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

The determination of road surface corrections for CNOSSOS-EU
model for the emission of road traffic noise
Fabienne ANFOSSO LEDEE1; Luc GOUBERT2
1IFSTTAR Nantes, France
2

BRRC, Belgium

ABSTRACTIn the CNOSSOS-EU noise emission model for road traffic noise, the calculation of sound power emissionof vehicles introduces a road surface correction term, which can be calculated from a set of values provided
for non-standard types of road surfaces. However, the data may be of limited use for the authorities in chargeof strategic noise mapping if they do not match with the national or local types of surfaces or if newly
developed types of surfaces are laid on their networks. A procedure for the acoustic characterization of roadsurface properties is currently discussed within the European standardization group CEN/TC227/WG5. This
procedure, based on CPX tire-road noise measurements, could serve in two ways for the evaluation of roadsurface corrections as input parameters in CNOSSOS-EU. The first is a direct snapshot measurement on an
existing network, providing a rapid and up to date evaluation of the road surface correction term, without
considering the nature of the surface. The second is a reference measurement to collect reference data toserve for calculations at later stage. The paper presents the proposed method and its compatibility with theexisting noise emission model.
Keywords: Rolling Noise, Road Surface, CNOSSOS-EU

1. INTRODUCTION
In the CNOSSOS-EU noise emission model for road traffic noise described in EU Directive
2015/996/EC [1], the calculation of sound power emission of vehicles introduces a road surface
correction term, which can be calculated from a set of values provided for non-standard types of roadsurfaces. The data provided may be of limited use for the authorities in charge of strategic noise
mapping if they do not match with the national or local types of surfaces or if newly developed typesof surfaces are laid on their networks.
A procedure for the acoustic characterization of road surface properties is being currently discussed
within the European standardization group CEN/TC227/WG5. This procedure was developed within
the EU FP7 project “ROSANNE” [2]. The main objective is to set up a harmonized system forcharacterizing the acoustic properties of pavements, to which road authorities and road companies canrefer in tenders for new pavements. However, this characterization procedure based on CPX tire-roadnoise measurements, could also be used to the benefit of strategic noise mapping, for the evaluationof road surface corrections. Obviously, this would enable road planners, road administrators,
contractors, and manufacturers of pavements to assess in a consistent way the acoustic performanceof road pavements.
More precisely, two ways of implementing the procedure for the purpose of noise mapping havebeen identified. The first approach consists in a direct measurement of the road surface corrections on
an existing network (acoustic “snapshot”). The second approach consists in collecting reference dataon specific types of road surfaces, to be used at later stage for the calculation of road surface
corrections. The paper presents the overall approach.

