SIST EN ISO 10534-2:2024

SLOVENSKI STANDARD
01-februar-2024
Nadomešča:
SIST EN ISO 10534-2:2002
Akustika - Ugotavljanje akustičnih lastnosti v Kundtovi cevi - 2. del:
Dvomikrofonska tehnika za določanje normalnega koeficienta absorpcije zvoka in
normalne površinske impedance (ISO 10534-2:2023)
Acoustics - Determination of acoustic properties in impedance tubes - Part 2: Two-
microphone technique for normal sound absorption coefficient and normal surface
impedance (ISO 10534-2:2023)
Akustik - Bestimmung der akustischen Eigenschaften in Impedanzrohren - Teil 2: 2-
Mikrofontechnik für Schallabsorptionsgrad und Oberflächenimpedanz bei senkrechtem
Einfall (ISO 10534-2:2023)
Acoustique - Détermination des propriétés acoustiques aux tubes d’impédance - Partie
2: Méthode à deux microphones pour le coefficient d’absorption acoustique normal et
l’impédance de surface normale (ISO 10534-2:2023)
Ta slovenski standard je istoveten z: EN ISO 10534-2:2023
ICS:
17.140.01 Akustična merjenja in Acoustic measurements and
blaženje hrupa na splošno noise abatement in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 10534-2
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2023
EUROPÄISCHE NORM
ICS 17.140.01 Supersedes EN ISO 10534-2:2001
English Version
Acoustics - Determination of acoustic properties in
impedance tubes - Part 2: Two-microphone technique for
normal sound absorption coefficient and normal surface
impedance (ISO 10534-2:2023)
Acoustique - Détermination des propriétés acoustiques Akustik - Bestimmung der akustischen Eigenschaften
aux tubes d'impédance - Partie 2: Méthode à deux in Impedanzrohren - Teil 2: 2-Mikrofontechnik für
microphones pour le coefficient d'absorption Standardschallabsorptionsgrad und
acoustique normal et l'impédance de surface normale Standardoberflächenimpedanz (ISO 10534-2:2023)
(ISO 10534-2:2023)
This European Standard was approved by CEN on 2 October 2023.

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
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Centre has the same status as the official versions.

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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
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 10534-2:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 10534-2:2023) has been prepared by Technical Committee ISO/TC 43
"Acoustics" in collaboration with Technical Committee CEN/TC 126 “Acoustic properties of building
elements and of buildings” 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 April 2024, and conflicting national standards shall be
withdrawn at the latest by April 2024.
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 ISO 10534-2:2001.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
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, 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.
Endorsement notice
The text of ISO 10534-2 has been approved by CEN as EN ISO 10534-2:2023 without any modification.

INTERNATIONAL ISO
STANDARD 10534-2
Second edition
2023-10
Acoustics — Determination of acoustic
properties in impedance tubes —
Part 2:
Two-microphone technique for
normal sound absorption coefficient
and normal surface impedance
Acoustique — Détermination des propriétés acoustiques aux tubes
d’impédance —
Partie 2: Méthode à deux microphones pour le coefficient d’absorption
sonore normal et l’impédance de surface normale
Reference number
ISO 10534-2:2023(E)
ISO 10534-2:2023(E)
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 10534-2:2023(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
4 Principle . 5
5 Test equipment .5
5.1 Construction of the impedance tube . 5
5.2 Working frequency range . 6
5.3 Length of the impedance tube . 7
5.4 Microphones . 7
5.5 Positions of the microphones . 7
5.6 Acoustic centre of the microphone . 8
5.7 Test sample holder. 8
5.8 Signal processing equipment . 9
5.9 Loudspeaker . 9
5.10 Signal generator. 9
5.11 Thermometer, barometer and relative humidity . 9
6 Preliminary test and measurements .10
7 Test specimen mounting .11
8 Test procedure .12
8.1 Specification of the reference plane .12
8.2 Determination of the sound velocity, wavelength and characteristic impedance .12
8.3 Selection of the signal amplitude . 13
8.4 Selection of the number of averages . 13
8.5 Correction for microphone mismatch . 13
8.5.1 General .13
8.5.2 Measurement repeated with the channels interchanged .13
8.5.3 Predetermined calibration factor . 14
8.6 Determination of the transfer function between the two locations .15
8.6.1 General .15
8.6.2 Cross- and autospectra-based estimate . 15
8.6.3 Frequency-domain deconvolution . 17
8.6.4 Impulse-response based estimate . 17
8.7 Determination of the reflection coefficient . 18
8.8 Determination of the sound absorption coefficient . 18
8.9 Determination of the specific acoustic impedance ratio . 18
8.10 Determination of the specific acoustic admittance ratio . 18
9 Precision .19
10 Test report .19
Annex A (normative) Preliminary measurements .22
Annex B (normative) Procedure for the one-microphone technique .24
Annex C (informative) Theoretical background .25
Annex D (informative) Error sources .27
Annex E (informative) Estimation of diffuse sound absorption coefficient α of locally
st
reacting absorbers from the results of this document .29
Annex F (informative) Estimation of intrinsic properties .30
Bibliography .32
iii
ISO 10534-2:2023(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 43 Acoustics, Subcommittee SC 2,
Building acoustics, in collaboration with the European Committee for Standardization (CEN) Technical
Committee CEN/TC 126, Acoustics properties of building products and of buildings, in accordance with
the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 10534-2:1998), which has been
technically revised.
