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Citation: Clar-Garcia, D.;
Campello-Vicente, H.; Campillo-Davo,
N.; Sanchez-Lozano, M.;
Velasco-Sanchez, E. A New CPX
Drum Test to Obtain Sound Pressure
Levels of Tyre Noise for Type
Approval. Acoustics 2024,6, 579–592.
https://doi.org/10.3390/
acoustics6030031
Academic Editors: Yat Sze Choy and
Andrew Y. T. Leung
Received: 15 May 2024
Revised: 3 June 2024
Accepted: 26 June 2024
Published: 28 June 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
acoustics
Article
A New CPX Drum Test to Obtain Sound Pressure Levels of
Tyre Noise for Type Approval
David Clar-Garcia * , Hector Campello-Vicente , Nuria Campillo-Davo, Miguel Sanchez-Lozano
and Emilio Velasco-Sanchez
Department of Mechanical Engineering, Miguel Hernandez University of Elche, Avda. de la Universidad, s/n,
03202 Elche, Alicante, Spain; hcampello@umh.es (H.C.-V.); ncampillo@umh.es (N.C.-D.);
msanchez@umh.es (M.S.-L.); emilio.velasco@umh.es (E.V.-S.)
*Correspondence: dclar@umh.es; Tel.: +34-630431514
Abstract: The primary cause of noise from vehicular traffic while travelling at speeds over 30 km/h
is tyre/road interaction. To reduce this noise source, tyre/road sound emissions research has been
carried out using different approaches. Most of this research has been centred around track tests,
leading to the development of various track and road-based methods for evaluating tyre/road noise
emissions. The CPX (Close-Proximity), along with the CPB (Controlled Pass-By), the CB (Coast-By)
and the SPB (Statistical Pass-By), methods are the most common ones. Nevertheless, since Reg.
(EC) 1222/2009 came into force, only the CB method, defined in Reg. (EC) 117/2007, can be used
to obtain tyre/road noise emission type approval values in Europe. However, current track test
methods have important limitations, such as the variability of the results depending on the test track
or the test vehicle, the repeatability, the influence of environmental variables or, the main aspect,
the limitation of the registered magnitude in these tests, which is the sound pressure level. The
Alternative Drum test method (A-DR) was developed in 2015 in order to avoid these disadvantages.
However, it involves a complex and time-consuming microphone array for each test. With the
purpose of improving the A-DR test method, a new methodology based on drum tests, the ISO
11819-2 and the ISO 3744 standards, was developed. This paper describes the new Alternative CPX
Drum test method (A-CPX-DR) and validates it by testing several tyres according to the CB, the A-DR
and the A-CPX-DR test methods and comparing their results. This research has demonstrated that all
three methods have equivalent sound spectra and obtain close equivalent sound pressure levels for
type approval of tyres in the EU, while drum tests have shown greater accuracy. For both reasons, the
new A-CPX-DR methodology could be used for tyre/road noise emission type approval in a more
precise and cheaper way.
Keywords: Alternative Drum method; Close Proximity method; tyre; noise; drum; sound power
1. Introduction
Tyre/road sound emissions are the main source of noise for an ICE vehicle travelling
at medium and high speeds. Over 30 km/h, the impact of other noise sources, such as
the engine, the exhaust system or the drivetrain, within the whole vehicle noise is very
limited [
1
]. This behaviour happens practically from a standstill in battery electric vehicles,
as electric motors, inverters and transmissions are almost silent [
2
]. Consequently, in order
to reduce road traffic noise, tyre/road sound emissions need to be addressed.
The EU published Reg. 117 [
3
] in 2007, Reg. 661 [
4
] and Reg. 1222 [
5
] in 2009, and Reg.
740 [
6
] in 2020, in order to reduce tyre/road sound emissions. Regulation 117 defines the
methodology to obtain sound pressure levels (SPLs) for tyre/road noise emissions, while
Reg. 661 sets limits on the external rolling noise of tyres to be on the market under CE type
approval. Finally, Reg. 1222 and Reg. 740 establish a classification of tyres according to
their noise emission.
Acoustics 2024,6, 579–592. https://doi.org/10.3390/acoustics6030031 https://www.mdpi.com/journal/acoustics
Acoustics 2024,6580
Several methods, such as the CPX (Close-Proximity) along with the CPB (Controlled
Pass-By), the CB (Coast-By) and the SPB (Statistical Pass-By) methods were developed to
evaluate tyre noise emissions. Nevertheless, as stipulated in Reg. 117, the CB method is the
only approved means to obtain values for tyre/road noise emissions in the EU.
However, these conventional track methods have several restrictions and disadvan-
tages. The first limitation involves not only the test track but also the test vehicle, as its
influence on the test results has been proven to be significant [
7
,
8
]. In fact, it is considerably
more difficult to obtain similar values when the tests are performed by several laboratories
on different tracks [
9
] or with different test vehicles [
10
]. Because of the wide range of
factors that can affect the test results, repeatability is also an issue, even when the tests are
carried out by the same laboratory with the same vehicle and on the same track. Moreover,
environmental factors such as background noise, temperature and wind, or variations in
the test vehicle or on the test track as time passes, make an important contribution to the
test results and cannot be quantified with ease [
7
,
11
,
12
]. Finally, several studies claim that
results can be considerably affected by real test speeds, vehicle categories or because of the
effect of pavement ageing or differences in surface roughness [13–15].
On the other hand, research on measurement uncertainty in track tests has found
that the limitations of each of these factors can lead to differences in the test results of
up to 4 dB [
8
,
16
]. These differences can be even much higher, as all—or most—of these
factors play an important role in each measurement at the same time. For these reasons,
the repeatability and reproducibility of conventional track test methods can be said to be
far away from a scientific method for type approval of tyres in the EU [7,17].
In this context, the Alternative Drum test method (A-DR) [
18
,
19
] was engineered to
overcome these limitations and was evidenced to be more accurate and repeatable than the
CB method. Even though the main objective was widely achieved, a complicated and time-
consuming microphone array setup is needed to carry out each A-DR test. For this reason,
a new drum methodology, called the Alternative CPX Drum (A-CPX-DR) test method
was developed. Based on the expertise of the A-DR test method, and the ISO 11819-2 [
20
]
and ISO 3744 [
21
] standards, it applies a standardized specific engineering method for
determining sound power level and consists of a much easier test configuration, while the
results are even more accurate than the ones obtained previously with the A-DR.
