SIST EN 1793-5:2016
(Main)Road traffic noise reducing devices - Test method for determining the acoustic performance - Part 5: Intrinsic characteristics - In situ values of sound reflection under direct sound field conditions
Road traffic noise reducing devices - Test method for determining the acoustic performance - Part 5: Intrinsic characteristics - In situ values of sound reflection under direct sound field conditions
This European Standard describes a test method for measuring a quantity representative of the intrinsic characteristics of sound reflection from road noise reducing devices: the reflection index.
The test method is intended for the following applications:
- determination of the intrinsic characteristics of sound reflection of noise reducing devices to be installed along roads, to be measured either on typical installations alongside roads or on a relevant sample section;
- determination of the in situ intrinsic characteristics of sound reflection of noise reducing devices in actual use;
- comparison of design specifications with actual performance data after the completion of the construction work;
- verification of the long-term performance of noise reducing devices (with a repeated application of the method).
The test method is not intended for the following applications:
- determination of the intrinsic characteristics of sound reflection of noise reducing devices to be installed in reverberant conditions, e.g. inside tunnels or deep trenches.
Results are expressed as a function of frequency, in one-third octave bands between 100 Hz and 5 kHz. If it is not possible to get valid measurements results over the whole frequency range indicated, the results shall be given in a restricted frequency range and the reasons of the restriction(s) shall be clearly reported.
Lärmschutzvorrichtungen an Straßen - Prüfverfahren zur Bestimmung der akustischen Eigenschaften - Teil 5: Produktspezifische Merkmale - In-situ-Werte der Schallreflexion in gerichteten Schallfeldern
Dispositifs de réduction du bruit du trafic routier - Méthode d'essai pour la détermination de la performance acoustique - Partie 5: Caractéristiques intrinsèques - Valeurs in situ de réflexion acoustique dans des conditions de champ acoustique direct
Protihrupne ovire za cestni promet - Preskusna metoda za ugotavljanje akustičnih lastnosti - 5. del: Bistvene lastnosti - Terenske vrednosti odboja zvoka z uporabo usmerjenega zvočnega polja
Ta dokument opisuje preskusno metodo za merjenje količine, ki predstavlja bistvene lastnosti na področju odboja zvoka od protihrupnih ovir za cestni promet: indeks odboja.
Preskusna metoda je namenjena:
– ugotavljanju bistvenih lastnosti na področju odboja zvoka od obcestnih protihrupnih ovir, ki se merijo na običajnih mestih namestitve ob cesti ali na ustreznem vzorčnem delu;
– ugotavljanju bistvenih lastnosti na področju odboja zvoka od protihrupnih ovir med dejansko uporabo na mestu uporabe;
– primerjavi specifikacij zasnove s podatki o dejanskih lastnostih po koncu izgradnje;
– preverjanje dolgoročnih lastnosti protihrupnih ovir (z večkratno izvedbo metode).
Preskusna metoda ni namenjena:
– ugotavljanju bistvenih lastnostih za odboj zvoka od protihrupnih naprav za namestitev v odmevnih pogojih, npr. v predorih.