The main changes are as follows:
— the introduction of the measurement procedure to estimate the characteristic properties of porous
materials (characteristic impedance, wavenumber, dynamic mass density, dynamic bulk modulus)
in an informative annex. The signal processing techniques have been updated since the first version
of this document.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
INTERNATIONAL STANDARD ISO 10534-2:2023(E)
Acoustics — Determination of acoustic properties in
impedance tubes —
Part 2:
Two-microphone technique for normal sound absorption
coefficient and normal surface impedance
1 Scope
This test method covers the use of an impedance tube, two microphone locations and a frequency
analysis system for the determination of the sound absorption coefficient of sound absorbing materials
for normal incidence sound incidence. It can also be applied for the determination of the acoustical
surface impedance or surface admittance of sound absorbing materials. As an extension, it can also be
used to assess intrinsic properties of homogeneous acoustical materials such as their characteristic
impedance, characteristic wavenumber, dynamic mass density and dynamic bulk modulus.
[1]
The test method is similar to the test method specified in ISO 10534-1 in that it uses an impedance
tube with a sound source connected to one end and the test sample mounted in the tube at the other
end. However, the measurement technique is different. In this test method, plane waves are generated
in a tube by a sound source, and the decomposition of the interference field is achieved by the
measurement of acoustic pressures at two fixed locations using wall-mounted microphones or an in-
tube traversing microphone, and subsequent calculation of the complex acoustic transfer function and
quantities reported in the previous paragraph. The test method is intended to provide an alternative,
[1]
and generally much faster, measurement technique than that of ISO 10534-1 .
Normal incidence absorption coefficients coming from impedance tube measurements are not
comparable with random incidence absorption coefficients measured in reverberation rooms according
[2]
to ISO 354 . The reverberation room method will (under ideal conditions) determine the sound
absorption coefficient for diffuse sound incidence. However, the reverberation room method requires
test specimens which are rather large. The impedance tube method is limited to studies at normal and
plane incidence and requires samples of the test object which are of the same size as the cross-section
of the impedance tube. For materials that are locally reacting only, diffuse incidence sound absorption
coefficients can be estimated from measurement results obtained by the impedance tube method (see
Annex E).
+ jtω
Through the whole document, a e time convention is used.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
ISO 10534-2:2023(E)
3.1
sound absorption coefficient at normal incidence
α
n
ratio of the sound power dissipated inside the test object to the incident sound power for a plane wave
at normal incidence
Note 1 to entry: “Plane wave” here describes a wave whose value, at any moment, is constant over any plane
perpendicular to its direction of propagation. “Normal incidence” describes the direction of the longest axis of
the impedance tube.
3.2
sound pressure reflection coefficient at normal incidence
r
complex ratio of the reflected wave sound pressure amplitude to that of the incident wave in the
reference plane for a plane wave at normal incidence
3.3
reference plane
cross-section of the impedance tube for which the reflection factor r or the impedance Z or the
admittance G are determined and which is usually the surface of the test object, if flat
Note 1 to entry: The reference plane is assumed to be at x = 0.
3.4
normal-incidence surface impedance
Z
ratio of the complex sound pressure p(x = 0) to the normal component of the complex sound particle
velocity v(x = 0) at an individual frequency in the reference plane defined as x = 0
Note 1 to entry: The particle velocity vector has a positive direction pointing towards the interior of the tested
object.
