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A review of the measurement procedure of the ISO 1996 standard.
Relationship with the European Noise Directive
Juan Miguel Barrigón Morillas
a,
⁎, David Montes González
a
, Guillermo Rey Gozalo
b
a
Departamento de Física Aplicada, E. Politécnica, Universidad de Extremadura, Avda. de la Universidad s/n, 10003 Cáceres, Spain
b
Universidad Autónoma de Chile, 5 Poniente 1670, 3460000 Talca, Chile
HIGHLIGHTS
•ISO 1996-2 standard and the accuracy
of estimations of noise doses were
reviewed.
•A wide variation among standard
corrections and experimental results
was published.
•Unexpected increases of sound level
with height have been reported in
literature.
•Detailed studies regarding the standard
and its Annex B (informative) are
necessary.
•Results of the application of the European
Noise Directive could be being affected.
GRAPHICAL ABSTRACT
abstractarticle info
Article history:
Received 1 February 2016
Received in revised form 8 April 2016
Accepted 27 April 2016
Available online 17 May 2016
Editor: D. Barcelo
Accuracy in the knowledge of the sound field incident on a façade is essential for proper planning of control ac-
tions. Independently of the chosen method for noise mapping, if we wish to know the exposed population, it is
essentialto measure the incident noiselevel on the façade. Regarding the geometryof the measuring point in re-
lation to the façade and otherelements of theenvironment, the normative part of the ISO 1996-2 standard only
makes reference to the distance between the microphone and the façade. The rest ofthe geometricaspects that
could influence the result of a measurement are not considered in the standard. Although some of these aspects
are considered in Annex B, the annex is only informative. The ISO 1996 standard is considered in the European
Noise Directive as a reference in the elaboration of strategic noise maps, the main tool for assessing the exposure
of the population to noise pollution.
This work presents a detailed review of the literature and proposes research strategies in order to study the re-
lationships between the ISO 1996-2 standard measurements procedure and the accuracy of the estimations of
noise dosesreceived by people obtained by theapplication of the European NoiseDirective. The published results
show significant relative differences with respect to the values proposed by the standard for the corrections and
indicate the possibility of the influence of these results on the accurate development of strategic maps.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords:
Noise pollution
Urban noise measurements
Environmental noise standards
Trafficnoise
Geometric urban configuration
Science of the Total Environment 565 (2016) 595–606
⁎Corresponding author.
E-mail address: barrigon@unex.es (J.M. Barrigón Morillas).
http://dx.doi.org/10.1016/j.scitotenv.2016.04.207
0048-9697/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
Contents
1. Introduction.............................................................. 596
2. TheISO1996-2standardandthemeasurementofnoisepollutioninurbanenvironments........................... 597
3. Themicrophonelocationwithrespecttothebuildingfaçade........................................ 598
3.1. The position with the microphone flush mounted on the reflectingsurface............................... 598
3.2. The position with the microphone located in front of the reflectingsurface............................... 599
4. Thepositionofthemicrophonewithrespecttothesoundsource...................................... 602
5. Theheightofthemicrophone...................................................... 602
6. Conclusions............................................................... 604
Acknowledgements ............................................................. 605
References.................................................................. 605
1. Introduction
Worldwide economic and social development occurred over the last
decades. Among other consequences, this has led to a significant in-
crease in the number of people living in cities (Buhaug and Urdal,
2013; Henderson and Gun, 2007; Mulligan and Crampton, 2005) and
in the use of transport infrastructure. As a result, a progressive increase
in noise levels in economically developing countries (Zannin and
Sant'Ana, 2011), and, possibly in other related environmental problems
(Can et al., 2011; Fernández-Camacho et al., 2015) has taken place. In
developed countries, the situation can be considered stabilized, as in
case of Europe, where estimations indicate that more than 125 million
people could actually be exposed to road traffic noise above 55 dB L
den
(day-evening-night–level indicator), including more than 37 million
exposed to noise levels above 65 dB L
den
(EEA, 2014).
In the 1990s, some studies detailed the harmful effect of acoustic
pollution on the health of human beings (Passchier-Vermeer and
Passchier, 2000). This includes annoyance (Arana and García, 1998;
Fidell et al., 1991; Fields, 1998; Guski, 1999) and sleep disturbance
(Carter, 1996; Öhrström, 1990, 1991, 1995; Thiessen, 1988). Such feel-
ings of displeasure show a relation with adverse effects on humanemo-
tions, leading to anger, disappointment (Fields, 1998) and even stress
(Evans et al., 1995, 2001). Stress hormones have the potential to in-
crease the incidence risk of cardiovascular diseases (Babisch et al.,
1990, 1994; Ising et al., 1999). Also, in the same years, the first studies
were published that disclosedthe approximate percentages of the Euro-
pean population who are exposed to day and night levels higher than
55 dBA (Berglund et al., 1999; Lambert and Vallet, 1994). In the margins
of uncertainty of these studies, they indicate that about 40% of the pop-
ulation in the European Union is exposed to road traffic noise with an
equivalent sound pressure level exceeding 55 dBA daytime; and 20% is
exposed to levels exceeding 65 dBA. More than 30% are exposed at
night to L
eq
exceeding 55 dBA which are disturbing to sleep. The emer-
gence of this large number of studies in different countries allowed car-
rying out a meta-analysis and some synthesis curves emerged, which
can be used for the prediction of the percentage of annoyed subjects
(Miedema and Oudshoorn, 2001; Miedema and Vos, 1998). According
to these curves, the estimations indicated approximately 23%, 18% and
10% of highly annoyed receivers for L
den
of 65 dB in the cases of aircraft,
road traffic and railway respectively.
Taking into account the evident adverse effects of environmental
noise, the European Commission recognized community noiseas an en-
vironmental problem, and an international focus on the problem was
initiated. Therefore, environmental noise emerged as a major issue in
environmental legislation and policy (EC, 1996).
The establishment of the European Noise Directive (EC, 2002)
represented a significant improvement in awareness among the general
public and policymakers about the knowledge of the acoustic situation
in the cities of the member states (Murphy and King, 2010). Neverthe-
less, the European Noise Directive has not only had an impact in
European countries (D'Alessandro and Schiavoni, 2015; Licitra and
Ascari, 2014; Kephalopoulos et al., 2014; Vogiatzis and Remy, 2014)
but has also been used as a reference by various studies made in cities
around the world (Chang et al., 2012; Suárez and Barros, 2014; Zuo
et al., 2014).
Accuracy in the knowledge of the acoustic situation is essential for
the identification of the sites concerned. And, as a consequence, it is
also very relevant for proper planning of control actions for each situa-
tion. Moreover, this knowledge of the acoustic situation can help us to
fight other serious environmental problems because of the relationship
of sound levels with other atmospheric pollutants (Allen et al., 2009;
Morelli et al., 2015; Vlachokostas et al., 2012). In order to conduct stud-
ies of the acoustic situationand its effects on theinhabitants of cities and
for the planning of possible solutions, an important option to consider is
noise mapping. In this
direction, according to the European Noise Directive, noise mapping is
the main tool for the assessment of human exposure to environmental
noise pollution. Consequently, searching for the obtention of better
noise maps means better assessments of exposed population and,
therefore, an improving in the design of action plans.