Rezultati so izraženi kot funkcija frekvence v tretjinskih oktavnih pasovih med 100 Hz in 5 kHz. Če ni mogoče dobiti veljavnih rezultatov meritev za celo opredeljeno frekvenčno območje, se rezultati navedejo za omejeno frekvenčno območje, razlogi za omejitev območja pa se jasno opredelijo.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-julij-2016
1DGRPHãþD
SIST-TS CEN/TS 1793-5:2004
3URWLKUXSQHRYLUH]DFHVWQLSURPHW3UHVNXVQDPHWRGD]DXJRWDYOMDQMHDNXVWLþQLK
ODVWQRVWLGHO %LVWYHQHODVWQRVWL7HUHQVNHYUHGQRVWLRGERMD]YRND]XSRUDER
XVPHUMHQHJD]YRþQHJDSROMD
Road traffic noise reducing devices - Test method for determining the acoustic
performance - Part 5: Intrinsic characteristics - In situ values of sound reflection under
direct sound field conditions
Lärmschutzvorrichtungen an Straßen - Prüfverfahren zur Bestimmung der akustischen
Eigenschaften - Teil 5: Produktspezifische Merkmale - In-situ-Werte der Schallreflexion
in gerichteten Schallfeldern
Dispositifs de réduction du bruit du trafic routier - Méthode d'essai pour la détermination
de la performance acoustique - Partie 5: Caractéristiques intrinsèques - Valeurs in situ
de réflexion acoustique dans des conditions de champ acoustique direct
Ta slovenski standard je istoveten z: EN 1793-5:2016
ICS:
17.140.30 Emisija hrupa transportnih Noise emitted by means of
sredstev transport
93.080.30 Cestna oprema in pomožne Road equipment and
naprave installations
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EN 1793-5
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2016
EUROPÄISCHE NORM
ICS 17.140.30; 93.080.30 Supersedes CEN/TS 1793-5:2003
English Version
Road traffic noise reducing devices - Test method for
determining the acoustic performance - Part 5: Intrinsic
characteristics - In situ values of sound reflection under
direct sound field conditions
Dispositifs de réduction du bruit du trafic routier - Lärmschutzvorrichtungen an Straßen - Prüfverfahren
Méthode d'essai pour la détermination de la zur Bestimmung der akustischen Eigenschaften - Teil
performance acoustique - Partie 5: Caractéristiques 5: Produktspezifische Merkmale - In-situ-Werte der
intrinsèques - Valeurs in situ de réflexion acoustique Schallreflexion in gerichteten Schallfeldern
dans des conditions de champ acoustique direct
This European Standard was approved by CEN on 23 January 2016.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 1793-5:2016 E
worldwide for CEN national Members.
Contents
European foreword . 4
Introduction . 6
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Symbols and abbreviations . 13
5 Sound reflection index measurements . 15
5.1 General principle . 15
5.2 Measured quantity . 15
5.3 Test arrangement . 18
5.4 Measuring equipment . 23
5.4.1 Components of the measuring system . 23
5.4.2 Sound source . 24
5.4.3 Test signal . 24
5.5 Data processing . 25
5.5.1 Calibration . 25
5.5.2 Sample rate . 26
5.5.3 Background noise . 27
5.5.4 Signal subtraction technique . 27
5.5.5 Adrienne temporal window . 30
5.5.6 Placement of the Adrienne temporal window . 32
5.5.7 Low frequency limit and sample size . 33
5.6 Positioning of the measuring equipment . 35
5.6.1 Maximum sampled area. 35
5.6.2 Selection of the measurement positions. 35
5.6.3 Reflecting objects . 42
5.6.4 Safety considerations. 42
5.7 Sample surface and meteorological conditions . 42
5.7.1 Condition of the sample surface . 42
5.7.2 Wind . 42
5.7.3 Air temperature . 42
5.8 Single-number rating of sound reflection DL . 42
RI
5.9 Measurement uncertainty . 43
5.10 Measuring procedure . 43
5.11 Test report . 44
Annex A (informative) Measurement uncertainty . 46
A.1 General . 46
A.2 Measurement uncertainty based upon reproducibility data . 46
A.3 Standard deviation of repeatability and reproducibility of the sound reflection index . 46
Annex B (informative) Template of test report on sound reflection of road noise barriers . 48
B.1 Overview . 48
B.2 Test setup (example) . 50
B.3 Test object and test situation (example) . 51
B.4 Test Results (example) . 53
B.4.1 Part 1 – Results in tabular form . 53
B.4.2 Part 2 – Results in graphic form. 54
B.5 Uncertainty (example) . 54
Annex C (informative) Near field to far field relationship . 56
Bibliography . 57
European foreword
This document (EN 1793-5:2016) has been prepared under the direction of Technical Committee
CEN/TC 226 “Road equipment”, by Working Group 6 “Anti-noise devices”, the secretariat of which is held by
AFNOR.