Note 2 to entry: Z is expressed in newton second per cubic meter (Ns/m )
3.5
normal-incidence surface admittance
G
inverse of the normal-incidence surface impedance Z
Note 1 to entry: G is expressed in cubic meter per newton per second (m /N/s)
3.6
wave number in air
k
variable, expressed in radian per metre, defined by
kc==ωλ//22ππfc = /
00 00
where
ω
is the angular frequency,
f
is the frequency,
c
is the speed of sound in the air,
λ
is the wavelength in air.
′ ′′ ′
Note 1 to entry: In general, the wave number is complex, so that kk= − jk where k is the real component
00 0 0
′′
and k is the imaginary component (which is the attenuation constant).
ISO 10534-2:2023(E)
′
Note 2 to entry: k is expression in radians per metre.
3.7
material characteristic wave number
k
c
variable, expressed in radian per meter, defined by
kc==ωω//2πfc= ρ /K
c eq eq
where
c is the speed of sound inside the material;
ρ
is the material dynamic mass density (defined in 3.9);
eq
K
is the material bulk modulus (defined in 3.10)
eq
3.8
material characteristic impedance
Z
c
variable, expressed in Newton second per cubic metre, defined by
ZK= ρ
c eq eq
3.9
material dynamic mass density
ρ
eq
variable describing the visco-inertial dissipation inside the tested material.
Note 1 to entry: The dynamic mass density can differ from the static (volume-averaged) value.
Note 2 to entry: It is expressed in kg/m .
3.10
material dynamic bulk modulus
K
eq
variable describing the thermal dissipation inside the tested material.
Note 1 to entry: The dynamic bulk modulus can differ from the static (volume-averaged) value.
Note 2 to entry: It is expressed in N/m (or equivalently in pascal).
3.11
complex sound pressure
p
frequency-domain spectrum of the sound pressure time signal
3.12
cross spectrum
S
product pp *, determined from the complex sound pressures p and p at two microphone positions
21 1 2
Note 1 to entry: * means the complex conjugate.
ISO 10534-2:2023(E)
3.13
cross spectrum
S
product pp *, determined from the complex sound pressures p and p at two microphone positions
12 1 2
Note 1 to entry: * means the complex conjugate.
3.14
auto spectrum
S
product pp *, determined from the complex sound pressure p at microphone position one
11 1
Note 1 to entry: * means the complex conjugate.
Note 2 to entry: S denotes the auto spectrum for pressure p at microphone position two.
22 2
3.15
transfer function
H
transfer function from microphone position one to two, defined by the complex ratio pp//=SS
21 12 11
12/
or SS/ , or ()SS//()SS
[]
22 21 12 11 22 21
3.16
calibration factor
H
c
factor used to correct for amplitude and phase mismatches between the microphones
Note 1 to entry: See 8.5.3.
3.17
locally reacting material
material for which the pressure and velocity fields at a given point on the surface are independent on
the behaviour at other points of the surface
Note 1 to entry: This local reaction behaviour infers specific properties for a material: its surface impedance is
independent on the incidence angle of a plane wave impinging the material. Homogeneous honeycomb structures
and perforated plates are examples of possible locally reacting materials (see Figure 1 a)). For a locally reacting
material, its absorption coefficient depends on the angle of incidence as its reflection coefficient does as well
a)  Locally reacting material sample b)  Non-locally reacting material sample
Key
1 rigid and impervious backing
2 plane wave impinging the sample
3 plane wave impinging the sample with a different angle
Figure 1 — Propagation of plane waves inside a locally reacting material sample and
comparison to a non-locally reacting material sample
ISO 10534-2:2023(E)
3.18
bulk or extended reaction material
material for which the reaction does not occur only normal to the surface.
Note 1 to entry: The reaction in each point of the material is hence dependent on the reaction of the neighbouring
points. Examples of materials experiencing bulk reactions are foams made of multiple pores and fibrous with
fibres not parallel to each other's (see Figure 1 b)).
4 Principle
The test sample is mounted at one end of a straight, rigid, smooth and airtight impedance tube.
Plane waves are generated in the tube by a sound source emitting a signal such as a random noise,
pseudo-random sequence, or a deterministic signal such as a chirp signal, and the sound pressures
are measured at two locations near to the sample. The complex acoustic transfer function of the two
microphone signals is determined and used to compute the normal-incidence complex reflection
coefficient (see Annex C), the normal-incidence absorption coefficient, and the normal incidence
surface impedance of the test material. From two distinct measurements, the intrinsic properties of the
material (characteristic wave number, characteristic impedance, dynamic mass density and dynamic
bulk modulus) can be assessed assuming this material is homogeneous.
