EN 1793-6:2018+A1:2021

SLOVENSKI STANDARD
01-julij-2021
Nadomešča:
SIST EN 1793-6:2018
Protihrupne ovire za cestni promet - Preskusna metoda za ugotavljanje akustičnih
lastnosti - 6. del: Bistvene karakteristike - Terenske vrednosti izolirnosti pred
zvokom v zraku pri usmerjenem zvočnem polju
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
Lärmschutzvorrichtungen an Straßen - Prüfverfahren zur Bestimmung der akustischen
Eigenschaften - Teil 6: Produktspezifische Merkmale - In-situ-Werte der
Luftschalldämmung in gerichteten Schallfeldern
Dispositifs de réduction du bruit du trafic routier - Méthode d'essai pour la détermination
de la performance acoustique - Partie 6 : Caractéristiques intrinsèques - Valeurs in situ
d'isolation aux bruits aériens dans des conditions de champ acoustique direct
Ta slovenski standard je istoveten z: EN 1793-6:2018+A1:2021
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 1793-6:2018+A1
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2021
EUROPÄISCHE NORM
ICS 17.140.30; 93.080.30
English Version
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
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 6 : Caractéristiques 6: Produktspezifische Merkmale - In-situ-Werte der
intrinsèques - Valeurs in situ d'isolation aux bruits Luftschalldämmung in gerichteten Schallfeldern
aériens dans des conditions de champ acoustique
direct
This European Standard was approved by CEN on 19 February 2018 and includes Amendment 1 approved by CEN on 17 August
2020.
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, Turkey 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
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 1793-6:2018+A1:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Sound insulation index measurements . 12
4.1 General principle . 12
4.2 Measured quantity . 12
4.3 Test arrangement . 13
4.4 Measuring equipment . 18
4.4.1 Components of the measuring system . 18
4.4.2 Sound source . 18
4.4.3 Test signal . 18
4.5 Data processing . 19
4.5.1 Calibration . 19
4.5.2 Sample rate . 19
4.5.3 Background noise . 20
4.5.4 Scanning technique using a single microphone . 20
4.5.5 Scanning technique using nine microphones . 20
4.5.6 Adrienne temporal window . 21
4.5.7 Placement of the Adrienne temporal window . 22
4.5.8 Low frequency limit and sample size . 23
4.6 Positioning of the measuring equipment . 24
4.6.1 Selection of the measurement positions. 24
4.6.2 Post measurements . 25
4.6.3 Additional measurements. 25
4.6.4 Reflecting objects . 25
4.6.5 Safety considerations. 25
4.7 Sample surface and meteorological conditions . 26
4.7.1 Condition of the sample surface . 26
4.7.2 Wind . 26
4.7.3 Air temperature . 26
4.8 Single-number rating . 26
4.8.1 General . 26
4.8.2 Acoustic elements . 26
4.8.3 Posts . 27
4.8.4 Global . 27
5 Measurement uncertainty . 28
6 Measuring procedure . 28
7 Test report . 29
Annex A (informative) Categorization of single-number rating . 31
Annex B (informative) Guidance note on use of the single-number rating . 32
Annex C (informative) Measurement uncertainty . 33
C.1 General . 33
C.2 Measurement uncertainty based upon reproducibility data . 33
C.3 Standard deviation of repeatability and reproducibility of the sound insulation
index . 33
Annex D (informative) Template of test report on airborne sound insulation of road traffic
noise reducing devices . 36
D.1 General . 36
D.2 Test setup (example) . 38
D.3 Test object and test situation (example). 39
D.4 Results (example) . 42
D.4.1 Part 1 – Results for ‘element’ in tabular form . 42
D.4.2 Part 2 – Results for ‘element’ in graphic form . 43
D.4.3 Part 3 – Results for ‘post’ in tabular form . 43
D.4.4 Part 4 – Results for ‘post’ in graphic form. 44
D.4.5 Part 5 – Results for global condition (average of ‘element’ and ‘post’) in tabular form . 45
D.4.6 Part 6 – Results for global condition (average of ‘element’ and ‘post’) in graphic form . 46
D.5 Uncertainty (example) . 46
Bibliography . 49