To obtain a noise map, different methods or strategies can be consid-
ered. Generally, we can differentiate between computerized methods
based on models of sound field propagation and studies carried out
with “in situ”measurements. These methods differ largely from each
other in methodological aspects associated with the selection of sam-
pling points. However, even when a computerized method is used,
“in situ”measurements are necessary for calibration or validation
(WG-AEN, 2007). In connection with this topic, the ISO 1996 interna-
tional standard (ISO 1996-1, 2003; ISO 1996-2, 2007) describes aspects
related to the calculation and measurement procedure of the sound
pressure level outdoors, and it is used as a reference for noise mapping
by the European Noise Directive. Although this paper focuses on the
study of the application of the corrections proposed in ISO 1996 stan-
dard, similar considerations for traffic noise measurements are included
in NT ACOU 039 (NT ACOU 039, 2002). In addition, some standards (ISO
140-5, 1998; ISO 16283-3, 2016; EN 12354-3, 2000) and papers
(Berardi, 2013; Berardi et al., 2011) take into consideration the reflec-
tions on thebuildingsurface for façade sound insulation measurements.
Independently of the chosen method for noise mapping, if we wish
to know the population exposed to noise, the fundamental question is
to evaluate the incident noise level on the façade to the desired height.
It is known that the incident sound level depends on manyfactors, both
temporaland spatial. Therefore, to get a suitable assessment, it is impor-
tant to consider not only the characteristics of the sound source but also
the situation of the evaluation point regarding the source and the specif-
ic urban environment of each street or façade that we intend to evalu-
ate. In this way, for each configuration, the sound energy incident on
the façade of the building under consideration is evaluated as accurately
as possible.
ISO 1996-2 guidelines are often followed to obtain measurement
noise mapping or for the calibration and validation of calculated noise
maps. But what is the level of accuracy that we can obtain with the
596 J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
use of the recommendations provided by the standard? Does the stan-
dard consider the variability that exists in urban environments? These
aspects are essential if we wish to obtain accurate noise maps and effec-
tively reduce the impact of noise pollution on the population.Note that
a recent publication by the World Health Organisation (WHO, 2011)
ranked noise pollution as second among a series of environmental
stressors for the public health impact in a selection of European coun-
tries. Indeed, contrary to the trend for other environmental stressors,
which are declining, noise exposure is actually increasing in Europe
(WHO, 2011).
In this way, some essential aspects, which could be interrelated,
must be taken into account, and they should be considered when the
measuring point is chosen and at the time of applying any corrections
to the value of the measured noise level:
1. The geometry of the measurement point in relation to the different
elements of the surroundings:
a. With respect to the façade, both in height and distance to it.
b. With respect to the sound source (distance, viewing angle…).
c. With respect to the different elements of the urban environ-
ment (street width, building height, terrain features, reflecting
surfaces…).
d. With respect to the geometry and the characteristics of the
façade (angle in relation to the source, building materials,
irregularities or presence of arcades, balconies…).
2. The characteristics of the sound source under evaluation (source
type, spectrum, temporality, intensity, geometry…).
Inthefollowingsections,howtheseaspectsareconsideredin
the ISO 1996-2 standard will be analysed. And a review of the
literature will be made to show the studies that different authors car-
ried out in relation to these issues in real measurement conditions
and the conclusions that have been reached. Section 2 discusses the
corrections proposed by the ISO 1996-2 standard in its normative
part for acoustic measurements in urban environments and the
conditions stipulated in Annex B (informative). In Section 3,each
of the cases of the corrections proposed by the standard is studied.
A literature review is carried out about the works related to these
topics. In Section 4, the possible relation between the corrections
to be applied and the distance between the microphone and
the sound source is analysed. Finally, Section 5 deals with a study
about the effect of the height of the microphone on the value of the
noise level.
2. The ISO 1996-2 standard and the measurement of noise pollution
in urban environments
Regarding the geometry of the measuring point in relation to the
façade and other elements of the environment, the normative part of
the ISO 1996-2 standard only makes reference to the distance between
the microphone and the façade. The rest of the previously mentioned
geometric aspects that could have an influence on the result of a mea-
surement are not considered in the standard. Also, it does not determine
the distance at whichthe microphone must be located in respect to the
building façade in a clear way, leaving this choice to scientific and tech-
nical criteria. In relation to this issue, in order to take into account the
effects of reflection for the building façade, the standard proposes
some corrections to be applied to the values of the measured noise
levels. The ISO 1996-2 standard makes a distinction between three
cases:
a) A position with the microphone flush mounted on the reflecting
surface: −6dB.
b) A position with the microphone located between 0.5 and 2 m in
front of the reflecting surface: −3dB.
c) A free field position (reference condition): 0 dB.
Note that, in this proposal for corrections, there may be some doubts
since:
•There is only one value, −3 dB, for a wide area of distance from the
façade, between 0.5 and 2 m.
•It is not clearwhat must be done if the measurement is performed fur-
ther than 2 m because the standard does not propose any correction.
Do these distances to the façade correspond to a free field?
In order for a proper understanding of thesethree issues, it is neces-
sary to consult Annex B of the standard. This appendix does not belong
to the normative part of ISO 1996-2; it is included in the standard with
an informative character. For the first case, some conditions are de-
scribed for which the indicated value is expected, and some situations
are mentioned for which it is not appropriate to measure this whole
range of distances. And, for the second case, some conditions for
which the measurement point can be considered in a free field are spec-
ified, but this range of distances to the evaluated building façade cannot
be considered as included in the standard.
Secondly, the standard points out that the proposed corrections may
not match the results in real measurement conditions in an urban envi-
ronment. Lower or higher deviations from the values indicated can be
obtained in practice. Again, although the normative part of ISO 1996-2
makes somereferences to the conditions for whichthe proposed correc-
tions are verified, in Annex B (informative), various considerations that
should be taken into account are listed in detail. However, as will be
shown, in many cases, these conditions cannot be verified in a real
urban environment.
In these areas, it is of great interest to conduct a detailed review of
the literature and to propose research strategies that allow to delimit
these uncertainties in order to improve the accuracy of the estimations
of noise doses received by people in their homes and workplaces and in
hospitals, nursing homes, schools, etc.
On the other hand, in connection with the characteristics of the
sound source under evaluation, ISO 1996-2 establishes, in its normative
part, some aspects to be considered, but all of them concern the repre-
sentativeness of the measure regarding the average conditions of the
source in the environment and the variations in weather conditions.
Nothing is indicated about the possibility that the corrections depend
on the features of the source. As we will see later, there are studies
that suggest a dependency in this regard.
It may also beof interest to note that, so far, the possibility of an in-
teraction between geometric and temporal aspects has not been raised.
This means that some geometrical elements influential on the final
value of the incident sound level on the façade may have significant var-
iations over time. These elements must also be considered for the mea-
surements or calculations to be representative of the average situation
of the environment under evaluation.