Tyre noise emissions have been assessed previously by measuring sound pressure
levels on drums [
16
,
22
,
23
]. The same magnitude is measured in the CB test method estab-
lished in Reg. 117. However, SPL is a magnitude that depends on several environmental
factors and the distance between the receiver and the noise source. For this reason, unless
test conditions are precisely defined, measured and controlled, it is impossible to assess
the sound power of the source by measuring sound pressure levels. This can be addressed
by obtaining the sound power level under laboratory-controlled conditions, which is an
inherent magnitude in the noise source that is independent of such external factors [24].
This paper explains the methodology and test configuration to carry out the new
A-CPX-DR test method and validates it by testing a wide range of tyres according to the
CB, the A-DR and the A-CPX-DR test methods and comparing their results. As previously
performed in the A-DR test method, to validate the new methodology, the standardized
ISO 9613 [
25
] sound propagation method was used to calculate the SPL at 7.5 metres from
the sound power level of a tyre measured under laboratory-controlled conditions when
rolling against a drum. Finally, the test results are compared and discussed, reaching
several interesting conclusions.
2. Materials and Methods
2.1. Regulation (EC) 117/2007 Standardized Coast-By Track Tests
This section summarizes the test conditions and set-up in order to obtain tyre/road
noise emissions for a series of tyres using the standardized Coast-By track test method.
For further explanations, please refer to Regulation (EC) 117/2007 and [
19
], where the
methodology is explained.
Acoustics 2024,6581
2.1.1. Coast-By Test Procedure
The CB method assesses the noise level of tyres fitted on a vehicle as it travels on a
standardized testing track. Two microphones (P-P’), placed at a distance of 7.5 m on both
sides of the track reference line CC’ and 1.2 m above the ground, register the maximum
sound pressure level when the vehicle is coasting (see Figure 1).
Acoustics 2024, 6, FOR PEER REVIEW 3
2. Materials and Methods
2.1. Regulation (EC) 117/2007 Standardized Coast-By Track Tests
This section summarizes the test conditions and set-up in order to obtain tyre/road
noise emissions for a series of tyres using the standardized Coast-By track test method.
For further explanations, please refer to Regulation (EC) 117/2007 and [19], where the
methodology is explained.
2.1.1. Coast-By Test Procedure
The CB method assesses the noise level of tyres fied on a vehicle as it travels on a
standardized testing track. Two microphones (P-P’), placed at a distance of 7.5 m on both
sides of the track reference line CC’ and 1.2 m above the ground, register the maximum
sound pressure level when the vehicle is coasting (see Figure 1).
Figure 1. Microphone position set-up on the test track. Source: Regulation (EC) 117/2007.
The reference speed for passenger vehicle tyres is 80 km/h. A minimum of four meas-
urements must be registered, on either side of the vehicle, at speeds over 80 km/h. The
same must be done at speeds under the above-mentioned speed. The result is then calcu-
lated by linear regression. As established in Reg. 117, the SPL (L
R
) measured in A-weighted
dB is obtained using the following Equation (1):
𝐿
= 𝐿
−𝑎· 𝑣
dB
(1)
where 𝐿
is the tyre/road SPL average value:
𝐿
=
∑
𝐿
dB
(2)
n is the number of measures (at least 16),
L
i
are the registered tyre/road SPLs,
𝑣 is calculated as the average of logarithms of speeds v
i
:
𝑣
=
∑
𝑣
𝑤𝑖𝑡ℎ 𝑣
= log
(3)
𝑎 is the slope of the regression line:
𝑎=
∑
·
∑
dB
(4)
Figure 1. Microphone position set-up on the test track. Source: Regulation (EC) 117/2007.
The reference speed for passenger vehicle tyres is 80 km/h. A minimum of four measure-
ments must be registered, on either side of the vehicle, at speeds over 80 km/h. The same
must be done at speeds under the above-mentioned speed. The result is then calculated by
linear regression. As established in Reg. 117, the SPL (L
R
) measured in A-weighted dB is
obtained using the following Equation (1):
LR=L−a·vdB (1)
where Lis the tyre/road SPL average value:
L=1
n∑n
i=1LidB (2)
nis the number of measures (at least 16),
Liare the registered tyre/road SPLs,
vis calculated as the average of logarithms of speeds vi:
v=1
n∑n
i=1viwith vi=log Vi
Vre f
(3)
ais the slope of the regression line:
a=∑n
i=1(vi−v)·(Li−L
∑n
i=1(vi−v)2dB (4)
2.1.2. Tested Tyres
All the tyres tested were class C1 tyres, as established by Regulation 117, which are
used in passenger vehicles (categories M1, O1 and O2). Table 1displays all the tyres which
were tested, categorized by their nominal dimensions, whilst the subsequent Figure 2
illustrates all the tested tyres, including a detailed view of one of them.
Acoustics 2024,6582
Table 1. Tested tyres and their nominal dimensions.
185/65R15 88H 195/50R15 82V 205/55R16 91V
Michelin Energy Michelin Pilot Michelin Energy
Nexxen CP641 Nexxen CP641 Nexxen CP641
Insa Turbo Sport Insa Turbo TVS Insa Turbo TVS
Insa Ecosaver Insa EcoEvolution Insa EcoEvolution
Insa Turbo RTD3 Insa Turbo RTD3 Insa Turbo RTD3
Acoustics 2024, 6, FOR PEER REVIEW 4
2.1.2. Tested Tyres
All the tyres tested were class C1 tyres, as established by Regulation 117, which are
used in passenger vehicles (categories M1, O1 and O2). Table 1 displays all the tyres which
were tested, categorized by their nominal dimensions, whilst the subsequent Figure 2 il-
lustrates all the tested tyres, including a detailed view of one of them.
Table 1. Tested tyres and their nominal dimensions.
185/65R15 88H 195/50R15 82V 205/55R16 91V
Michelin Energy Michelin Pilot Michelin Energy
Nexxen CP641 Nexxen CP641 Nexxen CP641
Insa Turbo Sport Insa Turbo TVS Insa Turbo TVS
Insa Ecosaver Insa EcoEvolution Insa EcoEvolution
Insa Turbo RTD3 Insa Turbo RTD3 Insa Turbo RTD3
Figure 2. Tyre tread paerns of some of the tested tyres.