This document supersedes CEN/TS 1793-5:2003.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by September 2016, and conflicting national standards shall be
withdrawn at the latest by September 2016.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
With respect to the superseded document, the following changes have been done:
— the rotating loudspeaker/microphone assembly has been replaced by a loudspeaker and a 9-
microphone square array (the measurement grid);
— the definition of RI has been changed;
— the geometrical divergence correction factor has been changed;
— a new correction factor for sound source directivity has been introduced;
— a new correction factor for gain mismatch has been introduced;
— the impulse response alignment for signal subtraction has been described in more detail;
— the lowest reliable one-third frequency band has been better defined;
— the way to evaluate the uncertainty of the measurement method from reproducibility data has been
introduced (Annex A);
— a detailed example is given (Annex B);
— information on the near-field to far-field relationship has been added (Annex C).
It should be read in conjunction with:
EN 1793-1, Road traffic noise reducing devices - Test method for determining the acoustic performance –
Part 1: Intrinsic characteristics of sound absorption under diffuse sound field conditions
EN 1793-2, Road traffic noise reducing devices - Test method for determining the acoustic performance –
Part 2: Intrinsic characteristics of airborne sound insulation under diffuse sound field conditions
EN 1793-3, Road traffic noise reducing devices - Test method for determining the acoustic performance –
Part 3: Normalized traffic noise spectrum
EN 1793-4, Road traffic noise reducing devices - Test method for determining the acoustic performance – Part
4: Intrinsic characteristics – In situ values of sound diffraction
EN 1793-6, Road traffic noise reducing devices - Test method for determining the acoustic performance – Part
6: Intrinsic characteristics – In situ values of airborne sound insulation under direct sound field conditions
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus,
Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany,
Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.
Introduction
This document describes a test method for determining the intrinsic characteristics of sound reflection of
noise reducing devices designed for roads in non-reverberant conditions (a measure of intrinsic
performance). It can be applied in situ, i.e. where the noise reducing devices are installed. The method can be
applied without damaging the surface.
The method can be used to qualify products to be installed along roads as well as to verify the compliance of
installed noise reducing devices to design specifications. Regular application of the method can be used to
verify the long term performance of noise reducing devices.
The method requires the average of results of measurements taken in different points in front of the device
under test and/or for specific angles of incidences. The method is able to investigate flat and non-flat
products.
The measurements results of this method for sound reflection are not directly comparable with the results
of the laboratory method (e.g. EN 1793-1), mainly because the present method uses a directional sound
field, while the laboratory method assumes a diffuse sound field. The test method described in the present
document should not be used to determine the intrinsic characteristics of sound reflection of noise reducing
devices to be installed in reverberant conditions, e.g. claddings inside tunnels or deep trenches.
For the purpose of this document reverberant conditions are defined based on the envelope, e, across the
road formed by the device under test, trench sides or buildings (the envelope does not include the road
surface) as shown by the dashed lines in Figure 1. Conditions are defined as being reverberant when the
percentage of open space in the envelope is less than or equal to 25 %, i.e. Reverberant conditions occur
when w/e ≤ 0,25, where e = (w+h +h )
1 2
This method introduces a specific quantity, called reflection index, to define the sound reflection in front of a
noise reducing device, while the laboratory method gives a sound absorption coefficient. Laboratory values
of the sound absorption coefficient can be converted to conventional values of a reflection coefficient taking
the complement to one. In this case, research studies suggest that some correlation exists between
laboratory data, measured according to EN 1793-1 and field data, measured according to the method
described in the present document [7], [10], [20], [21].
This method may be used to qualify noise reducing devices for other applications, e.g. to be installed nearby
industrial sites. In this case the single-number ratings should be calculated using an appropriate spectrum.
(a) Partial cover on both sides of the road; envelope, (b) Partial cover on one side of the road; envelope,
e = w+h +h . e = w+h .
1 2 1
(c) Deep trench; envelope, e = w+h +h . (d) Tall barriers or buildings; envelope, e = w+h +h .
1 2 1 2
Key
r road surface;
w width of open space
NOTE Figure 1 is not to scale.
Figure 1 —Sketch of the reverberant condition check in four cases
1 Scope
This European Standard describes a test method for measuring a quantity representative of the intrinsic
characteristics of sound reflection from road noise reducing devices: the reflection index.
The test method is intended for the following applications:
— determination of the intrinsic characteristics of sound reflection of noise reducing devices to be
installed along roads, to be measured either on typical installations alongside roads or on a relevant
sample section;
— determination of the in situ intrinsic characteristics of sound reflection of noise reducing devices in
actual use;
— comparison of design specifications with actual performance data after the completion of the
construction work;
— verification of the long term performance of noise reducing devices (with a repeated application of the
method).
