Numerical and experimental evaluation of a working environment on the basisof a speech intelligibility mapping
Marcus Maeder(1) , Monika Gatt(1) , Magdalena Scholz(1) , Steffen Marburg(1)(1) Chair of Vibroacoustics of Vehicles and Machines, Technical University of Munich, Germany, Marcus.Maeder@tum.deAbstract
In the area of workplace design, both the technical requirements of the workplace and inter-personal concerns of
the employees are of decisive importance in order to establish a suitable working environment. Often conflictsof interest arise due to the antagonism between disturbing noise and desired speech intelligibility. An employee
who has to concentrate a lot at his/her workplace can be distracted both by disturbing background noises (airconditioning, fans, aggregates etc.) caused by low sound absorption as well as disturbing high speech intelligibility of background conversations. When assessing such situations, the speech transmission index (STI) is
an important parameter. Influenced by the reverberation time and the signal-to-noise ratio, calculating the STI
is a powerful tool for assessing demanding work environments. The present work deals with the determinationof an STI mapping on the basis of numerical simulations using ray tracing methods as well as calculations of
the sound energy density distribution in room acoustics. The experimentally validated models can be used tooptimize the workplace configuration to meet technical requirements as well as social concerns of employees.
Keywords: STI mapping, work environment, numerical and experimental test

1INTRODUCTION

Computer-aided simulation models are used as a basis for the numerical evaluation of a working environment.
The benefits of these models lie in the virtual analysis of the room acoustic specifications of the respective
open-plan offices without having to resort to cost-intensive testings or structural measures without knowing thefinal performance. Within the simulation models, these measures can be realized, tested and their performancein relation to the desired criteria can be evaluated. This procedure makes it possible to analyze and optimize
a large number of both structural measures and possible seat configurations before they are implemented oravailable in real operation.
Fundamentally, it must be said that the engineering field of room acoustics is primarily satisfied with variables
such as background sound pressure level (background noise) and reverberation time (measure of reverberationin the room). These two parameters are not sufficient for the demands of a complex workplace design, since anevaluation of speech intelligibility and the associated potential distractions of the employees remain unnoticed.
Consequently, the models are extended so that the evaluation of speech intelligibility can be addressed within
the numerical environment[1, 2, 9, 11, 12, 13, 14, 16].

2SIMULATION

The general procedure for the creation and application of such simulation models is described as follows. Inorder to obtain a basic overview, all documents, i.e. architectural plans of the rooms as well as the furnishings
and the materials used, are first analyzed and studied. The first project-specific peculiarities arise here, whichmust be taken into account in the model. In addition to the geometric dimensions, which define the basic structure of the model, these include possibilities to scale and flexibly set up the model.
Based on the analyzed plans, first 3D models are created with which preliminary investigations regarding theexpected room acoustic parameters are carried out. Figure 1 shows the developed simulation model of a workstation configuration in a workplace environment of a major European air navigation service provider.

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Figure 1. Simulation model of a workplace environment
2.1 STI calculation
The Speech-Transmission-Index (STI) is standardized and defined in DIN EN 60268-16 and is based on theinvestigations of Houtgast and Steeneken[1, 14]. For the calculation of the STI it is necessary to determine
the modulation transfer function between transmitter and receiver with a total of 14 modulation frequencies
from f˜1 = 0.63 Hz to f˜14 = 12.5 Hz in the octave spectrum between 125 Hz ≤ f ≤ 8000 Hz. The resulting 14x7
matrix of the modulation transfer values m(k, f˜n ) can be calculated based on experiments as well as simulations.
Assuming a diffuse sound field with an exponential decrease of the sound energy of the reverberation, thecalculation m(k, f˜n ) can be simplified. The result ism(k, f˜n ) = r1+1

2π f˜n Tk
13.8

12 · 1 + 10(SNRk /10) .

(1)

The parameters in equation (1) are the modulation frequency f˜n , the reverberation time Tk and the signal-tonoise ratio SNRk for the respective octave band k. In order to derive the speech intelligibility index, additional
weighting factors and the consideration of different auditorical effects such as the hearing occlusion are accounted for. For a detailed description see DIN EN 60268-16 [1]. The resulting STI values range between

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zero and one, are location and room specific and contain information about the background noise level. This
makes it possible to derive an optimal seating configurations based on the speech intelligibility to be expectedor measured.
2.2 Determination of reverberation time Tk
Different models exist in room acoustics to estimate the reverberation time. The most widespread formulas arederived by Sabine or Eyring-Norris[5, 19]. However, these derivations are subject to special requirements orsimplifications. If the models for estimating the reverberation time are not applicable, numerical simulations canbe used. In the present case, the reverberation time was first estimated with the empirical formulas according
to Sabine and Eyring-Norris and further verified utilizing the software tool Comsol Multiphysics[7]. Here theRay-Tracing method was used, which is well known in the literature[4, 7, 15, 20, 21].
2.3 Determination of the signal-to-noise ratio SNRk
To determine the signal-to-noise ratio of the respective speaker positions, all sound sources, i.e. speech spectraof persons in the room, cf. Byrne[6], air conditioners, computer sounds, etc. were integrated into the model andevaluated using the acoustic diffusion equation in the room[7, 10, 18]. Comsol Multiphysics was also available
for this purpose.
Two simulations were carried out for each speaker configuration to determine the sound pressure level distribution throughout the room - once as a result of the speaker source and once as a result of all other sources inthe room.

