PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Hybrid joist floor constructions.
Evaluation of measurement results.
Anders HOMB1
1SINTEF Building & Infrastructure, NorwayABSTRACT

Lightweight building systems in general suffer from limited sound insulation, especially at low
frequencies. Furthermore, theoretical models have severe limitations regarding prediction ofthe impact sound insulation, and the design is to a high extent based upon experience andmeasurements. Recommendations in Norway includes the frequency range down to 50 Hz,
which means to include the spectrum adaptation term CI,50-2500 and C50-5000. One possible
solution to improve the properties at low frequencies is to add mass and increase the stiffness ofthe construction. We refer to lightweight building systems combining wood and cement-basedmaterials as hybrid constructions.
The paper will present analysis of relevant measurement results of hybrid wood joist floor
constructions from laboratory and field objects. The paper will discuss possible improvementsof such floor constructions. From the "Silent Timber Build project", some relevant laboratory
measurement data exist. Due to the impact sound insulation focus, some apartments with hybrid
joist floors have been built and data collected. This paper is a part of a project at SINTEF
Building & Infrastructure aiming to develop robust solutions also involving HVAC components
inside the partition structure. Further progress in this project will include new laboratory
measurements and a fire test.
Keywords: Impact sound insulation, floor, hybrid joist constructions

1. INTRODUCTION
Lightweight building systems generally suffer from limited sound insulation properties, especiallyin the low frequency range. A number of research studies and investigations confirm the lack ofcorrelation between perceived impact sound insulation and standard measurement objective L'n,w forlightweight systems, see [1] and [2]. The necessity of including low frequency evaluation in theobjectives and national requirements is conspicuously. The acceptability of lightweight wood frame
residential buildings is at risk if low frequencies aspects neither are formally integrated in building
codes with more specific acoustic requirements, nor made largely available in building design guides.
The lack of requirement seems also to prevent a more rapid development of lightweight solutions
including low frequency terms.
The process of developing new solutions could be more rapid up if reliable calculation tools wasavailable. Some development of mathematical models and numerical simulations predicting theimpact sound pressure level of the floor element is published, see for instance [3], but due to thecomplexity of such solutions, the development still need to be based upon experience andmeasurements more than "pure" mathematical models. In addition, the lack of tools to calculate thecontribution from flanking transmission is even more limited. Therefore, the development of new or
improved solutions need to include both theoretical investigations, measurements in the laboratory
and field- or prototype testing.
1

Anders.homb@sintef.no5013

The paper is focusing on the impact sound insulation and the low frequency properties in special.
When the final solution include some floor covering or parquet layer on an interlayer, the single
number rating will always be determined by frequencies below 400 Hz. Therefore, the measurement
spectra presented in this paper limits the frequency range upwards to 500 Hz.

2. OBJECTS
Traditional lightweight wood frame based buildings involves generally rigidly connected elements
for floors and walls except the ceiling and the top floor solutions. In Norway, the ceiling is normallyelastic suspended to the joists, more seldom completely separated. Concerning the floor covering,
floating floor solution (rigid topping on a resilient layer) are also commonly implemented. Over thepast years, many research projects and studies in different countries has been conducted; quite a large
number of laboratory measurements are now available, see for instance [4] or [5]. Results covering
hybrid joist floor solutions with resilient or independent ceiling solutions is more seldom. Thepresentation in this paper is therefore a first step on our work on this specific item.
Relevant alternatives for the top floor solution we divide into two main groups:- added Mass Below the Resilient layer, named MBR- added Mass Above the Resilient layer, named MAR
Figure 1 shows a sketch of the two principal solutions of the top floor. For the MBR solution,lightweight materials will be used above the resilient layer. For the MAR solution, we normally havesome additional floor covering or parquet product. In addition, we divide the resilient layer into twogroups, for simplicity:-Product with dynamic stiffness below 15 MN/m3, coded LDS
Products with dynamic stiffness above 30 MN/m3 , coded HDS

Figure 1 – Sketch of the MBR (left) and MAR (right) top floor solution
So far, the prioritizing in the project limits the added mass solutions to concrete products or somekind of wet screed. In this presentation, the concrete or wet screed have been installed without(stiffening) connectors between the mass and the wooden floor below.

