ISO/FDIS 13347-3

ISO/DISFDIS 13347-3:2025(en)
ISO/TC 117
Secretariat: BSI
Date: 2025-09-2611-25
Fans — Determination of fan sound power levels under standardized
laboratory conditions —
Part 3:
Enveloping surface methods
Ventilateurs — Détermination des niveaux de puissance acoustique des ventilateurs dans des conditions de
laboratoire normalisées — Partie 3: Méthodes de la surface enveloppante
Partie 3: Méthodes de la surface enveloppante
FDIS stage
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Published in Switzerland.
ii
ISO/DISFDIS 13347-3:2025(en)
Contents Page
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Acoustic environment and ducting . 3
5 Determination of measurement surface . 6
6 Test method . 25
7 Acoustic tests: determination of sound power level . 25
Annex A (normative) Determination of environmental correction K2 . 28

Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Acoustic environment and ducting . 2
4 Determination of measurement surface . 4
5 Test method . 17
6 Acoustic tests: determination of sound power level . 18
Annex A (normative) Determination of environmental correction K2 . 20
Bibliography . 21
© ISO 2025 – All rights reserved
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
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
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represent the latest information, which may be obtained from the patent database available at
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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.htmlwww.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 117, Fans.
This second edition cancels and replaces the first edition (ISO 13347-3:2004), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 13347-3:2004/Cor 1:2006 and the Amendment
ISO 13347-3:2004/Amd 1:2010.
The main changes are as follows:
The main changes compared with ISO 13347-2:2004 are as follows:
— the inclusion of acoustic methods for installation category E fans;
— symbols have been harmonized with those used in ISO 5801 and other ISO standards asdocuments listed
in the Normative references [2];
— editorial corrections have been incorporated.
A list of all parts in the ISO 13347 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.htmlwww.iso.org/members.html.
iv
ISO/DISFDIS 13347-3:2025(en)
Introduction
This document gives a method for determining the sound power level of a fan. The method is reproducible in
all laboratories which are qualified according to the requirements of this document.
The method employs standard sound measurement instrumentation, applied to rooms which are restricted in
certain acoustic properties. The test set-ups are generally designed to represent the physical orientation of a
fan as installed, in accordance with ISO 5801 or ISO 13350.
Since sound power levels are considered independent of the acoustic environment around the fan, a good
comparison can be made between two or more fans proposed for any specific air performance condition.
Moreover, these values establish an accurate base for estimating the acoustical outcome of the fan installation
in terms of sound pressure levels. A successful estimate of sound pressure levels requires extensive
information on the fan and the environment in which it is to be located.
It is often advantageous for the equipment user to employ acoustical consultation to ensure that all factors
which affect theaffecting final sound pressure levels are considered. More detailed information on the
complexity of this situation can be found in acoustic text books.
© ISO 2025 – All rights reserved
v
DRAFT International Standard ISO/DIS 13347-3:2025(en)

Fans — Determination of fan sound power levels under standardized
laboratory conditions — Part 3: Enveloping surface methods
Fans — Determination of fan sound power levels under standardized
laboratory conditions —
Part 3:
Enveloping surface methods
1 Scope
This document specifies a method for determining the sound power level of a fan. This document is applicable
to fans as defined in ISO 13349-1. The applicability of this document is limited to the determination of airborne
sound emission for the specified set-ups. Vibration is not measured, nor is the sensitivity of airborne sound
emission to vibration effects determined.
The sizes of fan which can be tested in accordance with this document are limited only by the practical aspects
of the test set-up. Dimensional limitations, test fan dimensions, and air performance will control the room size,
power and mounting requirements for the test fan.
NOTE Small fans can be tested according to this document or to ISO 10302-1, according to usage.
The test arrangements in this document establish the laboratory conditions necessary for a successful test.
Rarely will it be possible to meet these requirements in situ and this document is not intended for field
measurements.
NOTE Intending users are reminded that, in these situations, there can be additional acoustic system effects where
inlet and outlet conditions at the fan are less than ideal.
The enveloping surface methods can be used for the determination of open inlet and/or open outlet sound
power level of fans for the standardized installation category’s given in 4.1.4.1.
