ISO 23351-2:2026

International
Standard
ISO 23351-2
First edition
Acoustics — Measurement of
2026-08
speech level reduction of furniture
ensembles and enclosures —
Part 2:
Field method
Acoustique — Mesurage de la réduction du niveau de parole des
ensembles de meubles et des enceintes —
Partie 2: Méthode de terrain
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measurement conditions . 3
4.1 Overview of the method .3
4.2 Test environment .3
4.3 Test object .4
4.4 Equipment .5
4.4.1 Sound source .5
4.4.2 Measurement equipment .6
4.5 Microphone positions .6
4.5.1 Fixed positions .6
4.5.2 Scanning .7
4.6 Background noise correction .8
4.7 Environmental correction .8
4.8 Exception of Scenario 1 .8
5 Determination of apparent level reduction . 8
5.1 Apparent level reduction .8
5.2 Determination of apparent speech level reduction .9
5.3 Precision . . .9
5.4 Typical values .9
6 Information to be reported . 10
6.1 General .10
6.2 Test environment and test object .10
6.3 Instrumentation .10
6.4 Acoustical data .11
Annex A (informative) Example of the calculation of the main test results .12
Annex B (informative) Example of presentation of the main test results .13
Annex C (informative) Measurement uncertainty. 14
Bibliography .15

iii
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
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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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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This document was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 2, Building
acoustics.
A list of all parts in the ISO 23351 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
Introduction
Furniture ensembles and enclosures are used to improve the acoustic quality of workspaces, e.g. in offices,
education spaces, health-care premises, industrial spaces and public interiors.
Furniture ensembles (partial enclosures) are, for example, partially enclosed workstations, sofa groups, and
working zones with mobile screens, curtains, or other space dividers.
Enclosures are for example movable one-person phone booths (pods), movable one- or two-person working
booths, and demountable or configurable meeting booths for numerous occupants. Enclosures are typically
equipped at least with walls, ceiling, door, electric outlets, lighting, glazing, furniture and a ventilation fan.
An enclosure can also involve a floor element, but it is not necessary. Some enclosures can also be equipped
with air-conditioning systems or openable ceilings due to fire protection reasons.
Both furniture ensembles and enclosures (herein called "test objects") are usually assembled using elements
or modules, which can be transferred into any room through normal-sized passage doors. Test objects are
typically assembled in a finished room and not during the construction of the building. They are usually
purchased by the tenant or lessor, but they can also be leased. They are not considered as fixed parts of the
building, and their application does not require a building permit.
Both diversity and the market of the above-mentioned test objects have grown. Where possible, it is
recommended to test their acoustic performance according to ISO 23351-1 in laboratory conditions. However,
ISO 23351-1 gives an upper limit for the volume of the test object. Therefore, very large test objects cannot be
tested by ISO 23351-1. In addition, it would be beneficial to be able to determine the speech level reduction
in alternative environments. For example, the manufacturer should be able to conduct a preliminary test in
the product development environment, or the user (buyer) should be able to verify by measurement in situ
that the declared speech level reduction was fulfilled.
This document provides a method to determine the apparent speech level reduction of above-mentioned
test objects in rooms that do not fulfil the requirements of ISO 3741.
The aim of the method is to describe the acoustic performance in such a way that the result is comparable to
results obtained in any other room or to the result obtained by ISO 23351-1. The measurement uncertainty
of this method is small (engineering grade of accuracy). This way, the test results obtained for different
test objects can be reliably compared. This is important for buyers so that they can make product selection
reliably. Small measurement uncertainty is only possible by setting high requirements for the placement of
the test object in the room during the measurement. As a result, the actual position of the test object in the
room may not be acceptable and the test object may need to be moved during the test.
[1]
One study showed that ISO 23351-1 and this document produced similar results for an enclosure and for a
partial enclosure.
The aim of this method is not to determine how much the test object mounted in a specific location in the
room provides insertion loss to specific positions of the room. While speech level reduction is a constant
property of a test object, insertion loss depends on the position of the test object in the room.

