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

Quality assurance methods demonstrated with the calculation ofsound propagation with ISO 9613-2 and with CNOSSOS-EU
Wolfgang PROBST 1;
1

DataKustik GmbH, Germany

ABSTRACT
Many methods have been developed and where even implemented in legally binding rules how soundpropagation shall be calculated to support the minimization of environmental noise. Especially in the case ofsuch legal requirements it is necessary to ensure that the agreed algorithms are applied in the same way by
all experts independent from the software tool they use. With the series of the International Standard ISO
17534 we created a framework to ensure this necessary precision. One of the main pillar of the strategy is toclarify all ambiguous definitions and to close the gaps that will cause different results if programmed bydifferent software developers. With standard ISO 9613-2 examples are shown where even existing definitions
had to be modified. It can be stated that all calculation methods have their shortcomings that can only bedetected if the method is realized in software and then practically applied. The same is true with theharmonized European calculation method – here abbreviated as “CNOSSOS-EU”. Examples are shown
where it was necessary to modify and clarify some definitions before it was possible to create test cases tocheck the correct implementation of the method.
With this work on about 30 Test Cases for ISO 9613-2 and for CNOSSOS-EU we learned a lot about the
optimal strategy if a new method shall be implemented or an existing method shall be improved. Some
recommendations for future projects of that type will be given.
Keywords: Sound propagation, Prediction, Quality assurance

1. INTRODUCTION

In the past two decades a lot of effort was made to improve the accuracy of calculation models of
the “engineering type” for sound propagation. With all these engineering models like ISO 9613-2:1996[1], Harmonoise Engineering Model [2], or NMPB:2008 [3] the sound waves swapping over the
ground with all the objects above like vegetation, buildings and walls are replaced by somegeometrically exact defined ray paths. Many approximations are necessary to ensure an acceptableaccuracy – in this context accuracy means the agreement between calculated and measured soundpressure levels – but to keep the methods simple enough to support the necessary precision – thismeans that different experts applying the method shall get the same results for a given problem. These
two aspects are concurrent and we will never succeed to find the universal and final calculation
method. A severe problem is that the different physical phenomena like ground influence, attenuation
by vegetation, screening and meteorological influences are not independent but are treated in thecalculations as if they were. At the end, all these developments are a balancing act between the detailed
mathematical description of obviously relevant physical phenomena, on the one hand, and thepractical application in complex environments of the real world with many unknown
interdependencies and therefore necessary approximations, on the other hand. An example is theinclusion of meteorology in engineering calculation models: While the calculation method ISO 96132 is based on a single meteorological condition favorable to propagation it was a large step driven by
the European activities to include vertical temperature profiles and wind speeds in the Harmonoise
Engineering Model [4]. Taking into account the above mentioned balance it was finally decided to use
the calculation method NMPB:2008 as the Common European calculation method for noise mapping
under the acronym CNOSSOS-EU. This acronym is in the following applied as shortcut for the legallybinding text describing the calculation of sound propagation in the European directive 2015/996/EU
1

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Wolfgang.probst@datakustik.de[5].
With the International Standard ISO 17534-1 [6] measures to ensure the necessary correctness ifthese methods are implemented in software where defined and decided. Part 1 of this Standard defines
the general frame, Part 2 is a sort of library for Test Cases and for each calculation method one of thefollowing parts published as Technical Reports contains the method specific measures.
The main core of such a method specific documentation for quality assurance is a set of additional
specifications that are necessary to clarify open issues and – in some cases – to modify existing
definitions from the official publication that have proven to be faulty and further a set of test caseswith detailed step by step and final results.
The first step made in 2015 was the Quality Assurance of the calculation method ISO 9613-2:1996.
The method is applied worldwide and it was necessary to correct some algorithms and procedures thatwere obviously not designed to be implemented with nowadays available computer-power. These last
four years the main software-platforms applied ISO 9613-2 in connection with these “Additional
Recommendations” of ISO/TR 17534-3 [7] and it can be stated that this strategy improved thecommunication between all experts engaged in software development and implementation.
Based on this positive experience the next step was to start a similar project related to the Quality
Assurance of CNOSSOS-EU:2015. The method is by far more complicated than ISO 9613-2 and many
additional specifications and clarifications were necessary to ensure the unambiguous interpretationof the official text in the European Directive. During the work on this planned Part 4 of ISO 17534 in
Working Group 56 of ISO TC43 SC1 some implausible results were detected in the calculation ofmultiple diffraction with favorable propagation conditions. It was a lucky coincidence that the work
for Quality Assurance was not finished that time so that an improvement could be developed –published in [8] - and integrated as an additional specification. A special Test Case was constructedto check the correct implementation of this new strategy in different software applications.

