SIST EN 1793-4:2026

SLOVENSKI STANDARD
01-februar-2026
Nadomešča:
SIST EN 1793-4:2015
Protihrupne ovire za cestni promet - Preskusna metoda za ugotavljanje akustičnih
lastnosti - 4. del: Bistvene značilnosti - Notranja difrakcija zvoka
Road traffic noise reducing devices - Test method for determining the acoustic
performance - Part 4: Intrinsic characteristics - Intrinsic sound diffraction
Lärmschutzvorrichtungen an Straßen - Prüfverfahren zur Bestimmung der akustischen
Eigenschaften - Teil 4: Produktspezifische Merkmale - Intrinsische Schallbeugung
Dispositifs de réduction du bruit du trafic routier - Méthode d'essai pour la détermination
de la performance acoustique - Partie 4 : Caractéristiques intrinsèques - Diffraction
sonore intrinsèque
Ta slovenski standard je istoveten z: EN 1793-4:2025
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 1793-4
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2025
EUROPÄISCHE NORM
ICS 17.140.30; 93.080.30 Supersedes EN 1793-4:2015
English Version
Road traffic noise reducing devices - Test method for
determining the acoustic performance - Part 4: Intrinsic
characteristics - Intrinsic sound diffraction
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 4 : Caractéristiques 4: Produktspezifische Merkmale - Intrinsische
intrinsèques - Diffraction sonore intrinsèque Schallbeugung
This European Standard was approved by CEN on 17 November 2025.

