Structure-borne sound sources in timber buildings – prediction of machinery
noise using measured transmission functions
Fabian SCHÖPFER(1) ; Andreas R. MAYR(1) ; Ulrich SCHANDA(1)(1) Technical University of Applied Sciences Rosenheim, Germany, fabian.schoepfer@th-rosenheim.de

Abstract

Prediction of machinery noise in buildings involves two stages: (a) the emission of structure-borne sound and(b) the transmission (i.e. the propagation) across the building structure. This paper concerns the latter in timberbuildings. In previous work a practical approach to predict the equipment sound pressure level using measured
transmission functions was presented. A research project conducted at the Technical University of Applied
Sciences Rosenheim pursued this approach by measuring a range of transmission functions in various timberbuildings. In total a data set of 120 transmission functions measured in 19 timber buildings is now available.
This paper gives a survey of this data by grouping similar constructions and transmission paths. The approach
can be used to derive average transmission functions for similar situations. Those could potentially be provided
in a catalogue giving data in one-third-octave bands. The application of this empirical prediction method ispresented in a case study using a fictive structure-borne sound source in timber buildings.
Keywords: Building noise control; structure-borne sound sources; lightweight buildings

1

INTRODUCTION

To proof the requirements on sound insulation in residential buildings, prediction of sound emission from machinery and service equipment is required. EN 12354-5:2009 [1] provides a method for prediction in heavyweight buildings. However there are no validated prediction tools for lightweight timber constructions thatgained increasing attention in recent years. Hence it is only possible to proof the requirements on sound insulation using measurements in the actual building. To-date the design of constructions and choice of equipment
in terms of machinery noise is based on previous project experience. This leads to oversized constructions thatcan be unattractive in terms of the costs. EN 12354-5:2009 is currently under revision to include lightweight
constructions. In previous papers [2, 3] an empirical method was proposed that is based on a data-set of measured transmission functions. This paper presents the results of a research project at the Technical Universityof Applied Sciences Rosenheim that took the definition of transmission function [3] to collect data in fieldmeasurements. The presented results add on to the data presented in [4]. From this data-set representativetransmission functions for similar constructions and paths are derived that can be used to estimate the soundpressure level from service equipment.

2

DEFINITION OF TRANSMISSION FUNCTION

The transmission function DTF is defined as follows [3, 5].
DTF,k = Lav,k − LW,k(1)

LW,k is the input structure-borne sound power level at an excitation position, k, and Lav,k is the resulting spatial
average sound pressure level in a receiving room. This transmission function can be spatially averaged for K
excitation positions to give an average transmission function, DTF,av , by
DTF,av = 10 lg

1278∑Kk=1 100.1DTF,k
K


(2)

The standardized spatial-average transmission function, DTF,av,nT is TDTF,av,nT = DTF,av − 10 lg
T0(3)

where T is the reverberation time in the receiving room and T0 is the reference reverberation time of 0.5 s. To
determine the spatial average sound pressure level at low frequencies the procedure for field measurements ofsound insulation [6] is applied as described in [3].