1
2

fabienne.anfosso@ifsttar.fr
l.goubert@brrc.be3603

2. A PROCEDURE BASED ON CPX NOISE MEASUREMENTS
2.1 Rationale
The statistical pass-by (SPB) measurement of road surface effect on tire-road noise is a longstanding and widely used method described in the international standard ISO 11819-1 [3]. In thismethod, tire-road noise levels are measured at the side of the road and integrate all sorts of vehicles(tires) in the traffic. This tends to suggest that the method is representative of the real exposure ofpeople working or living close to the road. Over the last decades, large data base have been built of
SPB noise on many road surfaces, and in several countries, national road surface characterization
procedures make use of this measurement method. However, there are many disadvantages orlimitations in this SPB method, the main one being that the measurement result only represents a shortsection of the road. Another drawback is the sensitivity to sound reflected from structures such asbuildings or parked vehicles and to screening provided by guardrails or other objects affecting soundpropagation from vehicles to the microphone position. This drastically limits the applicability of themethod.
More recently, the close-proximity (CPX) measurement method was developed and published as
an international standard ISO 11819-2 in 2017 [4]. In this method, the noise generated by specified
reference tires is measured when running on the surface under test. Two test tires are specified in thetechnical specification ISO TS 11819-3 [5]: P1 typical of passenger car tire emission and H1
representing heavy vehicle noise emission. They are mounted on a vehicle, which may be self-poweredor on a towed trailer. The main advantages of the CPX method are that the results characterize longsections of road and that it is less sensitive than the SPB method to influences from variation in theroad environment. Among its disadvantages are that the measurement results are specific for theapplied reference tire which may not necessarily represent the tire/road noise from various categories
of vehicle. However, with this method, an entire network can be measured rapidly and efficiently,
with no site restriction. These advantages made the CPX method much more used over the last few
years than the SPB.
Finally, a quantification of the pros and cons of both methods was carried following a list of criteria.
The CPX method turned to be the more promising one for the acoustic characterization of pavements:
higher practicability, wider applicability (no site restriction) and coverage (network), flexibility forspeed range, representativeness for long sections, homogeneity checks, lower time consumption andlower cost, resistance to change in vehicle fleet, etc.
2.2 The relation between CPX and SPB
Although CNOSSOS-EU is not a measurement method, the noise calculation model is suppliedwith input parameters derived from measurements. The road surface correction coefficients aredefined as the difference in tire-road sound power emission on the surface and on a virtual reference
surface. This virtual reference road surface corresponds to an average between Dense Asphalt 0/11
and Stone Mastic Asphalt 0/11 road surfaces, all between 2 and 7 years old and in a representative
maintenance condition. The octave band coefficients provided for the reference tire-road sound power
are originated from the averaging of a large set of measured SPB noise levels during the EU FP6
project IMAGINE. The road surface correction coefficients provided are also originated from SPB
measurements in the Netherlands.
The consistency of an approach purely based on CPX measurements for deriving input parametersin CNOSSOS-EU was investigated in the EU FP7 project ROSANNE [6].
In a first theoretical approach assuming a simple point source and propagation model, the relation
between the sound pressure levels measured by CPX on two different surfaces (ΔLCPX ) and the roadsurface correction coefficient in CNOSSOS-EU ([ΔL WR]CNOSSOS-EU) was derived:(1)
ΔLCPX = [ ΔLWR]CNOSSOS-EU + ΔACPX - ΔASPBWhere ΔACPX and ΔASPB are the differences between the two surfaces of the ground attenuation
between the point source and the microphone measuring respectively CPX and SPB noise levels.
For reflecting road surfaces (dense surfaces), ΔACPX and ΔΑSPB are both equal to zero and therefore
ΔLCPX and [ ΔL WR]CNOSSOS-EU are fully equivalent.
For road surfaces with porosity, there is a difference due to the difference in ground effect between
the two measurement configurations. The simulations in the case of a typical porous pavement using
a method described in [7] showed that the discrepancy between the two sound attenuation differences
can be between 2 and 3 dB in the mid-frequency range (around the peak of sound absorption of the3604

porous pavement). It is illustrated in figure 1 as the difference between the two curves. It means that
according to the simulations, a CPX noise level difference between a porous and a dense surface canbe a few dB higher than the sound power level difference (road surface correction in CNOSSOS-EU)
between the same road surfaces. But it is clear that, by assuming point sources, the simulations areprobably overestimating the phenomena, in particular the interferential effect. In reality, the tire noiseradiation is no point source when heard from a 0.20 m distance and a vehicle passing-by at 7.5 m
distance is no stationary point source.

Figure 1 – Sound attenuation difference over a porous road surface with respect to a reflecting surface for
SPB and CPX positions (from [6] and [7])The relation between CPX and SPB noise levels was further investigated on a statistical perspective[8]. Measured data were collected from different EU countries (Austria, Denmark, Germany, Belgium,
UK, France) of CPX noise levels and SPB noise levels measured on the same road sections andpavements at approximately the same time. The analysis concluded that an average 1:1 relationshipbetween CPX noise levels measured with reference tire P1 (Uniroyal SRTT Tigerpaw tire) and SPB
noise levels from passenger cars (vehicle category m=1 in CNOSSOS-EU) can reasonably be derived,
as long as both types of noise levels are measured at the same reference speed, yielding a 20.5 dB
average difference between the two measured quantities and with almost 90 % of all data being within±1 dB around this trend line (figure 2). Considering truck tire noise at 80 km/h, the average SPB noiselevels from multi-axle trucks on dense pavements is 9.5 dB lower than the CPX noise levels measured
with H1 tire (Avon AV4). For two-axle trucks, the corresponding difference is 12.0 dB.