2. QUALITY ASSURANCE WITH ISO 17534 – A POWERFUL FRAMEWORK FOR
SOFTWARE-IMPLEMENTED CALCULATION METHODSOne of the main targets of quality assurance in the implementation of calculation methods insoftware is to ensure identical or with an acceptable frame of uncertainty comparable results.
Consultants or other noise experts applying one of the commercially available software-packages must
rely on the correct implementation even if they are not able to check this in each individual casethemselves. Checking the correctness of an implementation is easier with clear and consistentcalculation procedures, but on the other side there is a tendency to include more and more acoustically
relevant phenomena and thus to make calculation strategies more complex.
The drafts of ISO 17534 – series try to solve this balancing act by introducing adequate measures.
The structure of this series is shown in figure 1 – it takes into account that there is one group of basic
and generally prevailing measures and another group of method specific actions.
The core standard is ISO 17534-1. It describes 6 blocks of actions that set up requirements forcalculation methods that shall be implementable quality assured and for the software package as aplatform where even more calculation methods can be offered alternatively.
ISO 17534 - 1

Technical Report
ISO 17534 - 2
Test Cases and
QAI-Specification

Technical Report
ISO 17534 - 3
QA according toISO 9613-2---------------------------

Technical Report
ISO 17534 - xx
QA according toXXX1) Identification of the official documentation
2) Additional specifications
3) Test suite (set of test cases with results)
4) Forms to declare conformity
5) Additional QAI-format specifications

Figure 1: The structure of the series ISO 17534

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For each calculation method that shall be included in that framework of quality assurance an own
Technical Report is foreseen. As a first example the Technical Report ISO 17534–3 was drafted forthe calculation method ISO 9613 -2. It is required for a method specific Technical report to identify
clearly and unambiguously the official documentation, even if the relevant algorithms and strategies
are spread over more publications. Secondly the Technical Report may contain “Additional
recommendations” being a decided supplement if the method according to the official documentation
is not complete or ambiguous in some aspects. Such additional specifications may also be necessary
if open and undefined issues require all software developers to find an own solution – in such cases
the working group decides for a common solution as long as there is not a responsible standardizationgroup taking this action. Summing up the Additional recommendations are preliminary advices toensure the necessary precision (same results with the same test case) and finally to make a calculation
method fit to be quality-assured despite these undefined parts. A further important part of the method
specific Technical Report is a suite of test cases with step by step and final results. With these results
an interval with acceptable deviations to rank the calculated values as correct is given. It shall only
be mentioned that these test cases shall be as simple as possible to facilitate the necessary checks andonly as complex as necessary to prove the correct calculation of the algorithms under test. There isalso a form to be applied by the software developer to declare conformity. Finally the data exchange
by a common format may need some method specific supplements – these are also included in themethod specific Technical Report.

