ISO 13472-2:2010
(Main)Acoustics — Measurement of sound absorption properties of road surfaces in situ — Part 2: Spot method for reflective surfaces
Acoustics — Measurement of sound absorption properties of road surfaces in situ — Part 2: Spot method for reflective surfaces
ISO 13472-2:2010 specifies a test method for measuring in situ the sound absorption coefficient of road surfaces for the one-third-octave-band frequencies ranging from 250 Hz to 1 600 Hz under normal incidence conditions. For special purposes, the frequency range can be changed by modifying the dimensions of the system. The test method is intended for: a) determination of the sound absorption coefficient of semi-dense to dense road surfaces (and, if of interest, also the complex acoustical impedance); b) determination of the sound absorption properties of test tracks in accordance with standards such as ISO 10844 and test surfaces defined in national and international type approval regulations for road vehicles and vehicle tyres; c) verification of the compliance of the sound absorption coefficient of a road surface with design specifications or other requirements.
Acoustique — Mesurage in situ des propriétés d'absorption acoustique des revêtements de chaussées — Partie 2: Méthode ponctuelle pour les surfaces réfléchissantes
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 13472-2
First edition
2010-05-15
Acoustics — Measurement of sound
absorption properties of road surfaces in
situ —
Part 2:
Spot method for reflective surfaces
Acoustique — Mesurage in situ des propriétés d'absorption acoustique
des revêtements de chaussées —
Partie 2: Méthode ponctuelle pour les surfaces réfléchissantes
Reference number
ISO 13472-2:2010(E)
©
ISO 2010
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ISO 13472-2:2010(E)
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ii © ISO 2010 – All rights reserved
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ISO 13472-2:2010(E)
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .2
4 Principle.2
5 Test equipment.4
5.1 Components of the test system.4
5.2 Sound source.4
5.3 Test signal.4
5.4 Impedance tube .4
5.4.1 Tube diameter.4
5.4.2 Tube length and microphone positions.5
5.5 Microphones .6
5.6 In-situ test fixture between impedance tube and test surface.6
5.7 Signal-processing system.6
5.8 Thermometer and pressure measurement .6
6 Measurement and analysis procedure .6
6.1 Stabilizing the system.6
6.2 Calibration of the system.7
6.3 Reference measurement.7
6.4 Background noise measurement.7
6.5 Measurement of a road surface .7
6.6 Data analysis.8
7 Positioning of the equipment.8
7.1 Location of the measurement positions .8
7.1.1 Test surfaces such as those meeting ISO 10844 requirements .8
7.1.2 Regular roads.8
7.2 Condition of the road surface .8
7.3 Temperature.8
8 Measurement and analysis procedure .8
9 Measurement uncertainty.9
10 Test report.11
Annex A (normative) Correction on base of reference measurement .12
Annex B (informative) Measurement uncertainty .13
B.1 General.13
B.2 Expression for the calculation of the absorption coefficient .13
B.3 Sources of uncertainty.14
B.4 Expanded uncertainty of measurement.15
Annex C (informative) Sketch of in-situ test fixture .16
Annex D (informative) Example of a test report.18
Bibliography.20
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ISO 13472-2:2010(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 13472-2 was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 1, Noise.
ISO 13472 consists of the following parts, under the general title Acoustics — Measurement of sound
absorption properties of road surfaces in situ:
⎯ Part 1: Extended surface method
⎯ Part 2: Spot method for reflective surfaces
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ISO 13472-2:2010(E)
Introduction
This part of ISO 13472 specifies a test method for measuring in situ the sound absorption coefficient of road
surfaces as a function of frequency under normal incidence.
This method enables evaluation of the sound absorption characteristics of a road surface without damaging
the surface. It is intended to be used to qualify the absorption characteristics of road surfaces used for vehicle
and tyre testing. It may also be used during road construction, road maintenance, and other traffic noise
studies. However, the field of application is limited to low absorption surfaces.
The method specified in this part of ISO 13472 is based on propagation of the test signal from the source to
the road surface and back to the receiver through an impedance tube. The tube covers an area of
2
approximately 0,008 m and a frequency range, in one-third-octave bands, from 250 Hz to 1 600 Hz. It uses
the test procedure and signal processing specified in ISO 10534-2, but because of the defined frequency
range of application, the dimensions of the system are not adjustable, but fixed.
This method is primarily intended for smooth low absorption surfaces, such as those in accordance with
ISO 10844. The method is not reliable if the measured sound absorption coefficient exceeds 0,15. Surfaces
with values above 0,10 are not considered to be reflective.
[5]
This method is complementary to the extended surface method (ISO 13472-1 ) that covers an area of
2
approximately 3 m and a frequency range, in one-third-octave bands, from 250 Hz to 4 000 Hz.