Different sources of uncertainty should be considered in assessing
the exposure of the population to noise pollution. ISO 1996-2 standard
estimates a minimum uncertainty of 2 dB for measured noise levels,
which is associated with factors such as instrumentation, operating con-
ditions (repeatability), weather and terrain conditions and residual
sound. In the case of computerized noise maps, those uncertainties
due to the digital terrain model (Arana et al., 2011), the software used
(Arana et al., 2009), etc. will be added.
This paper focuses on the aspects related to the geometry of the
measurement point and road traffic as a sound source. Aspects associat-
ed with the temporality of the sound source represent an independent
and wide ranging line of work. For example, spatial and temporal pat-
terns of noise exposure due to road traffic in a city of a developing coun-
try (Pakistan) were studied by Mehdi et al. (2011). A measurement
methodology to know the evolution of daytime building façade noise
levels by road traffic in a city of a developed country (Belgium) was in-
vestigated by Van Renterghem et al. (2012). The effects of singularnoisy
597J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
events on long-term noise indicators were studied by Prieto Gajardo et
al. (2014). The relation between categorisation method and the tempo-
ral variability of urban noise was studied by Rey Gozalo et al. (2015).A
model for estimating annual levels of urban traffic based on Fourier
analysis noise was proposed by Barrigón Morillas et al. (2015).The
pilot project on the establishment of National Ambient Noise Monitor-
ing Network across some cities in a developing country (India) is de-
scribed by Garg et al. (2016).
In this field of work ISO 1996 standard, parts 1 and 2, are currently
under revision. Draft ISO 1996-2 (ISO 1996-2, 2011) recommend a
methodology for the calculation of uncertainty. Following the guide-
lines of draft ISO 1996-2, Alves and Waddington (2014) indicates that,
in their field measurements, the magnitude of the uncertainty associat-
ed with a short term measurement of L
Aeq,1h
is 4.2 dB for road traffic
noise (95% confidence). In connection with this topic, the influence of
short-term sampling parameters on the uncertainty of the L
den
environ-
mental noise indicator is studied according with draft ISO 1996-2 in
other work (Mateus et al., 2015a), which indicates that it is possible to
derive a two variable power lawrepresenting the uncertainty of the de-
termined values as a function of the two samplingparameters: duration
of sampling episode and number of episodes.
3. The microphone location with respect to the building façade
The ISO 1996-2 standard proposes corrections to be applied to the
values of the measured noise levels. These corrections are determined
depending on the distance between the microphone and the back sur-
face, as indicated in Section 2.
The aim of this proposal is to correct the effects of increased noise
levels due to sound reflections on the surface. In this way, the real
value of the incident sound field on the façade (free field) is obtained.
These corrections have been analysed by some authors in urban en-
vironments by “in situ”measurements or simulations. It is interesting to
indicate that the different papers published in this respect, in general,
have focused on studying the corrections, depending on the distance
to the building façade. But they have not carried out a detailed study
of whether the indications of Annex B (informative) are verified or not.
3.1. The position with the microphone flush mounted on the reflecting
surface
Although the standard establishes a correction of −6dBbetweena
microphone flush mounted on the façade and a microphone in a free
field, it also indicates that this is an ideal case, so lower deviations
from this value do occur in practice.
In respect of the mounting of the microphone on the reflective sur-
face, only what is previously indicated appears in the normative part.
It is necessary to look over Annex B (informative) to find two basic op-
tions to place the microphone:
a) On a plate placed on the surface.
b) On the surface itself.
In the first option, a microphone with a 13 mm (1/2 in.) diameter
should be used in the case of road traffic noise and broadband. The mi-
crophone can be mounted parallel to the plate or with the microphone
membrane flush with the surface of the mounting plate. For assembly,
certain conditions relating to the characteristics of the plate and the
mounting must be respected. In relation to the façade, it must be flat,
within 1.0 m of the microphone and with a tolerance of ±0.05 m, and
the distance from the microphone to the edges of the surface must be
higher than 1.0 m.
In the second case, it is indicated that the surface must be made of
concrete, stone, glass, wood or a similar hard material. In addition, the
reflecting surface must be flat, within 1.0 m of microphone and with a
tolerance of ±0.01 m. Annex B also states that, in this case, for octave-
band measurements, a microphone of 13 mm diameter or smaller
should be used. If the frequency range is expanded above 4 kHz, a
6 mm microphone should be used.
The indicated correction of −6 dB was analysed by different papers
in urban environments.
In the work done by Memoli et al. (2008), acoustic measurements
were carried out for a period of 15 min for streets with different geom-
etries: five urban roads with type U and two urban roads with type L.
Road traffic was considered as the sound source and four microphones
were used. They were placed at a 4.0 m height to simultaneously mea-
sure different distances from the façade. A range of distances from
6.6 m to 34.0 m between the source and the façade was used. In this
paper, a difference of 5.7 dB ± 0.8 (95%confidence) is obtained between
the measured sound level with one of the microphones placed on the
reflective surface on a plate and the measured sound level with one of
the microphones placed in free field conditions. Although the resulting
difference is globally consistent with the correction proposed by the
ISO 1996-2 standard, within the range indicated by the authors, differ-
ences higher than 1 dB between what it is indicated in the standard
and the measured values can be observed.
In connection with this topic, Mateus et al. (2015b) conducted si-
multaneous measurements for 47 months with three microphones:
one of them in a free field (3.5 m above the cornice of the building), an-
other flush mounted on the façade using a metal plate and the last one
placed on the glass of a window of the same wall. The distance between
the microphone in the free field and the horizontal line connecting the
other two devices on the façade was 6.3 m. In this case, an urban street
with an L profile was selected, and road traffic wastaken into account as
the sound source. The distance between the source and the sound level
meters was 150 m. Therefore, the results of two options to place the mi-
crophone flush mounted on the façade as indicated by ISO 1996-2 were
analysed in this paper. The results show that, if the microphone is
mounted directly on the window, the difference between the sound
levels varies from 4.0 dB to 4.4 dB, whereas, if a plate of reflective mate-
rial is used, the difference is 4.9 dB. Based on these results, it is stated
that if a −6 dB correction is applied following the standard guidelines,
significant errors could be introduced in some cases.
Therefore, studies that analyse the differences between the sound
levels measured in free field conditions and with the microphone locat-
ed on a reflective surface show disparity values that, depending on the
case, may involve differences of up to 2 dB regarding the correction of
−6 dB established by the ISO 1996-2 standard, as shown in Table 1.
These results may have an important impact on the results obtained
to date under the application of the European Noise Directive. As this
configuration is usually used to locate thereceptors in simulated strate-
gic noise maps, it is important to know what geometrical factors are
causing these results and whether these experimental results are
being considered or not in the application of the propagation models.
Consequently, it is essential to increase the number of studies in this
line of work by taking into account the urban reality of European cities.
Table 1
Differences between the sound levels measured in free field conditions and with the mi-
crophone located on a reflective surface in case of an extended source (see Fig. 1).
