Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
PROCEEDINGS of the 22nd International Congress on Acoustics
Calculation models for timber structures (Silent Timber
Build): Paper ICA2016-271
Modelling various floor and wall assemblies and
comparisons to measured values
Delphine Bard (a),*, Klas Hagberg (b), Tobias Augustsson (c)
(a)
Lund Univeristy, Engineering Acoustics, Sweden, delphine.bard@construction.lth.se
(b)
WSP Acoustics, Sweden, klas.hagberg@wspgroup.se
(c)
Chalmers University, Sweden, augustsson.tobias@gmail.com
Abstract
In the project Silent Timber Build a series of calculations have been made in order to find out
modelling difficulties using the software SEAWood, which is partly developed and refined in the
project. The calculations were carried out in order to achieve a better understanding of which
parts of the modelling that is most critical in order to arrive in a satisfactory prediction compared
to expected subjective evaluation, hence in order to take measures in the further development
of the prediction tool to arrive in better accuracy of the calculations. They are also made in order
to find out which building parts that are of less importance or even might be neglected in the
assemblies and finally which building parts that are of certain importance in terms of material
characteristics and similar, in order to further improve the optimization of wooden floor and wall
assemblies. The predicted floor and wall assemblies are typical from Europe (Scandinavia and
mid Europe), for which correct and verified laboratory measurements are available in order to
perform reliable comparisons and adequate conclusions. The results from the comparisons
(predictions vs measurements) show that additional work is needed in order to 1. Refine the
model in low frequencies; 2. Focus on improved material characteristics for intermediate layers
and also to avoid some of them; 3. Group the floor and wall assemblies due to their prediction
ability.
Keywords: SEAWood, Airborne sound, prediction, Impact sound
22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
MODELLING VARIOUS FLOOR AND WALL
ASSEMBLIES AND COMPARISONS TO MEASURED
VALUES
1 Introduction
In the project Silent Timber Build an SEA software called SEAWOOD [1] is refined and further
developed in order to facilitate prediction of any wooden construction currently used in Europe.
It might also be used in order to optimize future constructions in a wise manner based on
modern requirements for wooden constructions. In the master thesis work presented here, the
software has been used with rather small preparations and a number of randomly selected floor
assemblies have been modelled. All assemblies have been tested in an accredited laboratory in
order to compare the results. The calculations are primarily focused on airborne sound
insulation so far. The impact sound is also considered. However, the model is not yet fully
applicable due to shortcomings in the impact sound source model.
2 Method
The software SEAWOOD [1], which has been further developed within the research project
“Silent Timber Build”, is a specific version of SEA+ provided by the French company InterAC
(www.interac.fr). InterAC is research partner in Silent Timber Build. The software developed in
the project has restricted mathematical libraries limited to plane elements and is dedicated to
the wood building industry, to facilitate prediction of any wooden structure. SEAWOOD creates
SEA models out of Finite Element Models (Nastran preferably) using the SEA Virtual module.
Any Virtual SEA model can be coupled to any other analytical SEAWOOD subsystem reducing
analysis time in both SEA and FEM while improving accuracy.
Simulations have been carried out both for airborne sound insulation and for impact sound
levels. The simulations presented here do not currently make use of the virtual SEA module but
instead uses traditional SEA. The most critical part for wooden constructions is impact sound
levels, however still some work remains to model the source properly [2]. By measuring the
impact sound machine and importing a force time history signals to the software, a more
accurate prediction of the impact sound levels could be made.
The following requirements are used in the comparison:
·
Rw
·
Rw+C50-3150
·
Ln,w
·
Ln,w+CI,50-2500
All measures are described in [3] and [4].
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
3 Results
3.1
Airborne sound insulation
Five different floor assemblies have been modelled for airborne sound insulation and then
compared with measured results. Two of them have also been modelled and compared to
impact sound levels. The measurements were carried out according to ISO 10140-1 in various
accredited laboratories in Europe. Both bare structural CLT elements and floor assemblies
comprising several layers in addition to the structural element have been calculated and
measured. In figure 1, one bare CLT is modelled and the results are convincing. The element is
a CLT 140 L5s. The various values are according to the following:
Rw = 36 dB and Rw+C50-3150 = 35 dB (measured)
Rw = 36 dB and Rw+C50-3150 = 35 dB (calculated value using uniform, see below)
For modelling the CLT plates, three different approaches are used. The simplest way was to
model the CLT plate a solid plate called uniform section. In reality the CLT plates are made up
of laminated timber. In SEAWOOD this can be represented by either a static or dynamic
laminate. The static laminate section considers an orthotropic plate where each layer
contributes with their static inertia to calculate the material properties for the whole section. For
the dynamic laminate the different layers are cross coupled to each other. Depending on the
material properties involved the layers becomes uncoupled with the increase in frequency.