2.1.3. Coast-By Test Conditions
The Coast-By test environmental requirements are thoroughly described in Regula-
tion 117. Factors such as air or track temperature limit, maximum background sound level
or wind speed or the tyre’s loads and pressures are specified for the CB method. The fol-
lowing Table 2 summarizes the requirements and the test conditions in which the track
tests were carried out:
Table 2. Requirements and test conditions.
Requirement Test Conditions
Wind speed (m/s) <5 0–1.2
Air temperature (°C) 5 < T < 40 24.8–27.2
Surface temp (°C) 5 < T < 50 31.3–33.8
Background sound level (dB) <SPLMeasured −10 dB 12.2–18.9
2.1.4. Test Track Specifications
The test track, which spans a total distance of 700 m, is situated in the Miguel Her-
nández University of Elche’s campus. The measurement area is a 50 m section fulfilling
all the specifications and pavement requirements determined in Regulation 117. The test
track is an asphalt-paved surface which consists of four layers: a 200 mm thick subbase of
artificial gravel; 200 mm of artificial gravel base; a rolling layer consisting of a 50 mm sub-
base of G20; and a 40 mm base of S20 with porphyritic aggregate. Figure 3 shows the
measurement area, while the asphalt texture can be seen in Figure 4. For additional infor-
mation about the test track pavement, please refer to [26], where it is characterized.
Figure 2. Tyre tread patterns of some of the tested tyres.
2.1.3. Coast-By Test Conditions
The Coast-By test environmental requirements are thoroughly described in Regulation
117. Factors such as air or track temperature limit, maximum background sound level or
wind speed or the tyre’s loads and pressures are specified for the CB method. The following
Table 2summarizes the requirements and the test conditions in which the track tests were
carried out:
Table 2. Requirements and test conditions.
Requirement Test Conditions
Wind speed (m/s) <5 0–1.2
Air temperature (◦C) 5 < T < 40 24.8–27.2
Surface temp (◦C) 5 < T < 50 31.3–33.8
Background sound level (dB) <SPLMeasured −10 dB 12.2–18.9
2.1.4. Test Track Specifications
The test track, which spans a total distance of 700 m, is situated in the Miguel
Hernández University of Elche’s campus. The measurement area is a 50 m section fulfilling
all the specifications and pavement requirements determined in Regulation 117. The test
track is an asphalt-paved surface which consists of four layers: a 200 mm thick subbase
of artificial gravel; 200 mm of artificial gravel base; a rolling layer consisting of a 50 mm
subbase of G20; and a 40 mm base of S20 with porphyritic aggregate. Figure 3shows
the measurement area, while the asphalt texture can be seen in Figure 4. For additional
information about the test track pavement, please refer to [26], where it is characterized.
2.2. Alternative Drum Tests
This section briefly reviews the A-DR method, which was validated in 2015 to obtain
sound pressure levels for type approval of tyres under a controlled environment using
a 10-microphone array setup. It also explains in detail the new Alternative CPX Drum
(A-CPX-DR)
test methodology microphone array, considering that the rest of the measure-
ment procedure is exactly the same as in the A-DR test method. For further explanations,
please refer to [
18
], where the methodology and the measurement procedure are explained.
Acoustics 2024,6583
Acoustics 2024, 6, FOR PEER REVIEW 5
Figure 3. Test track measurement area.
Figure 4. Test track asphalt texture.
2.2. Alternative Drum Tests
This section briefly reviews the A-DR method, which was validated in 2015 to obtain
sound pressure levels for type approval of tyres under a controlled environment using a
10-microphone array setup. It also explains in detail the new Alternative CPX Drum (A-
CPX-DR) test methodology microphone array, considering that the rest of the measure-
ment procedure is exactly the same as in the A-DR test method. For further explanations,
please refer to [18], where the methodology and the measurement procedure are ex-
plained.
2.2.1. Drum Test Facilities, Instrumentation and Requirements for Acoustic Environment
The test facilities in which the Alternative Drum tests were carried out consist of a
testing chamber constructed from sound-absorbing materials. Its dimensions are 3.92 ×
9.35 × 4.84 m. The test drum surface is made of smooth steel with a diameter of 1700 mm.
Figure 3. Test track measurement area.
Acoustics 2024, 6, FOR PEER REVIEW 5
Figure 3. Test track measurement area.
Figure 4. Test track asphalt texture.
2.2. Alternative Drum Tests
This section briefly reviews the A-DR method, which was validated in 2015 to obtain
sound pressure levels for type approval of tyres under a controlled environment using a
10-microphone array setup. It also explains in detail the new Alternative CPX Drum (A-
CPX-DR) test methodology microphone array, considering that the rest of the measure-
ment procedure is exactly the same as in the A-DR test method. For further explanations,
please refer to [18], where the methodology and the measurement procedure are ex-
plained.
2.2.1. Drum Test Facilities, Instrumentation and Requirements for Acoustic Environment
The test facilities in which the Alternative Drum tests were carried out consist of a
testing chamber constructed from sound-absorbing materials. Its dimensions are 3.92 ×
9.35 × 4.84 m. The test drum surface is made of smooth steel with a diameter of 1700 mm.
Figure 4. Test track asphalt texture.
2.2.1. Drum Test Facilities, Instrumentation and Requirements for Acoustic Environment
The test facilities in which the Alternative Drum tests were carried out consist
of a testing chamber constructed from sound-absorbing materials. Its dimensions are
3.92
×
9.35
×
4.84 m. The test drum surface is made of smooth steel with a diameter of
1700 mm. Measuring instruments are calibrated yearly, while the laboratory has been
accredited annually since 2011 by an ILAC International Accreditation Body as fulfilling the
international standards ISO 17025 [
27
] for test laboratories and ISO 17020 [
28
] for inspection
bodies. The measuring instruments used in all the tests are shown in Table 3.
The criteria for background noise (K
1
) and suitability of the test environment (K
2
)
correction factors were obtained according to ISO 3744 as described in [
18
]. The test results
revealed that both K1and K2requirements were fulfilled, as can be seen in Table 4.
Acoustics 2024,6584
Table 3. Measuring instruments used in Alternative Drum and CB track tests.
Test Instrumentation Supplier Model
Tachometer RS 163-5348
Load cell Interface 1220AJ
Microphones Bruel & Kjaer 4935 1/4-inch
Pressure gauge Samoa 98-ND
Thermometer Omron E5CN-C2MT-500
Data acquisition system LMS International 16 channel LMS Scadas Mobile
Laser distance meter Bosch GLM80
Table 4. ISO 3744 requirements and test results for correction factors.