Reference Microphone RO
(m)
d′
(m)
a′
(m)
h (m) Correction
(dB)
Memoli et al. (2008) Façade 6.6–34 0 6.6–34 4.0 5.7 ± 0.8
Free –
Mateus et al. (2015b) Façade 150 0 150 15.2 4.9
Free –
Mateus et al. (2015b) Glass 150 0 150 15.2 4.0–4.4
Free –
598 J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
3.2. The position with the microphone located in front of the reflecting
surface
The normative part of the ISO 1996-2 standard excludes measures in
the range of distances from the façade up to 0.5 m. However, some re-
searchers have studied this range of distances. For instance, Memoli et
al. (2008) conducted a study that analysed the differences in sound levels
obtained between a microphone located on the façade and another one
situated at very small distances from it. For this purpose, a speaker with
an MLS signal was used as a sound source. The results show that the cor-
rection near the metal plate at distances between 0.01 and 0.02 m from it
changes very quickly with distance, variations of up to approximately 0.6
dB are obtained. In an analogous way, a study was conducted where the
range of distances to the façade was 0.25 to 0.5 m and in which two dis-
tances between the sound source and the façade were considered: 10.1
and 13.1 m. When the sound source was located at a distance of 10.1 m,
sound level differences between the two microphones were about
1.0 dB at 0.25 m and 0.4 dB at 0.5 m, whereas, for a distance of 13.1 m,
the results were approximately 1.9 dB at 0.25 m and 1.3 dB at 0.5 m.
In another work by Hopkins and Lam (2009) the range between 0.1
and 0.5 m for the distance between the microphone and the reflective
surface is analysed. A comparison of the variation of sound pressure
level predicted by the method of the integral equation (IEM) and that
measured in a scale model 1:5 in a semi-anechoic chamber with a
point sou rce for different si zes of reflective surface is shown. Differences
between the finite and semi-infinite reflectors are most noticeable at
frequencies below 300 Hz.
In this respect, the ISO 1996-2 standard states, “The difference be-
tween the sound pressure level at a microphone placed 2 m in front of
the façade and at a free-field microphone is close to 3 dB in an ideal
case where no other vertical reflecting obstacle influences sound prop-
agation to the studiedreceiver. In complex situations, e.g. high building
density on the site, canyon street, etc., this difference can be much
higher.”Therefore, the standard itself indicates the difficulty of accu-
rately knowing the value of the difference between the incident sound
field on the façade and the one effectively measured in these conditions.
Consequently, it indirectly indicates the need to develop research in this
line. The importance of this measurement configuration must be con-
sidered. It is quite used in the assessment of noise exposure in urban
areas and, also, as a reference to validate noise maps at selected sites.
Annex B (informative) of the ISO 1996-2 standard lists a series of spec-
ifications regarding the distances among the microphone, reflecting sur-
face and sound source for which a correction of −3 dB would be applied:
•The façade should be flat with a tolerance of ±0.3 m.
•In order to avoid the edge effects, minimum distances between the
image of the microphone on the reflective surface (point 0) and the
closest edges of the reflecting surface are set up: b (horizontal dis-
tance) and c (vertical distance) (see Fig. 1). These distances must sat-
isfy some conditions:
b≥4d ð1Þ
c≥2d ð2Þ
where d is the perpendicular distance from the microphone to the
façade.
•To guarantee that the incident and reflected sounds have the same
magnitude, in the case of the extended source (road traffic), the crite-
rion of Eq. (3) must be satisfied. This equation relates the distances a′
and d′, taken along the dividing line of viewing angle αas shown in
Fig. 1. Assuming that M’is the point on the dividing line of angle α
at a distance d from the façade, d′can be defined as the distance be-
tween M′and the façade, and a’can be defined as the distance be-
tween M’and the sound source.
d0≤0:1a0ð3Þ
•To ensure that the microphone is placed at an enough distance from
the area of the correction of −6 dB near the façade in the case of an
extended source (road traffic), Eq. (4) should be taken into consider-
ation when an analysis is performed on broadband, and Eq. (5) should
be taken into consideration when an analysis is performed on octave
bands.
d0≥0:5m ð4Þ
d0≥1:6m ð5Þ
•To guarantee that the microphone is in a free field, Eq. (6) should be
considered:
d0≥2a0:ð6Þ
Taking into account these considerations included in the informative
part of the standard, the distance between the façade and the sound
source limits the possibilities to place the microphone with respect to
the evaluated façade. In Fig. 2, different options for the microphone lo-
cation are presented for the distances façade-microphone and micro-
phone-sound source depending on the total distance between the
façade and the sound source. To develop these figures, a minimum dis-
tance of 2.0 m between the microphone and the sound source (the ref-
erence point of the sound source is the nearest vehicle wheel (Jonasson,
2006)) has been considered.
In Fig. 2:
–The solid line just on the axis X in Fig. 2 (a) represents the measure-
ment position on the façade, d′= 0 m. This measurement position is
represented by the solid line of the unit slope in Fig. 2(b).
–The shaded area at the bottom of Fig. 2(a) represents the options for
measuring from 0.5 to2 m, which corresponds to the shadedarea at
the top of Fig. 2(b).
–The shaded area at the top of Fig. 2(a) represents the measurements
in free field conditions. The measurements are also represented by
the shaded area at the bottom of Fig. 2(b).
Considering equations 3, 4 and 6, whose implications are shown in
Fig. 2, it is deduced that:
–The measurement at distances lower than 0.5 m from the façade is
explicitly excluded.
Fig. 1. Microphone near the reflecting surface (ISO 1996-2, 2007).
599J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
–For distances from the façade to the sound source below 5.5 m, only
the option for measuringwith the microphone flush mounted on the
façade guarantees that the correction indicated by the standard, in
this case of −6 dB, will be verified.
–For distances of between 5.5 m and 22 m from the façade to the traf-
fic line, in addition to the option for measuring on the façade, the ef-
fective range of distance from the façade to place the microphone in
the area of 0.5 to 2 m increases. However, only for a distance of 22 m
or higher from the façade to the sound source, a measurement car-
ried out with a microphone located 2 m of the façade ensure the cor-
rection of −3 dB indicated by the standard. For greater distances
than 22 m between the façade and the sound source being
evaluated, any option provided by the standard could be used to
place the microphone to guarantee the corrections indicated by the
standard. In this respect, it should be observed that, for the range
of distances from 0.5 to 2 m from the façade to the microphone, in
the work made by Memoli et al. (2008), a dependence of the correc-
tion due to reflection on the façade with respect to the distance be-
tween the sound source and the façade is found.
–For the distance between the microphone and the sound source in-
dicated, to find an area for measurements that verifies the free
field condition, it is necessary that the sound source is located at
least 6 m from the façade or other influential reflective surface be-
hind the microphone. It must be clarified that this area for measure-
ments is not valid in the ISO 1996-2 standard approach to assess the
incident sound field on the buildingfaçade, butit is valid on a façade
near the measuring point and placed at an equal distance from the
sound source. Naturally, in free field conditions, for distances further
than 4 m from the microphone to the building façade being evaluat-
ed, the value of the measured sound field does not correspond to the
value of the incident field on the façade because an attenuation by
geometric divergence would take place in the propagation from
the measuring point to the reflecting surface.