3. DIFFRACTED SOUND – THE APPROACH WITH ISO/TR 17534-3
3.1 Clarifications to improve the software application of ISO 9613-2
A typical example for necessary improvements of an existing standardized procedure in the frameof quality assurance with the framework of ISO 17534 is the calculation of screening with ISO 96132. Due to this standard the barrier attenuation Dz is a function of z, where z is the difference between
the pathlengths of diffracted and direct sound. It is calculated in case of a single barrier with1𝑧𝑧 = [(𝑑𝑑𝑠𝑠𝑠𝑠 + 𝑑𝑑𝑠𝑠𝑠𝑠 )2 + 𝑎𝑎2 ] �2 − 𝑑𝑑(1)and in case of double diffraction with1𝑧𝑧 = [(𝑑𝑑𝑠𝑠𝑠𝑠 + 𝑑𝑑𝑠𝑠𝑠𝑠 + 𝑒𝑒)2 + 𝑎𝑎2 ] �2 − 𝑑𝑑(2)where dss is the distance from the source to the first diffraction edge, dsr the distance from thesecond diffraction edge to the receiver, a the component distance parallel to the barrier edge between
source and receiver and e the distance between the two diffraction edges in the case of double
diffraction.
With more than two barriers it is recommended to apply the last equation choosing the two mosteffective barriers.
With software implementations of ISO 9613-2 many problems arose due to ambiguities of thesedefinitions. Equation (1) can be derived from the principle that the shortest path source-barrier edgereceiver is relevant, but equation (2) makes only sense if the two edges of a double barrier are parallel.
With software applications with many objects like buildings and barriers this condition is an extremely
seldom case and therefore it makes no sense to implement it.
This problem was solved by defining a more general procedure to find the propagation paths thatare regarded as representative as it is shown in Figure 2 for the propagation from a source S (red) toa receiver R (green). One path crossing the objects blocking the direct line S-R is the shortest “ribbonband-line” in a vertical plane containing S and R and up to two further paths are the shortest convex
polygon lines in a lateral plane that includes also S and R and is perpendicular to the first plane.

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Figure 2: Construction of the shortest paths from source to receiver in a vertical plane (blue) anda lateral plane (yellow)
3.2 Test cases to check the correct implementation
In ISO/TR 17534-3 a set of 19 Test Cases is included with about 68 tables with step-by-step andfinal results. The Test Cases are constructed to cover all the important algorithms and to check theircorrect implementation following the principle: as simple as possible and only as complicated asnecessary. An example is the Test case shown in Figure 3 that was constructed to check the
calculation of diffraction over top and laterally for a short barrier where the properties of the groundvaries along the propagation path.

Figure 3: Short barrier with varying heights and acoustic properties of the ground
Another example where more objects like buildings are blocking the line of sight is shown with
Figure 4. The aim of this Test Case is to include the construction of paths representing the laterally
diffracted sound in more complicated scenarios.

Figure 4: Three building blocking the direct line of sight between source and receiver
A further important part of this framework of quality assurance are the conformity sheets where
the developer of the software product under test declares that all the relevant parts of the official
documentation and the additional specifications are correctly included and that the results for all test
cases can be reproduced in the defined intervals.

4. QUALITY ASSURANCE FOR THE IMPLEMENTATION OF CNOSSOS-EU
4.1 Problems encountered and proposed solution
The calculation method for sound propagation CNOSSOS-EU is based on the French method
NMPB 2008. As described in the introduction above, the calculation method showed some strange
and not plausible results when it was applied in Noise-Mapping-Projects. From experts in the
Netherlands it was reported that screening by objects like buildings and embankments may be strongly
underestimated in special cases with favorable propagation conditions and therefore noise levels in1619

urban areas can be overestimated by 5 to 10 dB
The reason for these not plausible results is the method how the diffracted ray paths are constructed.
Figure 5 – scenario 1 with source S (height 3 m), receiver R (height 5 m) in a distance of roughly 1
km and barrier B with a height of 10 m - shows in the upper part the well-known ribbon-bandconstruction related to homogeneous propagation conditions to find the difference between thepathlengths of diffracted and direct sound and from that the resulting barrier attenuation.
homogeneous

S B

R

favorable

S B

R

Figure 5: The direct ray path (dash-dot) and the diffracted ray over top of the barrier for bothpropagation conditions as defined with CNOSSOS-EU:2015 (scenario 1)
With favorable propagation conditions the straight lines from homogeneous conditions are replacedby arcs with a radius Rcurv that is a function of the distance between source and receiver.
The attenuations due to diffraction calculated with CNOSSOS-EU:2015 are shown in the columns
with header “scenario 1” in Table 1.The attenuations are smaller with favorable compared tohomogeneous conditions in agreement with expectation.
Table 1: Attenuation due to diffraction for all scenarios and for both propagation conditions
Frequency
Hz

