Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Development of a compact test rig for impact sound in wood engineered products 

In order to determine the acoustic properties of wooden floor bases, the entire construction must be measured in a floor test rig without detours (DIN EN ESO 10140) or on-site (DIN EN ISO 140-7). For such measuring in a floor test rig, floors with an area of 20m² must be constructed. This involves an extensive amount of outlay, which makes it very difficult for small and medium-sized companies to fulfil innovative ideas.   

The Physikalisch Technische Bundesanstalt (German National Metrology Institute, PTB) has developed, in co-operation with the WKI, a compact test rig for lightweight floor bases. The aim of the compact method is a reduction of approximately 1/10 in the outlay required to determine the sound impact parameters, compared to the current standardised test methods.   

The approach taken within the project allows the inclusion of single components from the entire functional chain. This enables the determination of the radiation factor for wooden floor bases produced using standard construction methods; the results can then be stored in a database. When the radiation factor of a wooden floor base is known, the impact sound pressure level can be determined by measuring the sound particle velocity using a scale model. It can be expected that further potential ideas will be examined regarding their effectiveness, so that the results of this research project will primarily lead to a considerable surge of innovation. .

© Physikalisch-Technische Bundesanstalt Braunschweig
Figure 1: Grey: Measured entry mobility values of a laminate floor covering with underlay. Blue: Mobility of a mass. Red: Mobility of a spring. Black: Resultant mobility of a series connection of mass and spring.
© Physikalisch-Technische Bundesanstalt Braunschweig
Figure 2: Schematic diagram of the compact test rig.
© Physikalisch-Technische Bundesanstalt Braunschweig
Figure 3: Single values for 52 softly-sprung coverings determined on the compact test rig for lightweight floor bases compared to the corresponding data determined on the floor test rig.

The investigation began by modelling the major processes involved with measurement in the standardised test setup. These include, in particular, the excitation of a floor base construction using a hammering system, the diffusion of energy through the floor base as well as the sound radiation from the floor base. For the overall process, observations are made both with and without floor coverings, in order to provide insight into the various effects of the floor coverings. Amongst other tools, operational vibration analysis is used here.

Based on these findings, a compact method for the determination of impact sound reduction for lightweight covered floors can be developed. For this, it is extremely important that the receiving structure is reproducible in such as way as to enable standardisation. Furthermore, tests are carried out in order to establish whether the improvement values from board-shaped floor coverings on scaled-down constructions can be determined and which physical limitations exist for this.

The developed compact method for the determination of impact sound reduction is tested on real floor coverings and the results are compared to those achieved through standardised measuring procedures. During these measurements, diverse floor coverings are used in order to vary the impact sound reduction to such an extent that possible method limitations become evident. A draft standard is produced and incorporated into the standardisation process.    

In addition, investigations are made concerning the feasibility of determining the laboratory value of the impact sound pressure level of wooden floor bases using a scale model. The output emitted by the scale model is calculated using the velocity of the lower surface and the radiation factor of the structurally identical large floor. The velocity can be deduced from measurements taken on a reduced scale, provided the basic characteristics of the original construction are correctly reproduced in the compact test construction. For these models, clear project rules are required. Very little is currently known concerning the radiation factor of timber joist floors. Therefore, a databank for the radiation factors of timber joist floors will be compiled during the course of the project. For this purpose, intensity measurement technology and a laser scanning vibrometer are used, in order to determine the radiation factors of wooden floor bases in real buildings. As the impact sound pressure levels of the real buildings are also determined, the difference between the sound pressure levels and the sound intensity-based sound levels in the receiving area enables conclusions to be drawn concerning flank transfer.

The use of vibration analysis on laminate floor coverings on the compact test rig for solid floors enabled the implementation of models which depict the vibration behaviour of the coverings. With the help of electromechanical analogies, the laminate coverings being tested can be modelled as a mass-spring system. Such a system is characterised by a typical resonance characteristic in its mobility. The mobility depicts the relationship of the applied force and the resulting vibration velocity at the discharge point. Figure 1 shows ten entry mobility values measured on laminate. The mobility of an ideal spring is depicted in red whilst that of an ideal mass is shown in blue. A series connection of these elements, in combination with a resistance term, results in the mobility progression shown in black, which clearly reflects the vibration behaviour of the tested laminate coverings.

© Physikalisch-Technische Bundesanstalt Braunschweig
Figure 4: Degrees of radiation for various hammering system positions and excitation types, determined through measurements on a wooden beam floor.
© Physikalisch-Technische Bundesanstalt Braunschweig
Figure 5: Standardised impact sound level determined on test rig (black) and determined on a floor section using the previously determined degree of radiation at various hammering system positions on the large floor.
© Fraunhofer WKI
Figure 6: Schematic diagram of variations of conventional wooden beam floors.

The developed compact test rig for lightweight floor bases has a base area of approximately 1.2m x 0.8m and is mounted on a framework construction (Fig. 2).

The impact sound reduction of 52 softly-sprung coverings was determined on the compact test rig and the floor base test rig. In Figure 3, the resulting single values for the measurements taken on the floor base test rig are shown on the x-axis, and the measurements taken on the compact test rig are shown on the y-axis. For single values in excess of 7dB resulting from measurements taken on carpets, deviations arise which exceed the critical difference. This can be put down to the transient properties of the carpet, which are altered by the hammer blows. In Figure 4, the radiation factors determined on the PTB floor base test rig are shown. As mentioned earlier, these were obtained using a laser vibrometer and intensity measurement technology. The results obtained on various points of the floor base are similar for both acoustic excitation and excitation using a hammering system. On the basis of these measurements, the impact sound pressure level of a floor base section can be determined (see Figure 5). The black line shows the impact sound pressure level determined on the large test rig, and the blue, red and green lines show the impact sound pressure level measured on the floor base section. Despite increasing differences for the high frequencies, a good conformity can be observed for the single values. The determination of the impact sound pressure level for the floor base section would therefore appear to be possible. 

Using help obtained through the experience of the project partner, literature research and a market study, a classification of timber joist floors was undertaken, providing information on the frequency distribution of the variants. This showed that despite regional differences, wooden beam floors - as shown in Fig. 6 - are by far the most common building method. In properties such as multi-storey residential buildings and office buildings, planar floor bases made from plywood boards or wood-concrete composite are more common than in detached residential houses. Measurements on existing buildings are currently being carried out.

Execution

Physikalisch-Technische Bundesanstalt Braunschweig (National Metrology Institute)

J.-H. Schmidt, V. Wittstock, W. Scholl

 

Funding

Bundesministerium für Wirtschaft und Technologie (Federal Ministry of Economics and Technology, BMWi) via the Arbeitsgemeinschaften industrieller Forschungsvereinigungen (Federation of Industrial Research Associations, AiF), represented by the Internationaler Verein für Technische Holzfragen e.V.  (International Association for Technical Issues Related to Wood, iVTH)