The quantities are determined as functions of the frequency (or frequency bands as detailed in
[3]
ISO 266 ) with a frequency resolution which is determined from the sampling frequency and the record
length of the digital frequency analysis system used for the measurements. The usable frequency range
depends on the lateral dimensions or diameter of the tube and the spacing between the microphone
positions. An extended frequency range may be obtained from the combination of measurements with
different lateral dimensions (or diameter) and spacings.
The measurements may be performed by employing one of two techniques:
a) two-microphone method (using two microphones in fixed locations);
b) one-microphone method (using one microphone successively in two locations).
Technique 1: requires a pre-test or in-test correction procedure to minimize the amplitude and phase
difference characteristics between the microphones; however, it combines speed, high
accuracy, and ease of implementation. Technique 1 is recommended for general test
purposes.
Technique 2: has particular signal generation and processing requirements and may necessitate more
time; however, it eliminates phase mismatch between microphones and allows the se-
lection of optimal microphone locations for any frequency. Technique 2 is recommended
for measurements with higher precision, and its requirements are described in more
detail in Annex B.
5 Test equipment
5.1 Construction of the impedance tube
The apparatus is essentially a tube with a test sample holder at one end and a sound source at the other.
Microphone ports are usually located at two or three locations along the wall of the tube (depending on
the chosen microphone spacing).
The impedance tube shall be straight with a uniform cross-section (diameter or cross dimension within
±0,2 %) and with rigid, smooth, non-porous walls without holes or slits (except for the microphone
positions) in the test section. The walls shall be heavy and thick enough so that they are not excited to
vibrations by the sound signal and show no vibration resonances in the working frequency range of the
tube. For metal walls, a thickness of about 5 % of the diameter is recommended for circular tubes. For
rectangular tubes, the corners shall be made rigid enough to prevent distortion of the side wall plates.
ISO 10534-2:2023(E)
It is recommended that the side wall thickness be about 10 % of the cross dimension of the tube. Tube
walls made of concrete shall be sealed by a smooth adhesive finish to ensure air tightness. The same
holds for tube walls made of wood; these should be reinforced and damped by an external coating of
steel or lead sheets.
The shape of the cross-section of the tube is arbitrary, in principle. Circular or rectangular (if
rectangular, then preferably square) cross-sections are recommended.
If rectangular tubes are composed of plates, care shall be taken that there are no air leaks (e.g. by
sealing with adhesives or with a finish). Tubes should be sound and vibration isolated against external
noise or vibration.
5.2 Working frequency range
The working frequency range is given by Formula (1):
ff<< f (1)
lu
where
f
is the lower working frequency of the tube;
l
f is the operating frequency;
f
is the upper working frequency of the tube.
u
f is limited by the uncertainty of the signal processing equipment and the spacing between the two
l
microphone positions.
f is chosen to avoid the occurrence of non-plane wave mode propagation. The condition for f is
u u
given by Formula (2):
df<<05,:80 λ ⋅dc,58 (2)
uu 0
for circular tubes with the inside diameter d in metres and f in Hertz. The same condition, given by
u
Formula (3) is used:
df<<05,:00 λ ⋅dc,50 (3)
uu 0
for rectangular tubes with the maximum side length d in metres; c is the speed of sound in metres per
second given by Formula (4).
The spacing s in metres between the microphones shall be chosen to avoid singularities when the
distance of the two microphone positions is equal to a multiple of half the operating wavelength. The
first singularity is avoided when ensuring that
fs⋅< 04, 5 c (4)
u 0
The lower frequency limit is dependent on the spacing between the microphones and the uncertainty
of the analysis system but, as a general guide, the microphone spacing should exceed 1,5 % of the
wavelength corresponding to the lower frequency of interest, provided that the requirements of
Formula (4) are satisfied. A larger spacing between the microphones enhances the accuracy of the
measurements for these low frequencies but reduces the value of the upper working frequency.
Different microphone spacings can be used to cover a wider frequency range than the one allowed
for a single spacing. In this case, the working frequency ranges shall overlap by about one octave (as
ISO 10534-2:2023(E)
[3]
described in ISO 266 ). The averaging technique used to obtain the averaged and combined result
should be at least mentioned.
