PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Application of elastic interlayers at junctions in massive timberbuildings
Stefan SCHOENWALD1; Niko KUMER2; Sebastian WIEDERIN3;
Norbert Bleicher4, Bernhard FURRER5
1

Empa Materials Science and Technology, Switzerland
2

Stora Enso WP Bad St. Leonhard GmbH, Austria
3
4

Getzner Werkstoff GmbH, Austria

best wood SCHNEIDER GmbH, Germany
3

Lignum, Switzerland

ABSTRACTThe application of elastic interlayers at junctions in massive timber buildings was investigated in aparametric study in the Laboratory for Acoustics at Empa. Flanking sound transmission across a junction of afloor with an exterior wall was investigated for different construction details with and without interlayers, aswell as with rigid and decoupled connectors. The performance of the interlayers is discussed by comparing
the apparent airborne and impact sound insulation between two rooms one on top of the other. The benefit of
the elastic interlayers and connection variants is hereby gauged against other additional measures, forexample wall linings, suspended ceilings, floating floors required to achieve equal sound insulation and tofulfill acoustic requirements.
Keywords: Sound Insulation, structure-borne sound transmission, flanking, connections, timber buildings

1. INTRODUCTION
With the worldwide trend for sustainable buildings the demand for multi-storey wooden buildings
is increasing significantly. In many countries, including Switzerland, even high rise residential andoffice buildings are being designed and built out of wood. This was made possible due to massive
timber floors and walls, like Cross-Laminated-Timber (CLT) or Glulam plates, that are able to carryhigh structural loads and provide sufficient fire safety (1). Besides these advantages, the soundinsulation of the elements is limited due to their low mass in combination with a comparatively highbending stiffness. Therefore, for partitions between adjacent rooms in different units of the building
additional measures, like for example floating floors, gypsum board wall linings and suspended
ceilings, are required to fulfil sound insulation requirements for direct impact and/or airborne soundinsulation (2). Some of these measures are also beneficial for improving flanking sound insulation.
However, these additional measures are costly to install, reduce the room volume and also wooden
surfaces of the structure are being covered and not visible anymore. In this paper solutions withresilient interlayers at the junction between the massive timber elements and visible surfaces arepresented and their acoustical performance is gauged against wall linings. Further, consideration is
given to the connection details between the elements that are required for structural integrity.

2. BUILDING PERFORMANCEFirst, the methods for prediction of impact and airborne sound insulation in buildings using themethods of the EN 12354-series as well as the methods for the measurement of the necessary input
1

stefan.schoenwald@empa.ch
niko.kumer@storaenso.com
3
sebastian.wiederin@getzner.com
4
n.bleicher@schneider-holz.com
5
bernhard.furrer@lignum.ch25021

data are briefly outlined. In the following only vertically adjacent rooms, with one room on top of theother, are considered. For this situation, besides the direct sound transmission path, at each building
junction at the four floor edges additional three airborne and one impact flanking paths exists, where
structure-borne sound is transmitted from one element to the other.
2.1 Prediction according to EN 12354
The airborne and impact sound insulation performance of massive wood buildings can be predictedwith the methods of EN ISO 12354-1:2017 and EN ISO 12354-2:2017 respectively. As input data thesound insulation of the elements and the attenuation of structure-borne sound at the junction are
required. The flanking sound reduction index Rij can be calculated with equation 1 and the normalizedflanking sound pressure level Ln,ij with equation 2 respectively for each flanking path ij.
ܴ௜௝ ൌܴ௜ǡ௦௜௧௨ ൅ ܴ௝ǡ௦௜௧௨ܵௌ൅ οܴ௜ǡ௦௜௧௨ ൅ οܴ௝ǡ௦௜௧௨ ൅ ‫ܦ‬௩ǡ௜௝ǡ௦௜௧௨ ൅ ͳͲ݈݃ʹඥܵ௜ ܵ௝