An estimation (with increased uncertainty) of ducted sound power for fans too small, or otherwise
inconvenient, for testing by the in-duct method described in ISO 5136 can also be obtained by the addition of
end reflection corrections (see Annex C of ISO 13347-1:2025, Annex C).
2 Normative references
The following referenced documents are indispensable for the application 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 of noise sources using sound pressure — Engineering
method in an essentially free field over a reflecting plane
ISO 5136, Acoustics — Determination of sound power radiated into a duct by fans and other air-moving devices
— In-duct method
ISO 5801, Fans — Performance testing using standardized airways
ISO 10302-1, Acoustics — Measurement of airborne noise emitted and structure-borne vibration induced by
small air-moving devices — Part 1: Airborne noise measurement
ISO 13347--1, Fans — Determination of fan sound power levels under standardized laboratory conditions —
Part 1: General overview
ISO 13349--1, Fans — Vocabulary and definitions of categories — Part 1: Vocabulary
ISO 13350, Fans — Performance testing of jet fans
3 Terms and definitions
For the purposes of this document, the terms and definitions definedgiven in ISO 13347-1 shall 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/obphttps://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/https://www.electropedia.org/
3.1 3.2 Symbols and units
For consistency and mutual understanding, it is recommended that the symbols and units shown in Table 10
be used in reporting and calculation. Unless otherwise noted, the subscript number refers to the mid-
frequency of the octave band or one-third octave band number.
Table 1 — Symbols, units
Symbol Term SI unit
−12
𝐿 sound power level, re 1 pW (1 × 10 W) dB
𝑊
E duct end correction dB
𝐷 equivalent diameter m
e
𝐷 equivalent diameter m
e
𝐷 𝐷 the largest dimension of the circular or rectangular orifice m
O O
𝐷 𝐷 the measurement distance m
r r
𝐷 𝐷 the nominal diameter of the inlet configuration m
N N
h the height of centre of orifice above floor level or above another reflecting plane m
𝑙 , 𝑙 , 𝑙 , 𝑙 , 𝑙 dimensions of the reference box m
1 2 3 2 3
S the measurement surface area m
the cross-sectional area of a section through the fan in the measurement surface m
𝑆 𝑆 plane), only to be determined approximately; illustrated here for axial fans in
H H
examples a), c) and e) above
the portion of measurement surface in contact with the discharge flow, for fans with m
𝑆 𝑆
S S
outlet orifice
ISO/DISFDIS 13347-3:2025(en)
4 Acoustic environment and ducting
4.1 General
The fan installation conditions shall conform to the five installation categories, as specified in ISO 13349-1:
— installation category A: free inlet, free outlet with a partition;
— installation category B: free inlet, ducted outlet;
— installation category C: ducted inlet, free outlet;
— installation category D: ducted inlet, ducted outlet, (casing breakout noise may also be
determined for category D ducted inlet, ducted outlet);
— installation category E: free inlet, free outlet without a partition.

a) Category A b) Category B
c) Category C d) Category D e) Category E
Figure Figure 1 — Installation categories
The test environments that are suitable for measurements according to this procedure are specified in detail
in ISO 3744. They can be outdoors or indoors.
For fans generating large flowrates, measurements should be undertaken in a large space to minimise the
recirculation of turbulent airflow through the fan and to ensure that microphones are not positioned in high
velocity flowrates.
Care should be taken to conduct outdoor tests on still calm days where the effects of the wind on fan
aerodynamic and/or noise performance may be discounted. A maximum wind speed of 3 m/s is
recommended.
During the period of the tests, the variation in the ambient air temperature and in relative humidity should
not exceed ±5 %.
For tests according to this procedure, the environment correction K 𝐾 shall be determined according to
Annex A,Annex A, and should be less than or equal to 4 dB in any one-third octave band.
© ISO 2025 – All rights reserved
4.2 Test installation
4.2.1 General
The test arrangement shall comply with the general requirements of this clause and with the requirements
given in ISO 13347-1.
The ducting and anechoic termination and flow measurement and control device, when necessary, fitted to
the fan for tests for installation B, C and D shall be as specified in ISO 5136 and ISO 5801. Simplified anechoic
termination shall be fitted, see also ISO 13347-1.
