Acoustics - Determination of sound absorption coefficient and impedance in impedances tubes - Part 2: Transfer-function method (ISO 10534-2:1998)

This test method covers the use of an impedance tube, two microphone locations and a digital frequency analysis system for the determination of the sound absorption coefficient of sound absorbers for normal sound incidence. It can also be applied for the determination of the acoustical surface impedance or surface admittance of sound absorbing materials. Since the impedance ratios of a sound absorptive material are related to its physical properties, such as airflow resistance, porosity, elasticity and density, measurements described in this test method are useful in basic research and product development.
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 noise 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, the normal incidence absorption and the impedance ratios of the acoustic material. The test method is intended to provide an alternative, and generally much faster, measurement technique than that of ISO 10534-1.
Compared with the measurement of the sound absorption in a reverberation room according to the method specified in ISO 354, there are some characteristic differences. The reverberation room method will (under ideal conditions) determine the sound absorption coefficient for diffuse sound incidence, and the method can be used for testing of materials with pronounced structures in the lateral and normal directions. However, the reverberation room method requires test specimens which are rather large, so it is not convenient for research and development work, where only small samples of the absorber are available. The impedance tube method is limited to parametric studies at normal incidence but 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, diffuse incidence sound absorption coefficients can be estimated from measurement results obtained by the impedance tube method. For transformation of the test results from the impedance tube method (normal incidence) to diffuse sound incidence, see annex F.

Bauakustik - Bestimmung des Schallabsorptionsgrades und der Impedanz in Impedanzrohren - Teil 2: Verfahren mit Übertragungsfunktion (ISO 10534-2:1998)

Dieses Prüfverfahren behandelt die Bestimmung des Schallabsorptionsgrades von Schallabsorbern bei senkrechtem Schalleinfall unter Anwendung eines Impedanzrohres, zweier Mikrophonorte sowie eines digitalen Frequenzanalysesystems. Es kann auch zur Bestimmung der akustischen Oberflächenimpedanz oder Oberflächenadmittanz von schallabsorbierenden Werkstoffen eingesetzt werden. Da die Impedanzverhältnisse eines schallabsorbierenden Werkstoffs mit seinen physikalischen Eigenschaften, wie Luftströmungswiderstand, Porosität, Elastizität und Dichte in Beziehung stehen, sind die für dieses Prüfverfahren beschriebenen Messungen in der Grundlagenforschung und Produktentwicklung anwendbar.
Das Prüfverfahren ist dem in ISO 10534-1 beschriebenen insofern ähnlich, dass ein Impedanzrohr verwendet wird, das an einem Ende eine Schallquelle besitzt und am anderen Ende der Prüfling befestigt wird. Das Messverfahren ist jedoch ein anderes. Bei diesem Prüfverfahren werden mit einem Rauschgenerator ebene Wellen im Rohr erzeugt und die Zerlegung des Interferenzfeldes wird durch Messung des akustischen Druckes an zwei festen Orten erreicht, wobei an der Wand befestigte Mikrophone oder ein im Rohr querstehendes Mikrophon verwendet werden, sowie anschließender Berechnung der komplexen akustischen Übertragungsfunktion, der Absorption bei senkrechtem Einfall und der Impedanzverhältnisse des akustischen Werkstoffs. Dieses Prüfverfahren ist im Vergleich mit dem in ISO 10534-1 behandelten ein alternatives und im allgemeinen viel schnelleres Messverfahren.
Verglichen mit der Messung des Schallabsorptionsgrades in einem Hallraum nach ISO 354 bestehen einige charakteristische Unterschiede. Beim Hallraumverfahren wird (unter idealisierten Bedingungen) der Schallabsorptionsgrad bei diffusem Schalleinfall bestimmt und kann für die Prüfung von Werkstoffen mit ausgeprägter Struktur in seitlicher und senkrechter Richtung angewendet werden.