Different impedance tubes can also be used to cover a wider frequency range than the one allowed for a
single tube (see Clause 10 i).
5.3 Length of the impedance tube
The tube should be long enough to cause plane wave development between the source and the sample.
Microphone measurement points shall be in the plane wave field.
The loudspeaker generally will produce non-plane waves besides the plane wave. They will die
out within a distance of maximum three tube diameters or three times the lateral dimensions of
rectangular tubes for frequencies below the lower cut-off frequency of the first higher mode. Thus, it is
recommended that microphones be located no closer to the source than three tube diameters or three
times the lateral dimensions.
Test samples will also cause proximity distortions to the acoustic field. It is recommended to have a
minimum spacing between microphone and sample of ½ diameter or ½ maximum lateral dimension,
but this spacing should be increased to 2 diameters or 2 times the maximum lateral dimension for
non-planar materials or materials with a few small perforations (as perforated plates with a single
millimetric perforation).
5.4 Microphones
Microphones of identical type shall be used in each location. When side-wall-mounted microphones are
used, the diameter of the microphones shall be small compared to cf/ .
0 u
For side-wall mounting, it is recommended to use microphones of the pressure type. For in-tube
microphones, it is recommended to use microphones of the free-field type.
5.5 Positions of the microphones
When side-wall-mounted microphones are used, each microphone shall be mounted with the diaphragm
flush with the interior surface of the tube. A small recess is often necessary to prevent the microphone
to be inserted inside the tube (see Figure 2); the recess should be kept small and be identical for both
microphone mountings. The microphone grid shall be sealed tight to the microphone housing and there
shall be a sealing between the microphone and the mounting hole.
a)  Rectangular cross-section b)  Circular cross-section
Key
1 microphone
2 sealing
Figure 2 — Examples of typical microphone mounting for a tube
When using a single microphone in two successive wall positions, the microphone position not in use
shall be sealed to avoid air leaks and to maintain a smooth surface inside the tube.
ISO 10534-2:2023(E)
When using side-vented microphones, it is important that the pressure equalization vents are not
blocked by the microphone mounting.
All fixed microphone locations shall be known to a tolerance of ±0,2 mm or better, and their spacing s
(see Figure 3) shall be recorded.
Traversing microphone positions shall be known to a tolerance of ±0,5 mm or better.
Finally, it is recommended to set the microphone positions to a distance not larger than 250 mm
()x < 250 mm from the rigid backing of the impedance tube (i.e. the opposite end to the loudspeaker)
to reduce the impact of the first acoustic resonances in the tube on the microphone measurements.
Key
1 microphone A
2 microphone B
3 test specimen
s spacing between the two microphones
distance between the surface of the test specimen and the microphone closest to the sound source
x
Figure 3 — Microphone positions and distances
5.6 Acoustic centre of the microphone
For the determination of the acoustic centre of a microphone, or minimizing errors associated with a
difference between the acoustic and geometric centres of the microphones, see A.2.2.
5.7 Test sample holder
The test sample holder is either integrated into the impedance tube or is a separate unit, which is tightly
fixed to one end of the tube during the measurement. The length of the sample holder shall be large
enough to install test objects with air spaces behind them if required.
If the sample holder is a separate unit, it shall comply in its interior dimensions with the impedance
tube to within ±0,2 %. The mounting of the tube shall be tight, without insertion of elastic gaskets
(petroleum jelly or thread seal tape is recommended for sealing).
For rectangular tubes, it is recommended to integrate the sample holder into the impedance tube and to
make the installation section of the tube accessible by a removable cover for mounting the test sample.
The contact surfaces of this removable cover with the tube shall be carefully finished and the use of a
sealant (like a petroleum jelly or a thread seal tape) is recommended in order to avoid small leaks.
For circular tubes, it is recommended to make the test object accessible from both the front and the
back end of the sample holder. It is then possible to check the position and flatness of the front surface
and the back position.
Generally, in connection with rectangular tubes, it is recommended to install the test object from the
side into the tube (instead of pushing it axially into the tube). It is then possible to check the fitting and
the position of the test object in the tube, to check the position and the flatness of the front surface, and
to reposition the reference plane precisely in relation to the front surface. A sideways insertion also
avoids compression of soft materials.