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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 1793-4:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Symbols and abbreviations . 10
5 Sound diffraction index measurements . 12
5.1 General principle . 12
5.2 Dimensions and specifications . 13
5.2.1 Added devices . 13
5.2.2 Reference walls . 13
5.2.3 In situ tests . 14
5.3 Positions of the sound source . 14
5.4 Position of the microphone(s) . 14
5.5 Free-field measurements . 15
5.6 Measured quantity . 23
5.7 Measuring equipment . 24
5.7.1 Components of the measuring system . 24
5.7.2 Sound source . 25
5.7.3 Test signal . 26
5.8 Data processing . 26
5.8.1 Calibration . 26
5.8.2 Sample rate . 26
5.8.3 Background noise . 27
5.8.4 Measurement points . 27
5.8.5 Adrienne temporal window . 27
5.8.6 Placement of the Adrienne temporal window . 28
5.8.7 Low frequency limit and sample size . 30
5.9 Positioning of the measuring equipment . 30
5.9.1 Selection of the measurement positions . 30
5.9.2 Reflecting objects . 31
5.9.3 Safety considerations . 31
5.10 Sample surface and meteorological conditions . 31
5.10.1 Condition of the sample surface . 31
5.10.2 Wind. 31
5.10.3 Air temperature. 31
5.11 Sound diffraction index difference . 31
5.12 Single-number rating of sound diffraction index difference . 32
6 Measurement uncertainty . 33
7 Measuring procedure . 33
8 Test report . 34
Annex A (informative)  Low-frequency limit and window width . 36
Annex B (informative) Measurement uncertainty. 41
Annex C (normative) Template of test report on sound diffraction index difference of
added devices . 44
Annex D (normative) Indoor measurements for product qualification . 47
Bibliography . 49
European foreword
This document (EN 1793-4:2025) has been prepared by Technical Committee CEN/TC 226 “Road
equipment”, the secretariat of which is held by AFNOR.
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 June 2026, and conflicting national standards shall be
withdrawn at the latest by June 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 1793-4:2015.
— the title has been improved;
— the ‘References’ clause has been updated;
— the ‘Terms, definitions and abbreviations’ clause has been updated;
— the single-number rating DL is now reported with one decimal digit;
ΔDI
— Annex A “Low-frequency limit and window width” has been added;
— Annex C with a template of the test report has been added;
— previous Annex A has been shifted to Annex D;
— the ‘Bibliography’ clause has been updated.
The EN 1793 series, under the general title Road traffic noise reducing devices — Test method for
determining the acoustic performance, consists of the following parts:
— Part 1: Intrinsic characteristics — Sound absorption under diffuse sound field conditions;
— Part 2: Intrinsic characteristics — Airborne sound insulation under diffuse sound field conditions;
— Part 3: Normalized traffic noise spectrum;
— Part 4: Intrinsic characteristics — Intrinsic sound diffraction;
— Part 5: Intrinsic characteristics — Sound absorption under direct sound field conditions;
— Part 6: Intrinsic characteristics — Airborne sound insulation under direct sound field conditions.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Introduction
Part of the market of road traffic noise reducing devices is constituted of products to be added on the
top of road traffic noise reducing devices and intended to contribute to sound attenuation acting
primarily on the diffracted sound field. These products will be called added devices. This document has
been developed to specify a test method for determining the acoustic performance of added devices.
The test method can be applied in situ, i.e. where the road traffic noise reducing devices and the added
devices are installed. The method can be applied without damaging the road traffic noise reducing
devices or the added devices.
The method can be used to qualify products before the installation along roads as well as to verify the
compliance of installed added devices to design specifications. Repeated application of the method can
be used to verify the long-term performance of added devices.
No other national or international standard exists about the subject of this document.
This method can 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 can preferably be calculated using an
appropriate spectrum.
1 Scope
This document specifies a test method for determining the intrinsic characteristics of sound diffraction
of added devices installed on the top of road traffic noise reducing devices. The test method prescribes
measurements of the sound pressure level at several reference points near the top edge of a road traffic
noise reducing device with and without the added device installed on its top. The effectiveness of the
added device is calculated as the difference between the measured values with and without the added
devices, correcting for any change in height (the method specified gives the acoustic benefit over a
simple barrier of the same height; however, in practice the added device can raise the height and this
could provide additional screening depending on the source and receiver positions).
This document is applicable to:
— the preliminary qualification, outdoors or indoors, of added devices to be installed on road traffic
noise reducing devices;
— the determination of sound diffraction index difference of added devices in actual use;
— the comparison of design specifications with actual performance data after the completion of the
construction work;
— the verification of the long-term performance of added devices (with a repeated application of the
method);
— the interactive design process of new products, including the formulation of installation manuals.
The test method can be applied both in situ and on samples purposely built to be tested using the
method described here.
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 are given in the restricted frequency range and the reasons of the restriction(s) are clearly