Acoustique - Détermination du facteur d'absorption acoustique et de l'impédance des tubes d'impédance - Partie 2: Méthode de la fonction de transfert (ISO 10534-2:1998)

La présente méthode d'essai traite de l'utilisation du tube d'impédance, de deux emplacements de microphones et d'un système d'analyse de la fréquence numérique pour la détermination du facteur d'absorption acoustique des absorbants acoustiques sous incidence acoustique normale. Elle peut, de plus, être utilisée pour déterminer de l'impédance acoustique en surface ou l'admittance en surface des matériaux acoustiques absorbants. Dans la mesure où les rapports d'impédance d'un matériau acoustique absorbant sont liés à ses caractéristiques physiques, telles que la résistance à l'air, la porosité, l'élasticité et la densité, les mesurages décrits dans la présente méthode d'essai sont utiles pour la recherche fondamentale et le développement des produits.
La méthode d'essai est identique à la méthode d'essai ISO 10534-1 en ce sens qu'elle utilise un tube d'impédance avec une source sonore connectée à une extrémité et l'échantillon d'essai monté dans le tube au niveau de l'autre extrémité. Cependant, la technique de mesurage est différente. Dans cette méthode d'essai, les ondes planes sont générées dans un tube par une source de bruit, et la décomposition du champ d'interférence s'effectue par le mesurage des pressions acoustiques en deux emplacements fixes utilisant des microphones montés sur des parois ou un microphone transversal au tube, puis par le calcul de la fonction complexe de transfert acoustique, de l'absorption à incidence normale et des rapports d'impédance du matériau acoustique. La méthode d'essai est destinée à fournir une technique de mesurage alternative et plus rapide que celle décrite dans l'ISO 10534-1.
Il existe certaines différences caractéristiques par comparaison au mesurage de l'absorption acoustique dans une salle réverbérante selon la méthode d'essai ISO 354. La méthode en salle réverbérante déterminera, dans des conditions idéales, le facteur d'absorption acoustique sous incidence diffuse, et la méthode peut être utilisée pour l'essai des matériaux dont les structures dans le sens latéral et normal sont bien définies. Cependant, la méthode dite de la salle réverbérante nécessite des éprouvettes relativement grandes; elle ne convient donc pas aux travaux de recherche et de développement, pour lesquels seule une petite quantité d'échantillons de l'absorbant sont disponibles. La méthode du tube d'impédance est limitée aux études paramétriques sous incidence normale mais nécessite des échantillons de l'objet en essai, d'une taille équivalente à la section droite du tube d'impédance. Pour les matériaux à réaction locale, les facteurs d'absorption acoustique sous incidence diffuse peuvent être évalués à partir des résultats de mesurage obtenus par la méthode du tube d'impédance. Voir l'annexe F pour la transformation des résultats d'essai à partir de la méthode du tube d'impédance (incidence normale) pour la diffusion de l'incidence acoustique.

Akustika - Ugotavljanje koeficienta absorpcije in impedance zvoka v Kundtovi cevi – 2. del: Metoda s prenosno funkcijo (ISO 10534-2:1998)

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Publication Date
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SLOVENSKI STANDARD
SIST EN ISO 10534-2:2002
01-december-2002

Akustika - Ugotavljanje koeficienta absorpcije in impedance zvoka v Kundtovi cevi

– 2. del: Metoda s prenosno funkcijo (ISO 10534-2:1998)

Acoustics - Determination of sound absorption coefficient and impedance in impedances

tubes - Part 2: Transfer-function method (ISO 10534-2:1998)
Akustik - Bestimmung des Schallabsorptionsgrades und der Impedanz in
Impedanzrohren - Teil 2: Verfahren mit Übertragungsfunktion (ISO 10534-2:1998)

Acoustique - Détermination du facteur d'absorption acoustique et de l'impédance des

tubes d'impédance - Partie 2: Méthode de la fonction de transfert (ISO 10534-2:1998)

Ta slovenski standard je istoveten z: EN ISO 10534-2:2001
ICS:
17.140.01 $NXVWLþQDPHUMHQMDLQ Acoustic measurements and
EODåHQMHKUXSDQDVSORãQR noise abatement in general
SIST EN ISO 10534-2:2002 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST EN ISO 10534-2:2002
---------------------- Page: 2 ----------------------
SIST EN ISO 10534-2:2002
EUROPEAN STANDARD
EN ISO 10534-2
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2001
ICS 17.140.01
English version
Acoustics - Determination of sound absorption coefficient and
impedance in impedances tubes - Part 2: Transfer-function
method (ISO 10534-2:1998)

Acoustique - Détermination du facteur d'absorption Bauakustik - Bestimmung des Schallabsorptionsgrades

acoustique et de l'impédance des tubes d'impédance - und der Impedanz in Impedanzrohren - Teil 2: Verfahren

Partie 2: Méthode de la fonction de transfert (ISO 10534- mit Übertragungsfunktion (ISO 10534-2:1998)

2:1998)
This European Standard was approved by CEN on 13 May 2001.