The test method is not intended for the following applications:
— determination of the intrinsic characteristics of sound reflection of noise reducing devices to be
installed in reverberant conditions, e.g. inside tunnels or deep trenches.
Results are expressed as a function of frequency, in one-third octave bands between 100 Hz and 5 kHz. If it
is not possible to get valid measurements results over the whole frequency range indicated, the results
should be given in a restricted frequency range and the reasons of the restriction(s) should be clearly
reported.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
EN 1793-3, Road traffic noise reducing devices - Test method for determining the acoustic performance - Part
3: Normalized traffic noise spectrum
EN 61672-1, Electroacoustics – Sound level meters – Part 1: Specifications (IEC 61672-1)
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
3 Terms and definitions
For the purposes of this document the following terms and definitions apply:
3.1
noise reducing device (NRD)
device that is designed to reduce the propagation of traffic noise away from the road environment. This may
be a noise barrier, cladding, a road cover or an added device. These devices may include both acoustic and
structural elements
3.2
noise barrier
noise reducing device, which obstructs the direct transmission of airborne sound emanating from road
traffic
3.3
acoustic element
element whose primary function is to provide the acoustic performance of the device
3.4
structural element
element whose primary function is to support or hold in place acoustic elements
3.5
cladding
noise-reducing device, which is attached to a wall or other structure and reduces the amount of sound
reflected
3.6
cover
noise-reducing device, which either spans or overhangs the highway
3.7
added device
added component that influences the acoustic performance of the original noise-reducing device (acting
primarily on the diffracted energy)
3.8
roadside exposure
the use of the product as a noise reducing device installed alongside roads
3.9
sound reflection index
quantity, resulting from a sound reflection test, described by Formula (1)
3.10
measurement grid for sound reflection index measurements
vertical measurement grid constituted of nine equally spaced microphones in a 3x3 squared configuration
Note 1 to entry The orthogonal spacing between two subsequent microphones, either vertically or horizontally, is
s = 0,40 m.
Note 2 to entry See Figure 3 and 5.6.
Note 3 to entry Microphones are numbered like in Figure 3.b.
3.11
reference height
height h equal to half the height, h , of the noise barrier under test: h = h /2
S B S B
Note 1 to entry When the height of the device under test is greater than 4 m and, for practical reasons, it is not
advisable to have a height of the source h = h /2, it is possible to have h = 2 m, accepting the corresponding low
S B S
frequency limitation (see 5.5.7).
Note 2 to entry: See Figures 2 and 3.
3.12
(source and microphone) reference plane for sound reflection index measurements
plane facing the sound source side of the noise reducing device and touching the most protruding parts of
the device under test within the tested area
Note 1 to entry See Figures 2 and 4.
3.13
source reference position
position facing the side to be exposed to noise when the device is in place, located at the reference height h
S
and placed so that the horizontal distance of the source front panel to the reference plane is d = 1,50 m
S
Note 1 to entry See Figures 2 and 4.
3.14
measurement grid reference position
position of the measurement grid compliant with all the following conditions: i) the measurement grid is
vertical; ii) the measurement grid is on the noise reducing device side to be exposed to noise when the
device is in place; iii) the central microphone (microphone n. 5) is located at the reference height h ; iv) the
S
horizontal distance of the central microphone to the reference plane is d = 0,25 m; v) the line passing
M
through the centre plate of the loudspeaker and the central microphone is horizontal
Note 1 to entry See Figures 2, 3 and 4.
3.15
reference loudspeaker-measurement grid distance
distance between the front panel of the loudspeaker and the central microphone (microphone n. 5) of the
measurement grid (kept in vertical position)
Note 1 to entry The reference loudspeaker-measurement grid distance is equal to d = 1,25 m (see Figures 2 and
SM
4).
3.16
free-field measurement for sound reflection index measurements
measurement taken with the loudspeaker and the measurement grid in an acoustic free field in order to
avoid reflections from any nearby object, including the ground, keeping the same geometry as when
measuring in front of the noise reducing device under test
Note 1 to entry See Figure 5.