ISO 3744 Requirement Acoustic Environment
Limitation for background noise K1< 1.3 dB 0.22 dB
Suitability of the test environment K2< 2 dB 0.70 dB
2.2.2. A-DR and A-CPX-DR Microphone Array Setup
Ten microphones were used for the Alternative Drum (A-DR) tests. They were posi-
tioned on a measurement surface, of hemispherical shape and a one-meter radius, following
the coordinates established in ISO 3744, as explained in [
18
], using different stands arranged
around the tyre as shown in Figure 5.
Acoustics 2024, 6, FOR PEER REVIEW 6
Measuring instruments are calibrated yearly, while the laboratory has been accredited an-
nually since 2011 by an ILAC International Accreditation Body as fulfilling the interna-
tional standards ISO 17025 [27] for test laboratories and ISO 17020 [28] for inspection bod-
ies. The measuring instruments used in all the tests are shown in Table 3.
Table 3. Measuring instruments used in Alternative Drum and CB track tests.
Test Instrumentation Supplier Model
Tachometer RS 163-5348
Load cell Interface 1220AJ
Microphones Bruel & Kjaer 4935 1/4-inch
Pressure gauge Samoa 98-ND
Thermometer Omron E5CN-C2MT-500
Data acquisition system LMS International 16 channel LMS Scadas Mobile
Laser distance meter Bosch GLM80
The criteria for background noise (K
1
) and suitability of the test environment (K
2
) cor-
rection factors were obtained according to ISO 3744 as described in [18]. The test results
revealed that both K
1
and K
2
requirements were fulfilled, as can be seen in Table 4.
Table 4. ISO 3744 requirements and test results for correction factors.
ISO 3744 Requirement Acoustic Environment
Limitation for background noise K
1
< 1.3 dB 0.22 dB
Suitability of the test environment K
2
< 2 dB 0.70 dB
2.2.2. A-DR and A-CPX-DR Microphone Array Setup
Ten microphones were used for the Alternative Drum (A-DR) tests. They were posi-
tioned on a measurement surface, of hemispherical shape and a one-meter radius, follow-
ing the coordinates established in ISO 3744, as explained in [18], using different stands
arranged around the tyre as shown in Figure 5.
Figure 5. Microphone array around the tyre, according to ISO 3744, for the A-DR tests.
For the Alternative CPX Drum (A-CPX-DR) tests, three microphones were positioned
around the tyre, according to the ISO 11819-2 standard, using an aluminium Bosch profile
microphone stand, as can be seen in Figure 6.
Figure 5. Microphone array around the tyre, according to ISO 3744, for the A-DR tests.
For the Alternative CPX Drum (A-CPX-DR) tests, three microphones were positioned
around the tyre, according to the ISO 11819-2 standard, using an aluminium Bosch profile
microphone stand, as can be seen in Figure 6.
2.2.3. Alternative Drum Tests Configuration
To validate the new A-CPX-DR method, the tyres were tested according to the A-DR
and the A-CPX-DR test methods, while factors such as the tyre load, the test temperature
or the surface, were the same for all the tests [
29
]. A total of 135 tests were conducted on
15 different tyres, at speeds between 40 to 120 km/h in 10 km/h increments. Additionally,
18 measurements were carried out on the background noise while the whole drum equipment
was working within the same speed range with no tyre rotating against the drum.
The tested tyres were the same units which had been used before in the Coast-By track
tests. As prescribed in Regulation 117, each tyre’s nominal pressure was set to 200 kPa,
while its load was adjusted to 80% of its load capacity index.
Acoustics 2024,6585
The test temperature was 24.5
±
0.5
◦
C. Sound power spectra were registered in 5 s.
intervals, at an integration time of 125 ms, and analysed in third-octave bands.
Acoustics 2024, 6, FOR PEER REVIEW 7
Figure 6. Microphone array around the tyre for the A-CPX-DR tests.
2.2.3. Alternative Drum Tests Configuration
To validate the new A-CPX-DR method, the tyres were tested according to the A-DR
and the A-CPX-DR test methods, while factors such as the tyre load, the test temperature
or the surface, were the same for all the tests [29]. A total of 135 tests were conducted on
15 different tyres, at speeds between 40 to 120 km/h in 10 km/h increments. Additionally,
18 measurements were carried out on the background noise while the whole drum equip-
ment was working within the same speed range with no tyre rotating against the drum.
The tested tyres were the same units which had been used before in the Coast-By
track tests. As prescribed in Regulation 117, each tyre’s nominal pressure was set to 200
kPa, while its load was adjusted to 80% of its load capacity index.
The test temperature was 24.5 ± 0.5 °C. Sound power spectra were registered in 5 s.
intervals, at an integration time of 125 ms, and analysed in third-octave bands.
2.3. Procedure to Calculate Sound Pressure Level from Sound Power Level
Once sound power levels are registered in the drum tyre test facilities, the sound
pressure levels, in CB conditions, can be obtained using a sound propagation model. In
this case, ISO 9613-2: Aenuation of sound during propagation outdoors was used.
Even though there are several sound propagation models, the ISO 9613-2 method is
widely used in acoustic engineering applications. Although it can be less accurate than
other, more advanced empirical models, such as Rudnick [30] or Rasmussen [31], it is a
simple, reliable and easy model to implement. The ISO 9613-2 sound propagation model
is an engineering method for calculating the SPL from one or more, stationary or moving
sound sources, such as a tyre mounted on a test drum or a moving vehicle, as can be seen
in [32].
ISO 9613-2 defines Equation (5) to obtain the equivalent continuous SPL from the
sound power level.
𝐿
= 𝐿
+ 𝐷
−
𝐴
dB
(5)
where D
I
is the directivity correction. At the microphone’s height of 1.2 m, this factor is
zero [26]. The total aenuation factor can be obtained by the following Equation (6):
𝐴
=
𝐴
+
𝐴
+
𝐴
+
𝐴
+
𝐴
dB
(6)
Figure 6. Microphone array around the tyre for the A-CPX-DR tests.
2.3. Procedure to Calculate Sound Pressure Level from Sound Power Level
Once sound power levels are registered in the drum tyre test facilities, the sound
pressure levels, in CB conditions, can be obtained using a sound propagation model. In
this case, ISO 9613-2: Attenuation of sound during propagation outdoors was used.