Just as with the previous correction, this correction of 3 dB has been
analysed by different authors in urban environments.
Considering vehicles as a sound source on highways and main streets
of Toronto, Hall et al. (1984) conducted a study of the differences among
the sound pressure levels measured on the outside of 33 dwellings. To
this end, a comparison of the measurements performed at 2.0 m from
the façades and the surfaces was made, so the microphone was placed di-
rectly on the windows in the last case. The results show that, on average,
a correction of 3 dB between the two measuring points is appropriate, ex-
cept at low frequencies. In this work, there is no indication about the pos-
sible variability detected in this mean value. But it is specified that, for
frequencies below 200 Hz, the obtained values fluctuate significantly
above and below the 3 dB indicated, reaching values of 1.7 dB and
7.3 dB at the third octave bands of 40 Hz and 50 Hz respectively.
Quirt (1985) carried out a study to investigate the behaviour of the
sound field near the exterior surfaces of buildings. For this purpose, he
used a mathematical model to predict noise levels. In the verification,
a series of measurements was made in a semi-anechoic acoustic
chamber with a controlled sound source and another series of “in situ”
measurements were made with a controlled sound source and road
traffic noise. In this study, it is indicatedthat the assumptionthat the en-
ergy is doubled (+ 3 dB) at 2 m from thesurface ofthe buildingis a rea-
sonable approximation for an extended source such as road trafficand
for third octave bands above 100 Hz. This result is consistent with that
specified by the ISO 1996-2 standard in Annex B (informative) with re-
gard to the appearance of coherence effects at low frequencies and the
indication of a minimum distance of 1.6 m for measurements in octave
bands (Eq. (5)).
Both the studies of Hall et al. (1984) and Quirt (1985) were per-
formed before the development of the ISO 1996-2 standard (ISO 1996-
2, 1987), but they agree that, on average, a correction of −3dBinthe
range between 0.5 and 2 m in front of the reflecting surface is suitable.
After the development of the latest version of the ISO 1996-2 stan-
dard (ISO 1996-2, 2007), Memoli et al. (2008) tested the acoustic
corrections due to reflections from the back wall. In each of the measur-
ing points, the distance from the microphone to the façade (d) was var-
ied, establishing at least three values: 0.5, 1 and 2 m. The objective was
to compare the average of the values obtained in the range of 0.5 to 2 m
from the façade and the value established in ISO 1996-2 standard. Using
road traffic as a sound source, some values in the range of 6.6 to 34.0 m
were used for the distance from the source to the façade (D). However,
the values of the distance between the microphone and the sound
source (D-d) for each measurement point are not explicitly stated. The
Fig. 2. Relationship between the distances façade-microphone and façade-sound source
(a) and relationship between the distances microphone-sound source and façade
sound-source (b) according to the measurement areas regulated in Annex B
(informative) of ISO 1996-2 standard.
600 J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
results shown in the study are those obtained for the total streets stud-
ied, and there is no breakdown to distinguish the partial values for roads
with U and L typologies or to distinguish different values of the distance
between the sound source and the microphone. In this study, a differ-
ence of 3.0 ± 0.8 dB (95% confidence) is obtained between the micro-
phone located in the range 0.5–2.0 m and another microphone placed
in a free field. There is a difference of 2.7 ± 0.6 dB (95% confidence) be-
tween the microphone located in the range 0.5–2.0 m and a receiver
flush mounted on the reflecting surface. On average, the results show
a match with those values proposed by the ISO 1996-2 standard. But it
can be observed that there is a certain variability in the experimental
values for the measurement conditions used in this study involving de-
viations of up to 1 dB with respect to the proposed value. In fact, in this
work, using an MLS point source, it is found that, until distances greater
than 1.0 m, there are not differences of 3.0 dB between the microphone
flush mounted on the façade and the microphone located at some dis-
tance from the surface.
Hopkins and Lam (2009) also study the effects of diffraction on the
sound field in front of finite size reflectors in the range between 1 and
2 m for the distance between the microphone and the reflective surface.
Important fluctuations can be reached in connection with different
source-reflector-receiver geometry. The effects are significant until
100 Hz if reflector is larger than 4 × 4 m, but even up to 630 Hz for
2×2mreflector.
In another study, Jagniatinskis and Fiks (2014) realized noise mea-
surements for a year. In this case, as in the previous one, road traffic
was used as a sound source. A location with a high flow of vehicles
was selected where the distance between the microphone and the
sound source was 250 m. Two microphones were used to measure si-
multaneously and were connected to the same station. One of them
was located 2 m from the façade, and the other one was placed on a
plate in one window of the wall. The first of the conclusions drawn
from this study is that, in overall terms, the difference between the an-
nual values of the day-evening-night (L
den
) sound level registered by
both microphones is about −3 dB. In this way, the result matches the
correction proposed by the ISO 1996-2 standard in the case of a micro-
phone located between 0.5 and 2 m in front of a reflective surface. An-
other finding of this work is that the average difference in measured
sound levels between both sound level meters is up to 2 dB lower at
night than in the daytime. This fact could be related to the flow of vehi-
cles in both periods and, therefore, the characteristics of the studied
source.
Another paper that is of interest in this regard is the work of Montes
González et al. (2015) in which the effect of varying the distance be-
tween the microphone and the building reflective surface is studied in
urban environments. The work was carried out in different parts of a
city in a range of distances from 8.2 m to 28.4 m between the façade
and the centre of a set of traffic lanes (reference sound source). Two
microphones were used to measure simultaneously. The reference mi-
crophone was located 2 m from the building façade, and mobile micro-
phone was placed at different distances from it (0 m, 0.5 m, 1.2 m and
3.0 m). Analyses were conducted with microphones situated at the
heights that the ISO 1996-2 standard established for noise mapping:
1.5 m and 4.0 m. In the paper, an explicit reference is made to Annex
B (informative) of the standard and to compliance with some of the as-
pects mentioned in Annex. Also, the effect of the distance between the
microphone and the noise source is analysed. The results show that
the correction values for reflection in real measurement conditions in
urban areasare lower than those recommended by the ISO 1996-2 stan-
dard. In the case of microphones located at a 1.5 m height, the differ-
ences between sound levels obtained on the façade and 2.0 m from it
are 1.1 dB if a correction due to the distance to the sound source is not
applied and 1.7 dB if the correction is applied. In the case of the micro-
phone located 4.0 m high, thesedifferences are 2.0 dB and 2.6 dB respec-
tively. Therefore, the results obtained in this study show significant
differences between the corrections indicated in the standard and the
measured differences. Furthermore, in this range of façade-microphone
distances, an appreciable influence is observed on the outcome associat-
ed with the distance between the sound source and the façade under
evaluation. In addition, this study indicates the possibility that inevita-
ble urban configurations (parking lines) in the streets of our cities
could have a not insignificant effect on the results of the measurements
and, consequently,could have a result not considered at present in noise
maps elaborated under the European Noise Directive. It shouldbe noted
that, if this effect exists, it could involve a variability factor in time on the
setting of the calculation model.