Figure 2 – Relationship between CPXP80 (tire P1 at 80 km/h) and light vehicle pass-by Lveh at the same3605

reference speed. Grey data points are from measurements at different speed; the dashed red line is the trendline for these data (from [8])Figure 3 – Relation between CPXH80 noise levels and SPB noise levels for multi-axle heavy vehicles at 80km/h (from [8])From this analysis - based on overall noise levels - it was concluded that the CPX measurement
method can reasonably provide road surface correction coefficients consistent with those provided
with SPB measurements, provided that the test tire is adapted to the vehicle category (P1 for category
m=1 and H1 for m=2 and 3) and with a reference speed that should be as close as possible to theaverage speed of the traffic for the category of vehicles. Additional experiments would be necessary
to further validate and generalize this assumption for wider speed range and frequency range.
2.3

Reference conditions

2.3.1 Reference road surface
In the harmonized calculation method, the road surface correction coefficient is defined with regard
to a virtual reference surface. This reference corresponds to an average noise emission on a mix ofDense Asphalt Concrete (DAC) 0/11 and Stone Mastic Asphalt (SMA) 0/11 surfaces, all between 2
and 7 years old and in a representative maintenance condition.
Such surfaces have been measured with the CPX method with the two test tires P1 and H1 [9]. The
measured data from several EU countries (Belgium, Denmark, Poland and Sweden) were collected
and averaged. Data for DAC are for surface aged from 2 to 7 year old with an average of 4 years.
They were measured on 9 sections with tire P1 and 3 sections with tire H1 . Data for SMA surfaces are
aged from 2 months to 8 years with an average of 3.4 years. They were measured on 42 sections withtire P1 and 5 sections with tire H1 .
The resulting average noise levels at 80 km/h and expressed in octave bands are shown in figure 4
for tire P1 and figure 5 for tire H1 . The corresponding average values are and listed in Table 1.
Table 1 — Reference CPX octave band levels for test tires P1 and H1 at 80 km/h (in dB)
Octave63 Hz
125 Hz250 Hz500 Hz1000 Hz 2000 Hz 4000 Hz 8000 Hz
bands
LCPXPref,80--77.4
87.8
97.6
92.7
83.5
-(dB)
LCPXHref,80--76.7
89.4
96.9
90.3
81.1
-(dB)

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Figure 4 – CPX spectra measured at 80 km/h with tire P1 on DAC 0/11 (left) and SMA 0/11 (right).
Calculated average (thick black curve) (from [6])Figure 5 – CPX spectra measured at 80 km/h with tire H1 on DAC 0/11 and SMA 0/11. Calculated average(thick black curve) (from [6])2.3.2 Frequency range
In the current ISO 11819-2 standard, the CPX measurements are provided for one-third octave
bands between 315 Hz and 4 kHz, thus covering octave bands from 500 Hz to 4 kHz. This frequency
range is preservative with regard to possible contamination of the measurements by aerodynamic noiseand by near field effects (car body and tire vibration). However many measurement systems are moreimmune to this contamination (due to cover) and can perform at lower and higher frequencies.
Furthermore, in the lowest and highest frequency bands, the effect of the road surface is very low, as
the noise emission is dominated by propulsion noise for which road surface characteristics are notsignificant. Therefore, even for the most exposed measuring devices, the reference noise levels in theoctave band i = 250 Hz can be approximated as if the sound pressure levels measured in the thirdoctave band 315 Hz is constant in the three 1/3 octave band:(2)
LCPXPref,80 (i=250 Hz) = LCPXP;315,80 + 10 log10 (3)(3)
LCPXHref,80 (i=250 Hz) = LCPXH;315,80 + 10 log10 (3)For the other extreme octave bands (i = 63 Hz, 125 Hz and 8 kHz) the road surface correction
coefficients will be approximated to zero. The validation of such approximations still needs further
investigation.
2.3.3 Reference speed
Regarding the reference speed, the measurements shall be made as close as possible to the reference
speed v m (in km/h) corresponding to the average speed of the flow of vehicles of category m, asconsidered for the calculations. Usually, if local measurement data is unavailable, v m is taken as the
maximum legal speed for the vehicle category. Different reference speed may apply for light motor
vehicles and heavy vehicles, consequently for measurements with P1 and H1 test tires.
When the measurements are performed at a different speed than the reference speed for the3607

calculations, the noise levels must be corrected using a β log10(v/v ref) law, where β is the speed
correction coefficient. Usually, this coefficients ranges between 25 and 35. The statistical analysis of
the French database of SPB measurements showed that there is no correlation between the speed
coefficient and the surface age, the reference speed, the traffic intensity, etc. The following averageslopes were found [8]:• For light vehicles: β = 28 on porous and semi-porous (211 data), 30 on dense pavement (258data).
• For heavy trucks: β = 28 for porous (71 data), 30 for semi-porous (37 data) and 27 for dense
pavement (156 data).
By taking a coefficient of 30 is a reasonable approximation when the nature of the road surface isunknown. The effect of the coefficient on the calculation of the road surface correction coefficient is
illustrated in figure 6. The highest predicted error by taking a speed coefficient of 30 instead of 25 (or35) is estimated at 1.5 dB at 40 km/h.