‫ܮ‬௡ǡ௜௝ ൌ ‫ܮ‬௡ǡ௜ǡ௦௜௧௨ െ ο‫ܮ‬௦௜௧௨ ൅ܴ௜ǡ௦௜௧௨ െ ܴ௝ǡ௦௜௧௨ܵ௜െ οܴ௝ǡ௦௜௧௨ െ ‫ܦ‬௩ǡ௜௝ǡ௦௜௧௨ െ ͳͲ݈݃
ʹ
ܵ௝

(1)
(2)

For the two coupled elements i and j, the direct sound reduction index R and the normalized impact
sound pressure level Ln , the improvement due to additional layers ǻR and ǻLn for airborne and impact
sound, the direction averaged velocity level difference ‫ܦ‬௩ǡ௜௝ and the area S of the flanking elements S iand S j as well as of the partition Ss have to be known for the considered building situation situ. The
transfer of measurement data for massive timber elements from the lab to the building situation situ isdiscussed in detail in reference (3).
2.2 Junction transmission
The transmission of structure-borne sound at junctions of Type A elements according to
EN ISO 12354-1:2017 is expressed in terms of the so-called vibration reduction index K ij. The
structural reverberation time of Type A elements in a building is governed by the structuraltransmission to adjoining elements, in essence this is true for homogenous building elements with arelatively low internal damping, like for the CLT and Glulam elements with fully glued wooden
lamellas considered in this study. The Kij is determined from the measured direction averaged velocity
level difference of the two elements using equation 3.
തതതതതതതതതത
‫ܭ‬௜௝ ൌ ‫ܦ‬௩Ǥపఫǡ௟௔௕ ൅ ͳͲ ݈௜௝ǡ௟௔௕
ඥܽ௜ǡ௟௔௕ ܽ௝ǡ௟௔௕(3)

To determine തതതതതതതതതത
‫ܦ‬௩Ǥపఫǡ௟௔௕  one of the two coupled elements i and j is excited mechanically and thedifference between the spatial average surface velocity level on the excited and on the receiving
element is measured. The the test is repeated with excitation of the second element and the results for‫ܦ‬௩Ǥపఫǡ௟௔௕  is normalized with so-calledboth directions are arithmetically averaged afterwards. തതതതതതതതതത
absorption lengths ai and a j , that depend on the structural reverberation times Ts and surface area S of
the coupled elements, and with the junction length lij.
ܽ௜ǡ௦௜௧௨ ൌଶǤଶగమ ௌ೔௖బ ்ೞǡ೔ǡೞ೔೟ೠ

ට

௙ೝ೐೑
௙

మ

ௌೕ
௙ೝ೐೑
, and ܽ௝ǡ௦௜௧௨ ൌ ௖ଶǤଶగටrespectively௙బ ்ೞǡೕǡೞ೔೟ೠ(4)

For the prediction of the sound insulation in a building situation situ, the procedure for thenormalization in equation 3 is inversed. ‫ܦ‬௩ǡ௜௝ǡ௦௜௧௨ is obtained from K ij, the structural reverberation
times and the geometry of the building situation.

3. Measurement Setup
Experiments for this study were carried out in Empa’s Laboratory for Acoustics and Noise Control.
The structure-borne sound transmission at a T-junction of CLT walls and a Gluelam floor wasinvestigated for different connection details with and without elastic interlayers. The investigatedT-junction is a common solution for connecting a floor with exterior walls or with interior double walls,
when the floor is not continuous across this junction.

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3.1

Test Object

3.1.1 Junction
The measurement setup consists of a T-junction installed in the flanking facility of Empa’s
Lightweight Construction Laboratory, where building sections with a maximum of four rooms with
two on each floor can be installed. In the current study only two rooms were installed as shown in
Figure 1 separated by a 220 mm thick Glulam floor with a weight of 102 kg/m2 . The floor had a size of
4.20 m width by 5.60 m length consisted of seven approximately 60 cm wide planks. The planks had
tongue and groove joints along their length side and were joined together with diagonally driven
screws. The floor rested with the smaller edge on a 100 mm thick and 2.90 m high 3-ply CLT wall
(48.4 kg/m2 ) and on the opposite side on the permanent concrete structure of the facility. On the upperside of the junction a second100 mm thick and 2.90 m high 3-ply wall was on top of the Glulam floor.
The remaining two opposite edges of the floor were free and only sealed to prevent sound leaks.
Two walls in each room as well as the floor in the lower room were part of a permanent concrete
structure that gave the mock-up its structural integrity. The remaining two walls and the upper room’s
ceiling were movable massive wooden elements. Further, the heavy ceiling rested on the upper wall of
the test junction to load it like in a real building. At all permanent walls of the facility, as well as themovable walls measures were taken, like additional linings, elastic mountings or structural breaks, toprovide a high flanking sound insulation.

