Building Acoustics en miniature

Problems in building acoustics can be investigated excellently on small, downscaled models. They are more flexible, require less time and effort and are cheaper than the usual measurements carried out in large and standardized test facilities, and they are also more reliable and realistic than most computer simulations. A downscaled model of a building-acoustics wall test facility has been designed and validated according to the theory of similarity.

Usually, building-acoustical measurements in test facilities require much time and effort. The test objects (typically a heavy wall, a concrete ceiling or a large lightweight construction) have to be set up prior to the measurement and be demolished and disposed of afterwards to make room for the next test specimen.

Measurements on downscaled models have some advantages over measurements in full-scale test facilities: The costs of material and disposal are considerably reduced, it is not necessary to make much space for storage and erection available, and alterations of the experimental set-up are often accomplished very easily, with only a few steps.

To be able to translate to full scale the results obtained from model measurements, some scaling rules have, however, to be taken into account. For problems concerning room acoustics only - i.e. cases in which only airborne sound fields are involved - these rules are relatively simple: the measuring frequencies increase to the same degree to which the lengths are reduced. In building acoustics, besides airborne sound fields, also structure-borne sound fields come into play. A detailed study of the scaling of fundamental quantities in building acoustics leads to a number of additional scaling rules which concern, above all, the properties of the model material to be selected. It turns out, for example, that thanks to its material properties, acrylic glass is able to simulate quite well building materials that are typically used for heavy walls.

Based on these scaling studies, a midget version of a building-acoustics wall-test facility has been designed to the scaling factor 1:10, which meets the requirements for a standardised test facility of ISO 140-1. Solely the flanking transmission has - intentionally - not been suppressed to make a better studying of the damping and transmission effects possible. In the model test facility, the sound reduction index of acrylic plates of different thicknesses was measured in different mounting situations according to the standard. It could be demonstrated that the sound reduction index determined in the model on acrylic plates corresponds well with the damping of a comparable massive wall which was measured in a "real" wall-test facility. In particular, such typical effects as coincidence, thickness resonance and the behaviour of elastic mountings could be emulated very well both qualitatively and quantitatively (Figure 1).

The model cannot replace a real test facility, as certified measurements can only be carried out on a full scale. But a model is very well suited as a tool for fundamental research. As in all scientific disciplines, also in the field of building acoustics, computer-aided models are used for solving problems. However, in building acoustics, the different approaches such as FEM, BEM and SEA usually quickly reach the limits of the computing performance and/or validity. The translation of fundamental problems to downscaled models which were designed in compliance with the requirements of building-acoustical scaling rules is - if done with the required accuracy - a practical, cheap and reliable alternative to computer simulations and full-scale measurements.

Figure 1: Comparison between a) the sound reduction index of a sand-lime brick wall of 24 cm in thickness that was measured in full scale, and b) the sound reduction index of an acrylic glass wall of 25 mm in thickness that was measured on 1:10 model scale; both with rigid and elastic mounting.

Contact person:

Christoph Kling, FB 1.7, AG 1.71, Christoph.Kling@PTB.de