3. LABORATORY MEASUREMENTS
In the following, results from a number of laboratory measurements of hybrid wood joist floor
solutions will be presented. Some of the measurement were performed in the sound transmission
laboratory at SINTEF Building and Infrastructure in Oslo. Other measurements have been performed
at other European laboratories, collected within the Silent Timber Build project and published, see [4].
In all cases, the measurements have been conducted in accordance with NS-EN ISO 10140, part 3, see[6] and evaluated according to NS-EN ISO 717-1, see [7]. So far, there are a limited number of hybrid
floor solutions including resilient suspended ceiling available for our study. Among available results,
some of the most interesting data are presented in the following. The goal of this presentation is toquantify the effect of different hybrid floor solutions and secondly to present data relevant forverification of calculation tools. Table 1 shows an overview of relevant laboratory measurement
objects, and figure 2 shows the frequency spectrum of the normalized, impact sound pressure level.

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Table 1 – Overview, laboratory measurement objects
Main group

Resilient layer

Total mass pr. unit area, kg/m2Ln,w + CI,50-2500, dB

MAR

LDS

13554

MAR

LDS

166

55

MBRHDS

18046 1)

MBR

HDS

187

52

MBR

LDS

192

52

MAR
LDS
2241) High transverse stiffness of the joist system, see [4] or [8]51Normalized, impact sound pressure level, Ln (dB)80

70
NO: Ln,w = 42 + CI,50= +4(mpua=180), from ref. [8]60

VTT: Ln,w=44+CI,50 = +7(mpua=224), from ref.[4]DE: Ln,w=47 + CI,50=+8(mpua=166), from ref.[4]50LAB-A: Ln,w=50 + CI,50=+4(mpua=135)

40

LAB-B: Ln,w=46 + CI,50=+6(mpua=187)
LAB-C: Ln,w=43 + CI,50=+9(mpua=192)30

201050 63 80 100 125 160 200 250 315 400 500
Frequency (Hz)Figure 2 – Normalized impact sound level of laboratory measurement objects
Results presented in figure 2 shows a relatively large spreading, both with respect to single number
rating and the normalized impact sound level in the frequency range. Not unexpected, the two MBR
objects shows relatively low spectrum adaptation term, C I,50-2500 due to high dynamic stiffness of theresilient layer. For the MAR objects it is in general the opposite. The NO object differs significantly
from the other, probably due to the increased transverse stiffening of the joist floor.
A comment to the collection of results is that we have not data available for the combination ofMAR and HDS resilient products and solutions combining MBR and LDS resilient products.

4. FIELD MEASUREMENTS
Despite the fact that hybrid joist floor solution is relatively seldom, a number of field measurements has been performed the last years. The purpose has been to study the effect of additional massfor improving the impact sound insulation due to arguments presented in chapter 1. Collected data
include a number of relevant solutions and results. But collected data also include solutions not5015

relevant for further use and solutions without significant effect on the impact sound insulation
properties. Such data is not a part of the presentation below. All measurements carried out by SINTEF
Building & Infrastructure have been performed according to NS-EN ISO 16283 [9], from 2017 also
including low frequency procedure of small receiving room volumes, Some measurements have been
performed by consultancy companies, reported according to [9], but without the low frequency
procedure given for small receiving room volumes. Therefore, small receiving room volumes havebeen excluded in this paper.
Table 2 shows an overview of relevant objects, and figure 3 shows the frequency spectrum of thenormalized, impact sound pressure level. All of them with large receiving room volumes. Two
examples are from specific, field measurements, while the two others are from averaging of numbersof results from almost equal solutions.
Table 2 – Overview, field measurement objects
Main group