reported. A single-number rating is calculated from frequency data.
For indoor measurements, see Annex D.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. 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 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
EN 61672-1, Electroacoustics — Sound level meters — Part 1: Specifications (IEC 61672-1)
EN ISO 354, Acoustics — Measurement of sound absorption in a reverberation room (ISO 354)
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
road traffic noise reducing device
RTNRD
device designed to reduce the propagation of traffic noise away from the road environment
Note 1 to entry: A RTNRD can comprise acoustic elements (3.2) only or both structural (3.3) and acoustic
elements.
Note 2 to entry: Applications of RTNRDs include noise barriers (3.5), claddings (3.6), covers (3.7) and added
devices (3.8).
3.2
acoustic element
element whose primary function is to provide the acoustic performance of the device
3.3
structural element
element whose primary function is to support or hold in place the parts of the RTNRD
3.4
self-supporting acoustic element
acoustic element including its own structural element to support itself
3.5
noise barrier
road traffic noise reducing device which obstructs the direct transmission of airborne sound emanating
from road traffic
3.6
cladding
road traffic noise reducing device which is attached to a wall or other structure and reduces the amount
of sound reflected
3.7
cover
road traffic noise reducing device which either spans or overhangs the road
3.8
added device
additional component that influences the acoustic performance of the original road traffic noise
reducing device
Note 1 to entry: The added device is acting primarily on the diffracted sound energy.
3.9
roadside exposure
use of the product as a noise reducing device installed alongside roads
3.10
sound diffraction index
quantity representing the amount of sound diffracted by the device under test
Note 1 to entry: The sound diffraction index values in one-third octave bands are the result of a test according to
the present document.
Note 2 to entry: The symbol for the sound diffraction index includes information on the setup used during the
test: DI refers to measurements on a reflective reference wall. DI refers to measurements on an
x,refl x,abs
absorptive reference wall. DI refers to in situ measurements; where x is “0” when the added device is not on
x,situ
the top of the test construction and “ad” when the added device is on the top of the test construction (see Table 1).
Note 3 to entry: Formula (1) specifies how to calculate the sound diffraction index.
3.11
sound diffraction index difference
difference between the results of sound diffraction tests on the same reference wall with and without
an added device on the top
Note 1 to entry: This is described by Formulae (5), (6), (7).
3.12
test construction
construction on which the added device is placed
Note 1 to entry: For in situ measurements the test construction is an installed road traffic noise reducing device;
for qualification tests it is a reference wall (see 5.2).
3.13
reference plane of the test construction
vertical plane passing through the midpoint of the top edge of the construction (reference wall or
installed road traffic noise reducing device) on which the added device is placed
Note 1 to entry: See Figures 1, 2, 4, 5, 8.
3.14
reference height of the test construction without the added device
height of the highest point of the test construction in relation to the surrounding ground surface
Note 1 to entry: This highest point is not necessarily lying in the plane of longitudinal symmetry of the reference
test construction, if this symmetry exists (Figure 1).
3.15
reference height of the test construction with the added device on the top
height of the highest point of the added device installed on the test construction in relation to the
surrounding ground surface
Note 1 to entry: This highest point does not necessarily lie in the plane of longitudinal symmetry of the reference
test construction if this symmetry exists (Figure 4).
3.16
free-field measurement for sound diffraction index measurements
measurement taken with the loudspeaker and the measurement microphones in an acoustic free-field
in order to avoid reflections from any nearby object, including the ground and the noise reducing device
under test, keeping the same geometry as when measuring in front of the noise reducing device under
test
Note 1 to entry: For example, see Figure 7.
Note 2 to entry: Microphone and loudspeaker placement is as specified in 5.3, 5.4 and 5.5.
3.17
Adrienne temporal window
analysis window in the time domain to be used for the data processing according to the present
document
Note 1 to entry: The Adrienne temporal window is described in 5.8.5.
3.18
background noise
noise coming from sources other than the source emitting the test signal
3.19
signal-to-noise ratio
S/N ratio
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)
3.20
impulse response
time signal at the output of a system when a Dirac function is applied to the input
Note 1 to entry: 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 2 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.
4 Symbols and abbreviations
For the purposes of this document, the symbols in Table 1 apply.
Table 1 — Symbols and abbreviations
Symbol or abbreviation Designation Unit
α Sound absorption coefficient measured according to EN ISO 354 -
DI Sound diffraction index in the j-th one-third octave frequency dB
j
band
DI Sound diffraction index for the reflective reference wall without dB
0,refl
the added device
DI Sound diffraction index for the reflective reference wall with the dB
ad,refl
added device
DI Sound diffraction index for the absorptive reference wall without dB
0,abs
the added device
DI Sound diffraction index for the absorptive reference wall with dB
ad,abs
the added device
DI Sound diffraction index for the in situ test construction without dB
0,situ