3.17
maximum sampled area
surface area, projected on a front view of the noise reducing device under test for reflection index
measurements, which must remain free of reflecting objects causing parasitic reflections
3.18
Adrienne temporal window
composite temporal window described in 5.5.5
3.19
background noise
noise coming from sources other than the sound source emitting the test signal
3.20
signal-to-noise ratio, S/N
difference in decibels between the level of the test signal and the level of the background noise at the
moment of detection of the useful event (within the Adrienne temporal window)
3.21
impulse response
time signal at the output of a system when a Dirac function is applied to the input. The Dirac function, also
called δ function, is the mathematical idealisation of a signal infinitely short in time that carries a unit
amount of energy
Note 1 to entry: It is impossible in practice to create and radiate true Dirac delta functions. Short transient sounds
can offer close enough approximations but are not very repeatable. An alternative measurement technique, generally
more accurate, is to use a period of deterministic, flat-spectrum signal, like maximum-length sequence (MLS) or
exponential sine sweep (ESS), and transform the measured response back to an impulse response.
Key
1 Source and microphone reference plane 2 Reference height h [m]
S
3 Loudspeaker front panel 4 Distance between the loudspeaker front panel and
the reference plane d [m]
S
5 Distance between the loudspeaker front 6 Distance between the measurement grid and the
panel and the measurement grid d [m] reference plane d [m]
SM M
7 Measurement grid 8 Noise reducing device height h [m]
B
Figure 2 — (not to scale) Sketch of the sound source and the measurement grid in front of the noise
reducing device under test for sound reflection index measurements
Key
1 Noise reducing device height h [m]
B
2 Reference height h [m]
S
3 Orthogonal spacing between two subsequent microphones s [m]
Figure 3 (a) — (not to scale) Measurement grid Figure 3 (b) — (not to scale) Numbering of the
for sound reflection index measurements measurement points as seen from the sound
(source side). source.
Key
1 Source and microphone reference plane 2 Reference height h [m]
S
3 Loudspeaker front panel 4 Distance between the loudspeaker front panel
and the reference plane d [m]
S
5 Distance between the loudspeaker front 6 Distance between the measurement grid and the
panel and the measurement grid d [m] reference plane d [m]
SM M
7 Measurement grid 8 Noise reducing device height h [m]
B
Figure 4 — (not to scale) Placement of the sound source and measurement grid for sound reflection
index measurement for an inclined noise reducing device (side view)
Key
1 Reference height h [m] 2 Distance between the loudspeaker front panel and the
S
measurement grid d [m]
SM
3 Loudspeaker front panel 4 Measurement grid
Figure 5 — (not to scale) Sketch of the set-up for the reference “free-field” sound measurement for
the determination of the sound reflection index
4 Symbols and abbreviations
For the purposes of this document, the following symbols and abbreviations apply.
Table 1 – Symbols and abbreviations
Symbol or Designation Unit
abbreviation
a major axis of the ellipsoid of revolution used to define the maximum sampled m
area at oblique incidence
a0, a1, a2, a3 Coefficient for the expression of the four-term full Blackman-Harris window -
b Depth of the surface structure of the sample under test m
s
b Width of a portion of material of the sample under test m
m
c Speed of sound in air m/s
C Correction factor for the geometrical divergence -
geo,k
C Correction factor for the sound source directivity -
dir,k
C Correction factor for changes in the sound source gain -
gain,k
d Horizontal distance from the source and microphone reference plane to the m
M
measurement grid; it is equal to d = 0,25 m
M
dS Horizontal distance from the front panel of the loudspeaker to the source and m
microphone reference plane; it is equal to: d = 1,50 m
S
d Horizontal distance from the front panel of the loudspeaker to the m
SM
measurement grid; it is equal to: d = 1,25 m
SM
DL Single number rating of sound reflection dB
RI
δ Any input quantity to allow for uncertainty estimates -
i
Δf Frequency range encompassing the one-third octave frequency bands between Hz
g
500 Hz and 2 kHz
Δf Width of the j-the one-third octave frequency band Hz
j
Δt temporal step between the discrete points of the acquired data (linked to the ms
given sample rate by Δt = 1/f )
s
Δτ moving step of the free field impulse response in the adjustment procedure ms
included in the generalized signal subtraction technique (see 5.5.4)
Δt Time delay gap between the arrival of direct sound at microphone k (k ≠ 5) and s
