Acoustics - Test methods for the qualification of the acoustic environment - Part 1: Qualification of free-field environments (ISO 26101-1:2021)

This document specifies methodology for qualifying acoustic spaces as anechoic and hemi-anechoic spaces meeting the requirements of a free sound field.
This document specifies discrete-frequency and broad-band test methods for quantifying the performance of anechoic and hemi-anechoic spaces, defines the qualification procedure for an omni-directional sound source suitable for free-field qualification, gives details of how to present the results and describes uncertainties of measurement.
This document has been developed for qualifying anechoic and hemi-anechoic spaces for a variety of acoustical measurement purposes. It is expected that, over time, various standards and test codes will refer to this document in order to qualify an anechoic or hemi-anechoic space for a particular measurement. Annex D provides guidelines for the specification of test parameters and qualification criteria for referencing documents.
In the absence of specific requirements or criteria, Annex A provides qualification criteria and measurement requirements to qualify anechoic and hemi-anechoic spaces for general purpose acoustical measurements.
This document describes the divergence loss method for measuring the free sound field performance of an acoustic environment.

Akustik - Verfahren für die Qualifizierung von Freifeld-Prüfumgebungen - Teil 1: Qualifizierung von Freifeldumgebungen (ISO 26101-1:2021)

Acoustique - Méthodes d'essai pour la qualification de l'environnement acoustique - Partie 1: Qualification des environnements en champ libre (ISO 26101-1:2021)

Le présent document spécifie une méthodologie pour qualifier des espaces acoustiques comme des espaces anéchoïques ou semi-anéchoïques répondant aux exigences d’un champ acoustique libre.
Le présent document spécifie des méthodes d’essai en fréquences discrètes et en large bande pour quantifier la performance d’espaces anéchoïques et semi-anéchoïques. Il définit la procédure de qualification d’une source sonore omnidirectionnelle adéquate pour la qualification d’un champ libre, détaille le mode de présentation des résultats et décrit les incertitudes de mesure.
Le présent document a été conçu pour qualifier des espaces anéchoïques ou semi-anéchoïques pour une grande variété d’applications de métrologie acoustique. À terme, divers codes d’essai et normes devraient se référer au présent document pour qualifier un espace en tant qu’anéchoïque ou semi-anéchoïque en vue d’un mesurage spécifique. L’Annexe D énonce des lignes directrices pour la spécification de paramètres d’essai et de critères de qualification par les documents de référence.
En l’absence d’exigences ou de critères particuliers, l’Annexe A fournit les critères de qualification et les exigences de mesurage pour qualifier des espaces anéchoïques et semi-anéchoïques à des fins de mesurages acoustiques généraux.
Le présent document décrit la méthode de mesure de la performance d’un environnement acoustique en tant que champ acoustique libre, basée sur la perte par divergence.

Akustika - Preskusne metode za kvalifikacijo akustičnega okolja - 1. del: Kvalifikacija okolij prostega polja (ISO 26101-1:2021)

General Information

Status
Published
Public Enquiry End Date
27-Feb-2022
Publication Date
15-Jun-2022
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
02-Jun-2022
Due Date
07-Aug-2022
Completion Date
16-Jun-2022

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SLOVENSKI STANDARD
SIST EN ISO 26101-1:2022
01-julij-2022
Akustika - Preskusne metode za kvalifikacijo akustičnega okolja - 1. del:
Kvalifikacija okolij prostega polja (ISO 26101-1:2021)

Acoustics - Test methods for the qualification of the acoustic environment - Part 1:

Qualification of free-field environments (ISO 26101-1:2021)
Akustik - Verfahren für die Qualifizierung von Freifeld-Prüfumgebungen - Teil 1:
Qualifizierung von Freifeldumgebungen (ISO 26101-1:2021)

Acoustique - Méthodes d'essai pour la qualification de l'environnement acoustique -

Partie 1: Qualification des environnements en champ libre (ISO 26101-1:2021)
Ta slovenski standard je istoveten z: EN ISO 26101-1:2022
ICS:
17.140.01 Akustična merjenja in Acoustic measurements and
blaženje hrupa na splošno noise abatement in general
SIST EN ISO 26101-1:2022 en,fr,de

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

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SIST EN ISO 26101-1:2022
---------------------- Page: 2 ----------------------
SIST EN ISO 26101-1:2022
EN ISO 26101-1
EUROPEAN STANDARD
NORME EUROPÉENNE
May 2022
EUROPÄISCHE NORM
ICS 17.140.01
English Version
Acoustics - Test methods for the qualification of the
acoustic environment - Part 1: Qualification of free-field
environments (ISO 26101-1:2021)

Acoustique - Méthodes d'essai pour la qualification de Akustik - Verfahren für die Qualifizierung von Freifeld-

l'environnement acoustique - Partie 1: Qualification Prüfumgebungen - Teil 1: Qualifizierung von

des environnements en champ libre (ISO 26101- Freifeldumgebungen (ISO 26101-1:2021)

1:2021)
This European Standard was approved by CEN on 13 April 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 26101-1:2022 E

worldwide for CEN national Members.
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SIST EN ISO 26101-1:2022
EN ISO 26101-1:2022 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 26101-1:2022
EN ISO 26101-1:2022 (E)
European foreword

The text of ISO 26101-1:2021 has been prepared by Technical Committee ISO/TC 43 "Acoustics” of the

International Organization for Standardization (ISO) and has been taken over as EN ISO 26101-1:2022

by Technical Committee CEN/TC 211 “Acoustics” the secretariat of which is held by DIN.

