ISO 26101:2017

Acoustics — Test methods for the qualification of free-field environments

ISO 26101:2017 specifies methodology for qualifying acoustic spaces as anechoic and hemi-anechoic spaces meeting the requirements of a free sound field. ISO 26101:2017 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. ISO 26101:2017 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. 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. ISO 26101:2017 describes the divergence loss method for measuring the free sound field performance of an acoustic environment.

Acoustique — Méthodes d'essai pour la qualification des environnements en champ libre

General Information

• Status: Withdrawn
• Publication Date: 24-Apr-2017
• ICS: 17.140.01 - Acoustic measurements and noise abatement in general
• Technical Committee: ISO/TC 43/SC 1 - Noise
• Drafting Committee: ISO/TC 43/SC 1 - Noise
• Current Stage: 9599 - Withdrawal of International Standard
• Completion Date: 13-Jan-2022

Relations

• Revised: ISO 26101-1:2021 - Acoustics — Test methods for the qualification of the acoustic environment — Part 1: Qualification of free-field environments
• Effective Date: 17-Jul-2021
• Revises: ISO 26101:2012 - Acoustics — Test methods for the qualification of free-field environments
• Effective Date: 05-Nov-2015

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 26101
Second edition
2017-04
Acoustics — Test methods for
the qualification of free-field
environments
Acoustique — Méthodes d’essai pour la qualification des
environnements en champ libre
Reference number
ISO 26101:2017(E)
©
ISO 2017

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ISO 26101:2017(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, 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.
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Fax [phone]
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www.iso.org
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ISO 26101:2017(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 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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ISO 26101:2017(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 on 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 the following URL: www . i so .org/ iso/ foreword .html.
The committee responsible for this document is ISO/TC 43, Acoustics.
This second edition cancels and replaces the first edition (ISO 26101:2012), which has been technically
revised. The main changes are as follows:
— the term “acoustic centre” was replaced by “mathematical origin of the traverse” in several places in
the document to provide clarification of terminology;
— the minimum traverse path length was reduced from 1/2 wavelength to 1/4 wavelength;
— Figure B.1 has been added.
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ISO 26101:2017(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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INTERNATIONAL STANDARD ISO 26101:2017(E)
Acoustics — Test methods for the 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.
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:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1
free sound field
sound field in a homogeneous, isotropic medium free of boundaries
[SOURCE: ISO/TR 25417:2007, 2.17]
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ISO 26101:2017(E)

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 (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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ISO 26101:2017(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.
NOTE For measurements above 5 kHz, this method will normally require a microphone of diameter
[2]
equivalent 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 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 compliance 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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ISO 26101:2017(E)

′
LL= −+LL (1)
pi pi pi,ref,,p ref,0
where
L is the corrected sound pressure level at measurement point i, expressed in decibels (dB);
pi
L′ is the measured sound pressure level at measurement point i, expressed in decibels (dB);
pi
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 may 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 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 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 in Annex A shall be used to qualify anechoic and hemi-anechoic spaces for general
purpose acoustical measurements.
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ISO 26101:2017(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 sources 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
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
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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ISO 26101:2017(E)

In the absence of specific requirements for the spatial resolution of the measurement points, the
requirements 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
i
Lr() =−b 20lg   dB (2)
pi
 
r
 0 
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,
i
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.
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ISO 26101:2017(E)

NOTE 1 An iterative process can be used to determine b; a starting value is given by Formula (3):
N N
 
r
i
20lg  dB+ L
∑∑
pi
 
r
i==11 0  i
b = (3)
N
where
L is the measured sound pressure level (corrected for source stability) at measurement point i, ex-
pi
pressed in decibels (dB);
N is the number of measurement points along the measurement traverse.
NOTE 2 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);
pi
L is the measured sound pressure level (corrected for source stability) at measurement point
pi
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 compliance 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 in per cent and barometric pressure in
pascals;
f) equipment used for the measurements, including name, type, serial number and manufacturer;
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ISO 26101:2017(E)

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;
l) the test signal(s) and measurement bandwidth;
m) the frequency range of interest (see 3.7);
n) for each path, the number of measurement points and the averaging time at each measurement
point, or for continuous measurements, the speed of the traverse and the response time of the
instrumentation;
o) a table or chart of the sound pressure levels or deviations from the inverse square law in the
measurement band of interest and position relative to the test sound source, measured along each
traverse;
p) the qualification criteria for the allowable deviations from the inverse square law;
q) the dimensions and location of the anechoic or hemi-anechoic space, qualified in accordance with
the requirements of the referencing document or Annex A, as applicable.
5.3 Information to be reported
For measurements according to this document, the following information shall be reported:
a) the time and date of the measurements;
b) a description of the environment to be qualified, including dimensions, and a description of the
physical treatment of walls, ceiling and floor;
c) a description of the measuring instrumentation used;
d) a description of the sound source(s) used for the test, including a statement that the directionality
of the source(s) comply with this document;
e) the test signal(s) and measurement bandwidth;
f) the frequency range of interest (see 3.7);
g) clear identification of the traverse paths and the position of the source(s) used for the test;
h) the qualification criteria for the allowable deviations from the inverse square law;
i) the dimensions and location of the anechoic or hemi-anechoic space, qualified in accordance with
the requirements of the referencing document or Annex A, as applicable;
j) the measurement results of the divergence loss of the sound pressure level or deviations from
inverse square law versus distance;
k) a statement of the measurement uncertainty;
l) a statement whether the environment can be used for its intended use;
m) a statement that the qualification was made in accordance with this document.
Due to the quantity of measurement results, it can be impractical to present these in a table and
presentation in a graphical form is recommended.
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ISO 26101:2017(E)

Annex A
(normative)

Qualification criteria and measurement requirements
A.1 General
In the absence of specific requirements and criteria in a referencing document, the requ
...