k5
microphone 5
Δt Time delay gap between the arrival of direct and reflected sound at s
k
microphone k
Δd Path length difference between the arrival of direct sound at microphone k (k ≠ m
k5
5) and microphone 5
Δd Path length difference between the arrival of direct and reflected sound at m
k
microphone k
ε Tolerance on the path length difference at microphone k m
k
F Symbol of the Fourier transform -
f Frequency Hz
f Low frequency limit of sound reflection index measurements Hz
min
f Sample rate Hz
s
f cut-off frequency of the anti-aliasing filter Hz
co
h Noise reducing device height m
B
h Reference height m
S
h (t) Incident reference component of the free-field impulse response at the k-th -
ik
measurement point
h (t) Reflected component of the impulse response at the k-th measurement point -
rk
j Index of the j-th one-third octave frequency band (between 100 Hz and 5 kHz) -
k Coverage factor -
p
kf Constant used for the anti-aliasing filter -
L Sample period length of a non-homogeneous noise reducing device m
p
n Number of measurement points on which to average -
j
r Radius of the maximum sampled area at normal incidence m
R Reduction factor dB
sub
RI Sound reflection index in the j-th one-third octave frequency band
j
s Orthogonal spacing between two subsequent microphones m
s Standard deviation of repeatability -
r
s Standard deviation of reproducibility -
R
t Time s
T Length of the Blackman-Harris trailing edge of the Adrienne temporal window s
W,BH
T Total length of the Adrienne temporal window s
W,ADR
u Standard uncertainty -
U Expanded uncertainty -
w (t) Reference free-field component time window (Adrienne temporal window) at -
ik
the k-th measurement point
w (t) Time window (Adrienne temporal window) for the reflected component at the -
rk
k-th measurement point
5 Sound reflection index measurements
5.1 General principle
The sound source emits a transient sound wave that travels past the measurement grid (microphone)
position to the device under test and is then reflected on it (Figures 2 and 4). Each microphone, being placed
between the sound source and the device under test, receives both the direct sound pressure wave
travelling from the sound source to the device under test and the sound pressure wave reflected (including
scattering) by the device under test. The direct sound pressure wave can be better acquired with a separate
free field measurement (see Figure 5). The power spectra of the direct and the reflected components, gives
the basis for calculating the sound reflection index.
The measurement shall take place in an essentially free field in the direct surroundings of the device, i.e. a
field free from reflections coming from surfaces other than the surface of the device under test. For this
reason, the acquisition of an impulse response having peaks as sharp as possible is recommended: in this
way, the reflections coming from other surfaces than the tested device can be identified from their delay
time and rejected.
5.2 Measured quantity
The expression used to compute the reflection index RI as a function of frequency, in one-third octave bands,
is:
F h t ⋅w t df
( ) ( )
rk,,rk
∫
n
j
∆f
j
RI ⋅C⋅C∆⋅f C∆f (1)
( ) ( )
j ∑ geo,,k dir k j gain,k g
n
j
k=1 Fh t ⋅w t df
( ) ( )
ik,,ik
∫
∆f
j
where
h (t) is the incident reference component of the free-field impulse response at the k-th
i,k
measurement point;
h (t) is the reflected component of the impulse response taken in front of the sample under test
r,k
at the k-th measurement point;
w (t) is the time window (Adrienne temporal window) for the incident reference component of
i,k
the free-field impulse response at the k-th measurement point;
w (t) is the time window (Adrienne temporal window) for the reflected component at the k-th
r,k
measurement point;
F is the symbol of the Fourier transform;
j is the index of the one-third octave frequency bands (between 100 Hz and 5 kHz);
∆f is the width of the j-th one-third octave frequency band;
j
=
k is the microphone number according to Figure 3.b (k = 1, ., 9);
n is the number of microphone positions on which to average (n ≥ 6; see 5.6.2);
j
j
C is the correction factor for geometrical divergence at the k-th measurement point;
geo,k
C (Δf ) is the correction factor for sound source directivity at the k-th measurement point.
dir,k j
C (Δf ) is the correction factor to account for a change in the amplification settings of the
gain,k g
loudspeaker and in the sensitivity settings of the individual microphones when changing the
measurement configuration from free field to in front of the sample under test or vice versa,
if any (see 5.5.1 and Formula (4));
is the frequency range encompassing the one-third octave frequency bands between 500 Hz
∆f
g
and 2 kHz.