3

FIELD MEASUREMENTS

The main aim of the research project ’Übertragungsfunktionen im Holzbau’ (Transmission functions in timberbuildings) was to increase the data-set on measured transmission functions in timber constructions. So far measurements are available from 19 buildings including timber-frame constructions as well as constructions using
CLT (Cross-Laminated-Timber) elements. As usually more than one transmission path was considered in eachbuilding, the data-set contains 120 transmission functions. For each situation at least two excitation positions
were averaged although the range of different excitation positions is typically within ±4 dB [3] even for timberframe constructions. Each of these measured transmission functions is unique for the specific situation. Hence
representative spectra were derived to estimate the sound pressure level from machinery for similiar situations.
For this purpose groups were defined based on the following criteria:
Transmission path: This criterion describes the path from the excited building element to the considered receiving room. Direct transmission can be across a separating wall i. e. across a separating floor. Vertical transmission
describes basically the flanking path for an excited wall to the room above or the below the room where theexcitation took place. Similarly diagonal transmission considers the transmission to the room diagonally above
or below the room where the excitation took place. In addition to these examples other transmission paths
were defined from the exciting data-set to give in total 18 different transmission paths. These include alsotransmission across two or more junctions.
Type of excited building element: This criterion considers the purpose of the excited building element. Thiscan be an interior wall within a flat, a separating wall, a party wall or an exterior wall for example. In total
seven different building elements are available from the measured data-set.
Construction of excited building element: As measurements were carried in timber buildings the constructions
comprise mainly timber-frame walls, timber-joist floor or wall and floors built with CLT elements. However
measurements were also carried with excitation in the basement on the concrete or masonry walls. In total ninedifferent constructions are available from the field measurements.
Additional layers: This can be a floating screed or a facing or facing in front of the base element of theexcited wall or floor. It is distinguished between facings made from timber-frame constructions or using sanitarysystems with metal channels. Different types of floating screed were also considered.
Using these criteria, groups were formed that contain measured data for which all four criteria are the same.
From the data-set of 120 transmission functions 51 unique groups could be formed. However in some of thegroups only one measured transmission function is available so far. In the following, the results for groups
with at least three data-sets are presented. These 13 groups are presented as boxplots to give an indicationon the range within the groups. The data is presented in one-third octave bands for the frequency range from
20 Hz to 1000 Hz. From 20 Hz to 40 Hz, DTF,av,nT is shown as the reverberation time was not available for all
data-sets. At and above the 50 Hz one-third octave band, DTF,av,nT is shown (see dashed line in Figures 1 to 4).
Figure 1 shows four groups for horizontal transmission across different types of walls. Figure 2 shows fourgroups for vertical transmission (i. e. excitation of flanking wall) for different types of excited walls. Figure 3
shows three groups for diagonal transmission were cross-juntions and junctions with off-set are considered.
Figure 4 shows two groups that describe the transmission from the excitation in the basement to a rooms in the
ground floor or first floor with timber constructions.

1279

DTF,av-10

DTF,av,nT →DTF,av

DTF,av,nT →DTF,av

DTF,av,nT →DTF,av

DTF,av,nT →-20

DTF (dB)

-30
-40
-50
-60

Frequency (Hz)Frequency (Hz)Frequency (Hz)Frequency (Hz)

(a)

(b)

(c)

(d)

1k

500

125

250

63

31.5

1k

n=3
500

250

63

31.5

1k

500

250

125

63

31.5

1k

125

n=3

n=6
500

250

63

31.5

-80

125

n = 18-70

Figure 1. Transmission path: horizontal/direct(a) Single framework wall without additional facing, (b) Single framework wall with facing for pipes (alsotimber-frame construction), (c) Single framework wall with additional layer made from laths or resilient channels
perpendicular to the wall studs, (d) Party wall, separated framework
DTF,av-10

DTF,av,nT →DTF,av

DTF,av,nT →DTF,av

DTF,av,nT →DTF,av

DTF,av,nT →-20

DTF (dB)

-30
-40
-50
-60

Frequency (Hz)Frequency (Hz)Frequency (Hz)Frequency (Hz)

(a)

(b)

(c)

(d)

1k

500

125

250

63

31.5

1k

n=4
500

250

125

63

31.5

1k

n=4
500

250

125

63

31.5

1k

n=7
500

250

63

31.5

-80

125

n=3

-70

Figure 2. Transmission path: vertical(a) Exterior wall (T-junction), timber-frame construction, no facing, (b) Exterior wall (T-junction), timber-frame
construction, facing for electric installations, (c) Exterior wall (T-junction), CLT (Cross-Laminated-Timber), no
facing, (d) Interior wall (cross-junction), timber-frame construction, no facing

1280

DTF,av-10

DTF,av,nT →DTF,av
-10

-20

-20

-30

-30

-30

-60

Frequency (Hz)Frequency (Hz)Frequency (Hz)

(a)

(b)

(c)

1k

1k

-80

500

n=3

-70
500

63

31.5

1k

500

-80

250

n=2

-70

250

63

31.5

-80

125

n=6

-70

-50

250

-60

125

-60

-50

-40

125

-50

-40

DTF,av,nT →

63

-40

DTF (dB)

-20

31.5

DTF,av,nT →DTF (dB)DTF (dB)

DTF,av-10

Figure 3. Transmission path: diagonal(a) Cross-junction, timber-frame construction, no facing, (b) Cross junction with offset, timber-frame construction, no facing, (c) Cross junction with offset, timber-frame construction, with facing
DTF,av,nT →DTF,av
-10