Figure 6 – Effect of the speed coefficient on the correction as a function of speed
Alternatively, speed coefficients provided in CNOSSOS-EU (βm) can be used when the nature of
the road surface is known.

3. A TWO WAY APPROACH FOR NOISE MAPPING
Two ways of implementing the acoustic characterization procedure for the purpose of noise
mapping have been identified. The first approach is a direct measurement of the road surface
corrections on an existing network. The second approach is a measurement of spectral coefficients on
specific types of road surfaces to be stored and used as later stage for the calculation of road traffic
noise emission. They are described in the following sections.
3.1

Direct determination of the road surface corrections on a road network

3.1.1 Proposed measuring procedure
The first approach consists in a direct measurement of the road surface corrections on an existing
network (acoustic “snapshot”). It provides a rapid and up to date evaluation of the road surface
correction term on a specific network, without considering the nature of the surface. The use of CPX
method instead of pass-by method makes it possible to perform the evaluation of an entire network ina reasonable time frame
The measuring procedure is typical of a “monitoring” routine for the acoustic performance of roadsurfaces, aiming at understanding how a surface degrades acoustically over its working lifetime andinforming surface maintenance/replacement strategies. In this procedure, a single measurement run
may be satisfying the needs of the network provider as well as stated accuracy requirements. The
specificity in the present case of measuring road surface correction coefficients is the imperative needfor octave band noise levels.

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The road surface corrections can be measured directly on site, on segments or sections of an
existing network compatible with the road sections used for the calculation of source line emission in
CNOSSOS-EU, and without any consideration of the nature of the surface.
Measurements with test tire P1 will be used for evaluating the road surface correction for lightmotor vehicles (category m=1 in CNOSSOS-EU). Measurements with test tire H1 will be used forevaluating the road surface correction for medium heavy vehicles (category m=2 in CNOSSOS-EU)
and heavy vehicles (m=3 in CNOSSOS-EU).
CPX noise levels are expressed at the reference speed v m in octave bands, preferably from 63 Hz
to 8 kHz, in any case at least from 500 Hz to 4 kHz.
3.1.2 Processing of measurement results
For each road segments, the output of the measurements is a set of LCPXP;i;v1 and LCPXH;i;v2 , where:
i is the index of octave bands,
v 1 =v m for m=1 and v 2=v m for m=2 or 3.
LCPXP;i;v1 is the CPX noise level in octave band i, measured with tyre P at the speed v1 (in km/h),
LCPXH;i;v2 is the CPX noise level in octave band i, measured with tyre H at the speed v2 (in km/h).
For each octave bands i, the road surface correction can be expressed by:¨LWR,road,i,1 = LCPXP;i;v1 - LCPXPref;80 - 30 log10(v1/80) for m=1¨LWR,road,i,2 = ¨LWR,road,i,3 = LCPXH;i;v2 - LCPXHref;80 - 30 log10(v2/80) for m=2 and m=3
where LCPXPref;i;80 and LCPXHref;i;80 are the reference levels specified in Table 1.

(4)
(5)

In these formulas, a speed coefficient (i.e. the slope of the variation of noise level relative to thelogarithm of the speed) of 30 is assumed (see section 2.3.3).
In the case where CPX levels can’t be measured for the octave bands i = 63 Hz, 125 Hz and 8 kHz,the road surface correction can be approximated to zero:for m=1, 2 or 3(6)
¨LWR,road,i,1 = 0
3.2

The collection of reference input data

3.2.1 Proposed measuring procedure
The second approach consists in collecting reference data on specific types of road surfaces, to be
stored in a database and used at later stage for the calculation of strategic noise maps. In the Directive
2015/996/EC [1], the road surface correction coefficients ΔLWR,road can be calculated from input data
αi,m and βm representing respectively the spectral corrections in octave bands and the overall speed
effect, for a vehicle category m:‫ݒ‬௠ቇο‫ܮ‬ௐோǡ௥௢௔ௗǡ௜ǡ௠ ൌ ߙ௜ǡ௠ ൅ ߚ௠ ൈ ଵ଴ ቆ(7)‫ݒ‬௥௘௙
It is possible to supplement or update the available data base by using the procedure forcharacterizing the acoustic properties of a road surface type (procedure for acoustic labelling of roadsurfaces), with some specificities mentioned hereafter.
Because the measurements aims at evaluating a specific pavement type, a minimum number of fivetest sections shall be selected and tested to determine the acoustic parameters representative of thewhole type. The test sections should be representative of the lifetime of the road surface type,assuming proper maintenance. Some of the sections may present some discontinuities due to ageing.
The selected sections will only show discontinuities related to the pavement condition, with theexclusion of exceptional or specific discontinuities such as repairs, paintings, bumps, manholes, etc.
The length of each test section shall be a minimum of 200 m without interruption of the road surface;
ideally the length shall be 500 m or longer.
The CPX measurements shall be performed at least three times (meaning one measurement and
two repetitions) on each test section for each reference tire and for each applicable reference speed.
Measurements with test tire P1 will be used for evaluating the input data for light motor vehicles(category m=1) and measurements with test tire H1 will be used for medium heavy vehicles (categorym=2) and heavy vehicles (m=3).
In order to enable the determination of speed coefficients, measurements shall be performed at3609