Both methods should give similar results in the frequency range from 315 Hz to 1 600 Hz, but their fields of
[5]
application and therefore their accuracy differ strongly. The method described in ISO 13472-1 has limited
accuracy at small sound absorption values and is therefore unsuitable for checking compliance of surfaces
with the requirements of such documents as ISO 10844, while the method specified here fails at higher sound
absorption values.
Within their ranges, the methods are also applicable to acoustic materials other than road surfaces.
The measurement results of this method are comparable to the results of the impedance tube method,
[4]
performed on bore cores taken from the surface in accordance with documents such as ISO 10534-1 ,
[7]
ISO 10534-2 and ASTM E1050 .
The measurement results obtained with this method are in general not comparable to the results of the
[1]
reverberation room method (ISO 354 ), because the method described in this part of ISO 13472 uses a
plane progressive wave at perpendicular incidence, while the reverberation room method uses a diffuse sound
field.
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INTERNATIONAL STANDARD ISO 13472-2:2010(E)
Acoustics — Measurement of sound absorption properties of
road surfaces in situ —
Part 2:
Spot method for reflective surfaces
1 Scope
This part of ISO 13472 specifies a test method for measuring in situ the sound absorption coefficient of road
surfaces for the one-third-octave-band frequencies ranging from 250 Hz to 1 600 Hz under normal incidence
conditions. For special purposes, the frequency range can be changed by modifying the dimensions of the
system.
The test method is intended for:
a) determination of the sound absorption coefficient of semi-dense to dense road surfaces (and, if of interest,
also the complex acoustical impedance);
b) determination of the sound absorption properties of test tracks in accordance with standards such as
ISO 10844 and test surfaces defined in national and international type approval regulations for road
vehicles and vehicle tyres;
c) verification of the compliance of the sound absorption coefficient of a road surface with design
specifications or other requirements.
2 Normative references
The following referenced documents are indispensable for the application 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.
ISO 10534-2:1998, Acoustics — Determination of sound absorption coefficient and impedance in impedance
tubes — Part 2: Transfer-function method
ISO 10844, Acoustics — Specification of test tracks for measuring noise emitted by road vehicles and their
tyres
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
IEC 61260, Electroacoustics — Octave-band and fractional-octave-band filters
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ISO 13472-2:2010(E)
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
frequency range
frequency interval in which measurements are valid specified in one-third-octave bands in accordance with
IEC 61260
NOTE The frequency range is specified in one-third-octave bands. This means that its lower limit is the lower limit of
the lowest one-third-octave band specified and its upper limit is the upper limit of the highest one-third-octave band
specified. The frequency range specified in one-third-octave bands of 250 Hz to 1 600 Hz centre frequency implies a
frequency range specified in narrow bands of 220 Hz to 1 800 Hz.
3.2
sound absorption coefficient at normal incidence
α
ratio of sound power entering the surface of the test object (without return) to the incident sound power for a plane wave at
normal incidence
[ISO 10534-2:1998, 2.1]
3.3
sound pressure reflection factor at normal incidence
r
complex ratio of the pressure of the reflected wave to the pressure of the incident wave at the surface of the
test object for a plane wave at normal incidence
3.4
plane of reference for the road surface
hypothetical plane defined by the underside of the sealing device at which the sound pressure reflection factor
is calculated
3.5
signal-to-noise ratio level
difference, in decibels, between the level of the useful signal and the level of the background noise
3.6
normal surface impedance
Z
ratio of the complex sound pressure to the normal component of the complex sound particle velocity at an
individual frequency in the reference plane
NOTE 1 Adapted from ISO 10534-2:1998, 2.4.
NOTE 2 Although not used in specifications of road surfaces, calculating propagation over such a surface requires a
complex acoustic impedance.
4 Principle
[7]
The two microphone impedance tube method (see ISO 10534-2 or ASTM E1050 ) is adapted to a portable
apparatus that enables the normal incidence sound absorption coefficient of plane surfaces to be rapidly
measured over a broad frequency range without distortion of the surface. The procedure enables a single
skilled operator to perform such measurements. There is no need for a calibration for microphones as
required in typical acoustic measurements, but it does require a specific verification of the two microphone
apparatus for amplitude and phase relationship between microphones at the time of the measurement and a
determination of the internal energy loss of the system based on measurements on a totally reflecting plane.
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ISO 13472-2:2010(E)
The apparatus is a standard impedance tube utilizing the two microphone arrangement. A sound signal from a
loudspeaker located at one end propagates through the tube. The open end of the tube is placed on the
surface to be measured. The complex acoustic transfer function of the two microphone signals is determined
and used to compute the normal incidence sound absorption coefficient and related quantities.
The absorption coefficient covers the one-third-octave-band frequency range from 250 Hz to 1 600 Hz.
Figure 1 illustrates the system set-up.