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 November 2022, and conflicting national standards

shall be withdrawn at the latest by November 2022.

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.

Any feedback and questions on this document should be directed to the users’ national standards body.

A complete listing of these bodies can be found on the CEN website.

According to the CEN-CENELEC Internal Regulations, the national standards 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, Turkey and the

United Kingdom.
Endorsement notice

The text of ISO 26101-1:2021 has been approved by CEN as EN ISO 26101-1:2022 without any

modification.
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SIST EN ISO 26101-1:2022
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SIST EN ISO 26101-1:2022
INTERNATIONAL ISO
STANDARD 26101-1
First edition
2021-05
Acoustics — Test methods for
the qualification of the acoustic
environment —
Part 1:
Qualification of free-field
environments
Acoustique — Méthodes d'essai pour la qualification de
l'environnement acoustique —
Partie 1: Qualification des environnements en champ libre
Reference number
ISO 26101-1:2021(E)
ISO 2021
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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 2021 – All rights reserved
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

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

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Allowable deviations from inverse square law ..................................................................................................................... 2

5 Measurement of free sound field performance .................................................................................................................... 3

5.1 Divergence loss method .................................................................................................................................................................. 3

5.1.1 Principle .................................................................................................................................................................................. 3

5.1.2 Instrumentation and measuring equipment ........................................................................................... 3

5.1.3 Location of test sound sources and microphone traverses ......................................................... 4

5.1.4 Test procedure ................................................................................................................................................................... 5

5.1.5 Expression of results .................................................................................................................................................... 6

5.1.6 Measurement uncertainty ....................................................................................................................................... 7

5.2 Information to be recorded .......................................................................................................................................................... 7

5.3 Information to be reported .......................................................................................................................................................... 8

Annex A (normative) Qualification criteria and measurement requirements .........................................................9

Annex B (normative) General procedure for evaluation of test sound source directionality ...............12

Annex C (informative) Measurement uncertainty ...............................................................................................................................15

Annex D (informative) Guidelines for referring to this test method ................................................................................18

Bibliography .............................................................................................................................................................................................................................20

© ISO 2021 – All rights reserved iii
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(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 documents 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).

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. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

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 1, Noise.

This first edition of ISO 26101-1 cancels and replaces the second edition of ISO 26101:2017, of which it

constitutes a minor revision. The changes compared to the previous edition are as follows:

— The title of the document was changed from “Acoustics — Test methods for the qualification of free-

field environments” to “Acoustics — Test methods for the qualification of the acoustic environment —

Part 1: Qualification of free-field environments”, so that an additional part, "ISO 26101-2, Acoustics

— Test methods for the qualification of the acoustic environment —Part 2: Determination of the

environmental correction", can be introduced.
A list of all parts in the ISO 26101 series can be found on the ISO website.

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 © ISO 2021 – All rights reserved
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
Introduction

This document describes the divergence loss method of measurement of performance of an environment

designed to provide a free sound field or free sound field over a reflecting plane. An acoustical

environment is a free sound field if it has bounding surfaces that absorb all sound energies incident

upon them. This is normally achieved using specialized test environments, such as anechoic or hemi-

anechoic chambers. In practice, these provide a controlled free sound field for acoustical measurements

in a confined space within the facility.

The purpose of this document is to promote uniformity in the method and conditions of measurement

when qualifying free sound field environments.

It is expected that the qualification procedures outlined in this document will be referred to by other

International Standards and industry test codes. In such cases, these documents making reference

to this document may specify qualification criteria appropriate for the test method and may require

specific traverse paths.
© ISO 2021 – All rights reserved v
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SIST EN ISO 26101-1:2022
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SIST EN ISO 26101-1:2022
INTERNATIONAL STANDARD ISO 26101-1:2021(E)
Acoustics — Test methods for the qualification of the
acoustic environment —
Part 1:
Qualification of free-field environments
1 Scope

This document specifies methodology for qualifying acoustic spaces as anechoic and hemi-anechoic

spaces meeting the requirements of a free sound field.

This document specifies discrete-frequency and broad-band test methods for quantifying the

performance of anechoic and hemi-anechoic spaces, defines the qualification procedure for an omni-

directional sound source suitable for free-field qualification, gives details of how to present the results

and describes uncertainties of measurement.