different reference speeds – at least two - at which the surface is expected to be used, and with a total
range of at least 30 km/h. One of the measurement speeds shall be the reference speed of the Directive
2015/996/EC [1]: vrefCNOSSOS-EU = 70 km/h.
For each section, CPX noise levels for each tire configuration are expressed at the reference speedv refCNOSSOS-EU in octave bands, from 63 Hz to 8 kHz, or in any case at least from 500 Hz to 4 kHz.
Finally, for each road segments, the output of the measurements is a set of CPX noise levels
L(CPXP;i;vrefCNOSSOS-EU and LCPXH;i;vrefCNOSSOS-EU, measured with tire P1 (respectively H1 ) in octave bandi, at the speed vrefCNOSSOS-EU = 70 km/h.
3.2.6 Determination of coefficients αi,m and β m
For each octave bands i, the spectral coefficients αi,m are expressed by:for m=1
Įi,1 = LCPXP;i;vrefCNOSSOS-EU - LCPXPref;80 - 30 log 10 (70/80)
for m=2 and m=3
Įi,m = LCPXH;i;vrefCNOSSOS-EU - LCPXHref;80 -30 log10 (70/80)
LCPXPref;i;80 and LCPXHref;i;80 are the reference levels specified in Table 1

(8)
(9)

For the determination of the speed coefficient βm, a linear regression of the overall CPX levels
obtained versus log10 (v/vrefCNOSSOS-EU) must be performed and the slope sm of this regression calculated.
Then for each vehicle category m, the speed coefficient βm is expressed by:(10)
β m = s m - 30
The factors “30” in equations (8), (9) and (10) result from the speed correction approximation
discussed in 2.3.3. The estimated error is however small between 70 and 80 km/h as it can bevisualized in figure 6.

4. CONCLUSIONSThe paper has presented how a measurement procedure for the acoustic characterization of roadsurfaces based on CPX measurements could be used for deriving road surface correction coefficientsto be used for road traffic noise calculations as required by the European Directive 2015/996/EC. The
theoretical principles and the supporting data were presented and show that a consistent and multipurpose procedure can be established. Some approximations were necessary and still needexperimental validation. The established procedure is now sufficiently finalized to be tested on realcases in the future.

REFERENCES
1. Directive 2015/996/EC of 19 May 2015, establishing common noise assessment methods according to
Directive 2002/9/EC of the European Parliament and of the Council, Official Journal of the European
Union, 2015.
2. Conter M. et al., ROSANNE Project - a New Procedure for Noise Characterisation of Road Surfaces in
Europe, Proc Internoise 2016; Hamburg, Germany, 2016
3. ISO 11819-1, Acoustics – Measurement of the influence of road surfaces on traffic noise – Part 1: The
statistical pass-by method, 1997.
4. ISO 11819-2, Acoustics – Measurement of the influence of road surfaces on traffic noise – Part 2: The
close-poximity method, 2017.
5. ISO/TS 11819-3, Acoustics – Measurement of the influence of road surfaces on traffic noise – Part 3:
Reference tires, 2017.
6. Anfosso Lédée F. et al., Report on the compatibility of the proposed noise characterization procedure withCNOSSOS-EU and national calculation methods, ROSANNE Deliverable D2.5, 2016
7. F. Anfosso Lédée, Modeling the local propagation effects of tire-road noise: propagation filter between
CPX and CPB measurements, Proc. Internoise 2004, Prague, Czech Republic, 2004
8. Kragh J. et al., Report on the analysis and comparison of existing noise measurement methods for noiseproperties of road surfaces, ROSANNE Deliverable D2.3, version 3, 2015

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