European foreword
This document (EN 1793-6:2018+A1:2021) 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 September 2021, and conflicting national standards
shall be withdrawn at the latest by September 2021.
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 includes Amendment 1 approved by CEN on 06 January 2020.
This document supersedes !EN 1793-6:2018".
!Deleted text"
The start and finish of text introduced or altered by amendment is indicated in the text by tags !".
EN 1793-6 is part of a series of documents and will be read in conjunction with the following:
— 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 1793-5, 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 has been prepared, under the direction of Technical Committee CEN/TC 226
“Road equipment”, by Working Group 6 “Noise reducing devices”.
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, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Introduction
Noise reducing devices alongside roads should provide adequate sound insulation so that sound
transmitted through the device is not significant compared with the sound diffracted over the top. This
document specifies a test method for assessing the intrinsic airborne sound insulation performance for
noise reducing devices designed for roads in non-reverberant conditions. It can be applied in situ,
i.e. where the noise reducing devices are installed. The method can be applied without damaging the
surface of the noise reducing device.
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 averaging of results of measurements taken at different points behind the
device under test. The method is able to investigate flat and non-flat products.
The method uses the same principles and equipment for measuring sound reflection (see EN 1793-5)
and airborne sound insulation (the present document).
The measurement results of this method for airborne sound insulation are comparable but not identical
with the results of the EN 1793-2 method, mainly because the present method uses a directional sound
field, while the EN 1793-2 method assumes a diffuse sound field (where all angles of incidence are
equally probable). Research studies suggest that good correlation exists between laboratory data,
measured according to EN 1793-2 and field data, measured according to the method described in the
present document [4], [5], [6], [7], [15].
The test method described in this document should not be used to determine the intrinsic
characteristics of airborne sound insulation for noise reducing devices to be installed in reverberant
conditions, e.g. inside tunnels or deep trenches or under covers.
For the purpose of this document, reverberant conditions are defined based on the geometric envelope,
e, across the road formed by the barriers, 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 document introduces a specific quantity, called sound insulation index, to define the airborne
sound insulation of a noise reducing device. This quantity should not be confused with the sound
reduction index used in building acoustics, sometimes also called transmission loss.
NOTE 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.
a) Partial cover on both sides of the road; b) Partial cover on one side of the road;
envelope, e = w+h +h envelope, e = w+h
1 2 1
c) Deep trench; d) Tall barriers or buildings;
envelope, e = w+h +h envelope, e = w+h +h
1 2 1 2
Key
r road surface
w width of open space
h Developed length of element, e.g. cover, trench side, barrier or building
h Developed length of element, e.g. cover, trench side, barrier or building
NOTE Figure 1 is not to scale.
Figure 1 — Sketch of the reverberant condition check in four cases
1 Scope
This document describes a test method for measuring a quantity representative of the intrinsic
characteristics of airborne sound insulation for traffic noise reducing devices: the sound insulation
index.
The test method is intended for the following applications:
— determination of the intrinsic characteristics of airborne sound insulation of noise reducing devices
to be installed along roads, to be measured either in situ or in laboratory conditions;
— determination of the in situ intrinsic characteristics of airborne sound insulation 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);
— interactive design process of new products, including the formulation of installation manuals.
The test method is not intended for the determination of the intrinsic characteristics of airborne sound
insulation of noise reducing devices to be installed in reverberant conditions, e.g. inside tunnels or deep
trenches or under covers.
Results are expressed as a function of frequency in one-third octave bands, where possible, between
100 Hz and 5 kHz. If it is not possible to get valid measurement results over the whole frequency range
indicated, the results will be given in a restricted frequency range and the reasons for the restriction(s)
will be clearly reported.
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 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.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
noise reducing device
device that is designed to reduce the propagation of traffic noise away from the road environment
Note 1 to entry: 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
acoustical 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 acoustic elements
3.4
sound insulation index
result of airborne sound insulation test described by Formula (1)
3.5
reference height
height h equal to half the height, h , of the noise reducing device under test: h = h /2 (see Figures 2
S B S B
and 3)
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 4.5.8).
3.6
source reference plane
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
(see Figures 2, 4 and 9)
Note 1 to entry: The device under test includes both structural and acoustic elements.
3.7
microphone reference plane
plane facing the receiver side of the noise reducing device and touching the most protruding parts of the
device under test within the tested area (see Figures 4 and 9)
Note 1 to entry: The device under test includes both structural and acoustic elements.
3.8
source reference position
position facing the side to be exposed to noise when the device is in place, located at the reference
height h and placed so that its horizontal distance to the source reference plane is d = 1 m
S s
(see Figures 2, 5, 8 and 9)
Note 1 to entry: The actual dimensions of the loudspeaker used for the background research on which this
document is based are: 0,40 m x 0,285 m x 0,285 m (length x width x height).
3.9
measurement grid for sound insulation index measurements
vertical measurement grid constituted of nine equally spaced points
Note 1 to entry: A microphone is placed at each point (see Figures 3, 5, 6, 8, 9 and 4.5.4 and 4.5.5).
3.10
barrier thickness
distance t between the source reference plane and the
B
microphone reference plane at a height equal to the reference height h (see Figures 4, 8 and 9)
S
3.11
free-field measurement
measurement taken with the loudspeaker and the
microphone in an acoustic free field in order to avoid reflections from any nearby object, including the
ground (see Figure 6)
3.12
Adrienne temporal window
composite temporal window described in 4.5.6
3.13
background noise
noise coming from sources other than the source emitting the test signal
3.14
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 test signal (within the Adrienne temporal window)
3.15
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 idealization of a signal that is
infinitely short in time which carries a unit amount of energy.
Key
1 source reference plane 4 loudspeaker front panel
2 noise reducing device height, h [m] 5 distance between the loudspeaker front panel and source
B
reference plane, d [m]
S
3 reference height, h [m]
s
Figure 2 — Sketch of the loudspeaker-microphone assembly in front of the noise reducing device
under test for sound insulation index measurements (not to scale)