Key
1 loudspeaker
2 vibration isolation
3 microphones
4 in-situ test fixture
5 sound source and amplifier
6 frequency analyser
7 surface under test
Figure 1 — Configuration of the measuring device and related equipment
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ISO 13472-2:2010(E)
[7]
The signal processing is described in ISO 10534-2 and ASTM E1050 and consists basically of the
measurement of the complex transfer function between the two microphones in the presence of the sample
under test. This is then processed to obtain the complex pressure reflection factor from which the acoustic
[7]
absorption can be calculated. The procedure described in ISO 10534-2 and ASTM E1050 includes the
calibration of amplitude and phase properties of the two microphones.
In this method, the test sample holder specified in ISO 10534-2 is replaced by an in-situ test fixture that
enables an airtight connection between the inside of the test tube and the surface of the road under test. The
test tube and the fixture can be either integrated into a single piece or connected by some kind of fixing device
and an airtight seal such as a rubber O-ring.
On the underside of the in-situ test fixture, a ring of deformable material forms an airtight seal with the surface
texture of the road surface on one hand and with the fixture on the other. Sealing is improved with a small
groove made in the fixture (see Figures C.1 and C.2).
The sound absorption coefficient is determined in accordance with the procedure specified in ISO 10534-2.
5 Test equipment
5.1 Components of the test system
The test equipment comprises a signal generator, a sound source, a tube, two microphones mounted flush
with the inside wall of the tube at the specified positions, an in-situ test fixture device to maintain an airtight fit
to the surface, and a signal-processing unit capable of doing complex Fourier transforms in two channels
simultaneously.
Any measurement system that provides the characteristics and meets the criteria specified in ISO 10534-2 is
acceptable.
5.2 Sound source
The sound source shall meet the requirements defined in ISO 10534-2. It:
a) is sealed to and vibration isolated from the tube to minimize structure-borne sound excitation of the tube;
b) has a uniform power response over the frequency range of interest.
5.3 Test signal
The test signal shall be broad band with a uniform spectral density over the frequency range of interest.
A signal generator capable of producing a compatible test signal is often incorporated in a frequency analysis
system. When employing alternative signals, it is recommended that the time blocks in the frequency analysis
be synchronized with repetitions in the test signal pattern.
5.4 Impedance tube
5.4.1 Tube diameter
The diameter of the tube shall be (100 ± 1) mm. The tube shall have a circular cross-section, be straight with a
uniform cross-section (variations in diameter no greater than 0,2 %) and with smooth, non-porous walls,
without holes or slits and rigid so as to prevent unwanted loss of sound energy.
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ISO 13472-2:2010(E)
NOTE 1 Not meeting the diameter requirement affects the frequency range. The upper frequency at a given diameter,
f , is given by the equation:
u
c
0
f = 0,58
u
d
where
c is the speed of sound, in metres per second;
0
d is the diameter, in metres, of the tube.
NOTE 2 Loss of energy due to vibrations of the walls is generally prevented by using a metal tube with a thickness of
at least 5 % of the tube diameter.
The tube shall have a small ventilation hole in the vicinity of the loudspeaker so as to prevent build-up of static
pressure inside the tube.
5.4.2 Tube length and microphone positions
The length shall be sufficient to make a plane wave develop between the source and the position of the
microphone. This requirement is met when the microphones are at a distance not less than 3d, where d is the
tube diameter, from the sound source. Non-plane waves from the sample are generally suppressed within one
tube diameter. In the case of flat road surfaces and a tube diameter of 100 mm, this is realized by a tube with
a minimum length of 480 mm and with the lowest microphone mounted 100 mm from the plane of reference.
Microphones shall be mounted flush with the inner side wall. When a single pair of microphone positions is
used, the spacing shall be (81 ± 4) mm.
NOTE The minimum and maximum value of the microphones spacing, s, is defined by the upper and lower frequency
of interest as follows.
The maximum spacing is slightly less thаn half of the shortest wavelength and is given by the inequality:
c
0
s < 0,45
max
f
max
A maximum frequency of 1 800 Hz implies a maximum spacing of 85 mm.
The minimum spacing is larger thаn 5 % of the longest wavelength and is given by the inequality:
c
0
s > 0,05
min
f
min
A minimum frequency of 220 Hz implies a minimum spacing of 77 mm.
Reflective test objects cause at certain frequencies destructive interference at the position of the microphones
that jeopardizes the signal-to-noise ratio. The test result can be improved by using different microphone
positions and spacing. Several systems allow for three choices of microphone positioning to allow a wider
spacing for the lower frequency range and a narrow spacing for the higher frequency range.
The spacing shall be known within ± 0,5 mm.