This document has been developed for qualifying anechoic and hemi-anechoic spaces for a variety of

acoustical measurement purposes. It is expected that, over time, various standards and test codes

will refer to this document in order to qualify an anechoic or hemi-anechoic space for a particular

measurement. Annex D provides guidelines for the specification of test parameters and qualification

criteria for referencing documents.

In the absence of specific requirements or criteria, Annex A provides qualification criteria and

measurement requirements to qualify anechoic and hemi-anechoic spaces for general purpose

acoustical measurements.

This document describes the divergence loss method for measuring the free sound field performance of

an acoustic environment.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

measurement (GUM: 1995)

IEC 61260-1, Electroacoustics — Octave-band and fractional-octave-band filters — Part 1: Specifications

IEC 61672-1, Electroacoustics — Sound level meters — Part 1: Specifications
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
© ISO 2021 – All rights reserved 1
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
3.1
free sound field
sound field in a homogeneous, isotropic medium free of boundaries
[SOURCE: ISO/TR 25417:2007, 2.17]
3.2
anechoic space

volume which has been qualified as a sound field in a homogeneous, isotropic medium free of boundaries

3.3
hemi-anechoic space

volume above a reflecting plane which has been qualified as a sound field in a homogeneous, isotropic

medium free of boundaries
3.4
acoustic centre

position of the point from which approximately

spherical wave fronts appear to diverge
3.5
background noise
sum of all the signals except the one under investigation

Note 1 to entry: Background noise can include contributions from airborne sound, structure-borne vibration and

electrical noise in instrumentation.
3.6
divergence loss

reduction in sound pressure along a straight path due to the spreading of sound when a sound wave

propagates away from a source
3.7
frequency range of interest

contiguous one-third-octave-band frequencies from the lowest to the highest frequencies to be

qualified, inclusive
3.8
referencing document

standard or test code that refers to this document for the purpose of specifying the qualification method

of an anechoic space (3.2) or hemi-anechoic space (3.3)
4 Allowable deviations from inverse square law

The theoretical reduction in mean-square sound pressure along a straight path due to spherical

propagation of a sound wave in a free sound field shall be hereafter referred to as the inverse square

law.

For a space to be deemed anechoic or hemi-anechoic, as defined by criteria in a referencing document,

the deviations of the measured sound pressure levels from those estimated using the inverse square

law, obtained according to this document, shall not exceed the values specified by the referencing

document.

In the absence of specific criteria for the allowable deviations in a referencing document, the criteria

in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general purpose acoustical

measurements.

The allowable deviations specified by a referencing document may be more or less stringent than the

criteria given in Annex A.
2 © ISO 2021 – All rights reserved
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
5 Measurement of free sound field performance
5.1 Divergence loss method
5.1.1 Principle

The divergence loss method shall be used to quantify the performance of an anechoic or hemi-anechoic

space within a test environment and to determine the spatial limits of this qualified anechoic or hemi-

anechoic space.

The free sound field performance is evaluated by quantifying the contributions of both the direct and

the reflected components of acoustic energy.

The spatial decrease of sound pressure emitted from a test sound source shall be compared with the

decrease of sound pressure that would occur in an ideal free sound field.
5.1.2 Instrumentation and measuring equipment
5.1.2.1 General

The instrumentation system for measuring sound pressure level, including the microphone and cable,

shall be operated within the limit of the linearity errors specified for a Class 1 sound level meter

according to IEC 61672-1.

The microphone shall be nominally omni-directional (taking into account any supplementary equipment

connected to it, such as the protective grid and mounting arrangement).

For measurements in one-third-octave bands, the filters used shall meet the requirements for Class 1

specified in IEC 61260-1.

For measurements above 5 kHz, this method will normally require a microphone of diameter equivalent

[2]
to that of a WS2F microphone, as described in IEC 61094-4 , or less.
5.1.2.2 Test sound source

A sound source approximating a point source over the frequency range of interest shall be used for the

qualification measurement. The test sound source shall be

a) compact and of acoustical performance, such that the location of the acoustic centre of the source

is known to be located close enough to the origin of the microphone traverses specified in 5.1.3.2

to allow fitting of the sound pressure level versus distance data without an adjustment for the

acoustic centre of the source,

b) in conformity with the directionality criteria in Table B.1, when measured according to the

procedure in Annex B, so as to ensure the source radiates energy in all directions,

c) able to generate sufficient sound power over the frequency range of interest to yield sound pressure

levels at least 6 dB above the background noise levels for all points on each microphone traverse, or