Figure 1 – Drawing of the test setup in the Empa Flanking Facility with the concrete backbone (grey), test specimen (orange)
and movable walls (red)3.1.2 Connection details
In this article four of the investigated connection methods between the upper wall and floor are
discussed. For all these junctions the floor was always connected rigidly to the lower wall with 14
screws, 2 at each floor element, that were driven from the top through the Glulam-floor. The
connection of the upper wall was modified as follows:x Reference case:
No elastic interlayers were installed, the upper wall was connected with 3 metal angles
spaced 1.80 m at the inside of the junction.
This case is the most common rigid connection method for the walls and floors considered.
x Limit of interlayer:
The upper wall was put on an elastic PUR-foam strip (design frequency f0 § 20 Hz) without
any additional connectors.
This is not a realistic solution, but is used to identify the maximum achievablestructure-borne sound reduction of the interlayers.
x Interlayer with decoupled angles:
The elastically supported upper wall was connected with 3 decoupled angles spaced 1.80 m.
This is one recommended acoustically optimized connection method that fulfils also
common structural requirements.
x Interlayer with rigid angles:
The elastically supported upper wall was connected with 3 common rigid angles, like in thereference situation, at a spacing of 1.80 m.
This method is not recommended from an acoustical point of view, nevertheless often
applied due to structural reasons.

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3.1.3 Additional Linings
For the prediction of the building situation an additional floor topping and wall lining are
considered. The floor topping consists of a 70 mm thick concrete screed (147 kg/m2 ) floating on a
30 mm impact sound insulation (glass fibre, dynamic stiffness  6 MN/m3 ) and a 30 mm expanded
polystyrene foam (EPS; elastified) as installation layer. Underneath a 90 mm thick gravel layer (126kg/m2 ) was placed on the bare floor as additional ballast.
The wall lining for the CLT walls is an additional layer of 15 mm gypsum fibre board that is directly
attached to 40 mm thick wooden battens. The wooden battens are screwed to the CLT wall with aspacing of 62.5 cm. The cavity between the battens is filled with 40 mm lightweight glass fibre
insulation with a flow resistivity > 5 kPa s/m2 .
3.2 Measurement methods
All input data for the prediction in this study is based on measured data obtained as follows.
3.2.1 Vibration Reduction Index Kij
The vibration reduction index is measured according to EN ISO 10848-3:2017. Each element was
excited with an electro-dynamical shaker (B&K, Type 4809) driven by white noise at 3 positions for
30 seconds. The velocity levels were measured with 6 accelerometers (B&K, Type 4513B-002) oneach element simultaneously that were connected to a multi-channel analyser PAK MKII from VAS
Müller BBM. On each element and for each excitation position the levels were measured at 12
positions and corrected for background noise, if necessary. For each excitation position the velocity
levels were spatially averaged for each surface and the velocity level differences were calculated. The
velocity level differences then were averaged for each excited surface and finally direction averaged
for opposite transmission directions.
The structural reverberation times were measured with the same sensors and exciter, but connected
to a second 4-channel analyser Apollo box and the software SAMURAI from Sinus MesstechnikGmbH at all 12 receiver positions on each element. A sine sweep signal was used for excitation and theimpulse responses were band filtered and backward integrated to determine the structural
reverberation times from the decay curves.
From both the velocity level differences and the structural reverberation times of the elements K ij
was calculated as described in section 2.2.
3.2.2 Performance of elements and additional layers
The direct airborne and impact sound insulation measurements of the elements and additional
layers were conducted in two different facilities using both times the methods of
EN ISO 10140-2:2008 and EN ISO 10140-3:2010.
The floors were installed in the flanking facility described in section 3.1. To suppress flanking
transmission through the investigated junction additional shielding consisting of a double layer ofgypsum board on 20 cm cavity insulation, was installed without structural connections in front of theflanking CLT walls. The sound insulation performance of the walls was measured at a nominallyidentical test specimen that was installed in Empa’s wall facility with structurally separated rooms.
For airborne tests one room was excited with white noise for 60 s, the floors sequentially at two
fixed loudspeaker positions and for walls with a moving loudspeaker. The sound pressure in the bothrooms was measured simultaneously with two microphones on rotating booms. The sound pressurelevels are corrected for background noise. The reverberation times were measured in the receivingroom at six fixed microphone positions for two fixed loudspeaker positions using the interrupted noise
method in the flanking facility and the MLS-method in the wall facility. At Empa the sound reduction
index is usually measured twice in opposite directions and the reported result is the energeticallyaverage index of both tests.
For impact tests the floor is excited with a standard tapping machine for 60s sequentially at 6
positions and the sound pressure level is measured with a microphone on a rotating microphone boom.
The measured levels are corrected for background noise and normalized with the reverberation timeand room volume to obtain the normalized impact sound level.
The improvement of airborne and impact sound insulation is the difference of the one third octave
band results for the bare reference specimen and of a second test with the wall lining or the floor
topping installed.