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 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 Management Centre has the same status as the official

versions.

CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,

Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels

© 2001 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 10534-2:2001 E

worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN ISO 10534-2:2002
Page 2
EN ISO 10534-2:2001
Foreword

The text of the International Standard from Technical Committee ISO/TC 43 "Acoustics" of the

International Organization for Standardization (ISO) has been taken over as an European Standard

by Technical Committee CEN/TC 126 " Acoustic properties of building products 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 December 2001, and conflicting national standards

shall be withdrawn at the latest by December 2001.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Czech

Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg,

Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.

Endorsement notice

The text of the International Standard ISO 10534-2:1998 has been approved by CEN as a European

Standard without any modification.
---------------------- Page: 4 ----------------------
SIST EN ISO 10534-2:2002
INTERNATIONAL ISO
STANDARD 10534-2
First edition
1998-11-15
Acoustics — Determination of sound
absorption coefficient and impedance
in impedance tubes —
Part 2:
Transfer-function method
Acoustique — Détermination du facteur d’absorption acoustique
et de l’impédance des tubes d’impédance —
Partie 2: Méthode de la fonction de transfert
Reference number
ISO 10534-2:1998(E)
---------------------- Page: 5 ----------------------
SIST EN ISO 10534-2:2002
ISO 10534-2:1998(E)
Contents Page

1 Scope ....................................................................................... 1

2 Definitions and symbols ........................................................ 1

3 Principle.................................................................................... 3

4 Test equipment ........................................................................ 3

5 Preliminary test and measurements...................................... 7

6 Test specimen mounting ........................................................ 8

7 Test procedure ........................................................................ 9

8 Precision................................................................................... 13

9 Test report ............................................................................... 14

Annexes

A Preliminary measurements .................................................... 15

B Procedure for the one-microphone technique .................... 20
C Pressure-release termination of test sample........................ 21

D Theoretical background ......................................................... 22

E Error sources ........................................................................... 24

F Determination of diffuse sound absorption coefficient a
of locally reacting absorbers from the results of this part

of ISO 10534 ............................................................................

G Bibliography ............................................................................ 27

© ISO 1998

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced

or utilized in any form or by any means, electronic or mechanical, including photocopying and

microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
---------------------- Page: 6 ----------------------
SIST EN ISO 10534-2:2002
© ISO
ISO 10534-2:1998(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.
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.
International Standard ISO 10534-2 was prepared by Technical Committee
ISO/TC 43, Acoustics, Subcommittee SC 2, Building acoustics.
ISO 10534 consists of the following parts, under the general title
Acoustics — Determination of sound absorption coefficient and impedance
in impedance tubes:
— Part 1: Method using standing wave ratio
— Part 2: Transfer-function method
Annnexes A to C form an integral part of this part of ISO 10534. Annexes D
to G are for information only.
iii
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SIST EN ISO 10534-2:2002
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SIST EN ISO 10534-2:2002
INTERNATIONAL STANDARD © ISO ISO 10534-2:1998(E)
Acoustics — Determination of sound absorption coefficient
and impedance in impedance tubes —
Part 2:
Transfer-function method
1 Scope

This test method covers the use of an impedance tube, two microphone locations and a digital frequency analysis

system for the determination of the sound absorption coefficient of sound absorbers for normal sound incidence. It

can also be applied for the determination of the acoustical surface impedance or surface admittance of sound

absorbing materials. Since the impedance ratios of a sound absorptive material are related to its physical

properties, such as airflow resistance, porosity, elasticity and density, measurements described in this test method

are useful in basic research and product development.

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 noise 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, the normal incidence absorption and the impedance ratios of the acoustic

material. The test method is intended to provide an alternative, and generally much faster, measurement technique

than that of ISO 10534-1.

Compared with the measurement of the sound absorption in a reverberation room according to the method

specified in ISO 354, there are some characteristic differences. The reverberation room method will (under ideal

conditions) determine the sound absorption coefficient for diffuse sound incidence, and the method can be used for

testing of materials with pronounced structures in the lateral and normal directions. However, the reverberation

room method requires test specimens which are rather large, so it is not convenient for research and development

work, where only small samples of the absorber are available. The impedance tube method is limited to parametric

studies at normal incidence but 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, diffuse incidence sound absorption coefficients can

be estimated from measurement results obtained by the impedance tube method. For transformation of the test

results from the impedance tube method (normal incidence) to diffuse sound incidence, see annex F.