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ISO 13472-2:2010(E)
5.5 Microphones
A pair of nominally identical microphones shall be mounted at the specified positions. The microphone
diameter shall be small in comparison with the spacing between the microphone ports. It is recommended that
the microphone diameter be less than 20 % of the smallest spacing used. This in general implies the
application of “1/4 inch” microphones. For side-wall mounting, microphones of the pressure type are
recommended.
The microphone mountings shall give an airtight seal between the microphone housing and wall of the tube.
Take care when mounting the microphones to follow the manufacturer's recommendation that venting holes
not be blocked. Blocked venting holes can result in static pressure build-up over the diaphragm that alters the
phase response.
5.6 In-situ test fixture between impedance tube and test surface
Similar to a detachable holder (ISO 10534-2:1998, 4.7), an in-situ test fixture shall be fitted in such a way as to
avoid air flowing between the end of the tube opposite the sound source and the surface to be measured. Any
air leakage through this interface appears as absorption in the measurement results. The in-situ test fixture,
like the detachable holder, shall conform to the interior shape and dimensions of the main part of the
impedance tube. The connecting joint of the in-situ test fixture shall be finished carefully and shall exhibit no
slit or hole. The use of a sealant, such as an O-ring, is required for sealing it to the main part of the impedance
tube. Additionally, a groove shall be cut in the in-situ test fixture on the specimen side to accept a bead of
sealing material such as water-soluble modelling clay, for sealing the fixture to the road.
Practically, the in-situ test fixture should have a larger outer diameter than the main part of the tube. The
additional diameter is not used in the measurement, but this additional portion aids in stability when the
system is mounted upright (see Annex C).
The sealing material shall fill irregularities due to surface texture but shall not penetrate into the surface and
shall not spread out on the surface.
5.7 Signal-processing system
The signal-processing unit consists of a two-channel signal analyser capable of determining the narrowband
complex transfer function between the two microphones. The device shall meet the requirements stated in
ISO 10534-2.
NOTE In most cases, this is a multi-channel fast Fourier transform (FFT) signal analyser, but other solutions are
possible. Modern FFT analysers use special signals such as maximum length sequence (MLS) or swept (logarithmic)
sinus for better performance.
5.8 Thermometer and pressure measurement
The temperature shall be measured with a system readable to ± 1 °C. The atmospheric pressure shall be
measured with a system readable to ± 0,5 kPa.
6 Measurement and analysis procedure
6.1 Stabilizing the system
Since the measurement principle relies strongly on accurate phase and amplitude measurements, the system
shall be thermally stable, including the electronic parts, the loudspeaker coil, and the tube. Therefore, before
starting measurements, the system shall be switched on and operating for at least 15 min. Furthermore, the
system shall not be exposed to direct sunlight or other strong thermal sources.
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ISO 13472-2:2010(E)
6.2 Calibration of the system
The phase and amplitude relationship between the microphones shall be determined before each
measurement series and after each hour of measurements for each microphone position in accordance with
the procedure specified in ISO 10534-2:1998, 7.5.
Knowledge of the phase and amplitude response relationship between the microphones is necessary to
enable correction for it in the determination of the complex transfer function between the two microphone
positions.
All parts of the measuring system shall be checked at least once a year for proper functioning in accordance
with the manufacturer's specifications.
6.3 Reference measurement
A reference measurement on a totally reflective specimen, e.g. a steel plate of 10 mm thickness, shall be
conducted. Calculate the one-third-octave-band absorption coefficients of the reflective specimen by
averaging the narrowband absorption data. The reference level in each one-third-octave band shall be less
than 0,03.
The reference measurement shall take place at the same location and directly before or after the actual
measurement series. The results of the reference measurement shall be used to correct the measurement
results for internal energy loss in accordance with the procedure specified in Annex A.
6.4 Background noise measurement
A measurement shall be made with the sound signal switched off. Consecutive measurements shall be
checked against this level for a minimum signal-to-noise level of 10 dB in each one-third-octave band.
To prevent possible interference by non-stationary background noise, measurements shall be performed at
least at 25 m from passing heavy vehicles or motorcycles. Any measurement that comprises potential
disturbance by background noise shall be omitted.
6.5 Measurement of a road surface
Apply a small bead of sealant into the groove made in the in-situ test fixture. The size of bead depends on the
surface texture. Smooth surfaces allow a small bead extending only a few millimetres above the underside of
the fixture; large texture depths require a thicker bead. One has used the right amount when, after pressing
the fixture on the road, little material is pressed out from under the fixture. If nearly no material is pressed out,
remove the fixture and check for a complete circular impressing of the sealing material on the road. If that is
not the case increase the amount of sealant and redo the mounting of the fixture. Remove any pressed-out
sealing material.
Carefully mount the tube, without moving the fixture. Turn on the source and check for possible air leakage.
NOTE This can be done for instance by a stethoscope with an open tube, scanning the perimeter of the coupler.
Perform the measurement, and check the signal-to-noise ratio. If it is too low, increas
...
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