The differences between the sound levels measured with the micro-
phone located on a reflective surface and near façade in the case of an
extended source are summarized in Table 2.
Another situation of a lack of definition that arises in the application
of the corrections proposed by the ISO 1996-2 standard is the existence
of a single correction value for a very wide area between thedistances of
0.5 and 2 m. Perhaps, this is the reason why most of the above men-
tioned studies compare only the mean values obtained in this range of
distances. Although, for example, in the study of Memoli et al. (2008),
the average correction of the positions of 0.5, 1 and 2 m show a coeffi-
cient of variation of approximately 22%. In the work of Montes
González et al. (2015), a comparison is made between the sound values
obtained at 0.5 and 1.2 m from the reflecting surface with respect to
those registered at 2 m. The most significant differences were found at
a height of 4.0 m. At this point, in the measurements performed be-
tween 0.5 and 2 m from the façade, differences of 0.6 ± 0.2 dB (without
correction for distance from the source) and 1.1 ± 0.2 dB (with correc-
tion for distance from the source) were obtained.
Accordingly, for the results obtained in different studies, a wide var-
iation wasfound regarding the correction that would correspond when
the measurement is made between 0.5 and 2 m from the façade under
evaluation. This variation could be motivated by very diverse circum-
stances, and it seems to be associated with the complex urban environ-
ment of our cities. This can be caused because the urban environment
implies the existence of distances between source and façade that, for
certain measurement configurations, does not allow compliance with
the recommendations of ISO 1996-2, or the sound field can be influ-
enced in its propagation by urban configurations (size and shape of
the façades) or by urban elements (parking lanes) that, in some cases,
could become variable in time. The sound source might be rather close
to the façade under evaluation or be influenced in its propagation by
urban configurations. And, in some cases, it could become variable in
time. Both aspects can have repercussions on the accuracy of the noise
maps developed up to now under the European Noise Directive.
Therefore, it is concluded that it is necessary to increase the number
of studies, which check the correction to be made in the case of
Table 2
Differences between thesound levels measured with the microphone located on a reflec-
tive surface and near façade in case of an extended source (see Fig. 1).
Reference Microphone RO
(m)
d′
(m)
a′
(m)
h
(m)
Correction
(dB)
Hall et al. (1984) Near façade No data 2 No data No
data
3.2 ± 0.2
Façade 0 No data
Memoli et al.
(2008)
Near façade 6.6–34 0.5,
1.0,
2.0
4.6–33.5 4.0 3.0 ± 0.8
Free –6.6–34
Memoli et al.
(2008)
Near façade 6.6–34 0.5,
1.0,
2.0
4.6–33.5 4.0 2.7 ± 0.6
Façade 0 6.6–34
Jagniatinskis and
Fiks (2014)
Glass 250 0 250 No
data
≈3
Near façade 2 248
Montes González
et al. (2015)
Near façade 8.2–28.4 2 6.2–26.4 1.5 1.1–1.7
(±0.2)Façade 0 8.2–28.4
Montes González
et al. (2015)
Near façade 8.2–28.4 2 6.2–26.4 4.0 2.0–2.6
(±0.4)Façade 0 8.2–28.4
601J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
measurements performed at a distance from the façade between 0.5
and 2 m depending on the variety of urban configurations and distances
to the sound source that can be found. It is very necessary that, if
analysing the specific effectsof different geometric urban configurations
is wanted, new studies analyse and indicate results independently for
the different configurations of the environment and the different micro-
phone positions, which have been used.
Finally,situations that may be of interestis what must be done if the
measurement is made more than 2 m from the building façade but fails
to fulfil the free field condition (Eq. (6)). This area is not considered in
the ISO 1996 standard and has not been previously studied in detail.
However, it can be of great interest to measure noiselevels in urban en-
vironments. Since this area does not meet the free field condition, it is
still influenced by the building façade. So, perhaps some correction
term will allow evaluating the free sound field incident on a façade.
Therefore, it is of interest to conduct studies in this new line of
work. This possibility has been analysed by Montes Gonzalez et al.
(2015) in a study of the differences between two microphones located
2 and 3 m from the façade, using road traffic as the reference noise
source. The results show a slight increase in the sound level in the
microphone situated at 3 m, although it becomes negligible when
applying a correction due to the difference in distance to the source
between these two positions. These resultsmay indicate the possibility
of using distances between the façade under evaluation and a measur-
ing point larger than 2 m to evaluate the incident sound field on the
façade.
4. The position of the microphone with respect to the sound source
Annex B (informative) of the ISO 1996-2 standard, as has been men-
tioned above, in the case of a microphone in a free field (Eq. (6)), as
when it is positioned at a distance between 0.5 and 2 m from a reflective
surface (Eq. (3)), established relations between the distances micro-
phone-sound source and microphone-reflective surface (see Fig. 2). In
this regard, the standard does not take into account any kind of depen-
dence of the proposed corrections on the distance between the micro-
phone and the sound source, probably because it considered an
effective compliance with the conditions indicated by these equations.
However, due to the great variability in the geometry of streets in real
conditions, it is not possible to verify the condition stated in Eq. (3).
For this reason, it is interesting to analyse the effect that the distance be-
tween the façade and the sound source has on the corrections to be ap-
plied. This would provide checks of the calculation models that are
made through measures in this range of distances.
In relation to this aspect, Memoli et al. (2008) refers to the impor-
tance of registering the distance between the sound source and micro-
phone as well as the distance between the façade and the sound
source (parameter D). The variation of parameter D is associated with
a variation in the distance between the sound source and the micro-
phone, and, due to the different distances between sound sources and
dwellings that exist between northern Europe and southern Europe, it
is considered necessary to take it into account in these types of studies.
In this way, Memoli et al. (2008), using a loudspeaker with an MLS
signal as a sound source, check the differences of 3.0 dB with respect
to a microphone located on the façade. A very interesting aspect was
found, the dependence of these differences on the distance between
the sound source and the measurement point. Differences of 3.0 dB
were found when the sound source was located at a distance of
13.1 m from the reflecting surface. However, if the source was placed
at a distance of 10.1 m, the average difference did not exceed 2.5 dB.
Picaut et al. (2005) analyse the sound propagation in urban areas in
an experimental study. They use an impulsive sound source and an
array of microphones located at heights between 1.2 and 6.0 m on a
street with a U profile whose buildings are approximately 18 m high.
The obtained values during testing indicate a decrease in sound level
as the distance between the source and the array increases, reaching
approximately 11 dB at the 1 kHz octave band between the microphone
positions located 6 and 50 m from the source.
Lee and Kang (2015) conducted a simulation work in order to study
the behaviour of the sound field in urban streets. In particular, they used
a technique based on a calculation method that combines ray tracing
and modelling by source image. The results show, for the case of a
point source, an attenuation of the sound pressure level as the distance
between the source and receiver increases. It is more significant in a
near field, especially in the case of narrow streets. However, in the
case of a line source, for representing road traffic noise, the obtained
values of sound pressure level are relatively constant as the distance be-
tween source and receiver increases, both in narrow and wide streets.
