Fredrik Ljunggren

Development of a new damper to reduce resonant vibrations in lightweight steel joist floors

Floor vibrations annoying to humans often occur in lightweight constructions. A number of methods to solve the problem of resonant vibrations are reported in the literature. Tuned mass damper, semi ...

Variations in Sound Insulation in Nominally Identical Prefabricated Lightweight Timber Constructions

Variations in sound insulation necessitate higher safety margins to the legal requirements, which results in higher production costs. Increased knowledge about variations leads to lowered costs and better sound quality. In-situ measurements of 30 nominally identical apartments of a lightweight timber construction were performed, to assess and quantify the variations in airborne sound reduction and impact sound pressure level. The construction is an industrially prefabricated system of complete volumes. Different sound insulation was found between floor numbers as the apartments on the highest floor achieved significantly better sound insulation. This difference was assumed to be due to the extra weight on lower floors affecting the elastic connections used to structurally connect the apartments. The variation between apartments on the same floor was therefore evaluated using the Root Mean Square Error, resulting in a standard deviation of 0,9 dB and 1,4 dB for the airborne and impact sound insulation, respectively. The measurement variance was subtracted from the total variance. The remaining, unexplainable, variation of 0,8 dB in airborne sound insulation can be attributed to workmanship.

Dynamic and subjective analysis of a lightweight/semi-heavyweight floor in a laboratory

A lightweight steel framed floor was tested in a laboratory for eight configurations: continuous/lumped main support, support/unsupported free long sides and concrete/plasterboard as top. The dynamic properties were revealed by modal analysis and subjective evaluations in terms of body perception and perception of vibrating articles. The results showed that the tested parameter that affects the floor most was the top layer. The semi-heavy weight floor with concrete layer was in general judged to be of higher acceptance than the lightweight floor having plaster-boards on the top. Additionally, the lightweight floor was used in a series of experiments where the effect of modified support and the effect of adding line loads of various degree of mechanical connection were tested. This part aims to simulate how the dynamics of a floor stepwise might change going from a bare floor to a floor installed inside a real building. The results show that the line load, mass and stiffness influence the floor in terms of changed natural frequencies. Depending on where on the floor the loads are located, different modes were affected and, depending on how the loads were connected to the floor, the gained stiffness varied slightly.

Uncertainty of in situ low frequency reverberation time measurements from 20 Hz - An empirical study

Measuring reverberation time is normally one of the steps within the procedure of determining sound insulation in dwellings where 100 or 50 Hz usually serves as the lower frequency limit. However, ...

Elastic Layers to Reduce Sound Transmission in Lightweight Buildings

To obtain satisfactory sound insulation is a challenging task when designing lightweight buildings. Poor performance at low frequencies as well as severe flanking transmission has traditionally often been more pronounced compared to heavier constructions. In the present casestudy based paper, various aspects of using elastic layers to improve sound insulation in lightweight buildings are considered. The effect on impact and airborne sound insulation by using two different kinds of vibration insulators between floor plans was examined together with the effect of using glues of various degree of elasticity in the construction. In situ measurements were performed inside a four-storey wooden frame based residential building and statistically significant variations in sound insulation were found. The efficiency of the two vibration insulators was further evaluated by vibration reduction measurements over the junctions. The difference in vibration reduction was found to be nearly constant in the frequency range 50–1000 Hz while the improvement of impact sound insulation increased by frequency. A long term test of elastic glues was also conducted, during three years, for stability over time. The best glues preserved a significantly higher damping ratio over time compared to the main part of the glues.

Variations in sound insulation from 20 Hz in lightweight dwellings

The objective of this study is to quantify the total variations in impact and airborne sound insulation from 20 and 50 Hz, respectively, within a wood-based building system. Field measurements in 70 rooms have been statistically analyzed, in weighted metrics and in third octave bands. Since 1994, the Swedish building code permits wood-based high-rise multifamily dwellings. However, the low frequency sound insulation soon turned out to be insufficient, and complaints were more common in lightweight buildings compared to traditional heavy constructions. Therefore, in 1999, the extended frequency range 50-3150 Hz was introduced in the requirements, an action that mitigated the problem, but only to some extent. Consequently, the lightweight industry often aims one sound class (4 dB) higher than the minimum requirements to obtain satisfactory performance. The Swedish research program Aku20 (2014-2017) showed that the correlation between measured and perceived impact sound insulation in lightweight constructions was significantly improved, when even lower frequencies (20-50 Hz) were included. In this context, the variations of low frequency field measurements and corresponding safety margins need to be investigated.

An Improved Prediction Model for the Impact Sound Level of Lightweight Floors: Introducing Decoupled Floor-Ceiling and Beam-Plate Moment

To better understand the complex acoustic behaviour of lightweight building structures both experimental and theoretical approaches are necessary. Within the theoretical approach developing theoretical models is of great importance. The aim here is to further develop an existing method to predict the impact sound pressure level in a receiving room for a coupled floor structure where floor and ceiling are rigidly connected by beams. A theoretical model for predicting the impact sound level for a decoupled floor structure, which has no rigid mechanical connections between the floor and the ceiling, is developed. An analytical method has been implemented, where a spatial Fourier transform method as well as the Poissons sum formula is applied to model transformed plate displacements. Radiated sound power was calculated from these displacements and normalized sound pressure levels were calculated in one-third octave frequency bands. The predicted results from the model are compared with the results from the experiments on the decoupled floor-ceiling construction. The results gave agreements in line with comparisons regarding previous model. The effect of introducing beam-plate moment in the model is studied and is found to be dependent on frequency, showing significant improvement in predicting impact sound level at high frequency region.

A new technique to reduce vibration in lightweight floors

Lightweight floor constructions often suffer from annoying vibration. Long span floor are especially vulnerable with dynamic properties that yield both springiness due to low stiffness and resonant vibration due to poor damping. A new method, that treats resonant vibration, applicable to floors with resilient ceiling, is presented in this study. By using a piece of strategically located visco-elastic material connected to the ceiling joists, which is designed using a finite element model for an optimal position and size, it is demonstrated that the damping of the floor is increased. The results show that the damper is extremely efficient to reduce vibration of the type where floor and ceiling are oscillating out of phase to each other. The device also affects in phase modes like the fundamental one, but to a lower extent.