The appropriate operating condition shall be determined from a measurement of the air volume flowrate and
fan pressure in accordance with ISO 5801, small fans shall be tested in accordance with ISO 10302-1.
The sound pressure level in the test area due to the noise generated by the flow measurement or control device
shall be at least 10 dB below the measured sound pressure level from the fan under test.
When an air vent is necessary for air to recirculate into or out of the test area, it shall be silenced and placed
on the room surface. It may also be used to regulate the air flow.
If practicable, all auxiliary equipment necessary for the fan under test shall be located outside the test area
which shall be cleared of all objects which may interfere with the measurements.
Care should be taken to ensure that any electrical conduits and fittings, piping, or air ducts connected to the
equipment do not radiate significant amounts of sound energy into the test area.
4.2.2 Installation category A
The fan shall be placed in an aperture over the reflecting plane such that the fan inlet or outlet orifice (as
appropriate) is axially located in relation to the reflecting plane in a manner representative of its
recommended installation. The fan orifice shall be placed coaxially with the wall aperture, and, to ensure
satisfactory air flow conditions, the aperture shall be larger all round than the fan orifice by an amount at least
as great as the thickness of the reflecting plane structure, and the intervening gap shall be sealed by a panel.
Provision shall be made for determining and adjusting the fan air flowrate.
Care should be taken to avoid vibration of the sealing panel.
NOTE The reverberant conditions of the non-measured side can affect the measurements made on the measured
side of the fan.
4.2.3 Installation category B
The inlet of the fan shall be placed in the vicinity of the centre of the test area. Ducting connected to the fan
discharge shall either be of sufficiently massive construction or be treated externally so as to avoid
transmission of undesired noise from within the duct.
The duct shall be terminated with a simplified anechoic device as described in ISO 13347-1. The outlet duct
shall comprise an intermediate duct, a transition duct if necessary, a test duct or terminating duct and an
anechoic termination (see ISO 13347-1:—,:2025, Annex D).
Precautions should be taken to ensure that noise from the flow control or flow measurement device does not
interfere with noise from the fan at any of the measurement locations.
NOTE Unless precautions are made to ensure otherwise, the sound pressure levels measured will include noise
radiated from the fan casing and the fan drive motor (see ISO 13347-1).
ISO/DISFDIS 13347-3:2025(en)
4.2.4 Installation category C
The outlet of the fan shall be placed in the vicinity of the centre of the test area. Ducting connected to the fan
inlet shall either be of sufficiently massive construction or be treated externally so as to avoid transmission of
undesired noise from within the duct.
The duct shall be terminated with a simplified anechoic device as described in ISO 13347-1. The inlet duct
shall comprise an intermediate duct, a transition duct if necessary, a test duct or terminating duct and an
anechoic termination (see ISO 13347-1:—,:2025, Annex D).
Precautions should be taken to ensure that noise from the flow control or flow measurement devices does not
interfere with noise from the fan at any of the measurement locations.
NOTE Unless precautions are made to ensure otherwise, the sound pressure levels measured will include noise
radiated from the fan casing and the fan drive motor (see ISO 13347-1).
4.2.5 Casing sound power, Installation category D
The sound power radiated from the external surface of the fan casing and, if appropriate, from the drive, may
be determined by this method provided that noise radiated from the associated ducting is minimal. The
ducting connected to the fan inlet or outlet shall be terminated with a simplified anechoic device as described
in ISO 13347-1 to ensure that the reflection coefficients are within the limits specified in Table 4 of ISO 13347-
1, and shall also comply with 4.2.1, 4.2.3 and 4.2.44.2.1, 4.2.3 and 4.2.4 of this document.
4.2.6 Installation category E
The fan shall be placed in the vicinity of the centre of the test area.
Performance of jet fans shall be measured in accordance with ISO 13350.
NOTE The sound pressure levels measured on the inlet can include noise radiated from the casing and a contribution
of noise from the outlet. The same applies to sound pressure levels measured on the outlet.
4.2.7 Ducted small fans
Fans with test ducts smaller in diameter than those specified in ISO 13347-1:—,:2025, Clause 8, cannot be
tested by the in-duct method.