Resilient layer

Total mass pr. unit area, kg/m2Ln,w + CI,50-2500, dB

MAR

LDS

109-124

55

MARLDS

124

53

MAR

HDS

125-140

57

MARHDS

148

59

Normalized, impact sound pressure level, L'n (dB)

80

70

60

50

40
FELT, AVG n=8: L'n,w=50 + CI,50=+5(mpua=109-124)
FELT, AVG n=4: L'n,w=53 + CI,50=+4(mpua=125-140)
FELT-A: L'n,w=49 + CI,50=+4(mpua=124)
FELT-B: L'n,w=54 + CI,50=+5(mpua=148)30201050 63 80 100 125 160 200 250 315 400 500
Frequency (Hz)Figure 3– Normalized impact sound level of field measurement objects
Results presented in figure 3 shows a 6 dB spreading of the single number rating, L'n,w + C I,50-2500,
and relatively large spreading of the normalized impact sound level in the frequency range. All results
show spectrum adaptation term, C I,50-2500 in the range 4 to 5 dB, positively influenced by large
receiving room volumes. All constructions fulfill national minimum requirements (sound class C) ofL'n,w  53 dB. Except one construction, the example object don't fulfil recommended limit ofL'n,w+C I,50-2500  53 dB or the stricter requirements for sound class B according to present Norwegian
requirements in NS 8175:2012.
A comment to the collection of results, is that we have not field measurement results available forthe combination of MBR independent of the type of resilient layer.

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5. DELTA ACOUSTIC CALCULATIONS
Predicting the impact sound insulation improvement by a floating floor is not an easy task. It isnecessary to take into account both forced and resonant transmission. The resonant transmission will
depend on the boundary conditions for the floating as well as for the primary floor. Theoretical
calculation of the impact sound insulation improvements has been studied by several researchers. The
most well-known work dealing with floating floor constructions was performed by Cremer, seereference [10]. From reference [11], the following equation from this work regarding continuously
elastic layer will be used.
݂݆ ή 2ߨ ή ݂ ή ݉௛ο‫ܮ‬௡ = 40 ή ݈‫ ݃݋‬൬ ൰ + 20 ή ݈‫ ݃݋‬ฬ1 +ฬ݂௢ܼଵ

(1)

Simple, one-dimensional theory based on mechanical impedance also ends up with the samefrequency dependent improvement above the resilient floor resonance frequency, fo . Theory presented
in [10] especially developed for lightweight floating floors also give a similar slope of theimprovement according to equation (2). Below the fo frequency, the improvement will be set to 0 dB.
௙௙ ଶ௙೚

௙೥ο‫ܮ‬௡ = 40 ή ݈‫ ݃݋‬ቀ ቁ + 20 ή ݈‫ ݃݋‬ฬ1 + ቀ ቁ ฬ݂௭ =(2)4ඥ‫ܤ‬ଵ ή ݉ଵߨ ή ݉௛The textbook also conclude that it is not possible to fulfil some important assumptions, and themeasured improvement may therefore differ from the calculated one. Improvements achieved frommeasurement objects in LAB and FIELD will be compared with results from these equations.

6. COMPARISON OF MEASURED AND CALCULATED IMPROVEMENT
Calculation of the impact sound insulation improvement will be compared with measured
improvements due to different top floor solution added to a basic structure. The basic structure consistsof a traditionally wooden joist floor with resilient suspended ceiling with data from the basic structureof the LAB measurement series. It means that the comparison should be reliable for the other
LAB-examples if we assume a low influence on the modal behavior of the floor due to the added topfloor. The comparison with improvements measured from field objects may of course be less accurate
due to obviously different modal behavior. Table 3 shows an overview of essential data for comparison
between calculations and measurements. Exact data of the dynamic stiffness of the resilient layer is notavailable, but from general information 10 MN/m3 have been used for the LDS layers and 40 MN/m3for the HDS layers.
Table 3 – Objects for comparison between calculations and measurements
Mass above/below resilient