63
125
250
500
1000
2000
4000
8000

Attenuation due to pure diffraction Δdiff dB calculated withCNOSSOS-EU:2015
proposed modification
scenario 1 scenario 2 scenario 3 scenario 2 scenario 3
hom fav hom fav hom fav hom fav hom fav
6.9 6.5 6.3 0.0 9.6 3.8 6.3 3.7 9.6 6.5
8.2 7.8 7.5 0.0 11.8 2.6 7.5 2.4 11.8 7.8
10.1 9.5 9.2 0.0 14.3 0.0 9.2 0.0 14.3 9.5
12.5 11.7 11.3 0.0 17.1 0.0 11.3 0.0 17.1 11.7
15.1 14.3 13.8 0.0 20.0 0.0 13.8 0.0 20.0 14.3
17.9 17.0 16.5 0.0 22.9 0.0 16.5 0.0 22.9 17.0
20.8 19.9 19.4 0.0 25.9 0.0 19.4 0.0 25.9 19.9
23.8 22.9 22.3 0.0 28.9 0.0 22.3 0.0 28.9 22.9

With scenario 2 shown in Figure 6 the barriers B1 – B6 with heights between 6 m and 12 m areinserted. The detailed coordinates are not reported here – they can be taken from the original
publication [8].

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homogeneous

S

B1

B2

B3

B4 B5

B6

R

B1

B2

B3

B4 B5

B6

R

favourable

S

Figure 6: Scenario 2 with barriers B1 – B6 and relevant ray paths in agreement with thedefinition in CNOSSOS-EU:2015
The upper part in figure 5 (homogeneous conditions) defines on basis of the ribbon-bandconstruction the relevant diffraction edges B2, B3 and B5 and in the lower part (favorable conditions)these same edges are connected with arcs. As it is shown in the columns with the header “scenario 2”
in Table 1, no attenuation due to diffraction is calculated for this case and the curved direct ray is
assumed to propagate undisturbed by the barriers.
Now a scenario 3 is created combining scenarios 1 and 2 as it is shown in Figure 7.
homogeneous

S B

B1

B2

B3

B4 B5

B6

R

B1

B2

B3

B4 B5

B6

R

favorable

S B

Figure 7: Scenario 3 with all barriers of scenario 1 and scenario 2 combined
The attenuations with this scenario 3 calculated for favorable conditions with CNOSSOS-EU:2015
are with exception of the lowest two frequency bands 0 dB, as it is shown in Table 1. This means thatthe high attenuations of barrier B in scenario 1 are reduced to 0 dB if the barriers B1 – B6 are inserted
additionally – a result not plausible and not in agreement with expectation even taking into accountthe approximation of the sound wave by geometrically defined ray-paths.
The reason for this unexpected result is the strategy to regard the same barrier edges as relevant
under favorable conditions as they were found for homogeneous conditions applying the ribbon-bandmethod. The proposed and tested solution is to construct the path for favorable conditions completely
analogous as for homogeneous conditions as a convex envelope but with arcs instead of straight lines.
Connecting source S with all barrier edges and the receiver R the first diffracting edge is defined by
the maximal gradient of the arc at source S – if this gradient is the largest for the arc from source S to
the receiver R the curved direct ray is not blocked. In case a barrier edge is found that way this istaken as the starting point for the next arc and this is repeated till the receiver R is reached.