Even though there are several sound propagation models, the ISO 9613-2 method is
widely used in acoustic engineering applications. Although it can be less accurate than
other, more advanced empirical models, such as Rudnick [
30
] or Rasmussen [
31
], it is a
simple, reliable and easy model to implement. The ISO 9613-2 sound propagation model
is an engineering method for calculating the SPL from one or more, stationary or moving
sound sources, such as a tyre mounted on a test drum or a moving vehicle, as can be
seen in [32].
ISO 9613-2 defines Equation (5) to obtain the equivalent continuous SPL from the
sound power level.
Lp=LW+DI−AdB (5)
where D
I
is the directivity correction. At the microphone’s height of 1.2 m, this factor is
zero [26]. The total attenuation factor can be obtained by the following Equation (6):
Atotal =Adiv +Aatm +Agr +Abar +Amisc dB (6)
where
Adiv,Aatm,Agr,Ab ar
and
Amisc
are the attenuation due to geometric divergence,
atmospheric absorption, ground attenuation, barriers or screening, and miscellaneous
effects, respectively, and can be obtained according to ISO 9613-2. Only
Aatm
is different
for each octave band. The rest of the attenuation factors are frequency-independent. For
this reason, there are no significant differences between each third-octave band in the total
attenuation factor, Atotal.
Moreover, it must be taken into account that CB track tests, which register the
sound emission of 4 tyres, vary considerably from the Alternative Drum and Alterna-
tive CPX Drum tests, where only one tyre is tested. The following Equation (7) takes
this into consideration:
Lp=LW+10·log 4 −Ato tal dB (7)
Acoustics 2024,6586
This Equation (7) permits obtaining the propagation model spectra that allows to
calculate the continuous equivalent SPL, from the sound power level obtained in both the
A-DR and A-CPX-DR tests.
Table 5shows the influence of having four tyres and the total value of attenuation at
the same time. To obtain the sound pressure values L
p
, the values on this table must be
subtracted to the sound power values Lwfor each third-octave band.
Table 5. Sound propagation model spectrum to obtain Lpfrom Lw.
f (Hz) 125 160 200 250 315 400 500 630
10·log 4 −Atotal (dB) −
20.36
−
20.36
−
20.36
−
20.37
−
20.37
−
20.38
−
20.39
−
20.39
f (Hz) 800 1000 1250 1600 2000 2500 3150 4000
10·log 4 −Atotal (dB) −
20.40
−
20.41
−
20.42
−
20.43
−
20.44
−
20.46
−
20.48
−
20.52
3. Results and Analysis
3.1. Coast-By Track Test Results
Tyre/road rolling noise level values L
R
were calculated from the data measured by
the microphones in accordance with Reg. 117. Additionally, the SPL spectrum was also
calculated for the values registered on the track. The sound spectra were registered in
5-second intervals with an integration time of 125 ms. The maximum SPL spectra were
then determined for each tyre.
This approach offers several significant advantages compared to the information
provided by the CB test procedure established in Regulation 117. Firstly, a sound spectrum
provides much more data than the SPL by itself. Background noise is easily identified in a
whole sound spectrum, whereas it can be masked in a value such as L
R
. Furthermore, it is
more accurate to compare sound spectra than to compare sound pressure levels. Finally,
obtaining L
eq
from the SPL spectra, which is a time-independent magnitude, enables the
comparison of CB with A-DR and A-CPX-DR test results, as these values are obtained in
this way in accordance with ISO 3744.
The SPL spectra of several tyres, obtained using the CB track tests, can be seen in the
following Figure 7:
Acoustics 2024, 6, FOR PEER REVIEW 9
Figure 7. Different 185/65R15 88H tyre maximum sound pressure level spectra (test speed: 80 km/h).
The characteristic tyre/road rolling noise spectrum is shown in Figure 7, as described
in [1,33]. Noise values show a gradual increase with frequency, reaching a peak at 1 kHz
and decreasing thereafter.
3.2. Alternative Drum and Alternative CPX Drum Tests Results
The results in this section for both the Alternative Drum and the Alternative CPX
Drum test methods correspond to A-weighted sound power levels.
Figure 8 shows the sound power level obtained in both the Alternative Drum tests,
L
w A-DR
(orange line with rhomboidal marker type), and in the Alternative CPX Drum tests,
L
w A-CPX-DR
(green line with square marker type), for an INSA Turbo Sport 185/65R15 88H
tyre at 80 km/h. The typical tyre sound power spectra with the characteristic peak around
1 kHz, as described in the literature and registered in the Coast-By track tests, were also
obtained in both Drum tests. However, there is a significant difference in the lower third-
octave bands between the spectra of the A-DR and the A-CPX-DR methods. There are also
differences in other frequencies, such as in 1 and 2 kHz. The results seem to be affected by
the tyre’s sound emission directivity, which is caused by the difference between micro-
phone positions and their distance to the noise source for each Alternative Drum test
method.
Figure 7. Different 185/65R15 88H tyre maximum sound pressure level spectra (test speed: 80 km/h).
The characteristic tyre/road rolling noise spectrum is shown in Figure 7, as described
in [
1
,
33
]. Noise values show a gradual increase with frequency, reaching a peak at 1 kHz
and decreasing thereafter.
Acoustics 2024,6587
3.2. Alternative Drum and Alternative CPX Drum Tests Results
The results in this section for both the Alternative Drum and the Alternative CPX
Drum test methods correspond to A-weighted sound power levels.
Figure 8shows the sound power level obtained in both the Alternative Drum tests,
L
w A-DR
(orange line with rhomboidal marker type), and in the Alternative CPX Drum
tests, L
w A-CPX-DR
(green line with square marker type), for an INSA Turbo Sport 185/65R15
88H tyre at 80 km/h. The typical tyre sound power spectra with the characteristic peak
around 1 kHz, as described in the literature and registered in the Coast-By track tests, were
also obtained in both Drum tests. However, there is a significant difference in the lower
third-octave bands between the spectra of the A-DR and the A-CPX-DR methods. There
are also differences in other frequencies, such as in 1 and 2 kHz. The results seem to be
affected by the tyre’s sound emission directivity, which is caused by the difference between
microphone positions and their distance to the noise source for each Alternative Drum
test method.
Acoustics 2024, 6, FOR PEER REVIEW 10
Figure 8. Sound power levels registered in both the Alternative Drum test (L
w A-DR
) and in the Alter-
native CPX Drum test (L
w A-CPX-DR
) for an INSA Turbo Sport 185/65R15 88H tyre at 80 km/h.