[3]
while the microphone is moving for continuous traverse systems , and

d) of high stability so that the radiated sound power (due to the source, associated signal generation

and amplification electronics) as measured by a monitor microphone located at an arbitrary fixed

position in the test environment does not vary significantly at the frequency of measurement

during the time taken to complete the measurements for each microphone traverse. If the stability

of the source varies by more than ±0,2 dB then the monitor microphone shall be used to apply a

correction according to Formula (1):
© ISO 2021 – All rights reserved 3
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
LL= −+LL (1)
pp,,ii pp,,ref,i ref,0
where

L is the corrected sound pressure level at measurement point i, expressed in decibels (dB);

p,i

L′ is the measured sound pressure level at measurement point i, expressed in decibels (dB);

p,i

L is the sound pressure level measured by the monitor microphone at the reference

p,ref,i
location for measurement point i, expressed in decibels (dB);

L is the sound pressure level measured by the monitor microphone at the reference

p,ref,0
location for the initial measurement point 0, expressed in decibels (dB).

Since, in general, two or more sources can be required to cover the overall frequency range of interest,

the requirements given above shall be met for each source over its applicable frequency range.

NOTE It is possible to estimate the acoustic centre of a source by evaluating it in an anechoic space already

known to meet the requirements given in Annex A.
Care should be taken:

— to ensure that the sound pressure levels are more than 6 dB, and preferably more than 15 dB, above

the background noise levels;

— in positioning the monitor microphone to avoid acoustic interference with the traversing mechanism

affecting the results;

— to ensure that changes in atmospheric conditions over the duration of the traverse are not confused

with those related to the source stability.
5.1.3 Location of test sound sources and microphone traverses
5.1.3.1 Test sound source location

Referencing documents may specify the test sound source location(s) to be used in order to qualify the

anechoic or hemi-anechoic space.

In the absence of specific requirements for the sound source location in a referencing document, the

requirements given in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general

purpose acoustical measurements.

The test sound source should be placed in a chosen orientation and held in that orientation for all

microphone traverses.
An environment may be qualified for more than one source location.
5.1.3.2 Microphone traverses

Microphone traverses shall be made along paths that will characterize and qualify the anechoic or

hemi-anechoic space for the types of acoustical measurements to be conducted in the test environment.

The origin of the microphone traverse shall be within the physical volume occupied by the test sound

source.

Referencing documents may specify the traverse paths to be conducted in order to qualify the anechoic

or hemi-anechoic space.

In the absence of specific requirements for the traverse paths in a referencing document, the

requirements given in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general

purpose acoustical measurements.
4 © ISO 2021 – All rights reserved
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)
Sound reflection from the microphone support system should be carefully avoided.
5.1.4 Test procedure
5.1.4.1 Qualification bandwidth

The qualification measurements of the anechoic or hemi-anechoic space shall be made using a

bandwidth that is typical of the spectral characteristics of the type of devices that will be measured or

evaluated.

Discrete-frequency qualification may be accomplished by using a test source that generates discrete

tone(s) or by using a test source that generates broad-band noise and a measurement system that

[3]
provides discrete-frequency measurement capabilities, such as an FFT analyser .

Broad-band qualification may be accomplished by using a test source that generates broad-band noise

and a measurement system that provides one-third-octave-band filtering.

Referencing documents may specify the bandwidth for the qualification measurement.

In the absence of specific requirements for the bandwidth in a referencing document, the requirements

given in Annex A shall be used for the selection of the appropriate qualification measurement bandwidth

for their intended purpose.
5.1.4.2 Generation of sound

The test sound source described in 5.1.2.2 may be operated with a test signal of pure tones, multiple

pure tones, band-limited or broad-band noise.

If pure tones or multiple pure tones are used for discrete-frequency qualification, the measured signal

after any filtering shall not contain energy at frequencies not being characterized that are within 15 dB

of the frequencies being characterized. If broad-band noise is used as a test signal for either broad-band

or discrete-frequency qualification, then the test signal shall consist of either random noise or broad-

band test signals derived from random noise.

In the absence of specific requirements for the test signal in a referencing document, the requirements

given in Annex A shall be used for the selection of the appropriate test signal for qualification of anechoic

or hemi-anechoic spaces for their intended purpose.

NOTE Use of a mix of pure tones spaced apart by more than a one-third-octave band can be much more rapid

than sequential traverses, each at a single pure tone.

When using tonal or mixed tone signals, care should be taken to avoid distortion due to excessive signal

levels.
5.1.4.3 Measurement of sound pressure level

The sound pressure levels shall be measured using fractional octave-band filters or FFT analysis.

The microphone shall be moved along the paths described in 5.1.3.2 for each test signal. The

measurement of sound pressure level shall be carried out starting, at most, a quarter of a wavelength

(at the lowest frequency to be qualified) from the origin of the traverse, traversing at least a quarter of

a wavelength (at the lowest frequency to be qualified) and to the hypothetical boundary of the anechoic

or hemi-anechoic space to be qualified.