Figure 1: Modelling of airborne sound insulation of CLT 140 L5s
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
By adding a floating floor on top of the CLT element, comprising 60 mm Weber cement screed
on a 20 mm impact sound isolating board (ISOVER TDPS 20) and then modelling this with two
subsystems gives a good agreement to the measured value. In both cases the Rw becomes 49
dB and Rw+C50-3150 becomes 47 dB.
Figure 2: Modelling of airborne sound insulation for a floor assembly comprising CLT 140 L5s
Isover TDPS 20 impact sound plate and 60 mm Weber floor. Blue dotted curve equals measured
curve and red curve is predicted. Rw becomes 49 dB in both cases.
Additionally, a 320 mm CLT was modelled and the result is shown in figure 3. When plates get
thicker their behaviour change and they are not a plate anymore. They act more like a “block”. It
seems that this transition appear at the thickness from 220 mm. Therefore the same theory
cannot apply and that might also be the reason for bad conformity between the measured 1/3
octave band values and the predicted 1/3 octave band values. However, the single numbers
corresponds well to each other and exhibit the following results:
Rw = 45 dB and Rw+C50-3150 = 43 (measured)
Rw = 46 dB and Rw+C50-3150 = 44 (calculated value using uniform, see below)
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
Figure 3: Modelling of airborne sound insulation of CLT 320 L5s
Another attempt to model a more complex system comprising two separated layers of CLT of different
thicknesses (exact layup is not presented due to secrecy) and mineral wool in the cavity. In one of the
two cases four layers of 12,5 mm gypsum boards are added on top (total thickness 50 mm). The
measured values and the predicted values are less consistent in these cases. In high frequencies there
are big deviations probably due to laboratory issues, see figure 4. Corresponding adaptation terms for
the different cases are
1. with Gypsum boards
o C50-3150= -5 dB (measured)
o C50-3150= -3 dB (SEAWood)
2. without Gypsum boards
o C50-3150= -4 dB (measured)
o C50-3150= -2 dB (SEAWood)
Figure 4: Simulation of two different floor assemblies comprising two structural elements divided
with no connection at all, however mineral wool in the cavity. Case one is pure CLT and case two
four layers of gypsum boards are added.
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
3.2
Impact sound levels
Two simulations have been made for impact sound levels on bare CLT structural elements. The
single number show good agreement, see table 1, however the third octaves do not fit perfectly,
see figure 4. For CLT 140 the calculated values are generally higher than the measured,
especially in the low frequency region and for the CLT 320 it is the opposite.
Table 1: Weighted normalized impact sound levels for CLT 140 and 320
Structural element
Ln,w / Ln,w+CI,50-2500
Calculated
Measured
CLT 140 Ls
90 / ..
86 /..
CLT 320 Ls
80 /..
80 /..
The model for impact sound source is still under development and improvements are expected
in the near future. Current modelling is carried out by using a force time history signal from the
impact sound machine placed on a concrete floor. The signal was then averaged and modelled
as a point force in the software.
Figure 5: Calculated and measured values for bare floor structural element. Red curves
correspond to predicted values and blue curves are measured values. Left diagram: CLT 140 Ls;
Right diagram: CLT 320 Ls.
4 Conclusions
Wooden structures might be complex and their behaviour can vary. So far only few structures
have been modelled in this master thesis project. However, in the project Silent Timber Build a
lot more modelling and comparisons have been carried out. The modelled floor assemblies are
structural homogeneous CLT elements of different thicknesses and in some cases with
additional layers to improve the sound insulation. However if the material characteristics are
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
known it should be possible to create accurate models for any other structural elements with
different lay ups.
The modelling is rather convincing for the airborne sound insulation so far, all the way down to
50 Hz. However, still there are a number of assumptions that have to be carried out which is not
very easy. For going further in prediction we have to experimentally characterize both panel loss
factor (DLF) and their related mechanical coupling loss factor (CLF).
For impact sound level, additional information regarding the tapping machine is also needed
(apart from what is described above), to create and reliable predictions in the full frequency
range.
Acknowledgments
The authors would like to acknowledge all R&D partners of Silent Timber Build project
representing the following countries: Sweden, France, Germany, Austria, Norway and
Switzerland.
References
[1] Borello, G. “SEAWood”, Software for calculating floor and wall assemblies for wooden structures.
www.interac.fr, 2016.
[2] Borello, G. Force identification of the shock machine – Test Campaigns in FCBA, Internal Report for
the Silent Timber Build Project, 2015.
[3] ISO, International Standard ISO 717-1: Acoustics - Rating of sound insulation in buildings and of
building elements - Part 1; Airborne sound insulation. 2013.
[4]
ISO, International Standard ISO 717-2: Acoustics - Rating of sound insulation in buildings and of
building elements - Part 1; Impact sound insulation. 2013.
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