3MEASUREMENTS

In order to be able to compare the simulation models and test their validity, extensive experimental investigations
were carried out. In addition to the identification of the workplace noise sources (computer, fan, mobile table)
and the air conditioning system as an acoustic source, measurements of the reverberation time at different
positions where conducted. The results were used to correct the absorption coefficients of the acousticallyeffective surfaces and thus link the numerical model to the real system behavior of the room. This way, thequality of the simulation results could be verified.
Further, the STI was measured. Extensive measurement technology from Brüel & Kiær was available for theinvestigations. The following list is a compilation of hardware and software used.
• Sound level meter Type 2270
• Omnidirectional sound source Type 4292-L• Calibration instrument Type 4231
• Microphone Type 4231
• Echo Speech Source Type 4720
• Software Type BZ5503• Software Dirac6

4RESULTS

In order to deliver reliable results, a model comparison with the aid of experimental investigations is required.
Figure 2 shows the comparison of the numerically calculated reverberation times according to Sabine andEyring-Norris and the measured reverberation times. There is a satisfactory agreement between the results of2455

Figure 2. Comparison of the reverberation time of simulation and experiment
the simulation and the experiment. Furthermore, the calculation of the STI values could also be validated using
experiments. For this purpose, a corresponding test scenario was set up and examined in the simulation model
as well as in the experiment for two different positions. The result of this investigation are shown in Table 1.

Simulation

Measurements
Relative Error

Frequency
125
250
500
1000
2000
4000
8000
STISTIε in %SNR
-4.72
3.83
3.95
3.49
2.29
1.42-1.23

Position 1
Noise Signal
28.18 23.46
39.30 43.12
41.08 45.03
33.44 36.9229.91 32.21
27.11 28.53
25.95 24.71
0.52
0.55
5.45

SNR-5.85
2.33
2.42
1.97
0.90
-0.07
-2.84

Position 2
Noise Signal
28.37 22.53
39.81 42.14
41.62 44.04
33.95 35.92
30.34 31.24
27.52 27.45
26.31 23.47
0.47
0.55
6.00

Table 1. Comparison of STI values from simulation an experiments for two positions; SNR, Noise and Signal
values in [dB]; Relative error calculated with respect to the measurements
In this table, the relative error is calculated with respect to the measurements utilizing the following equation:ε=(STIsim,i − STIexp,i )2.
2STIexp,i(2)In equation (2), STIsim,i represents the STI values calculated from the simulation and STIexp,i from the experiment at position i, respectively. It can be seen that the results are in good agreement.
To illustrate the performance of the model, the results for a test scenario are presented below. The question
was relevant to what distance employees in the working environment are influenced by other conversations in2456

Figure 3. Sound sources in the example scenario; Speech source left, noise sources right

Figure 4. STI mapping of the example scenario; STI color coded left and two discrete values rightthe room or whether a possible distraction can occur due to a conversation. Figure 3 shows in the left part theposition of the source of interference from which a potentially disturbing noise in the form of a conversationemanates. The right half of the picture shows the main sound sources in the room. These include conversations
at the workplaces or in the room and all other acoustic sources emanating from technology as well as the air
conditioning system.
Figure 4 shows the results of the STI mapping, where on the left side the mapping is realized in color gradations and on the right side delimitations are set with an STI of STI = 0.5 and STI = 0.2. These deliminations,known as Distraction Distance (STI = 0.5) and Privacy Radius (STI = 0.2), provide information to which extent speech can be used as an information carrier (STI > 0.5), up to which distance it can be considered as apotential disturbance (0.5 ≤ STI ≤ 0.2) and from which distance (STI < 0.5) speech is lost in the backgroundnoise and thus no longer causes interference[1]. It can be seen that within the working environment a natural
decrease of the STI value with respect to distance is present for this room scenario. Additionally a naturalconversation can be carried out within a group of three workplaces. Beyond this group, conversations will not
disturb other members in the working environment.

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5CONCLUSION

With the developed and experimentally validated models it is possible to identify room-specific peculiarities
in speech intelligibility at an early stage and, if necessary, to derive and test measures. For this purpose, an
example scenario was identified and examined. Future measures can be derived from the presented results andthe know-how regarding room acoustic factors can be increased.

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