2 Definitions and symbols
For the purposes of this part of ISO 10534 the following definitions apply.
2.1
sound absorption coefficient at normal incidence

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
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SIST EN ISO 10534-2:2002
ISO 10534-2:1998(E) © ISO
2.2
sound pressure reflection factor at normal incidence

complex ratio of the amplitude of the reflected wave to that of the incident wave in the reference plane for a plane

wave at normal incidence
2.3
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 The reference plane is assumed to be at x = 0.
2.4
normal surface impedance

ratio of the complex sound pressure p(0) to the normal component of the complex sound particle velocity v(0) at an

individual frequency in the reference plane
2.5
normal surface admittance
inverse of the normal surface impedance Z
2.6
wave number
variable defined by
k = ω /c = 2pf/c
0 0 0
where
w is the angular frequency;
f is the frequency;
c is the speed of sound.
NOTE In general the wave number is complex, so
k = k ¢ – jk †
0 0 0
where
k ¢ is the real component (k ¢ = 2π/l );
0 0
l is the wavelength;

k † is the imaginary component which is the attenuation constant, in nepers per metre.

2.7
complex sound pressure
Fourier Transform of the temporal acoustic pressure
2.8
cross spectrum

product p ⋅p *, determined from the complex sound pressures p and p at two microphone positions

2 1 1 2
NOTE * means the complex conjugate.
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SIST EN ISO 10534-2:2002
© ISO
ISO 10534-2:1998(E)
2.9
auto spectrum

product p ⋅p *, determined from the complex sound pressure p at microphone position one

1 1 1
NOTE * means the complex conjugate.
2.10
transfer function

transfer function from microphone position one to two, defined by the complex ratio p /p = S /S or S /S , or

2 1 12 11 22 21
1/2
[(S /S )(S /S )]
12 11 22 21
2.11
calibration factor

factor used to correct for amplitude and phase mismatches between the microphones

NOTE See 7.5.2.
3 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 (random, pseudo-random sequence, or chirp), 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 factor (see annex C), the

normal-incidence absorption coefficient, and the impedance ratio of the test material.

The quantities are determined as functions of the frequency 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 width of the tube and the spacing between the microphone positions.

An extended frequency range may be obtained from the combination of measurements with different widths and

spacings.
The measurements may be performed by employing one of two techniques:
1: two-microphone method (using two microphones in fixed locations);

2: 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 require more time; however, it

eliminates phase mismatch between microphones and allows the selection of optimal microphone

locations for any frequency. Technique 2 is recommended for the assessment of tuned resonators

and/or precision, and its requirements are described in more detail in annex B.
4 Test equipment
4.1 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, but variations involving a

centre mounted microphone or probe microphone are possible.
---------------------- Page: 11 ----------------------
SIST EN ISO 10534-2:2002
ISO 10534-2:1998(E) © ISO

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. 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.

4.2 Working frequency range
The working frequency range is
f < f < f (1)
l u
where
f is the lower working frequency of the tube;
f is the operating frequency;
f is the upper working frequency of the tube.
f is limited by the accuracy of the signal processing equipment.
f is chosen to avoid the occurrence of non-plane wave mode propagation.
The condition for f is:
d < 0,58 l ; f ·d < 0,58 c (2)
u u 0
for circular tubes with the inside diameter d in metres and f in hertz.
d < 0,5 λ ; f ·d < 0,50 c (3)
u u

for rectangular tubes with the maximum side length d in metres; c is the speed of sound in metres per second given by

equation (5).
The spacing s in metres between the microphones shall be chosen so that
f ·s < 0,45 c (4)

The lower frequency limit is dependent on the spacing between the microphones and the accuracy of the analysis

system but, as a general guide, the microphone spacing should exceed 5 % of the wavelength corresponding to the

lower frequency of interest, provided that the requirements of equation (4) are satisfied. A larger spacing between

the microphones enhances the accuracy of the measurements.
4.3 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.
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SIST EN ISO 10534-2:2002
© ISO
ISO 10534-2:1998(E)

The loudspeaker generally will produce non-plane modes besides the plane wave. They will die out within a

distance of about three tube diameters or three times the maximum 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 suggested above, but in any case no closer than one diameter or one

maximum lateral dimension, as appropriate.