the added device
DI Sound diffraction index for the in situ test construction with the dB
ad,situ
added device
δ Any input quantity to allow for uncertainty estimates -
i
Δf Width of the j-th one-third octave frequency band Hz
j
f Frequency Hz
F Symbol of the Fourier transform -
f Low frequency limit of sound diffraction index measurements Hz
min
f Sample rate Hz
s
f Cut-off frequency of the anti-aliasing filter Hz
co
h Noise barrier height m
B
h Reference height of the test construction m
ref
h Reference height of the test construction without the added m
ref,0
device
h Reference height of the test construction with the added device m
ref,ad
h (t) Incident reference component of the free-field impulse response -
i,k
at the k-th measurement point
h (t) Diffracted component of the impulse response at the k-th -
d,k
measurement point
j Index of the j-th one-third octave frequency band (between -
100 Hz and 5 kHz)
k Index of the k-th measurement point (k = 1… n) -
k Coverage factor -
p
k Constant used for the anti-aliasing filter -
f
Symbol or abbreviation Designation Unit
L Relative A-weighted sound pressure level of the normalized dB
i
traffic noise spectrum in the i-th one-third octave band (see
EN 1793-3)
L Minimum length of the reference wall m
b
L Minimum length of the added device under test m
d
n Number of measurement points -
SI Sound insulation index measured according to EN 1793-6 dB
t Time s or
ms
t Air temperature °C
C
T Length of the Blackman-Harris trailing edge of the Adrienne ms
W,BH
temporal window
u Standard uncertainty -
U Expanded uncertainty -
w (t) Width of the time window (Adrienne temporal window) for the ms
i,k
component of the free-field impulse response received at the k-th
measurement point
w (t) Width of the time window (Adrienne temporal window) for the ms
d,k
component of the impulse response diffracted by the top edge of
the test construction and received at the k-th measurement point
5 Sound diffraction index measurements
5.1 General principle
The sound source emits a transient sound wave that travels toward the road traffic noise reducing
device under test and is partly reflected, partly transmitted and partly diffracted by it. The microphone
placed on the other side of the road traffic noise reducing device receives both the transmitted sound
pressure wave travelling from the sound source through the road traffic noise reducing device and the
sound pressure wave diffracted by the top edge of the road traffic noise reducing device under test (for
the test to be meaningful the diffraction from the vertical edges of the test construction shall be
sufficiently delayed in order to be outside the Adrienne temporal window). If the measurement is
repeated without the added device and the test construction between the loudspeaker and the
microphone, the direct free-field wave can be acquired. The power spectra of the direct and the top-
edge diffracted components, corrected to take into account the path length difference of the two
components, give the basis for calculating the sound diffraction index.
The final sound diffraction index shall be a weighted average of the diffraction indices measured at
different points (see Figures 1-6).
When the test method is applied in situ, the measurement procedure and sound diffraction index
calculation shall be carried out two times, with and without the added device placed on the test
construction.
When the test method is applied on samples purposely built to be tested according to the present
standard, the added device shall be subsequently placed on the top of two reference walls (reflective
and absorptive), or of the same reference wall in two different configurations, (see 5.2) and the
measurement procedure and sound diffraction index calculation shall be carried out for both walls, with
and without the added device on the top.
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.
For measurements carried out indoors or under geometric site conditions that may create a
reverberant field on the emission or reception side (tunnel exits, building facades, etc.), the
requirements of Annex D shall be verified.
5.2 Dimensions and specifications
5.2.1 Added devices
The added device shall have a minimum length L of 10 m. The reference wall shall have a minimum
d
length L of 10 m and a minimum height of 4 m. The reference wall shall be vertical, flat and fixed firmly
b
and without any air gaps on a supporting construction (foundation, floor etc.). The top surface of the
supporting construction shall be level with the surrounding ground surface.
The maximum size of the added device measured perpendicularly from the reference plane either in the
direction of the source or in the direction of the microphones shall not exceed a value of 1,0 m (see
Figure 8).
5.2.2 Reference walls
Two versions of the reference wall shall be used in the tests:
a) A reflective reference wall, constructed of homogeneous panels with a smooth surface finish. The
wall shall be free of air leaks and shall have a thickness not greater than 0,20 m. The reference wall
shall have the minimum values of sound insulation index measured according to EN 1793-6
specified in Table 2, in order that the sound transmission through the reference wall is negligible.
Table 2 — Minimum values of the sound insulation index of the reference wall, measured
according to EN 1793-6. Tolerance ± 0,5 dB
Octave centre frequency (Hz) 125 250 500 1000 2000 4000
SI (dB) 21,0 22,0 24,0 26,0 29,0 32,0
b) An absorptive reference wall, constructed as mentioned under A, lined on the source side with an
absorptive flat layer of a single porous material having the minimum values of sound absorption
coefficient measured according to EN ISO 354 specified in Table 3.
Table 3 — Minimum values of the sound absorption coefficient for the absorptive treatment
of the reference wall, measured in reverberation room. Tolerance ± 0,05
Octave centre frequency (Hz) 125 250 500 1000 2000 4000
α 0,20 0,50 0,85 0,95 0,95 0,95
5.2.3 In situ tests