v
International Standard ISO 23351-2:2026(en)
Acoustics — Measurement of speech level reduction of
furniture ensembles and enclosures —
Part 2:
Field method
1 Scope
This document specifies a method to determine the apparent speech level reduction of furniture ensembles
and enclosures (hereinafter referred to as test objects) in field conditions meeting engineering grade of
accuracy.
The method is applicable to furniture ensembles or enclosures, which can be used by one or more occupants.
This method is not intended for single components used in workstations, such as a screen, a storage unit, a
table, a luminaire, a cupboard, a bookshelf, a standard chair, a wall absorber, or a ceiling absorber.
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 3744, Acoustics — Determination of sound power levels and sound energy levels of noise sources using sound
pressure — Engineering methods for an essentially free field over a reflecting plane
ISO 3382-2, Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary
rooms
IEC 60942, Electroacoustics — Sound calibrators
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 terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
apparent level reduction
D’
i
frequency-dependent measure describing how much the sound power level is reduced by the test object in
1/1-octave frequency bands from 125 Hz to 8 000 Hz

3.2
apparent speech level reduction
D’
S,A
reduction of A-weighted sound power level of speech caused by the test object
3.3
artificial mouth
AM
instrument primarily used as the sound source for measuring the apparent level reduction (3.1) of furniture
ensembles
[2]
Note 1 to entry: Requirements for an AM are specified in ITU-T P.51 .
3.4
enclosure
single object that is enclosed with walls, ceiling and door
Note 1 to entry: Examples for enclosures are given in Figure 1 d) and e).
Note 2 to entry: Enclosures can also have their own floor structure.
EXAMPLE Examples of enclosures are mobile phone booths, mobile working booths and mobile meeting booths.
3.5
furniture ensemble
setup of furniture consisting of items such as tables, screens, storage units, connected sofas or any specified
combination considered as a single object
Note 1 to entry: Furniture ensembles are also called as partial enclosures.
Note 2 to entry: Examples for furniture ensembles are given in Figure 1 a) to c).
EXAMPLE Furniture ensembles can be conventional workstations consisting of, for example, screen(s), storage
unit(s) and a table, working booths, meeting booths, partially enclosed sofa groups, and partially enclosed chairs.
3.6
measurement surface
box-shaped imaginary envelope within which the test object is located during Scenario 2 and on which the
microphone positions or scanning paths are located in Scenarios 1 and 2
Note 1 to entry: The measurement surface is not a tangible object.
Note 2 to entry: Scenarios 1 and 2 are defined in Clause 4.
3.7
omnidirectional sound source
OSS
instrument used as the sound source for measuring the apparent level reduction (3.1) of enclosures (3.4)
[8]
Note 1 to entry: The directivity of OSS fulfils the specifications given in ISO 3382-1 .
3.8
sound power level of speech
L
W,S,1
sound power level of normal effort genderless speech
Note 1 to entry: Genderless speech is the mean of female and male speech and the values are reported in Table 1.