Test samples will also cause proximity distortions to the acoustic field and the following recommendation is given for

the minimum spacing between microphone and sample, depending upon the sample type:

non-structured: ½ diameter or ½ maximum lateral dimension
semi-lateral structured: 1 diameter or 1 maximum lateral dimension
strongly asymmetrical: 2 diameters or 2 times the maximum lateral dimension
4.4 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 c /f . In addition, it is recommended that the microphone

0 u
diameters be less than 20 % of the spacing between them.

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.
4.5 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 as shown in figure 1; 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.

Key
1 Microphone
2 Sealing
Figure 1 — Examples of typical microphone mounting

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.

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 an accuracy of ± 0,2 mm or better, and

their spacing s (see figure 2) shall be recorded. Traversing microphone positions shall be known to an accuracy

of ± 0,5 mm or better.
---------------------- Page: 13 ----------------------
SIST EN ISO 10534-2:2002
ISO 10534-2:1998(E) © ISO
Key
1 Microphone A
2 Microphone B
3 Test specimen
Figure 2 — Microphone positions and distances
4.6 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.2.3.
4.7 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 as 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 (vaseline 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 (vaseline) 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.

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 20 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.
4.8 Signal processing equipment

The signal processing system shall consist of an amplifier, and a two-channel Fast Fourier Transform (FFT)

analysing system. The system is required to measure the sound pressure at two microphone locations and to

calculate the transfer function H between them. A generator capable of producing the required source signal

(see 4.10) compatible with the analysing system is also required.
---------------------- Page: 14 ----------------------
SIST EN ISO 10534-2:2002
© ISO
ISO 10534-2:1998(E)

The dynamic range of the analyser should be greater than 65 dB. The errors in the estimated transfer function H

due to nonlinearities, 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 from

the generator signal and the two microphone signals measured consecutively.
4.9 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.

4.10 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 and a periodic pseudo-random

sequence is well suited for this method, although special signal processing will be required. The processing first

involves an m-sequence correlation via the fast Hadamard transform to produce an impulse response. The

frequency response is subsequently obtained by Fourier transform of the impulse response.

Discrete-frequency generation and display are necessary for tube calibration purposes (see annex A). Discrete-

frequency generation and display shall have an uncertainty of less than ± 2 %.
4.11 Loudspeaker termination

Resonances of the air column in the impedance tube will always arise. These should be suppressed by lining the

inside of the impedance tube near the loudspeaker with at least a 200 mm length of an effective sound-absorbent

material.
4.12 Thermometer and barometer

The temperature in the impedance tube 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.
5 Preliminary test and measurements

The test equipment shall be assembled, typically as shown in figure 3, 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 considered to

be in two categories: prior to or following each test, and periodic calibration tests. In each case the loudspeaker

should be operated for at least 10 min prior to a measurement to allow the temperature to stabilize.

Checks prior to and following each test involve microphone response constancy, temperature measurement and a

test of the signal-to-noise ratio.
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SIST EN ISO 10534-2:2002
ISO 10534-2:1998(E) © ISO

Periodic calibrations are performed with a rigid termination of the empty 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 B.
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 3 — Example of layout for test equipment
6 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 vaseline or plasticine is recommended. The test

sample can be held more firmly, if necessary, by taping and greasing the entire edge. For example, samples such

as carpet material should be firmly attached to the back plate using double-sided adhesive tape to prevent

vibrational motion and unwanted air gaps.

The front surface of flat test samples shall be mounted normal to the tube axis. Their positions shall be specified

with minimum tolerances: for objects with flat and smooth surfaces, to within ± 0,5 mm. With porous materials of low

bulk density, it may be helpful to fix and to define the surface by a thin, non-vibrating wire grid with wide mesh.

If the specimen has an uneven or irregular face, surface microphone locations shall be chosen to be sufficiently far

away so that the measured transfer function is in the plane wave region. When the specimen has an uneven back

which would introduce an unintended backing air space, a layer of putty-like material should be placed between it

and the sound-reflective back plate to seal the back of the specimen and to add enough thickness to make the front

surface parallel to the back plate.

A minimum of two specimens, more if the sample is not uniform, should be tested in repeated measurements using

the same mounting conditions.

If the test object has a regular lateral structure (e.g. perforated cover sheets, resonator arrays, etc.), the cuts of the

test samples shall be along lines of symmetry of that structure. If the dimensions of multiple structural units of the

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SIST EN ISO 10534-2:2002
© ISO
ISO 10534-2:1998(E)

test object do not fit with the cross dimensions of the impedance tube, the measurements shall be performed with

several test samples with varying positions of the cuts relative to the structure. Repetition of the measurements with

test samples cut from different places o
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