When applying the test method in situ on existing noise reducing devices, with the intention of
obtaining results valid over the entire frequency range specified in 5.6, the test construction shall
satisfy the requirements in 5.2.2.
If these requirements cannot be fulfilled by the existing noise reducing device, the obtained results shall
only be valid over a restricted frequency range (see 5.8.7) and for the type of noise reducing device
being tested.
5.3 Positions of the sound source
Two angles of incidence, 90° and 45°, shall be used (see Figures 2 and 5).
For execution of the diffraction test at a right angle to the test construction the sound source shall be
placed as follows (see Figures 1, 2, 4 and 5):
— in the vertical plane containing the perpendicular bisector plane to the reference plane;
— horizontally: at 2 m distance from the reference plane of the test construction;
— vertically: in relation to the reference height h of the test construction,
ref
— for the obligatory source position S1: centre of the source 0,50 m lower than h ;
ref
— for the obligatory source position S2: centre of the source 0,15 m lower than h ;
ref
— oriented towards the microphone position M1 (see 5.4 and Figures 1 and 3).
For execution of the diffraction test at an angle of 45° with the reference plane of the test construction
the sound source shall be placed as follows (see Figures 2 and 5):
— in a vertical plane that makes an angle of 45° with the reference plane of the test construction,
passing through its mid-point;
— horizontally: at 2 m distance from the reference plane of the test construction;
— vertically in relation to the reference height h of the test construction,
ref
— for the obligatory source position S3: centre of the source 0,50 m lower than h ;
ref
— for the obligatory source position S4: centre of the source 0,15 m lower than h ;
ref
— oriented towards the microphone position M6 (see 5.4 and Figures 2 and 3).
5.4 Position of the microphone(s)
For execution of the diffraction test at a right angle to the test construction the microphone(s) shall be
placed as follows (see Figures 1-6):
— in the vertical plane containing the perpendicular bisector plane to the reference plane;
— horizontally: at 2 m distance from the reference plane of the test construction;
— vertically in relation to the reference height h of the test construction,
ref
for the obligatory microphone positions M1, M2, M3, M4 and M5:
• microphone M1: 0,50 m higher;
• microphone M2: 0,25 m higher;
• microphone M3: equal to the reference height;
• microphone M4: 0,25 m lower;
• microphone M5: 0,50 m lower;
— making an angle in the horizontal plane so as to be oriented toward the sound source.
For execution of the diffraction test at an angle of 45° with the reference plane of the test construction
the microphone(s) shall be placed as follows (see Figures 1, 2, 3, 5 and 6):
— in a vertical plane that makes an angle of 45° with the reference plane of the test construction,
passing through its mid-point;
— horizontally: at 2 m distance from the longitudinal axis of the test construction;
— vertically in relation to the reference height h of the test construction,
ref
for the obligatory microphone positions M6, M7, M8, M9 and M10:
• microphone M6: 0,50 m higher;
• microphone M7: 0,25 m higher;
• microphone M8: equal to the reference height;
• microphone M9: 0,25 m lower.
• microphone M10: 0,50 m lower.
— making an angle in the horizontal plane so as to be oriented toward the sound source.
5.5 Free-field measurements
For each set of measurements done placing the sound source according to 5.3 (90° and 45°), a “free-
field” impulse response shall be measured for each microphone position, keeping the sound source and
the microphone positions with the same geometrical configuration of the set-up and without the
reference wall or supporting barrier present (see for example Figure 7).
A whole set of measurements shall be carried out within two hours. Otherwise, a new free-field
measurement has to be carried out.
No obstacle shall be present within a distance of 3 m from the microphone(s).
NOTE Figure not to scale.
Key
RP reference plane h reference height of the test construction without the
ref,0
added device [m]
Figure 1 — Source and microphone positions in a vertical cross section of the test construction
without added device.
NOTE Figure not to scale.
Key
RP reference plane
Figure 2 — Source and microphone positions in a top view of the test construction without
added device.
NOTE Figure not to scale.
Key
h reference height of the test construction without the added device [m]
ref,0
Figure 3 — Microphone positions in a vertical back view from receiver side of the test
construction without added device.
NOTE Figure not to scale.
Key
AD added device h reference height of the test construction with the added device [m]
ref,ad
RP reference plane
Figure 4 — Source and microphone positions in a vertical cross section of the test construction
with added device.
NOTE Figure not to scale.
Key
AD added device RP reference plane
Figure 5 — Source and microphone positions in a top view of the test construction with added
device.
Key
AD added device h reference height of the test construction with the added device [m]
ref,ad
Figure 6 — Microphone positions in a vertical back view from receiver side of the test
construction with added device.
NOTE Figure not to scale.
Key
href reference height of the test construction [m]
Figure 7 — Source and microphone positions for the free-field measurement in a vertical cross
section (example given for source position S1 and microphone position M1).
NOTE Figure not to scale.
Key
AD added device h reference height of the test construction with the added device [m]
ref,ad
RP reference plane
Figure 8 — Maximum horizontal dimension of the added device.
5.6 Measured quantity
The expression used to compute the sound diffraction index DI for all loudspeaker locations and
measuring frequencies, in one-third octave bands, is:
2