Sound pressure levels shall be measured along each microphone traverse using equally spaced

measurement points at each frequency. Referencing documents may specify the maximum spacing

of the measurement points in order to qualify the anechoic or hemi-anechoic space for their intended

purpose.
© ISO 2021 – All rights reserved 5
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SIST EN ISO 26101-1:2022
ISO 26101-1:2021(E)

In the absence of specific requirements for the spatial resolution of the measurement points, the

requirements given in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general

purpose acoustical measurements.

Alternatively, for discrete-frequency measurements using pure tone signals, the microphone may be

[3]

moved slowly and continuously along the traverse and the sound pressure levels recorded . Sound

pressure level versus distance data should then be determined using the spatial sampling guidelines for

discrete measurements.

If broad-band test signals are used, measurement times should be of sufficient duration to achieve

stable levels.
5.1.5 Expression of results
5.1.5.1 Method of calculation
5.1.5.1.1 General

Measured sound pressure levels are compared with the theoretical sound pressure level decay

according to the inverse square law in a free sound field.

5.1.5.1.2 Formula for estimation of sound pressure levels based on the inverse square law

From
...

SLOVENSKI STANDARD
oSIST prEN ISO 26101-1:2022
01-februar-2022
Akustika - Preskusne metode za kvalifikacijo akustičnega okolja - 1. del:
Kvalifikacija okolij prostega polja (ISO 26101-1:2021)

Acoustics - Test methods for the qualification of the acoustic environment - Part 1:

Qualification of free-field environments (ISO 26101-1:2021)
Akustik - Verfahren für die Qualifizierung von Freifeld-Prüfumgebungen - Teil 1:
Qualifizierung von Freifeldumgebungen (ISO 26101-1:2021)

Acoustique - Méthodes d'essai pour la qualification de l'environnement acoustique -

Partie 1: Qualification des environnements en champ libre (ISO 26101-1:2021)
Ta slovenski standard je istoveten z: prEN ISO 26101-1
ICS:
17.140.01 Akustična merjenja in Acoustic measurements and
blaženje hrupa na splošno noise abatement in general
oSIST prEN ISO 26101-1:2022 en,fr,de

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

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oSIST prEN ISO 26101-1:2022
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oSIST prEN ISO 26101-1:2022
INTERNATIONAL ISO
STANDARD 26101-1
First edition
2021-05
Acoustics — Test methods for
the qualification of the acoustic
environment —
Part 1:
Qualification of free-field
environments
Acoustique — Méthodes d'essai pour la qualification de
l'environnement acoustique —
Partie 1: Qualification des environnements en champ libre
Reference number
ISO 26101-1:2021(E)
ISO 2021
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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ii © ISO 2021 – All rights reserved
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

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

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Allowable deviations from inverse square law ..................................................................................................................... 2

5 Measurement of free sound field performance .................................................................................................................... 3

5.1 Divergence loss method .................................................................................................................................................................. 3

5.1.1 Principle .................................................................................................................................................................................. 3

5.1.2 Instrumentation and measuring equipment ........................................................................................... 3

5.1.3 Location of test sound sources and microphone traverses ......................................................... 4

5.1.4 Test procedure ................................................................................................................................................................... 5

5.1.5 Expression of results .................................................................................................................................................... 6

5.1.6 Measurement uncertainty ....................................................................................................................................... 7

5.2 Information to be recorded .......................................................................................................................................................... 7

5.3 Information to be reported .......................................................................................................................................................... 8

Annex A (normative) Qualification criteria and measurement requirements .........................................................9

Annex B (normative) General procedure for evaluation of test sound source directionality ...............12

Annex C (informative) Measurement uncertainty ...............................................................................................................................15

Annex D (informative) Guidelines for referring to this test method ................................................................................18

Bibliography .............................................................................................................................................................................................................................20

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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(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 documents 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).

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. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

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 1, Noise.

This first edition of ISO 26101-1 cancels and replaces the second edition of ISO 26101:2017, of which it

constitutes a minor revision. The changes compared to the previous edition are as follows:

— The title of the document was changed from “Acoustics — Test methods for the qualification of free-

field environments” to “Acoustics — Test methods for the qualification of the acoustic environment —

Part 1: Qualification of free-field environments”, so that an additional part, "ISO 26101-2, Acoustics

— Test methods for the qualification of the acoustic environment —Part 2: Determination of the

environmental correction", can be introduced.
A list of all parts in the ISO 26101 series can be found on the ISO website.

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 © ISO 2021 – All rights reserved
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
Introduction

This document describes the divergence loss method of measurement of performance of an environment

designed to provide a free sound field or free sound field over a reflecting plane. An acoustical

environment is a free sound field if it has bounding surfaces that absorb all sound energies incident

upon them. This is normally achieved using specialized test environments, such as anechoic or hemi-

anechoic chambers. In practice, these provide a controlled free sound field for acoustical measurements

in a confined space within the facility.

The purpose of this document is to promote uniformity in the method and conditions of measurement

when qualifying free sound field environments.