3.3. Coast-By Track vs. Alternative Drum and Alternative CPX Drum Tests Results Comparison
Previous sections show the CB track test results, and both the A-DR and the A-CPX-
DR test results separately. Figure 9 now shows the results of the three different tests, for
the same tyre, in one graph. The sound power level spectrum, L
w A-DR
(orange line with
rhomboidal marker type), and the sound power level spectrum, L
w A-CPX-DR
(green line with
square marker type), of a 185/65R15 88H Insa Sport tyre, measured by means of the A-DR
and the A-CPX-DR test methods at 80 km/h, respectively, are shown at the top of the
graph. Beneath them, there are several sound pressure level spectra obtained by different
CB track tests (L
pTrack
) for the same tyre.
Figure 9. Sound power level spectra obtained in A-DR and A-CPX-DR tests at 80 km/h, and sound
pressure level spectra obtained on Coast-By track tests.
Figure 8. Sound power levels registered in both the Alternative Drum test (L
w A-DR
) and in the Alterna-
tive CPX Drum test (Lw A-CPX-DR) for an INSA Turbo Sport 185/65R15 88H tyre at 80 km/h.
3.3. Coast-By Track vs. Alternative Drum and Alternative CPX Drum Tests Results Comparison
Previous sections show the CB track test results, and both the A-DR and the A-CPX-
DR test results separately. Figure 9now shows the results of the three different tests, for
the same tyre, in one graph. The sound power level spectrum, L
w A-DR
(orange line with
rhomboidal marker type), and the sound power level spectrum, L
w A-CPX-DR
(green line
with square marker type), of a 185/65R15 88H Insa Sport tyre, measured by means of the
A-DR and the A-CPX-DR test methods at 80 km/h, respectively, are shown at the top of the
graph. Beneath them, there are several sound pressure level spectra obtained by different
CB track tests (LpTrack) for the same tyre.
When comparing all these spectra, it is important to take into account that the spectra
shown in both the A-DR and the A-CPX-DR tests correspond to the SPL at a distance of
1 m, while the spectrum registered in the Coast-By track tests corresponds to the SPL at a
distance of 7.5 m. This explains the difference of 20 dB between the A-DR and A-CPX-DR
tests and the Coast-By track tests. For this reason, to be able to carry out a comparison, the
SPL at 7.5 m has to be calculated, from the sound power level, using ISO 9613-2.
Acoustics 2024,6588
Acoustics 2024, 6, FOR PEER REVIEW 10
Figure 8. Sound power levels registered in both the Alternative Drum test (L
w A-DR
) and in the Alter-
native CPX Drum test (L
w A-CPX-DR
) for an INSA Turbo Sport 185/65R15 88H tyre at 80 km/h.
3.3. Coast-By Track vs. Alternative Drum and Alternative CPX Drum Tests Results Comparison
Previous sections show the CB track test results, and both the A-DR and the A-CPX-
DR test results separately. Figure 9 now shows the results of the three different tests, for
the same tyre, in one graph. The sound power level spectrum, L
w A-DR
(orange line with
rhomboidal marker type), and the sound power level spectrum, L
w A-CPX-DR
(green line with
square marker type), of a 185/65R15 88H Insa Sport tyre, measured by means of the A-DR
and the A-CPX-DR test methods at 80 km/h, respectively, are shown at the top of the
graph. Beneath them, there are several sound pressure level spectra obtained by different
CB track tests (L
pTrack
) for the same tyre.
Figure 9. Sound power level spectra obtained in A-DR and A-CPX-DR tests at 80 km/h, and sound
pressure level spectra obtained on Coast-By track tests.
Figure 9. Sound power level spectra obtained in A-DR and A-CPX-DR tests at 80 km/h, and sound
pressure level spectra obtained on Coast-By track tests.
3.4. Validation of the Alternative CPX Drum Test Methodology
The results registered using the CB track test method and the A-DR and A-CPX-DR
laboratory test methods have been presented before. Note that the Coast-By track test
results show the SPL at 7.5 m (L
p Track
), while the values registered in the A-DR and A-
CPX-DR tests are sound power levels, (L
w A-DR
) and (L
w A-CPX-DR
), obtained by the sound
propagation model, described in ISO 3744, using a hemispherical measurement surface of 1
m in diameter and the CPX microphone positions, respectively, as explained in Section 2.2.2.
In this section, the sound pressure levels at 7.5 m (L
p A-DR
) and (L
p A-CPX-DR
), are obtained
from the sound power levels, (L
w A-DR
) and (L
w A-CPX-Drum
), according to ISO 9613-2, to
validate the Alternative CPX Drum test method explained in Section 2.2.
Figure 10 shows the sound pressure level, L
p A-DR
(orange dotted line), calculated
from L
w A-DR
, and the sound pressure level, L
p A-CPX-DR
(green dashed line), calculated
from L
w A-CPX-DR
on the Alternative Drum tests, for a 185/65R15 88H Insa Sport tyre at 80
km/h, considering the attenuation in accordance with ISO 9613-2, and the effect of four
tyres instead of just one. We can see a comparison between them as well as the CB sound
pressure level, L
p Track
. Both sound pressure level spectra, L
p A-DR
and L
p A-CPX-DR
, resemble
the sound pressure level spectrum, L
p Track
(red line with circular marker type), especially
in the third-octave band frequencies around 1000 Hz, where most of the sound energy is
contained.
On the other hand, Table 6shows L
p A-DR
and L
p A-CPX-DR
third-octave band sound
pressure values for an Insa Turbo Sport 185/65R15 88H tyre, calculated from the sound
power levels, L
w A-DR
and L
w A-CPX-DR
, respectively. It also shows the difference between
these values and the sound pressure values, Lp Track, obtained using Reg. 117.
As can be seen, important differences with the conventional CB track test results were
registered in the lower frequencies for the A-CPX-DR method, and in the higher frequencies
for both the Alternative Drum test methods. This behaviour is most probably caused by
the different test surfaces (asphalt test track vs. smooth steel drum). However, further
evaluation has demonstrated that these differences are not as important when comparing
the overall sound pressure levels, LR, which are the EU tyre type approval values.
The overall sound pressure level (L
R
) can be calculated from sound pressure level
spectra according to Equation (8):
LR=10·logh∑N
i=1100.1L′
pi idB (8)
Acoustics 2024,6589
where L′pi are the SPLs for each third-octave band.