In the study of Montes González et al. (2015), a correction due to the
distance to the sound source is applied in the analysis of each of the
blocks of acoustic measurements (Harris, 1991). These normalized
sound values due to the distance to the sound source were compared
with those sound values not normalized. Overall, it appears that nor-
malized sound values show a qualitative behaviour accordingto expec-
tations and are closer to the results indicated in the ISO 1996-2
standard.
Therefore, no detailed study of the impact that the distance between
the source and façade has on the correction to apply has been made,
whether complying with the conditions established in AnnexB or omit-
ting them. But, according to results published so far, the existence of an
effect due to distance on these corrections seems to be detected. Owing
to normal urban configurations that exist in Europe, this fact could have
significant effects on the assessment of noise impact on the population
in the application ofthe EuropeanNoise Directive if the calibration pro-
cess of the simulated results with the measured sound levels is
considered.
5. The height of the microphone
The ISO 1996-2 standard provides that, for noise mapping, the fol-
lowing microphone heights must be used:
a) 4.0 ± 0.5 m in residential areas with multistorey buildings.
b) 1.2 ± 0.1 m or 1.5 ± 0.1 in residential areas with one floor buildings
and recreational areas.
In relation to this topic, the European Noise Directive states
that, when calculations are carried out for developing strategic
noise maps in relation to noise exposure, the assessment points must
be 4.0 m ± 0.2 m in height above ground level. Similarly, it states
that, when measurements for noise mapping are made, other heights
may be chosen, but they must not be lower than 1.5 m above the
ground, and results should be corrected in accordance with an equiva-
lent height of 4.0 m. However, no correction method is proposed in
this regard.
In this way, the ANSI S12.18. (1994) standard proposes a micro-
phone height between 1.2 and 1.8 m above ground level to perform
acoustic measurements outdoors while the ANSI S12.9-3 (1993) stan-
dard establishes a height between 1.0 and 2.0 m. On the other hand,
the FHWA-PD-96-046 (1996) report of the US Department of Transpor-
tation proposes a microphoneheight of 1.5 m as a preferred position, es-
tablishing other possible options of from between 4.5 m and 7.5 m for
areas of multistorey buildings.
The actual measurement conditions in an urban environment do not
always allow placing the measuring device at the height of 4.0 m as
specified by the ISO 1996-2 standard. Therefore, as neither the Europe-
an Noise Directive nor the ISO 1996-2 standard make any mention of
the use of possible corrections if the measures are carried out at differ-
ent heights, this is considered an area to investigate and analyse that
is of great interest.
602 J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
In connection with this aspect, the “Guide du Bruit des Transports
Terrestrial: Prevision des Niveaux Sonores”(CETUR, 1980)fixes the fol-
lowing corrections (K
h
)forUprofile streets:
kh¼−2h−4ðÞ
lif hN4mð7Þ
kh¼0if h≤4mð8Þ
where “l”is the distance between the façades of both sides ofthe street,
and “h”is the height above ground at which the measuring microphone
is located.
The corrections proposed by the “Guide du Bruit des Transports Ter-
restrial: Prevision des Niveaux Sonores”, which provide a decrease in
sound level as the microphone height increases above 4.0 m, have
been taken, among others, as a reference in different studies (Rey
Gozalo et al., 2013, 2014) to normalize the long-term sound measure-
ments made on balconies of apartments located higher than 4.0 m.
However, this guide does not propose any corrections for microphones
situated between 1.5 and 4.0 m.
In relation to this matter, in Nicol and Wilson (2004), the vertical
variation of the noise level is analysed in urban streets with a U profile.
To do this, taking road traffic as the reference sound source, several
streets of the city of Athens were selected with different relationships
between the average height of buildings and the width of the street. Si-
multaneous measurements of 15 min were made with three micro-
phones at a distance of 1 m from the building façade. One of the
microphones was placed on the street and the others, with different
configurations, were placed on two floors of the building. The results
show a decrease in sound level as the height increases. Based on the
data reported in this study, an average wasmade of the obtained differ-
ences among the sound value registered by the microphone located at
street level and those registered by microphones located on different
floors of the building at heights of 8, 11.5, 15, 18.5 and 22 m. The results
show a decrease of 2.3 dB, 3.1 dB,3.5 dB, 2.1 dB and 7.8 respectively with
height, so the trend is in line with what is established in the “Guide du
Bruit des Transports Terrestrial: Prevision des Niveaux Sonores”in rela-
tion with a decrease of sound level as height increase above 4 m al-
though the measured decrease in this paper is results greater than
that proposed in the standard. The results found by the authors for
two of the streets studied in this work are shown in Fig. 3.
Shortly after the publication of the work of Nicol and Wilson (2004),
Soler Rocasalbas et al. (2005) focused their analysis on the differences of
microphones located at heights between 1.5 and 4.0 m. They assessed
noise levels in different circumstances based on the slope of the street,
the distance from the building façade and trafficflow. The results
show that, on average, the microphone situated at 1.5 m registered
0.2 dB more than the microphone at 4.0 m (see Fig. 3). So, the difference
is very small between the two locations according to what is indicated
in the “Guide du Bruit des Transports Terrestrial: Prevision des Niveaux
Sonores”(CETUR, 1980).
Also, in the same direction, some studies have been conducted by
combining simulation software and experimental measurements in
order to study the behaviour of the sound level on the façade in streets
with road trafficconditions(Janczur et al. 2006a, 2006b, 2009; Walerian
et al., 2011). Generally, in these works, the field test confirmed the val-
idation software for the range of higher floors. However, for the range of
lowest floors, an overestimation of the sound level is observed.
Firstly, Janczur et al. (2006a) conducted a study to predict the distri-
bution of noise levels on the façade of buildings by simulation software
(PROP11), and these estimates were experimentally verified (Janczur et
al., 2006b). The agreement between measurement and simulation re-
sults was tested for different directivity characteristics of an equivalent
point source representing the vehicles. The study was made in an urban
street with a width of 43.4 m with buildings on both sides with heights
of 25.8 and 32.4 m. The microphones were placed at heights of 2.0, 5.3,
8.6, 14.6, 19.1, 22.4and 25.7 m and at a distance of 0.5 m from the façade
of the highest building. The experimental results due to current traffic
show that, between 2.0 and 5.3 m, there is an average increase of
noise levels of 0.5 dB. This increase, not foreseen in the standard,
could have an influence on noise mapping. For heights between 5.3
and 8.6 m and 8.6 and 14.6 m, there are mean decreases of 0.5
and 0.4 dB respectively, which are in line with the estimates of Eq. (7)
(see Fig. 3).
In an analogous way to the previous work, Janczur et al. (2009)
analysed a new urban environment by carrying out a comparison of
the data obtained through simulation and acoustic measurements. In
this case, a series of microphones were placed in each of the 10 floors
of the façade of a 34 m high building located in the vicinity of a road
in a street with an L typology. Measurements of 10 min were performed
to determine the equivalent noise level by a four channel digital
analyser. The measurements were divided into three groups of
Fig. 3. Sound level variation depending on the height of microphone.