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4. Results
First, the results for Kij for the junctions and the effects due to the connection method are presented.
Then the sound insulation is predicted for three building situations showing the effect of elastic
interlayers.
4.1 Vibration reduction index Kij for connection methods
For the junction without interlayer the measured Kij and for the junctions with interlayer the
improvement ǻK ij relative to this reference situation are shown for the three transmission paths Ff, Fd
and Df, which are depicted in the Figures 2-4 in the middle. For K ij higher values indicate a bettersound insulation and for ǻK ij positive values indicate an improvement relative to the reference
situation. Besides the one third octave band data also single number ratings evaluated according to
EN ISO 10848-1 as the arithmetic average of K ij from 200 Hz to 1.25 kHz are presented.
4.1.1 Reference – rigid connection
For the reference case with the rigid connection and no interlayer K ij are shown in Figure 2. The
values are almost constant over frequency and the Ff-path (wall-wall) has with a single number valueof about 24 dB the highest insulation. The Fd-path (upper wall-floor) and Df-path (floor-lower wall)are both about 12 dB lower and below 1 kHz equal, but above 1 kHz the angles at the upper walltransmitted slightly less sound than the screws at the lower wall. The higher K ij of the Ff path isplausible as structure-borne sound has to be transmitted twice from the wall to the floor and to theother wall, whereas for path Fd and Df transmission occurs only between the floor and wall.

K ij (200 Hz – 1.25 kHz):
Ff

23.7

dB

Fd

12.3

dB

Df

11.4

dB

Figure 2 – Vibration reduction index Kij of path Ff, Fd and Df for the rigid connection with 3 steel angles - Left: Kij-Spectrum,
middle: Single number rating Kij, left: Connection details