The correction factor for geometrical divergence, C , are given by:
geo,k
d
rk,
C = (2)
geo,k
d
ik,
where
d is the distance from the front panel of the loudspeaker to the k-th measurement point;
i,k
d is the distance from the front panel of the loudspeaker to the source and microphone
r,k
reference plane and back to the k-th measurement point following specular reflection;
k is the microphone number according to Figure 3.b (k = 1, ., 9).
NOTE 1 For the microphone n. 5, d = d = 1,25 m
i,5 SM
The distances d , d and the correction factors C are given in Table 2.
geo,k
i,k r,k
Table 2 – Distances d , d and correction factors C
geo,k
i,k r,k
k d , m d , m C
i,k r,k geo,k
1 1,37 1,84 1,80
2 1,31 1,80 1,87
3 1,37 1,84 1,80
4 1,31 1,80 1,87
5 1,25 1,75 1,96
6 1,31 1,80 1,87
7 1,37 1,84 1,80
8 1,31 1,80 1,87
9 1,37 1,84 1,80
NOTE 2 The reflections from different portions of the surface of the device under test arrive at the microphone
position at different times, depending on the travel path from the loudspeaker to the position of each test surface
portion and back. The longer the travel path from the loudspeaker to a specific test surface portion and back, the
greater the time delay. Thus, the amplitude of the reflected sound waves from different test surface portions, as
detected at the microphone positions, is attenuated in a manner inversely proportional to the travel time. For non flat
complex devices it is difficult to predict the exact travel path of each wave, considering also non specular scattering;
therefore the geometrical divergence correction factors are calculated on the basis of specular reflection on an ideal flat
reflecting surface.
The correction factors for sound source directivity are given by:
F h t ,α ⋅w t df
( ) ( )
ik,,k ik
∫
∆f
j
(3)
C ∆f =
( )
dir ,k j
F h t ,β ⋅w t df
( ) ( )
ik,,k ik
∫
∆f
j
where
α is the angle between the line connecting the centre of the front panel of the loudspeaker to
k
microphone 5 and the line connecting the centre of the front panel of the loudspeaker to
microphone k (see Figure 6.a);
β is the angle between the line connecting the centre of the front panel of the loudspeaker to
k
microphone 5 and the line connecting the centre of the front panel of the loudspeaker to the
specular reflection path to microphone k (see Figure 6.a);
h (t,α ) is the incident reference component of the free-field impulse response at the k-th
k
i,k
measurement point;
h (t,β ) is the incident reference component of the free-field impulse response at a point on the
k
i,k
specular reflection path for microphone k and at distance d from the centre of the front
i,k
panel of the loudspeaker;
w (t) is the time window (Adrienne temporal window) for the incident reference component of
i,k
the free-field impulse response at the k-th measurement point;
F is the symbol of the Fourier transform;
j is the index of the one-third octave frequency bands (between 100 Hz and 5 kHz);
is the width of the j-th one-third octave frequency band;
∆f
j
k is the microphone number according to Figure 3.b (k = 1, ., 9).
When measuring the sound source directivity correction factors, the numerator and denominator of the
ratio in Formula (3) shall be measured in two different points at the constant distance d (see Table 2) from
i,k
the centre of the front panel of the loudspeaker. The first point is placed at the microphone position and the
second point is placed on the specular reflection travel path of the sound emitted by the loudspeaker (see
Figure 6).
NOTE 3 For non flat complex devices it is difficult to predict the exact travel path of each wave, considering also non
specular scattering; therefore the correction factors for sound source directivity are calculated on the basis of specular
reflection on an ideal flat reflecting surface.