a) Measurement grid for sound insulation index b) Numbering of the measurement points
measurements as seen from the receiver (not to as seen from the receiver (not to scale)
scale)
Key
1 noise reducing device height, h (m)
B
2 reference height, h (m)
S
3 orthogonal spacing between two adjacent microphones, s (m)
Figure 3 — Measurement points as seen from the receiver
Key
1 sound source reference plane 4 noise reducing device height, h [m]
B
2 microphone reference plane 5 reference height, h [m]
S
3 noise reducing device thickness, t , at height h [m]
B S
Figure 4— Sound source and microphone reference planes (side view, not to scale)

Key
M measurement grid h noise reducing device height (m)
B
s distance between two vertical or horizontal d horizontal distance [loudspeaker - source
S
microphones in the grid reference plane] at height h
s
h reference height d horizontal distance [microphone 5 - microphone
S M
reference plane] at height h
S
Figure 5 — Placement of the sound source and measurement grid for sound insulation index
measurement (side view, not to scale)
Key
S loudspeaker front panel t noise reducing device thickness at height h
B S
M measurement grid d horizontal distance [microphone 5 - microphone
M
reference plane] at height h
S
h reference height d horizontal distance [loudspeaker –
S T
microphone 5] at height h
S
d horizontal distance [loudspeaker - source
S
reference plane] at height h
S
NOTE d= dt++ d ; see Formula (3).
T SB M
Figure 6 — Sketch of the set-up for the reference “free-field” sound measurement for the
determination of the sound insulation index (not to scale)
4 Sound insulation index measurements
4.1 General principle
The sound source emits a transient sound wave that travels toward the device under test and is partly
reflected, partly transmitted and partly diffracted by it. The microphone placed on the other side of the
device under test receives both the transmitted sound pressure wave travelling from the sound source
through the device under test, and the sound pressure wave diffracted by the top edge of the device
under test (for the test to be meaningful the diffraction from the lateral edges should be sufficiently
delayed). If the measurement is repeated without the device under test between the loudspeaker and
the microphone, the direct free-field wave can be acquired. The power spectra of the direct wave and
the transmitted wave give the basis for calculating the sound insulation index.
The sound insulation index shall be the logarithmic average of the values measured at nine points
placed on the measurement grid (scanning points). See Figure 3 and Formula (1).
The measurement shall take place in a sound field free from reflections within the Adrienne temporal
window. 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 can be identified from their delay
time and rejected.
4.2 Measured quantity
The expression used to compute the sound insulation index SI as a function of frequency, in one-third
octave bands, is:


F h t w t df

( ) ( )
tk tk
∫


n
∆ f
j
SI =−⋅10 lg (1)

∑
j
n

k=1

F h 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
ik th
scanning point;
h (t) th
tk is the transmitted component of the impulse response at the k scanning point;
w (t) is the time window (Adrienne temporal window) for the incident reference component
ik
th
of the free-field impulse response at the k scanning point;
w (t) is the time window (Adrienne temporal window) for the transmitted component at the
tk
th
k scanning point;
F is the symbol of the Fourier transform;
j th
is the index of the j one-third octave frequency band (between 100 Hz and 5 kHz);
Δf is the width of the jth one-third octave frequency band;
i
n = 9 is the number of scanning points.
4.3 Test arrangement
The test method can be applied both in situ and on barriers purposely built to be tested using the
method described here. In the second case, the specimen shall be built as follows (see Figure 7):
— a part, composed of acoustic elements;
— a post (if applicable for the specific noise reducing device under test);
— a part, composed of acoustic elements.
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.
The tested area is a circle having a radius of 2 m centred on the middle of the measurement grid. The
sample shall be built large enough to completely include this circle for each measurement.
For qualifying the sound insulation index of posts only, it is only necessary to have acoustic elements
that extend 2 m or more on either side of the post (see Figure 7).
If the device under test has a post to post distance less than 4 m, the distance between posts should be
reduced accordingly but the overall minimum width of the construction should be the same as shown in
Figure 7.
a) Sound insulation index measurements for b) Sound insulation index measurements in
elements and posts front of a post only

c) Sound insulation index measurements in front of a sample having a post to post distance
smaller than 4 m
Key
Thin circles: tested area for elements
Dotted circles: tested area for posts
L actual horizontal length of the acoustic elements having a post to post distance smaller than 4 m
L minimal horizontal length of the sample if the post to post distance is smaller than 4 m
TOT
Figure 7 — Sketch of the minimum sample required for measurements in laboratory conditions
Key
S loudspeaker front panel d horizontal distance [loudspeaker - source reference plane] at height
S
h
S
M measurement grid t barrier thickness at height h
B S
h reference height d horizontal distance [microphone 5 - microphone reference plane] at
S M
height h
S
h barrier height d horizontal distance [loudspeaker - microphone 5] at height h
B T S
NOTE d= d++t d ; see Formula (3).
T SB M
Figure 8 — Sketch of the set-up for the sound insulation index measurement – Normal incidence
of sound on the sample – Transmitted component measurement in front of a flat noise reducing
device (not to scale)
a) Transmitted component measurements in front of a concave noise reducing device