Acoustics 2024, 6, FOR PEER REVIEW 11
When comparing all these spectra, it is important to take into account that the spectra
shown in both the A-DR and the A-CPX-DR tests correspond to the SPL at a distance of 1
m, while the spectrum registered in the Coast-By track tests corresponds to the SPL at a
distance of 7.5 m. This explains the difference of 20 dB between the A-DR and A-CPX-DR
tests and the Coast-By track tests. For this reason, to be able to carry out a comparison, the
SPL at 7.5 m has to be calculated, from the sound power level, using ISO 9613-2.
3.4. Validation of the Alternative CPX Drum Test Methodology
The results registered using the CB track test method and the A-DR and A-CPX-DR
laboratory test methods have been presented before. Note that the Coast-By track test re-
sults show the SPL at 7.5 m (L
p Track
), while the values registered in the A-DR and A-CPX-
DR tests are sound power levels, (L
w A-DR
) and (L
w A-CPX-DR
), obtained by the sound propaga-
tion model, described in ISO 3744, using a hemispherical measurement surface of 1 m in
diameter and the CPX microphone positions, respectively, as explained in Section 2.2.2.
In this section, the sound pressure levels at 7.5 m (L
p A-DR
) and (L
p A-CPX-DR
), are obtained from
the sound power levels, (L
w A-DR
) and (L
w A-CPX-Drum
), according to ISO 9613-2, to validate the
Alternative CPX Drum test method explained in Section 2.2.
Figure 10 shows the sound pressure level, L
p A-DR
(orange doed line), calculated from
L
w A-DR
, and the sound pressure level, L
p A-CPX-DR
(green dashed line), calculated from L
w A-
CPX-DR
on the Alternative Drum tests, for a 185/65R15 88H Insa Sport tyre at 80 km/h, con-
sidering the aenuation in accordance with ISO 9613-2, and the effect of four tyres instead
of just one. We can see a comparison between them as well as the CB sound pressure level,
L
p Track
. Both sound pressure level spectra, L
p A-DR
and L
p A-CPX-DR
, resemble the sound pressure
level spectrum, L
p Track
(red line with circular marker type), especially in the third-octave
band frequencies around 1000 Hz, where most of the sound energy is contained.
Figure 10. CB sound pressure level L
pTrack
vs. sound pressure levels L
p A-DR
and L
p A-CPX-DR
, obtained
from the sound power levels L
w A-DR
and L
w A-CPX-DR
for an Insa Turbo Sport 185/65R15 88H tyre.
On the other hand, Table 6 shows L
p A-DR
and L
p A-CPX-DR
third-octave band sound pres-
sure values for an Insa Turbo Sport 185/65R15 88H tyre, calculated from the sound power
levels, L
w A-DR
and L
w A-CPX-DR
, respectively. It also shows the difference between these values
and the sound pressure values, L
p Track
, obtained using Reg. 117.
Figure 10. CB sound pressure level L
pTrack
vs. sound pressure levels L
p A-DR
and L
p A-CPX-DR
, obtained
from the sound power levels Lw A-DR and Lw A-CPX-DR for an Insa Turbo Sport 185/65R15 88H tyre.
Table 6. Difference between Lp A-DR and Lp A-CPX-DR, and LpTrack sound pressure values.
f (Hz) 125 160 200 250 315 400 500 630
Lp Track (dB) 44.85 48.06 49.73 51.70 54.90 55.48 60.75 62.82
Lp A-DR (dB) 40.76 48.67 50.94 51.65 54.19 55.10 59.16 64.45
Lp A-CPX-DR (dB) 53.49 55.52 56.11 57.13 57.60 60.80 64.12 67.41
Lp A-DR −Lp Track (dB) −4.10 0.61 1.21 −
0.05
−0.71 −
0.38
−1.58 1.63
Lp A-CPX-DR −Lp Track (dB) 8.64 7.47 6.38 5.43 2.70 5.32 3.37 4.59
f (Hz) 800 1000 1250 1600 2000 2500 3150 4000
Lp Track (dB) 67.50 69.94 67.51 65.13 63.29 58.34 55.76 51.87
Lp A-DR (dB) 68.43 72.67 67.34 68.18 67.50 64.00 60.52 58.20
Lp A-CPX-DR (dB) 68.00 69.88 67.60 66.18 64.07 61.64 61.44 58.53
Lp A-DR −Lp Track (dB) 0.93 2.73 −0.17 3.04 4.21 5.65 4.76 6.32
Lp A-CPX-DR −Lp Track (dB) 0.50 −
0.06
0.09 1.05 0.77 3.30 5.68 6.66
The overall sound pressure values are then calculated with the values shown in
Figure 10 and Table 6for both the A-DR and A-CPX-DR tests. For the 185/65R15 88H Insa
Sport tyre, these values are
LR_A−DR =
77.2
dB
and
LR_A−CPX −DR =
76.4
dB
, respectively,
while the overall sound pressure level, L
R_Track
, calculated from the track tests by means of
the CB track method, is LR_Track =75.9 dB.
Figure 11 shows the L
R_Track
sound pressure level results measured during the Coast-
By track tests, in addition to the L
R_A-DR
and the L
R_A-CPX-DR
results calculated using the
A-DR and the A-CPX-DR test methods. Even though the correlation between track and
drum tests is not sufficient to consider equivalent methods, the results are quite similar, and
when the overall sound pressure values obtained with the Drum methods are compared
with the CB method, the mean values of these differences are
∆LR=
1.61
dB
for the A-DR
method, and
∆LR=
0.52
dB
for the A-CPX-DR method. These differences are significantly
lower than other deviations obtained using the CB track tests according to Reg. 117 [
13
,
16
].
These deviations have been classified into these factors: test track (3–9 dB), test temperature
Acoustics 2024,6590
(2 dB) and the vehicle (1.6 dB). None of them affect either the A-DR or the A-CPX-DR test
methods, as both of them are carried out under laboratory-controlled conditions.
Acoustics 2024, 6, FOR PEER REVIEW 13
Figure 11. Comparison between type approval values obtained in the Drum and the CB track tests.
4. Discussion
The conventional Coast-By method described in Regulation 117 needs a vehicle, four
tyres, a considerable amount of time and fuel, and two technicians to be carried out. More-
over, no vehicle can fit every tyre size on the market, so different vehicles need to be used
depending on the tyre’s size to be tested. For these reasons, the CB method is more expen-
sive than the A-DR or the A-CPX-DR methods described in this paper.