4.1.2 Elastic interlayer – no connection
For this “ideal” case with only the elastic interlayer the maximum structural decoupling is obtained.
ǻK ij increases with frequency up to 20 dB for the Ff- and Fd-path that cross the elastic interlayer.
Below 100 Hz the interlayer gives only 3-4 dB improvement, however, flanking transmission is notvery important in this frequency range as it is shown in the predictions.
Between 200 Hz and 1 kHz the improvement for Ff is smaller than for Fd. Here the measurement
setup reaches its limits as very high Kij are measured for the Ff path. To obtain their absolute values
ǻK ij has to be added to the Kij in Figure 2, where KFf is already much bigger than KFd . The single
number K Ff is 36 dB in comparison to K Fd with 29.2 dB. Likely airborne sound transmission occurs
between the upper to the lower wall. Both have their coincidence frequency in this range and radiate
and receive very well sound waves travelling parallel to their surface. As expected for path Df ǻK ij isapproximately zero since the interlayer does not affect this path.
4.1.3 Elastic interlayer – 3 decoupled angles
As next step 3 decoupled angles are added to provide the necessary structural stability to thejunction. ǻKij in Figure 4 show a similar trend as without connectors in Figure 3. Ff- path is still
limited due to airborne sound insulation between 125 Hz and 500 Hz. Therefore only the Fd-path isdiscussed in detail and ǻKij from 100 Hz to 2 kHz is except of a dip at around 800 Hz only 3 dB lessthan for the ideal case before. The dip is likely caused by a resonance, e.g. between the different steel

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plates and interlayers of the connecting angle, but needs further investigation. The single number
ratings are only reduced by approximately 3 dB and 5 dB for the Ff-path and Fd-path respectively.

K ij (200 Hz – 1.25 kHz):
Ff

36.0

dB

Fd

29.2

dB

Df

10.4

dB

Figure 3 – Improvement of vibration reduction index ǻKij of path Ff, Fd and Df for elastic interlayer without connectors - Left:ǻKij -spectrum, middle: Single number Kij, left: Connection details

K ij (200 Hz – 1.25 kHz):
Ff

33.4

dB

Fd

24.5

dB

Df

10.8

dB

Figure 4 – Improvement of vibration reduction index ǻKij of path Ff, Fd and Df for elastic interlayer with 3 decoupled angles Left: ǻKij-Spectrum, middle: Single number Kij, left: Connection details

K ij (200 Hz – 1.25 kHz):
Ff

29.0

dB

Fd

17.8

dB

Df

10.5

dB

Figure 5 – Improvement of vibration reduction index ǻKij of path Ff, Fd and Df for elastic interlayer with 3 rigid angles - Left:ǻKij -spectrum, middle: Single number Kij, left: Connection detail4.1.4 Elastic interlayer – 3 rigid angles
Finally, ǻKij is shown in Figure 5 for three common rigid steel angles that connect the upper
elastically supported wall. Often this is a compromise with structural requirements. The obtained ǻKijfor both paths Ff and Fd are now equal and much smaller than with decoupled angles. Below 250 Hz
results are even equal to the case without elastic interlayer. Above ǻK ij is increasing to a maximum of
10 dB above 1 kHz. Also the single number K ij of path Ff and path Fd give only a small improvementof about 5 dB in comparison without interlayer.

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4.2 Prediction of sound insulation in buildings
To underpin the benefit of elastic interlayers in massive timber buildings, the in-situ airborne andimpact sound insulation performance between two vertically adjoining rooms, that are separated by a
3 m by 4 m Glulam-floor as described in Section 3.1, is predicted based on measured input data. For all
scenarios the floor topping of section 3.2.2 is considered. This is necessary to achieve an acceptabledirect sound insulation. Along the 4 m long edges of the floor load bearing T-junctions with 100 mm
CLT walls are assumed. The remaining two junctions are considered to be cross-junctions with
non-loadbearing lightweight frame walls that do not contribute to flanking transmission and therefore
are neglected in the following. The sound insulation of the single flanking paths are predicted withequation (1) and (2) using K ij from section 4.1 as input data. For the matter of simplicity the in-situ
structural reverberation times used for the transfer of K ij , R and Ln are assumed to be equal to the Empa
flanking test facility. Hereby slightly conservative results were obtained for all situations, as the usedreverberation times are slightly longer than in real buildings.