-20

-20

-30

-30
DTF (dB)

-40
-50

-60

1k

500

63

1k

-80

250

n=9

-70
500

250

63

31.5

125

n=3

-70
-80

-50

125

-60

DTF,av,nT →

-40

31.5

DTF (dB)

DTF,av-10Frequency (Hz)Frequency (Hz)(a) Basement to ground floor(b) Basement to first floor

Figure 4. Transmission path: Vertical from the basement to the ground floor or first floor with timber construction.
From the data shown in Figures 1 to 4, representative spectra were derived that follow the 75 % quantile (i. e. theupper bound of the boxes) of the data within each group. Figure 5 shows eight representative spectra fortransmission within a storey or across one or more junctions. For group (VII) and (VIII) there was insufficient
singal-to-noise ratio for most measurements below 50 Hz and therefore the reprentative spectra are available for
one-third octave bands at and above 50 Hz. These representative transmission functions indicate that there is atypical spectral shape for each of the groups.

1281

−10

−10

−20

−20

DTF (dB)

(II)

I)

(II

DTF (dB)

(I)
−30

−40

−50

−60

−60

31.5

63

125
250
Frequency (Hz)(a) Direct transmission:(I) see Figure 1a(II) see Figure 1b(III) see Figure 1c

500

1k

(IV
I)

)
(VII)

)

−50

(V

(V

−40

−30

(VIII)
31.5

63

125
250
Frequency (Hz)

500

1k

(b) Transmission across one or more junctions:(IV) see Figure 2c(V) see Figure 2a(VI) see Figure 3a(VII) see Figure 4a(VIII) see Figure 4b

Figure 5. Representative transmission functions.

4

APPLICATION

To show the application of the proposed empirical method, this section uses the representative transmissionfunctions to estimate the sound pressure for a fictive structure-borne sound source that is fictively installed ineach of the representative situations. This source was introduced and described in [4], where timber-frame andconcrete receiving structures were considered. For the source and receiver mobility a constant value is assumed.
For the source this value is in the order of magnitude of a compact wall-hanging ventilation unit that wasinvestigated earlier [7]. The value for the receiver structures is in the order of magnitude of the infinte plate
mobilities for a concrete wall, a CLT wall and a timber frame wall. The activity of the source is based on
experience from investigations on source characterization. Scheck et al. [8] showed for various sources, that thepower input from typically drops significantly with frequency. Figure 6 shows the installed power of this fictive
source installed on three receiving structures which occur in the representative transmission functions shown in
Figure 5.
Using the installed power and the representative transmission functions, the sound pressure level for this fictive
source can be calculated for the eight presented representative situations. The results are shown as A-weighted
sound pressure levels in Figure 7. However it has to be noted that focus should not be on the absolute value ofthe sound pressure level because a fictive source was considered. Hence the A-weighted sum in the figure caption is shown normalized to group (I) for the groups (II) to (VI) and normalized to group (VII) for group (VIII).
The A-weighted sum was determined from 20 Hz to 1000 Hz for groups (I) to (VI) and from 50 Hz to 1000 Hz
for groups (VII) and (VIII).

1282

80

on

60

all
e w allram T w
r-f
CL
be
on

tim
te
cre

n
co
on
ll
wa

Structure-borne sound power level, LW (dB)

100

40

31.5 63 125 250 500
Frequency (Hz)1k

Figure 6. Installed structure-borne sound power level, LW,inst , of a fictive source [4], fictivly attached to threereceiving structures.

(c)

The sound pressure levels indicate the spectral shape for the same source attached in different situations. It
can be seen that the A-weighted sum does not differ for direct transmission across an interior wall with orwithout an additional facing as the strength of the source is at low frequencies. Similarly the the sum is only
7 dB lower for a party wall with separated framework although this construction provides a major improvement
towards high frequencies. For vertical transmission, the spectral shape differs for timber-frame walls or CLT
walls but the A-weighted sum is very similar (2 dB difference). Compared to horizontal transmission, the Aweighted sum of the sound pressure level for vertical transmission is ≈20 dB lower. For diagonal transmission
it is ≈25 dB lower. Similar observations can be made for the transmission from the basement to the groundfloor compared to the transmission to the first floor. Adding one junction (storey) int he transmission path,the sum of the sound pressure level reduces by 17 dB. Interestingly the sound pressure level for transmission
from the basement to the ground floor is in the same range compared to vertical or diagonal transmission inpurely timber constructions <125 Hz, although the installed power is significantly lower. This is because thesame power is transmitted more effectively from the heavy basement wall to the light timber construction (seeFigure 5).