ISO 10534-2:2023(E)
The back plate of the sample holder shall be rigid and shall be fixed tightly to the tube since it serves
as a rigid termination in many measurements. A metal plate of thickness not less than 10 mm is
recommended.
For some tests, a pressure-release termination of the test object by an air volume behind it is needed
(this is described in Annex C).
5.8 Signal processing equipment
The signal processing system shall consist of an amplifier and an analysing system which is able to
determine the transfer function H between the two microphone locations. The transfer function can
be determined via a two-channel fast Fourier transform (FFT) analysing system, or via an impulse
response measuring system and a subsequent Fourier transformation of the impulse responses. The
impulse response measuring system can use two-channel FFT or cross-correlation. If the measurement
signal is of the m-sequence type, a cross-correlation-based analysis system, which for instance uses the
Fast Hadamard Transform, shall be used.
A generator capable of producing the required source signal (see 5.10) compatible with the analysing
system is also required.
The dynamic range of the analyser should be greater than 65 dB. The errors in the estimated transfer
function H due to non-linearities, resolution, instability and temperature sensitivity of the signal
processing equipment shall be less than 0,2 dB.
Using the one-microphone technique, the analysing system shall be able to calculate the transfer
function H from the generator signal and the two microphone signals measured consecutively.
5.9 Loudspeaker
A membrane loudspeaker (or a pressure chamber loudspeaker for high frequencies with a horn as a
transmission element to the impedance tube) should be located at the opposite end of the tube from
the test sample holder. The surface of the loudspeaker membrane shall cover at least two-thirds of the
cross-sectional area of the impedance tube. The loudspeaker axis may be either coaxial with the tube,
or inclined, or connected to the tube by an elbow.
The loudspeaker shall be contained in an insulating box in order to avoid airborne flanking transmission
to the microphones. Elastic vibration insulation shall be applied between the impedance tube and the
frame of the loudspeaker as well as to the loudspeaker box (preferably between the impedance tube
and the transmission element also) in order to avoid structure-borne sound excitation of the impedance
tube.
5.10 Signal generator
The signal generator shall be able to generate a stationary signal with a flat spectral density within
the frequency range of interest. It may generate one or more of the following: random, pseudo-random,
periodic pseudo-random, or chirp excitation, as required.
In the case of the one-microphone technique, a deterministic signal is recommended. A periodic pseudo-
random sequence is also well suited for this method.
Discrete-frequency generation and display are necessary for tube calibration purposes (according to
Annex A). Discrete- frequency generation and display shall have an uncertainty of less than ±2 %.
5.11 Thermometer, barometer and relative humidity
The temperature shall be measured and kept constant during a measurement with a tolerance of ±1 K.
The temperature transducer shall be accurate to ±0,5 K or better.
The atmospheric pressure shall be measured with a tolerance of ±0,5 kPa.
ISO 10534-2:2023(E)
If available, the information about the relative humidity shall be reported with a tolerance of ±2 %.
It is recommended to place the sensors for these measurements in the room where the tube lies rather
than inside the tube unless the tube and the sensors are designed so that they can work properly
without having a noticeable influence on the wave field inside the tube.
6 Preliminary test and measurements
The test equipment shall be assembled, typically as shown in Figure 4, and checked before use by a
series of tests. These tests help to exclude error sources and secure the minimum requirements. The
checks may be in two categories: prior to or following each test, and periodic calibration tests before
each measurement session of half a day or a day maximum. In each case, the loudspeaker should be
operated for at least 5 min prior to a measurement to allow the temperature to stabilize.
Checks prior to and following each test involve microphone response consistency, temperature
measurement and a test of the signal-to-noise ratio.
Periodic calibrations are performed with a rigid termination of the empty impedance tube and an
absorbent material (to reduce the impact of the air-column resonances inside the impedance tube).
Their aim is the determination of the acoustic centre of a microphone, and/or the corrections for
attenuation in the impedance tube.
These preliminary measurements are described in Annex A.
ISO 10534-2:2023(E)
Key
1 microphone A 4 impedance tube 7 signal generator
2 microphone B 5 sound source 8 frequency analysis system
3 test specimen 6 amplifier
Figure 4 — Example of layout for test equipment
7 Test specimen mounting
The test specimen shall fit snugly in the holder. However, it shall not be compressed unduly nor fitted
so tightly that it bulges. Sealing of any crack about the edge of the sample with a petroleum jelly or a
thread seal tape is recommended. Samples such
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