F h t w t df

( ) ( )
n
d,,k d k
∫

∆ f
1 j

DI =−⋅10 lg (1)

j 
∑
n


k=1
F h t w t df
( ) ( )
ik,,ik
∫

∆ f
j


where
h (t) is the component of the free-field impulse response received at the k-th measurement
i,k
point (k = 1…n);
h (t) is the component of the impulse response diffracted by the top edge of the test
d,k
construction and received at the k-th measurement point (k = 1…n);
w (t) is the time window (Adrienne temporal window) for the component of the free-field
i,k
impulse response received at the k-th measurement point (k = 1…n);
w (t) is the time window (Adrienne temporal window) for the component of the impulse
d,k
response diffracted by the top edge of the test construction and received at the k-th
measurement point (k = 1…n);
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);
k is the index of the k-th measurement point (k = 1…n);
is the width of the j-th one-third octave frequency band (between 100 Hz and 5 kHz);

∆ f
j
n is the number of measurement points (source-microphone combinations), n = 20.
The sound diffraction index shall be calculated two times:
— for the test construction without added device;
— for the test construction with added device.
For each set of measurements, at least one free-field measurement shall be carried out, as described in
5.5.
5.7 Measuring equipment
5.7.1 Components of the measuring system
The measuring equipment shall comprise an electro-acoustic system, consisting of an electrical signal
generator, a power amplifier and a loudspeaker, a microphone with its microphone amplifier and a
signal analyser capable of performing transformations between the time domain and the frequency
domain.
NOTE 1 Some of these components can be integrated into a frequency analyser or a personal computer
equipped with specific add-on board(s).
The essential components of the measuring system are shown in Figure 9.
The microphones shall meet at least the requirements for type 2 in accordance with EN 61672-1 and
have a diameter of 12,7 mm (0,5 inch) maximum.
NOTE 2 The measurement procedure described here is based on ratios of the power spectra of signals
extracted from impulse responses sampled with the same equipment in the same place under the same conditions
within a short time. In addition, a high accuracy in measuring sound levels is not of interest here. Therefore, strict
requirements on the absolute accuracy of the measurement chain are not needed.
The microphones should be sufficiently small and lightweight in order to be fixed on a frame to
constitute the microphone grid without moving.
Figure 9 — Sketch representing the essential components of the measuring system.
5.7.2 Sound source
The electro-acoustic sound source shall meet the following characteristics:
— have a single loudspeaker driver;
— be constructed without any port, e.g. to enhance low frequency response;
— be constructed without any electrically active or passive components (such as crossovers) which
can affect the frequency response of the whole system;
— have a smooth magnitude of the frequency response without sharp irregularities throughout the
measurement frequency range, resulting in an impulse response under free-field conditions where
more than 90 % of the energy is in the first 3 ms.
NOTE As the sound diffraction index is calculated from the ratio of energetic quantities extracted from
impulse responses taken using the same loudspeaker and microphone array within a short time period, the
characteristics of the loudspeaker frequency response are not critical, provided a good quality loudspeaker
meeting the above prescriptions is used.
All the measurements (diffraction and free-field) shall be made with the same amplification gain.
5.7.3 Test signal
The electro-acoustic source shall receive an input electrical signal that is deterministic and exactly
repeatable.
The use of a loudspeaker typically introduces nonlinear distortion in the system, which strictly speaking
violates the requirement for linearity in this method. Distortion due to the loudspeaker increases with
the excitation level. In such cases the user shall be aware of the problem and experiment with the
excitation level to obtain the optimum S/N ratio. Sometimes the S/N ratio may be increased by reducing
the excitation level. With certain types of signals the S/N ratio may be improved by repeating the same
test signal and synchronously averaging the microphone response.
Generally speaking, any kind of excitation signal may be used to determine the impulse response and
respective frequency response function of any linear and time-invariant system, provided that it
contains enough energy at every frequency of interest. The impulse response can be obtained from the
response to the excitation by deconvolution, or the frequency response function can be obtained by
dividing the output spectrum of the system under test by the spectrum of the input. The latter implies
Fourier transformation of the input and output signal in order to perform the division in the spectral
domain.
This document recommends the use of a period of a deterministic, flat-spectrum signal, like maximum-
length sequence (MLS) or exponential sine sweep (ESS), and transform the measured response back to
an impulse response [5], [8], [9], [10], [11].
It shall be verified that
— the generation of the test signal is deterministic and repeatable;
— impulse responses are accurately sampled (without distortion) on the whole frequency range of
interest (one-third octave bands between 100 Hz and 5 kHz);
— the test method maintains a good background noise immunity, i.e. the effective S/N ratio can be
made higher than 10 dB over the whole
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