Calculatedlayer, kg/m2fo (Hz)LDS

86 / 49~ 90

MBR

HDS

30 / 157~ 202

LAB-C 8MBR

LDS

38 / 154~ 91

FIELD-A B

MAR

LDS

86 / 38~ 98

FIELD-B E

MAR

HDS

102 / 46~ 179

ObjectMain group

Resilient layer

LAB-A 7MAR

LAB-B 35017

In figure 4, comparison between measured and calculated improvement of LAB example objects
are presented. Figure 4a shows the result for the solution with the mass above the resilient layer, while
figure 4b show the result when the added mass is positioned below the resilient layer. In figure 5,comparison between measured and calculated improvement of FIELD example objects are presented,
both with mass above the resilient layer.

Figure 4a – Comparison of calculated and measured improvement of object LAB-A

Impact sound insulation improvement (dB)

50403020100-10102103

Frequency (Hz)Figure 4b – Comparison of calculated and measured improvement of object LAB-B and LAB-C

Figure 5 – Comparison of calculated and measured improvement of object FIELD A and FIELD-B

Results presented in figure 4a show that the calculated improvement is overestimated for the MAR
example influenced by the low f o value from the calculations. Results presented in figure 4b show thatthe calculated improvement is slightly underestimated for the MBR examples, and the resonancefrequencies, fo coincide to a high degree. Results presented in figure 5 show that the calculated
improvement is overestimated for both MAR examples, influenced by the low f o value from thecalculations. Generally, the improvement slope coincides relatively well except the FIELD-Bexample.
The comparison also, partly shows an improvement at low frequencies, which deviate from thegeneral assumption of zero improvement in this frequency range. The modal behavior plays obviously
a role in this frequency range, together with effects of the field object solutions and may be flanking

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transmission. The overestimation for all MAR examples is probably caused by vibration transmission
because of low impedance of the basic structure compared with the top floor impedance. Too low
impedance of the basic structure gives additional or increased resonance frequencies of the system,compared to the calculated fo value.

7. CONCLUSIONS
Different hybrid floor solutions have been measured experimentally with Ln,w + C I,50-2500 results
between 46 and 55 dB. Three of those solutions may perform sufficient for the recommended impact
sound insulation level in residential housing of Ln,w + C I,50-2500  53 dB.
Object with increased transverse stiffening of the joist solution perform significantly better thanother objects at low frequencies. Until now, object measured in ejected buildings have more uniformsolutions, but still the results varies between 53 and 59 dB. Only one of those example objects fulfil therecommended impact sound insulation level. Therefore, other solutions need to be developed andtested. Both analyses and further development of solutions is a part of the ongoing research project"Hybrid joist floors with integrated ducts".
From a laboratory setup recently measured, we had an option to analyse measurement data of thebasic floor and with additional hybrid solutions on top. Different combinations of the position of theresilient layer and added weight was a part of this. Data from the basic floor have also been used forcomparison with field measurement results with hybrid top floor solutions.
Results from the measured improvement have been compared to calculation according to wellknown analytical equations. Results from this comparison show that the prediction of MAR solutions
overestimate the improvement, mainly due to the low fo value from the calculations. The reason forthis deviation is obviously uncomplete prediction tools. Results from the comparison show that theprediction of MBR solutions slightly underestimate the improvement, but the resonance frequencies,
fo coincide to a high degree in these cases.
Generally, the improvement slope coincides relatively well except one object. From these studies,
we so far conclude that a positioning of the mass below the resilient layer is beneficial, at least when
the basic floor has relatively low stiffness in the transverse direction.
Results presented in this paper shows that more research need to be done regarding theoretical
calculations of such timber floor constructions. Up to now, too little focus has been put into the impact
sound insulation of hybrid floor solutions, partly due to the complexity on modelling and partly due toexperimental costs.

ACKNOWLEDGEMENTS
This study has been carried out within a project funded by Regional Research Fund of Norway. The
owner of this " Hybrid joist floors with integrated ducts " project is Norgeshus, The project involve
research activities at SINTEF Building & Infrastructure and applied activities and preparation for fieldobjects by the project owner and partners.

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