firms, and that enables them to develop facades for a quieter city in challenging urban situations? To answer this question, both disciplines and their respective tools were studied. In the scope of a literature research, the results of acoustic research related to the topic of reflection on urban surfaces were summarised and examined. The architectural relevance of the results of the individual researches was determined by use of the examined parameters. Several case studies were conducted to record existing acoustic urban spaces with acoustic measuring methods. The on-site measurements were used to examine the possibility of generating architecturally relevant measurement values for the effect of facades in dependence of locally present sound sources. The laboratory method of scaled acoustic measurement was not only examined to determine the transferability of the results from the on-site measurements to the model scale, but also the possibility of applying them in an architectural design process. Studies were executed on the design planning scales of 1:50 and 1:100. Design studies were used to develop proposals for acoustically effective surfaces on a detailed scale as well. Another question was how and based on which parameters surfaces can be designed, and how they can be determined in terms of measurement technology. The essence of the here conducted studies around the topic of implementing an intended acoustical effect of a façade construction is the development of a procedural guideline for future projects. The results in short: – The disciplines architecture and acoustics must develop new paths to be able to execute acoustically driven building projects in a mutually fertile collaboration. Building projects must be thought acoustically and architecturally by both parties. – This requires a fundamental paradigm change in building acoustics. The transmission path of the sound energy must be considered from the exterior space to the exterior space and not, as until now, from the exterior space to the interior space. – In order to plan the acoustic effect of a façade it is necessary to define a targeted acoustic quality for specific uses at specific locations in the exterior space. – The effect of the noise sources defining the building site can only be determined in dependence of the position of the planned use. – The acoustic values as the basis for acoustic façade planning can only be reliably determined with the help of a multi-channel measurement on-site. – The impact of a façade area can only be described in a meaningful manner with individual sound level values per frequency band, in dependence of a specific planning-relevant angle of reflection. – All known and available façade elements can be used as reflection changing elements to reduce the sound impact if they lie at a certain angle to the sound source and the planned exterior use. – Applying suitable dimensioning of building elements such as lamellas or sills, for example, allows specifying a certain frequency range of the reflection change. – The frequency range of the greatest reflection changing impact must be attuned to the sensitive range of the human ear to achieve the longest possible effective reflection change with an acoustically effective façade. – An acoustic façade design can only be applied successfully if, at the same time it fulfils all other basic requirements of a façade construction such as tightness, transparency and light. Only then is it possible that the acoustic input leads to new, exciting façade constructions.