In addition, factors such as the test vehicle, the track, background noise and temper-
ature or wind, among others, have been demonstrated to considerably affect the repeata-
bility and reproducibility of conventional track test methods. In fact, it has been shown
that they introduce significant uncertainties in the results [7,8]. Additionally, the meas-
ured magnitude, the sound pressure level, is the most important limitation of conven-
tional track methods, as it depends on aenuation, environmental factors or the distance
between the noise source and the data acquisition system. For this reason, unless all these
factors are precisely quantified and controlled, which does not occur in the CB method, it
is impossible to obtain the sound power of the source.
Both the A-DR and the A-CPX-DR methods have been demonstrated to be reliable
and consistent in terms of repeatability and reproducibility, and their results resemble
those obtained using the conventional CB track test method. Additionally, the limitations
of the acoustic test environment and background noise of the drum tyre test facilities were
not only fully resolved, but even exceeded the standards of ISO 3744, which requires that
results have a typical deviation lower than 1.5 dB, as established in Table 0.1 of ISO 3744.
Furthermore, by using the ISO 9613-2 sound propagation model, sound pressure levels L
p
A-DR
and L
p A-CPX-DR
can be calculated from the sound power levels L
w A-DR
and L
w A-DR
obtained
in the Drum tests with the Alternative Drum and the Alternative CPX Drum test methods,
respectively.
On the other hand, the sound pressure values L
R_A-DR
and L
R_A-CPX-DR
, obtained in the
laboratory Drum tests, are very similar to the sound pressure values, L
R_Track
, calculated
Figure 11. Comparison between type approval values obtained in the Drum and the CB track tests.
4. Discussion
The conventional Coast-By method described in Regulation 117 needs a vehicle, four
tyres, a considerable amount of time and fuel, and two technicians to be carried out.
Moreover, no vehicle can fit every tyre size on the market, so different vehicles need to be
used depending on the tyre’s size to be tested. For these reasons, the CB method is more
expensive than the A-DR or the A-CPX-DR methods described in this paper.
In addition, factors such as the test vehicle, the track, background noise and tempera-
ture or wind, among others, have been demonstrated to considerably affect the repeatability
and reproducibility of conventional track test methods. In fact, it has been shown that
they introduce significant uncertainties in the results [
7
,
8
]. Additionally, the measured
magnitude, the sound pressure level, is the most important limitation of conventional track
methods, as it depends on attenuation, environmental factors or the distance between the
noise source and the data acquisition system. For this reason, unless all these factors are
precisely quantified and controlled, which does not occur in the CB method, it is impossible
to obtain the sound power of the source.
Both the A-DR and the A-CPX-DR methods have been demonstrated to be reliable
and consistent in terms of repeatability and reproducibility, and their results resemble those
obtained using the conventional CB track test method. Additionally, the limitations of
the acoustic test environment and background noise of the drum tyre test facilities were
not only fully resolved, but even exceeded the standards of ISO 3744, which requires that
results have a typical deviation lower than 1.5 dB, as established in Table 0.1 of ISO 3744.
Furthermore, by using the ISO 9613-2 sound propagation model, sound pressure levels
L
p A-DR
and L
p A-CPX-DR
can be calculated from the sound power levels L
w A-DR
and L
w A-DR
obtained in the Drum tests with the Alternative Drum and the Alternative CPX Drum test
methods, respectively.
Acoustics 2024,6591
On the other hand, the sound pressure values L
R_A-DR
and L
R_A-CPX-DR
, obtained in
the laboratory Drum tests, are very similar to the sound pressure values, L
R_Track
, calculated
from CB track tests, which are used for type approval of tyres. Their differences have
proven to be lower than the uncertainty introduced by factors such as the vehicle or the test
track, especially in the A-CPX-DR method, which stands out as a more accurate, cheaper
and simpler alternative test for obtaining sound pressure-approved values.
The lack of correlation between the conventional track test (CB) and the Alternative
Drum tests (A-DR and A-CPX-DR) has demonstrated that results are heavily influenced by
the test surface. For this reason, it is not possible to compare the type approval values of
tyres which have been obtained with different methods, as rolling against a smooth steel
drum is not comparable to rolling over an asphalt paved surface. The steel drum introduces
completely unrealistic conditions of interaction between the tyre tread and the road surface.
In order to improve the correlation between the track and drum tests, solutions such as
mounting replica road surface sections on the drum have been considered. However, the
effect of centrifugal forces or the joints between sections causes significant parasitic noises,
which is an important—but not the only—problem caused by this approach. Moreover,
small differences in the characteristics of the surface that occur on the measurement sections
covered with the ISO surface cause measurable differences in tyre noise. For this reason,
the study of tyre noise using smooth steel drums using the presented Alternative Drum
test methods and considering sound power levels instead of sound pressure levels, which
has not yet been done, should not be rejected.
The test environment and the fundamental parameters of the Alternative CPX Drum
test have been shown to be more controllable than those of the CB track test. In addition,
the A-CPX-DR test methodology has been validated and demonstrated to be more accurate
and repeatable and to have lower uncertainty than Reg. 117. Moreover, the information
provided by the A-CPX-DR method is more accurate, as the noise spectrum along with the
overall sound pressure level is calculated, unlike the CB method, where only L
R
is measured.
For these reasons, the Alternative CPX Drum test methodology could be considered as an
alternative simpler option, but in no way comparable or equivalent, to the track method
established in Regulation 117 for type approval of tyres.
Author Contributions: Conceptualization, N.C.-D. and E.V.-S.; data curation, D.C.-G. and H.C.-V.;
formal analysis, D.C.-G., H.C.-V. and N.C.-D.; funding acquisition, M.S.-L. and E.V.-S.; investigation,
D.C.-G.; methodology, D.C.-G.; project administration, M.S.-L. and E.V.-S.; supervision, M.S.-L. and
E.V.-S.; validation, D.C.-G.; writing—original draft, D.C.-G.; writing—review and editing, H.C.-V.
and N.C.-D. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Engineering Research Institute I3E and the Vehicles’
Laboratory of the University Miguel Hernandez.
Data Availability Statement: The datasets presented in this article are not readily available because the
data are part of an ongoing study. Requests to access the datasets should be directed to dclar@umh.es.
Conflicts of Interest: The authors declare no conflicts of interest.
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