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favorable

B1

S

B2

B3

B4 B5

B6

R

Figure 8: Ray paths for scenario 2 (favorable) calculated with the proposed methodfavorable

S B

B1

B2

B3

B4 B5

B6

R

Figure 9: Ray paths for scenario 3 (favorable) calculated with the proposed method
Applying this proposed solution we get for scenario 2 under favorable conditions the ray pathshown in Figure 8 and for scenario 3 the ray path shown in Figure 9. The attenuations calculated on
the basis of the proposed strategy shown in the last 4 columns of Table 4 are now in agreement with
our expectation taking into account the behavior of sound waves in a downward refracting atmosphere.
4.2 Test cases to check the correct implementation of CNOSSOS-EU
The above described modification is only one aspect that had to be taken into account in the
preparation of a draft for a new ISO/TR 17534-4 related to Quality assurance of CNOSSOS-EU. Someother additional specifications were necessary to make the official text unambiguous and precise forthe implementation in different software platforms.
The set of 19 Test Cases developed with ISO/TR 17534-3 (related to ISO 9613-2) was extended
and now this complete set contains 28 cases.

Figure 10: Test Case with 8 buildings and calculation-rays for homogeneous conditions

Figure 11: Test Case with 8 buildings and calculation-rays for favourable conditions
Figures 10 and 11 show the last Test Case No. 28 constructed so that the “correct” calculation of
the ray paths can be checked – correct in the sense that the above explained solution is assumed to be
integrated in a revised version of CNOSSOS-EU.
At the time this paper is written a working draft of the planned ISO/TR 17534-4 is just finalized
by ISO TC43 SC1 WG 56. Therefore all the above is only a report from ongoing work and far frombeing decided and accepted. But it was the aim to give an impression of the many activities that arenecessary to reach the target – a calculation method unambiguously defined and described that can be
implemented in different software platforms and used by different experts even in different countries

1622with the intended high precision.

5. FINAL REMARKS

The authors experience from these and other cases where it was tried to integrate more physics inthe algorithmic scheme of calculation methods that shall be applied even in legally controlled areas
to prevent people from unwanted and not acceptable effects is a clear dominance of precision over the
aspects of accuracy. We should accept that these methods applied are – and must be – relatively crude
approximations. In the case of environmental noise we want to predict the sound caused by complex
sources like industrial facilities, propagating in an atmosphere with timely varying temperature
profiles roughly simulated by the applied algorithms and over vegetated and/or built up and often
complex shaped ground surfaces. We must accept that the methods of geometrical acoustics applied
need a thorough balancing of all the effects influencing the accuracy of the final result. What we needis a robust and transparent engineering method where thousands of consultants applying them have achance to make plausibility controls and to check the reason for unexpected results.

ACKNOWLEDGEMENTS

I acknowledge the work of the colleges in Netherlands – especially the acousticians in RIVM – fortheir critical work with CNOSSOS-EU where they detected the problems of the diffraction calculation
with favorable propagation conditions. It was the basis for my own analysis and gave me and mycolleges the possibility to contribute the solution described above to the working group on CNOSSOSEU installed by DG ENVIRONMENT. We hope that it will soon be integrated in a revised Annex IIof the Directive.

REFERENCES

1. ISO 9613-2: 1996, Acoustics - Attenuation of sound during propagation outdoors - Part 2: General methodof calculation
2. E. Salomons, D. van Maercke, J. Defrance, F. de Roo: The Harmonoise Sound Propagation Model. Acta
Acustica united with Acustica 97 (2011) 62 - 74.
3. NF S31-133:2011, Acoustique - Bruit dans l'environnement - Calcul de niveaux sonores (Acoustics –Outdoor noise calculation of sound levels)
4. European Directive on Environmental Noise, 2002/49/EC,URL: http://ec.europa.eu/environment/noise/home.htm
5. COMMISSION DIRECTIVE (EU) 2015/996 of 19 May 2015 establishing common noise assessment
methods according to Directive 2002/49/EC of the European Parliament and of the Council
6. ISO 17534-1:2015 Acoustics – Software for the calculation of sound outdoors –Part 1: Quality requirements and quality assurance
7. ISO 17534-3:2015 Acoustics – Software for the calculation of sound outdoors –Part 1:Recommendations for quality assured implementation of ISO 9613-2 in software according to ISO
17534-1
8. W. Probst, L. Schade: Calculation of Multiple Diffraction in a Downward Refracting Atmosphere with
Ray-Based Engineering Models. Acta Acustica united with Acustica 105 (2019) 226 - 230

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