The sound source directivity correction factors shall be measured only once for each sound source,
assuming that the source directivity patterns don’t change. For the sake of accuracy they may be measured
again from time to time (e.g. once a year).
a) Sketch showing microphone positions 4, 5, and 6 (white circles), the angles α and β for
4 4
microphone 4 and the point, at a distance d from the loudspeaker centre plate, where
i,4
measurements to get the correction factor Cdir,4 shall be done (grey circles)
b) Sketch of the front view of the nine microphone positions (white circles) and the nine positions
where the specular reflection travel path of the sound emitted by the loudspeaker intersect the
plane of the measurement grid (black circles)
Key
1 distance d from the loudspeaker centre plate, 2 angle α between the line connecting the
i,4 4
where measurements to get the correction factor centre of the front panel of the loudspeaker to
C shall be done [m] microphone 5 and the line connecting the centre of
dir,4
the front panel of the loudspeaker to microphone n.
3 angle β between the line connecting the 4 Microphone n. 4
centre of the front panel of the loudspeaker to
microphone 5 and the line connecting the centre of
the front panel of the loudspeaker to the specular
reflection path to microphone n. 4
5 Microphone n. 5 6 Microphone n. 6
7 Orthogonal spacing between two subsequent 8 Horizontal distance from the source and
microphones s [m] microphone reference plane to the
measurement grid d [m]
M
Figure 6 (not to scale)
5.3 Test arrangement
The test method can be applied both in situ and on real-size samples purposely built to be tested using the
method described here.
For applications on real-size samples purposely built to be tested using the method described here the
specimen shall be built as follows:
— a part, composed of acoustic elements, that extends at least 4 m and is at least 4 m high.
The test specimen shall be mounted and assembled in the same manner as the manufactured device is used
in practice with the same connections and seals between components parts.
For in situ applications the test specimen shall be constructed as follows:
— for in situ applications using single acoustic elements to achieve full height:
• the test specimen shall be constructed as a single element which is representative of the in situ
application;
• where the test specimen cannot be constructed as a single element or where the in situ application
is lower than 4 m, the test specimen shall be centred on the loudspeaker axis (at reference
height h above the ground) and built up to 4 m high using smaller height acoustic elements at
S
the base and top as appropriate;
— for in situ applications using stacked elements to achieve full height:
• the test specimen shall be constructed as used in situ.
For the results to be valid on the full frequency range, the minimum dimensions of the sample shall be as
follows (see Figure 7):
— a part, composed of acoustic elements, 4 m wide and 4 m high;
— two posts 4 m high at both sides (if applicable for the specific noise reducing device under test);
In all cases, if the sample under test is non-flat with a periodic spatial corrugation in the vertical direction,
then whenever possible the sample shall be extended in the vertical direction with one full period of the
corrugation (see Figure 8).
Figure 7 — Sketch of the minimum flat sample required for reflection index measurements in the
200 Hz – 5 kHz frequency range (see 5.5.7). The nine white dots represent the measurement grid.
The thin circle represents the maximum sampled area for the central microphone (5.6.1)
Key
1 Source and microphone reference plane 2 Reference height h [m]
S
3 Loudspeaker front panel 4 Distance between the loudspeaker front panel
and the reference plane d [m]
S
5 Distance between the loudspeaker front panel 6 Distance between the measurement grid and
and the measurement grid d [m] the reference plane d [m]
SM M
7 Measurement grid 8 Noise reducing device height h [m]
B
9 Spatial period length of the corrugation in the
vertical direction L [m]
p
Figure 8 — (not to scale) Sketch of the set-up for the reflection index measurement in front of a non-
flat sample with a spatially periodic corrugation in the vertical direction (period length Lp in the
vertical direction); one additional period of the structure, added on the top of the sample, is shown
in lighter grey colour
(a) in front of an inclined flat noise reducing device
(b) in front of an inclined non-flat noise reducing device
Key
1 Source and microphone reference plane 2 Reference height h [m]
S
3 Loudspeaker front panel 4 Distance between the loudspeaker front panel and
the reference plane d [m]
S
5 Distance between the loudspeaker front 6 Distance between the measurement grid and the
panel and the measurement grid d [m] reference plane d [m]
SM M
7 Measurement grid 8 Noise reducing device height h [m]
B
Figure 9 — (not to scale) Sketch of the set-up for the reflection index meas
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