It is expected that the qualification procedures outlined in this document will be referred to by other

International Standards and industry test codes. In such cases, these documents making reference

to this document may specify qualification criteria appropriate for the test method and may require

specific traverse paths.
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oSIST prEN ISO 26101-1:2022
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oSIST prEN ISO 26101-1:2022
INTERNATIONAL STANDARD ISO 26101-1:2021(E)
Acoustics — Test methods for the qualification of the
acoustic environment —
Part 1:
Qualification of free-field environments
1 Scope

This document specifies methodology for qualifying acoustic spaces as anechoic and hemi-anechoic

spaces meeting the requirements of a free sound field.

This document specifies discrete-frequency and broad-band test methods for quantifying the

performance of anechoic and hemi-anechoic spaces, defines the qualification procedure for an omni-

directional sound source suitable for free-field qualification, gives details of how to present the results

and describes uncertainties of measurement.

This document has been developed for qualifying anechoic and hemi-anechoic spaces for a variety of

acoustical measurement purposes. It is expected that, over time, various standards and test codes

will refer to this document in order to qualify an anechoic or hemi-anechoic space for a particular

measurement. Annex D provides guidelines for the specification of test parameters and qualification

criteria for referencing documents.

In the absence of specific requirements or criteria, Annex A provides qualification criteria and

measurement requirements to qualify anechoic and hemi-anechoic spaces for general purpose

acoustical measurements.

This document describes the divergence loss method for measuring the free sound field performance of

an acoustic environment.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

measurement (GUM: 1995)

IEC 61260-1, Electroacoustics — Octave-band and fractional-octave-band filters — Part 1: Specifications

IEC 61672-1, Electroacoustics — Sound level meters — Part 1: Specifications
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
3.1
free sound field
sound field in a homogeneous, isotropic medium free of boundaries
[SOURCE: ISO/TR 25417:2007, 2.17]
3.2
anechoic space

volume which has been qualified as a sound field in a homogeneous, isotropic medium free of boundaries

3.3
hemi-anechoic space

volume above a reflecting plane which has been qualified as a sound field in a homogeneous, isotropic

medium free of boundaries
3.4
acoustic centre

position of the point from which approximately

spherical wave fronts appear to diverge
3.5
background noise
sum of all the signals except the one under investigation

Note 1 to entry: Background noise can include contributions from airborne sound, structure-borne vibration and

electrical noise in instrumentation.
3.6
divergence loss

reduction in sound pressure along a straight path due to the spreading of sound when a sound wave

propagates away from a source
3.7
frequency range of interest

contiguous one-third-octave-band frequencies from the lowest to the highest frequencies to be

qualified, inclusive
3.8
referencing document

standard or test code that refers to this document for the purpose of specifying the qualification method

of an anechoic space (3.2) or hemi-anechoic space (3.3)
4 Allowable deviations from inverse square law

The theoretical reduction in mean-square sound pressure along a straight path due to spherical

propagation of a sound wave in a free sound field shall be hereafter referred to as the inverse square

law.

For a space to be deemed anechoic or hemi-anechoic, as defined by criteria in a referencing document,

the deviations of the measured sound pressure levels from those estimated using the inverse square

law, obtained according to this document, shall not exceed the values specified by the referencing

document.

In the absence of specific criteria for the allowable deviations in a referencing document, the criteria

in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general purpose acoustical

measurements.

The allowable deviations specified by a referencing document may be more or less stringent than the

criteria given in Annex A.
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
5 Measurement of free sound field performance
5.1 Divergence loss method
5.1.1 Principle

The divergence loss method shall be used to quantify the performance of an anechoic or hemi-anechoic

space within a test environment and to determine the spatial limits of this qualified anechoic or hemi-

anechoic space.

The free sound field performance is evaluated by quantifying the contributions of both the direct and

the reflected components of acoustic energy.

The spatial decrease of sound pressure emitted from a test sound source shall be compared with the

decrease of sound pressure that would occur in an ideal free sound field.
5.1.2 Instrumentation and measuring equipment
5.1.2.1 General

The instrumentation system for measuring sound pressure level, including the microphone and cable,

shall be operated within the limit of the linearity errors specified for a Class 1 sound level meter

according to IEC 61672-1.

The microphone shall be nominally omni-directional (taking into account any supplementary equipment

connected to it, such as the protective grid and mounting arrangement).

For measurements in one-third-octave bands, the filters used shall meet the requirements for Class 1

specified in IEC 61260-1.