a)  Workstation b)  Workstation c)  Sofa group for d)  Mobile work- e)  Meeting booth
without ceiling with closed ceiling meetings ing booth for one for 6 persons
person
Figure 1 — Examples of furniture ensembles and enclosures
4 Measurement conditions
4.1 Overview of the method
In this method, the sound power level produced by the sound source is determined in two scenarios:
a) without the test object;
b) with the test object.
In both scenarios, the measurement method of sound power level is developed from that described in
ISO 3744.
Scenario 1 is needed to determine the sound power level of the sound source. Scenario 2 is needed to
determine the sound power level of the sound source when it is surrounded by the test object. In both
scenarios, the sound source should have the same position, which corresponds to the occupant’s position
inside the test object in Scenario 2. Apparent level reduction is the difference in the sound power levels
measured in the two scenarios in 1/1-octave frequency bands from 125 Hz to 8 000 Hz. Apparent speech
level reduction is a single-number quantity that expresses the corresponding reduction in A-weighted sound
power level of standard speech within the entire frequency range from 125 Hz to 8 000 Hz.
The sound power level of the sound source shall be the same during Scenario 1 and Scenario 2.
Audio amplifiers and signal generators that have step-by-step volume control should be used instead of
stepless volume control to avoid unintentional change of sound power level between Scenarios 1 and 2.
4.2 Test environment
Reference [1] and [3] showed that the effect of room volume and room reverberation time on D’ are
S,A
negligible. Therefore, the test can be conducted in all kinds of rooms provided there is sufficiently space
around the test object. The requirement of ISO 3744 for the environmental correction K of the room
(K ≤4 dB) is not applied.
During the measurement of Scenario 2, the test object shall be located so that the distance to the walls of
the room is at least 1,5 m. In addition, the distance to nearby furniture items not belonging to the test object
shall be at least 1,5 m.
Deviations from these minimum distance requirements shall be clearly documented in the report, along
with an assessment of their impact on measurement uncertainty.
If the widths of the walls of the test object are larger than 1,5 m, the room walls and test object walls should
not be parallel to avoid reactive sound field. This is especially important if the room walls are strongly
sound-reflective.
The room height shall be at least 0,5 m larger than the test object. Otherwise, another room shall be chosen
for the measurement.
Two options (see Figure 2) are possible for determining the measurement surface.
— Option A: low room. The room height is 0,5 to 1,5 m larger than the test object. The measurement surface
has four sub-surfaces surrounding the test object. The height of the box is equivalent to the room height.
The sound absorption coefficient of the ceiling shall be small (such as hard unperforated building board
made of wood, gypsum or steel, concrete slab, or wood slab) to guarantee that all sound radiated by the
test object’s ceiling is reflected by the room ceiling and propagates to the vertical sub-surfaces, where all
measurements are undertaken.
— Option B: high room. The room height is at least 1,5 m larger than the test object. The measurement
surface has five sub-surfaces surrounding the test object. There are no requirements for the sound
absorption properties of the ceiling.
In both options, the measurement surface is located 0,5 m away from the outer envelope of the test object.
The minimum distance between midpoint of sound source and microphone position is 1,0 m.
Option A does not have a horizontal measurement surface above the test object unlike Option B.
The positions of microphones should be the same in both Scenarios 1 and 2 if engineering grade of accuracy
is desired. Therefore, the test object should be absent during Scenario 1. The exception to this requirement
is detailed in 4.8.
Any elements of the room, within which the test is undertaken which may affect the performance of the
test object (e.g. ventilation grilles within a raised access floor of the room) should be mitigated prior to the
tests. Where this is not possible, this should be clearly described within the test report. The room shall be
unoccupied during the measurements excluding the test operator.
The floor of the room under the measurement surface shall be sound reflective (such as concrete, laminate,
vinyl or parquet) if the test object does not contain a fixed floor element. If the enclosure has a fixed floor
element, textile carpet thinner than 12 mm is allowed because of the evidence of Reference [1].
4.3 Test object
Test object tested according to this document shall have a predefined size, geometry, and material listing.
The position(s) of the occupant(s) shall be clearly identifiable.
The test object shall be tested at least in one position in the room. Additional tests in another position in the
same or another room may be used.
The test object shall be assembled according to manufacturer’s installation instructions. Operable parts,
such as doors or windows, shall be in normal operable condition. The pressure loss of ventilation openings
shall be the same as in normal operation conditions. However, the position of the test object in the room
shall fulfil the requirements given in 4.2.
If the test object involves operable components such as doors or windows, these components shall be opened
and closed at least five times before the testing to ensure that they operate properly.
If the test object consists of several components such as a user-specific workstation involving, for example,
a table, a chair, screens, and a storage unit, all the components shall be installed according to the specified
way, which conforms to the intended configuration of the workstation.
If the test object involves fixed accessories, which may affect the sound propagation inside the test object,
such as computer monitors or chairs, they shall be installed during the test. A chair may be removed at the
position of the omnidirectional sound source (OSS) if it impedes the positioning (e.g. where it is fixed upon
a tripod). Additional accessories not included with the typical installation of the test object (i.e. accessories
typically brought by the occupant) are not installed during the test.
The test object shall be unoccupi
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