R’ w (C, C tr ) =48 (-1, -4) dB
L’ n,w (C I, C I,50 ) =44 (-1, 5) dB

R’ w (C, C tr ) =61 (-3, -10) dB
L’ n,w (C I, C I,50 ) =44 (-1, 5) dB

R’ w (C, C tr ) =58 (-2, -7) dB
L’ n,w (C I, C I,50 ) =45 (-1, 4) dB
Figure 6 – Predicted airborne (left) and impact (middle) sound insulation performance with single number ratings (right)
between two vertically adjacent rooms for three situations; top: without any measures at flanking walls, middle: with wall
linings, and bottom: with elastic interlayer and isolated steel angles

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For the first situation the walls at the junctions are assumed rigidly connected as for the reference
situation. The CLT walls and ceiling are not equipped with additional linings. The results are presented
in Figure 6 in the top row. The results are given for direct transmission through the floor (red), theresultant for all flanking paths (blue) as well as the overall transmission for the building situation(black). On the left the sound reduction index is presented and it is obvious that, except for thefrequency range below 100 Hz, flanking is dominating due to the two dashed blue paths in the rightdrawing (Ff and Fd) that are not reduced by the floor topping. For the normalized impact soundpressure level in the middle, direct transmission (red) is dominating in the whole frequency range, as
the Df-path (solid blue in the right drawing) is reduced well by the floor topping. Looking at the in-situ
single number ratings in the right column of Figure 6 the L’n,w-values for impact sound insulation
likely fulfil building requirements, whereas the R’w does not.
The improved results with the additional wall linings of Section 3.1 at the flanking walls arepresented in Figure 6 in the middle row. The in-situ sound reduction index on the left is increasing, as
flanking transmission is further reduced and R’w = 61 dB is achieved. The in-situ impact soundinsulation does not change, as direct transmission is not affected and flanking not relevant.
In the bottom row of Figure 6 the results are shown when the additional linings are substituted by
the elastic interlayer and decoupled steel angles of section 4.1.3. Hereby, similar in-situ soundinsulation performance with less effort for installation of additional wall linings is obtained.

5. CONCLUSIONS
Results of a study on the performance of elastic interlayers for reducing flanking transmission at awall floor T-junction in massive timber construction are presented and the influence of typicalconnectors necessary for the structural stability of the building is investigated. For vertical soundtransmission at the T-junction the most critical flanking paths are from upper wall to the lower wall andfrom the upper wall to the floor as the remaining floor-wall flanking path is usually sufficiently
suppressed by a floor topping that is necessary to provide sufficient direct impact sound insulation.
These two paths can be sufficiently improved by using an elastic interlayer between the upper wall andfloor and provide a very effective alternative to the installation of wall linings at the flanking walls.
However, the type of used connectors at the decoupled wall, that are necessary for structural
integrity, play a crucial role for the achievable sound insulation. Therefore, the performance of theconnectors is gauged against the maximum performance of the interlayer without any connectors.
Common decoupled steel angles achieve a structure-borne sound insulation that is only about 5 dB lessthan the maximum performance without connectors. Further optimization of the connectors is limited,as only a maximum of 2-3 dB further improvement can realistically be expected. Every connector will
add additional rigidity to the ideal and increase structure-borne sound transmission respectively. The
use of common rigid steel angles for the connection of the elastically mounted wall cannot berecommended, as only flanking sound reduction is improved only above 250 Hz and the single number
vibration reduction index was only 5 dB better than the rigid connection without interlayer. This
usually is not sufficient to replace additional wall linings.

ACKNOWLEDGEMENTS
The study was part of the Lignum project “Noise control in wooden buildings” funded by Wood
Action Plan of the Swiss Federal Office of Environment (FOEN) and industry partners. We gratefully
acknowledge the support.

REFERENCES
1. Mayo J, Solid Wood - Case Studies in Mass Timber Architecture, Technology and Design. 1st ed.
London: Routledge; 2015.
2. Homb A, Guigou-Carter C, Rabold A, Impact sound insulation of cross-laminated timber/massive woodfloor constructions: Collection of laboratory measurements and result evaluation. Building Acoustics.
2016; 24(1), 35-52.
3. Schoenwald S, Zeitler B., Sabourin I, King F, Sound insulation performance of Cross Laminated Timber
Building Systems. InterNoise 2013: Proceedings of 42nd international congress and exposition on noise
control engineering; 2013 Sep 15-18; Innsbruck, Austria.

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