For measurements above 5 kHz, this method will normally require a microphone of diameter equivalent

[2]
to that of a WS2F microphone, as described in IEC 61094-4 , or less.
5.1.2.2 Test sound source

A sound source approximating a point source over the frequency range of interest shall be used for the

qualification measurement. The test sound source shall be

a) compact and of acoustical performance, such that the location of the acoustic centre of the source

is known to be located close enough to the origin of the microphone traverses specified in 5.1.3.2

to allow fitting of the sound pressure level versus distance data without an adjustment for the

acoustic centre of the source,

b) in conformity with the directionality criteria in Table B.1, when measured according to the

procedure in Annex B, so as to ensure the source radiates energy in all directions,

c) able to generate sufficient sound power over the frequency range of interest to yield sound pressure

levels at least 6 dB above the background noise levels for all points on each microphone traverse, or

[3]
while the microphone is moving for continuous traverse systems , and

d) of high stability so that the radiated sound power (due to the source, associated signal generation

and amplification electronics) as measured by a monitor microphone located at an arbitrary fixed

position in the test environment does not vary significantly at the frequency of measurement

during the time taken to complete the measurements for each microphone traverse. If the stability

of the source varies by more than ±0,2 dB then the monitor microphone shall be used to apply a

correction according to Formula (1):
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
LL= −+LL (1)
pp,,ii pp,,ref,i ref,0
where

L is the corrected sound pressure level at measurement point i, expressed in decibels (dB);

p,i

L′ is the measured sound pressure level at measurement point i, expressed in decibels (dB);

p,i

L is the sound pressure level measured by the monitor microphone at the reference

p,ref,i
location for measurement point i, expressed in decibels (dB);

L is the sound pressure level measured by the monitor microphone at the reference

p,ref,0
location for the initial measurement point 0, expressed in decibels (dB).

Since, in general, two or more sources can be required to cover the overall frequency range of interest,

the requirements given above shall be met for each source over its applicable frequency range.

NOTE It is possible to estimate the acoustic centre of a source by evaluating it in an anechoic space already

known to meet the requirements given in Annex A.
Care should be taken:

— to ensure that the sound pressure levels are more than 6 dB, and preferably more than 15 dB, above

the background noise levels;

— in positioning the monitor microphone to avoid acoustic interference with the traversing mechanism

affecting the results;

— to ensure that changes in atmospheric conditions over the duration of the traverse are not confused

with those related to the source stability.
5.1.3 Location of test sound sources and microphone traverses
5.1.3.1 Test sound source location

Referencing documents may specify the test sound source location(s) to be used in order to qualify the

anechoic or hemi-anechoic space.

In the absence of specific requirements for the sound source location in a referencing document, the

requirements given in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general

purpose acoustical measurements.

The test sound source should be placed in a chosen orientation and held in that orientation for all

microphone traverses.
An environment may be qualified for more than one source location.
5.1.3.2 Microphone traverses

Microphone traverses shall be made along paths that will characterize and qualify the anechoic or

hemi-anechoic space for the types of acoustical measurements to be conducted in the test environment.

The origin of the microphone traverse shall be within the physical volume occupied by the test sound

source.

Referencing documents may specify the traverse paths to be conducted in order to qualify the anechoic

or hemi-anechoic space.

In the absence of specific requirements for the traverse paths in a referencing document, the

requirements given in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general

purpose acoustical measurements.
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
Sound reflection from the microphone support system should be carefully avoided.
5.1.4 Test procedure
5.1.4.1 Qualification bandwidth

The qualification measurements of the anechoic or hemi-anechoic space shall be made using a

bandwidth that is typical of the spectral characteristics of the type of devices that will be measured or

evaluated.

Discrete-frequency qualification may be accomplished by using a test source that generates discrete

tone(s) or by using a test source that generates broad-band noise and a measurement system that

[3]
provides discrete-frequency measurement capabilities, such as an FFT analyser .

Broad-band qualification may be accomplished by using a test source that generates broad-band noise

and a measurement system that provides one-third-octave-band filtering.

Referencing documents may specify the bandwidth for the qualification measurement.

In the absence of specific requirements for the bandwidth in a referencing document, the requirements

given in Annex A shall be used for the selection of the appropriate qualification measurement bandwidth

for their intended purpose.
5.1.4.2 Generation of sound

The test sound source described in 5.1.2.2 may be operated with a test signal of pure tones, multiple

pure tones, band-limited or broad-band noise.

If pure tones or multiple pure tones are used for discrete-frequency qualification, the measured signal

after any filtering shall not contain energy at frequencies not being characterized that are within 15 dB

of the frequencies being characterized. If broad-band noise is used as a test signal for either broad-band

or discrete-frequency qualification, then the test signal shall consist of either random noise or broad-

band test signals derived from random noise.

In the absence of specific requirements for the test signal in a referencing document, the requirements

given in Annex A shall be used for the selection of the appropriate test signal for qualification of anechoic

or hemi-anechoic spaces for their intended purpose.

NOTE Use of a mix of pure tones spaced apart by more than a one-third-octave band can be much more rapid

than sequential traverses, each at a single pure tone.

When using tonal or mixed tone signals, care should be taken to avoid distortion due to excessive signal

levels.
5.1.4.3 Measurement of sound pressure level

The sound pressure levels shall be measured using fractional octave-band filters or FFT analysis.