5

SUMMARY

This paper presented some of the results from a field survey of measured transmission functions. In total 120
transmission functions, measured in 19 timber buildings are available to-date. The proposed empirical approachfor prediction suggests to form representative transmission functions for similar situations from measured data.
Therefore the data was grouped based on the transmission path, the type and the construction of the excitedbuilding element as well the type of an optional additional layer like a floating screed. From these groups
representative spectra were derived. In an example using a fictive source, which was fictivly installed in thesesituations, the sound pressure level was calculated. This allows to get insight into the expected sound pressure
for the same source in various transmission situations.

1283

30

30

20

A-weighted sound pressure level, L p,A (dB)A-weighted sound pressure level, L p,A (dB)

40

(I)
20
(II)
10

0

−10

(III)

(V)

10

(IV)

(VI)
0

−10

(VII)

−20
(VIII)

−20

31.5

63

125
250
Frequency (Hz)

500

(c)

(a) Direct transmission:(I) L p,A,sum = 43 dB(A) (20 kHz to 1 kHz)(II) 0 dB normalized to (I)(III) −7 dB normalized to (I)

1k

−30

31.5

63

125
250
Frequency (Hz)

500

1k

(b) Transmission across one ore more storeys:(IV) −20 dB normalized to (I)(V) −18 dB normalized to (I)(VI) −25 dB normalized to (I)(VII) L p,A,sum = 14 dB(A) (50 kHz to 1 kHz)(VIII) −17 dB normalized to (VII)

Figure 7. A-weighted sound pressure level for the fictive source installed in situations from Figure 5.

ACKNOWLEDGEMENTS
The work reported in this paper is funded by the German Federal Institute for Research on Building, Urbain
Affairs and Spatial Development within the program Future Building (grant reference SWD-10.08.18.7-16.39).
The authors are grateful to the industrial partnerns within this project, Bau-Fritz GmbH & Co. KG, Huber &Sohn GmbH & Co. KG, Regnauer Hausbau GmbH & Co. KG, Züblin Timber Aichach GmbH, for providing
buildings for the field measurements. The authors are also grateful to the company Müller-BBM VibroAkustik
Systeme GmbH for the support with the measurement equipment.

REFERENCES

1284

[1]

EN 12354-5:2009: Building acoustics – Estimation of acoustic performance of building from the performance of elements:
Part 5: Sound levels due to the service equipment.

[2]

Schöpfer, F. et al.: Measured transmission functions from structure-borne sound sources in a timber-frame construction. In:
Proceedings of Euronoise 2015. (May 31–June 3, 2015). Maastricht, Netherlands, 2015

[3]

Schöpfer, F. et al.: Measurement of transmission functions in lightweight buildings for the prediction of structure-borne
sound transmission from machinery. Acta Acustica united with Acustica 103 (2017), pp. 451–464

[4]Mayr, A. R. et al.: Measurement and prediction of structure-borne sound transmission from machinery in timber-framebuildings. In: Proceedings of Euronoise 2018. (May 27–31, 2018). Crete, Greece, 2018[5]

ISO 10848-1:2017: Acoustics – Laboratory and field measurement of flanking transmission for airborne, impact and building
service equipment sound between adjoining rooms: Part 1: Frame document[6]

ISO 16283-1:2014: Acoustics – Field measurement of sound insulation in buildings and of building elements. Part 1: Airborne sound insulation[7]

Schöpfer, F. et al.: Prediction of noise from machinery in timber-frame buildings using transmission functions. In: Proceedings of ICSV24. (July 23–27, 2017). London, UK, 2017[8]

Scheck, J. et al.: Messung und Prognose der Luft- und Körperschallübertragung von gebäudetechnischen Anlagen im Massivbau. In: Fortschritte der Akustik - DAGA. (Mar. 14–17, 2016). Ed. by Deutsche Gesellschaft für Akustik e.V. Aachen,
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