603J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
simultaneous measures. In the first one, receivers in floors 1 to 4 are in-
cluded; in thesecond one, floors 4 to 7 areincluded and, in the third one,
receivers in floors 7 to 10 are included. To this end, the microphones
were placed 1 m from the building façade and 1.5 m above the corre-
sponding floor. The experimental results show an increase in noise
levels of approximately 1.5 dB between the heights of 5.6 and 13.9 m
and 0.7 dB between the heights of 13.9 and 22.2 m. This increase in
sound level is opposite what is expected, even just for reasons of geo-
metrical divergence. Above 22.2 m, sound levels begin to decrease as
altitude increases (see Fig. 3).
Walerian et al. (2011) carried out a new study similar to the prior
one. In this instance, the urban environment is the same as that used
in the work of Janczur et al. (2009), but, instead of placing the micro-
phones next to the façade of the building, they were located in a zone
near the road. In this study, four microphones were situated at the re-
spective heights of 1.4, 2.8, 4.2 and 5.6 m in two vertical lines next to
a pedestrian bridge, one on each side of the road. Measurements of
10 min were performed to determine the equivalent noise level by a
four channel digital analyser and were divided into twogroups of simul-
taneous measurements, one on each vertical line. The experimental re-
sults show that, for one of the vertical lines, the measured noise level
increases with heightfrom 1.4 m to 4.2 m for a total of 0.5 dB, decreasing
by about 1.0 dB between 4.2 and 5.6 m. However, for the second line,
located 1 m closer to the road (5.95 m), the sound level values remain
nearly constant from 1.4 m to 4.2 m high, showing a fall of 1.0 dB be-
tween 4.2 and 5.6 m (see Fig. 3).
In this respect, the work of Montes González et al. (2015) studies the
effect of varying the height of the microphone at different points in a
city with two sound level meters using simultaneous measurements.
For this purpose, a reference microphone was placed 4.0 m high and an-
other microphone was placing at different heights (1.2 m, 1.5 m, 2.5 m
and 6.0 m), performing measurements of 15 min. In all cases, the micro-
phones were placed at 3.0 m from the building façade. The values ob-
tained in broadband for the differences of sound levels measured by
both microphones, with and without the application of a correction
due to the distance to the sound source (Harris, 1991), indicate that,
just considering the proximity to the source as the height of measure-
ments decreases, the obtained values have different signs. The results
achieved for the differences of sound level between the mobile micro-
phone located at heights of 1.2 m, 1.5 m, 2.5 m and 6.0 m and the refer-
ence microphone located at 4.0 m are −0.7 dB, −0.8 dB, −0.2 dB and
0.4 dB respectively. Therefore, the microphone registered, on average,
higher sound values as the height increased in spite of being at a greater
distance from the source. In the case of applying a correction due to the
distance to the source, the values obtained are −0.9 dB, −1.0 dB,
−0.4 dB and 0.7 dB respectively. Thus, the sound level increase with
height is kept. As this paper does not show measured sound levels, the
results cannot be included in Fig. 3.
In addition to variation of sound level depending on the heightof mi-
crophonein analysed studies, the correction proposed in the “Guide du
Bruit des Transports Terrestrial: Prevision des Niveaux Sonores”in case
of U profile streets are shown in Fig. 3.
The fact that recent studies show an increase in noise levels between
1.5 and 4.0 m can lead to underestimations of sound exposure levels
represented in the strategic noise maps of cities around the world
following the instructions of the European Noise Directive and the
ISO-1996-2 standard. Furthermore, this trend appears to exceed 4.0 m,
which would contradict the corrections due to the height of the micro-
phoneproposedbythe“Guide du Bruit des Transports Terrestrial: Pre-
vision des Niveaux Sonores”.
6. Conclusions
This work presents a detailed review of the literature and proposes
research strategies in order to study the relationships between the ISO
1996-2 standard measurements procedure and the accuracy of the
estimations of noise doses received by people obtained by the applica-
tion of the European Noise Directive.
The ISO 1996-2 standard proposes corrections to be applied to the
values of themeasured noise levels. The aim of this proposal is to correct
the effects of increased noiselevels dueto sound reflections on surfaces.
In this way, the real value of the incident sound field on the façade
(free field) is obtained. These corrections have been analysed by some
authors in urban environments using “in situ”measurements or
simulations.
The different papers published in this respect, in general, have fo-
cused on studying the corrections depending on the distance to the
building façade, but they have not carried outa detailed study regarding
to what extent the indications of Annex B (informative) are verified or
not.
The most relevant results published, which may have a significant
impact on the results obtained up to now for the implementation of
the European Noise Directive, are summarized below:
–The studies conducted to analyse the differences between the mea-
sured sound level in the free field and with the microphone located
on the reflective surface present adisparity in values. Depending on
the case, this may involve differences of up to 2 dB relative to the
−6 dB correction indicated by the ISO 1996-2 standard. It should
be remembered that this configuration is usually employed in the
realization of strategy noise maps through simulation to locate re-
ceivers.
–In the studies realized for analysing the correction, which would be
applied when the measurement is made between 0.5 and 2 m
from the façade under evaluation, the results of different works
have a wide variation. This variation can be greater than 1 dB rela-
tive to the −3 dB proposed in the standard.
–The studies carried out with respect to the sound level variation de-
pending on the height of the microphone also show quite different
results. In some cases, they correspond with that expected, and, in
other cases, increases of sound level with heighthave been detected,
which would directly contradict the expected results considering
the geometric divergence of the sound wave.
Besides the mentioned results, some possibilities are not considered
until the moment arises. On the one hand, the corrections applied could
be related to the flow of vehicles and, therefore, tothe characteristicsof
the sound source to be studied. On theother hand, it may be that the in-
cident sound field in the façade can be studied directly by measure-
ments at larger distances than 2 m.
The differences found between the corrections proposed by the
standard and the experimental results could be caused by very diverse
circumstances, and they seem to be associated with the quite complex
configuration of the urban environment of our cities. The sound source
can be rather close to the façade under evaluation or influenced in its
propagation by urban configurations. It could even, in some cases, be-
come variable in time. Therefore, considering the results shown above,
different lines of research arise:
–It is of great importance to know what geometric factors cause the
differences found between the correction values proposed by the
standard and the experimental results and to what extent these ex-
perimental results are being considered in the application of the
propagation models.
–It is essential to increase the number of studies in this line of work by
taking into account the urban reality of European cities, that is, the
wide variety of urban configurations and distances to the sound
source that can be found.
–It is necessary that, if analysing the specific effects of different geo-
metric urban configurations is wanted, that new studies analyse
and indicate results independently for the different configurations
604 J.M. Barrigón Morillas et al. / Science of the Total Environment 565 (2016) 595–606
of the environment and the different microphone positions, which
have been used.
Acknowledgements
The authors wish to thank the funded project TRA2015-70487-R
(MINECO/FEDER, UE). This work was also partially supported by the Na-
tional Commission for Scientific and Technological Research (CONICYT)
through Nacional Fund for Scientific and Technological Development
(FONDECYT) for research initiation No 11140043.
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