The microphone shall be moved along the paths described in 5.1.3.2 for each test signal. The

measurement of sound pressure level shall be carried out starting, at most, a quarter of a wavelength

(at the lowest frequency to be qualified) from the origin of the traverse, traversing at least a quarter of

a wavelength (at the lowest frequency to be qualified) and to the hypothetical boundary of the anechoic

or hemi-anechoic space to be qualified.

Sound pressure levels shall be measured along each microphone traverse using equally spaced

measurement points at each frequency. Referencing documents may specify the maximum spacing

of the measurement points in order to qualify the anechoic or hemi-anechoic space for their intended

purpose.
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)

In the absence of specific requirements for the spatial resolution of the measurement points, the

requirements given in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general

purpose acoustical measurements.

Alternatively, for discrete-frequency measurements using pure tone signals, the microphone may be

[3]

moved slowly and continuously along the traverse and the sound pressure levels recorded . Sound

pressure level versus distance data should then be determined using the spatial sampling guidelines for

discrete measurements.

If broad-band test signals are used, measurement times should be of sufficient duration to achieve

stable levels.
5.1.5 Expression of results
5.1.5.1 Method of calculation
5.1.5.1.1 General

Measured sound pressure levels are compared with the theoretical sound pressure level decay

according to the inverse square law in a free sound field.

5.1.5.1.2 Formula for estimation of sound pressure levels based on the inverse square law

From the sound pressure levels measured at positions specified in 5.1.4.3, the estimation of sound

pressure levels based on the inverse square law shall be determined for each measurement traverse

using Formula (2):
 r 
Lr()=−b 20lg dB (2)
 
 
where

L (r ) is the sound pressure level at distance, r , estimated by the inverse square law, expressed in

p i i
decibels (dB);

r is the distance of measurement point i from the mathematical origin of the traverse,

expressed in metres (m);
r is the reference value, r = 1 m;
0 0

b is the source strength parameter that is adjusted to optimize the fit of the measured sound

pressure levels into the tolerance range, to maximize the qualified distance from the test

sound source, expressed in decibels (dB).

If a continuous traverse is used, an “analogue” recording of level versus distance is obtained. To use the

formulae in this clause, sound pressure levels at a large number of points at regularly spaced intervals

shall be derived from the records. The selection of point spacing shall be based on the criteria of 5.1.4.3.

An iterative process may be used to determine b; a starting value is given by Formula (3):

N N
 
20lg dB+ L
 
pi,
 0 
i==11i
b= (3)
where
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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)

L is the measured sound pressure level (corrected for source stability) at measurement point i,

p,i
expressed in decibels (dB);
N is the number of measurement points along the measurement traverse.

NOTE Over long traverses and especially at high frequency, air absorption might not be negligible, it might be

necessary to correct the measured sound pressure level for absorption of sound by the atmosphere in accordance

[4]
with ISO 9613-1 . For example, atmospheric absorption can be 0,3 dB/m at 10 kHz.
5.1.5.1.3 Deviations from the inverse square law

Using the estimation of sound pressure levels based on the inverse square law, the deviation of the

measured sound pressure level from the inverse square law is determined at each measurement point

using Formula (4):
ΔLL=− Lr (4)
pi,,pi pi
where
ΔL is the deviation from the inverse square law, expressed in decibels (dB);
p,i

L is the measured sound pressure level (corrected for source stability) at measurement point

p,i
i, expressed in decibels (dB).
5.1.6 Measurement uncertainty

The uncertainty of the results obtained from measurements according to this document shall be

evaluated, preferably in accordance with ISO/IEC Guide 98-3. The expanded uncertainty together

with the corresponding coverage factor for a stated coverage probability of 95 % as defined in

ISO/IEC Guide 98-3 shall be given. Guidance on the determination of the expanded uncertainty is given

in Annex C.
5.2 Information to be recorded

For measurements according to this document, the following information shall be recorded:

a) the time and date of the measurements;
b) the person responsible for the measurements and calculations;

c) a description of the environment to be qualified, including dimensions and a description of the

physical treatment of walls, ceiling and floor;

d) a sketch showing the location of the test sound source and any unique features or non-uniformities;

e) air temperature in degrees Celsius, relative humidity as a percentage and barometric pressure in

pascals;

f) equipment used for the measurements, including name, type, serial number and manufacturer;

g) the sound source(s) used for the test;

h) position of the mathematical origin of the traverse for each test sound source used;

i) clear identification of the traverse paths used for the test;

j) the locations and orientation of the traverse paths, any reflecting planes, bounding surfaces and

the assumed mathematical origin of the traverse (a sketch shall be included, if necessary);

k) for each path, the start location relative to the test source and the path length;

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oSIST prEN ISO 26101-1:2022
ISO 26101-1:2021(E)
l) the test signal(s) an
...

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