b) Transmitted component measurements in front of a convex noise reducing device

c) Transmitted component measurements in front of an inclined noise reducing device
Key
S loudspeaker front d horizontal distance [loudspeaker - source reference plane] at height h
S S
panel
M measurement grid t barrier thickness at height h
B S
h reference height d horizontal distance [microphone 5 - microphone reference plane] at
S M
height h
S
h barrier height d horizontal distance [loudspeaker - microphone 5] at height h
B T S
NOTE d= d++t d ; see Formula (3).
T SB M
Figure 9 — Examples of the set-up for the sound insulation index measurement – Normal
incidence of sound on the sample (not to scale - informative)
Key
1 device under test
2 microphone
3 loudspeaker
d horizontal distance [microphone 5 - microphone reference plane] at height h
M S
Figure 10 — Sketch representing the essential components of the measuring system
4.4 Measuring equipment
4.4.1 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 10.
The complete measuring system shall meet the requirements of at least a type 1 instrument in
accordance with EN 61672-1, except for the microphone which shall meet the requirements for type 2
and have a diameter of 1/2” maximum.
NOTE 2 The measurement procedure here described 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. Nevertheless, the requirement
for a type 1 instrument is maintained for compatibility with other European Standards.
The microphones should be sufficiently small and lightweight in order to be fixed on a frame to
constitute the microphone grid without moving. In addition, they should be not too expensive. For these
reasons, the microphones are allowed to meet the requirements for type 2.
4.4.2 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 with a
length not greater than 3 ms.
4.4.3 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 deterministic, flat-spectrum signal, like maximum-
length sequence (MLS) or exponential sine sweep (ESS), and transform the measured response back to
an impulse response [8], [9], [10], [11], [12].
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 frequency range of interest within a short measurement
time (no more than 5 min per impulse response);
— the sample rate can be chosen high enough to allow an accurate correction of possible time shifts in
the impulse responses between the measurement in front of the sample and the free-field
measurement due to temperature changes;
4.5 Data processing
4.5.1 Calibration
The measurement procedure here described 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. Therefore, an absolute calibration of the measurement chain with regard to the sound
pressure level is not needed. Nevertheless, it is recommended to check the correct functioning of the
measurement chain from the beginning to the end of the measurement exercise.
4.5.2 Sample rate
The frequency at which the microphone response is sampled depends on the specified upper frequency
limit of the measurement and on the anti-aliasing filter type and characteristics.
The sample rate f shall have a value greater than 43 kHz.
s
NOTE Although the signal is already unambiguously defined when the Nyquist criterion is met, higher sample
rates facilitate a clear reproduction of the signal and the knowledge of the exact wave form. Therefore, with the
prescribed sample rates, errors can be detected and corrected more easily, such as time shifts in the impulse
responses between the measurement in front of the sample and the free-field measurement due to temperature
changes.
The sample rate shall be equal to the clock rate of the signal generator.
The cut-off frequency of the anti-aliasing filter, f , shall have a value:
co
f ≤ kf (2)
co s
where
k = 1/3 for the Chebyshev filter and k = 1/4 for the Butterworth and Bessel filters.
For each measurement, the sample rate, the type and the characteristics of the anti-aliasing filter shall
be clearly stated in each test report.
4.5.3 Background noise
The effective signal-to-noise ratio S/N, taking into account sample averaging, shall be greater than
10 dB over the frequency range of measurements.
NOTE Coherent detection techniques, such as the MLS cross-correlation, provide high S/N ratios.
4.5.4 Scanning technique using a single microphone
The sound source shall be positioned as described in 3.8.
The measurement grid shall be square, with a side length 2 s of 0,80 m. Its centre shall be located at the
reference height h . The grid shall be placed facing the side of the noise reducing device under test
S
opposite to the side to be exposed to noise when the device is in place, so that its horizontal distance to
the microphone reference plane is d = 0,25 m (see Figures 3, 5, 6, 8 and 9). The grid shall be placed at
M
a distance as large as possible from the edges of the noise reducing device under test.
A single microphone shall be subsequently placed at the nine scanning points; the nine resulting
impulse responses shall be then measured. Each of these consists of the direct component, the
transmitted component through the device under test, diffracted components and other parasitic
reflections (Figure 12).
A “free-field” impulse response shall be measured for each microphone position, keeping the supporting
frame with the same geometrical configuration of the set-up and without the barrier present.
In particular, the distance d of the microphone position n. Five from the sound source shall be kept
T
constant (see Figure 6):
d= dt++ d= 1,25+ t (3)
T sB M B
where
t is the barrier thickness (see 3.10).
B
Care shall be taken that the supporting frame does not alter the measurement result.
4.5.5 Scanning technique using nine microphones
As an alternative to the procedure described in 4.5.4, the procedure described below may be used,
leading to the same results.
The sound source shall be positioned as described in 3.8.
The measurement grid shall be square, with a side length 2 s of 0,80 m. Its centre shall be located at the
reference height h . The grid shall be placed facing the side of the noise reducing device under test
S
opposite to the side to be exposed to noise when the device is in place, so that its horizontal distance to
the microphone reference plane is d = 0,25 m (see Figures 3, 5, 6, 8 and 9). The grid shall be placed at
M
a distance as large as possible from the edges of the noise reducing device under test.
A set of nine microphones supported by a rigid frame shall be placed at the nine scanning points
corresponding to the measurement grid and the nine impulse responses are measured simultaneously
or in sequence. Each of these consists of the direct component, the transmitted component through the
device under test, diffracted components and other parasitic reflections (Figure 12).
A “free-field” impulse response shall be measured for each microphone position, keeping the supporting
frame with the same geometrical configuration of the set-up and without the barrier present.
In particular, the distance d of the microphone position n° 5 from the sound source shall be kept
T
constant (see Figure 6):
(4)
d= dt++ d= 1,25+ t
T sB M B
where
t is the barrier thickness (see 3.10).
B
Care shall be t
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