4.2.8 End Reflection correction
If an indication of the in-duct sound power level is required, it may be obtained for these fans by applying the
end reflection correction to the sound pressure level determined in the room. This resultant sound power
level is not a true in-duct sound power level, but may be taken as a characteristic in-duct sound power level
for fans for the purposes of noise control calculations. The end reflection correction E can be found in
ISO 13347-1.
𝐿 = 𝐿 𝐿 = 𝐿 + 𝐸
𝑊(𝑖𝑛 𝑑𝑢𝑐𝑡) 𝑊 𝑊(𝑖𝑛𝑑𝑢𝑐𝑡) 𝑊
4.3 Fan-powered exhaust ventilators
Fan-powered exhaust ventilators may be tested by the free field method. When it is not possible to mount a
unit with gravity-controlled shutters in its correct mounting attitude, the shutters shall be locked in the correct
mounting operating position. The units shall be mounted in as representative a manner as possible with only
that degree of vibration isolation recommended for a normal site installation. The opening in the test room
shall be dimensionally similar to the recommended opening for the installed unit. The powered ventilator
© ISO 2025 – All rights reserved
shall not be separated from the external face of the test room by a connecting duct but additional fittings may
be used to simulate accurately the mounting conditions specified by the manufacturer.
4.4 Drive and transmission noise
In cases where the fan drive and its inlet and outlet are in the same measurement space, the noise will be
partly aerodynamic and partly due to the drive and transmission. If the drive is representative of that supplied
with the fan, this noise shall be taken to be the noise from the fan assembly. In cases where drive and
transmission are not supplied as standard, the contribution to the noise made by these shall be checked by
removing the fan impeller and substituting an equal and smooth surfaced mass. The noise thus obtained shall
be measured. If this is 10 dB or more below the level in any octave band due to the complete assembly,
transmission noise may be ignored. For differences between 6 dB and 10 dB, background noise corrections in
accordance with ISO 3744 shall be made. Otherwise, some action shall be taken to reduce noise from this
source. If the drive and transmission are considered to make a significant contribution to the total noise, this
fact shall be included in the test report.
5 Determination of measurement surface
5.1 General
This document recognises a number of different methods for defining the positions at which microphones
shall be placed for measuring the fan noise. The general principle is to define a hypothetical surface enveloping
some or all of the source, or if there is a reflecting plane, enveloping the source and terminating on the
reflecting plane.
5.2 Measurement surfaces
5.2.1 General
The different measurement surfaces are as shown in 5.2.2 to 5.2.4,5.2.2 to 5.2.4, to be used with installation
category A, B, C, D and E as appropriate.
5.2.2 A rectangular parallelepiped enclosing the sound source
As shown in Figure 2.0. This surface is easily and accurately measured. It may include part of the casing
radiated noise, in which case this shall be clearly identified. Directivity of the noise is not easily deduced.
5.2.3 A sphere, or hemisphere over a reflecting plane
As shown in Figures 3, 4 and 5.0, 0 and 0. Directivity of the sound may be deduced by plotting curves of equal
dB value through the levels at the measuring points.
5.2.4 A small-radius hemisphere, the centre of which is located at the intersection of the axis of
symmetry of the inlet(s) and of the plane normal to this axis at the inlet under consideration
As shown in Figure 6,0, this method is suitable for use in more restricted spaces, but is generally confined to
the measurement of inlet noise on fans having a ducted outlet. For absolute comparisons, the use of a
Reference Sound Source is necessary.
5.3 Arrangement of measuring points (rectangular parallelepiped) (installation category A
to E)
A hypothetical reference surface in the form of a rectangular parallelepiped (reference box) shall enclose the
fan, any protruding elements which are not significant radiators of sound energy beingare disregarded.
ISO/DISFDIS 13347-3:2025(en)
For the determination of sound power level 𝐿  𝐿  𝑜𝑟 𝐿  𝐿 the reference box
𝑊 (𝐵,𝑖𝑛 + 𝑐𝑎𝑠) 𝑊(𝐵,𝑖𝑛+𝑐𝑎𝑠) 𝑊 (𝐵,𝑜𝑢𝑡) 𝑊(𝐵,𝑜𝑢𝑡)
shall be selected in such a way that it encloses a portion of the space at the orifice, and in the case of outlet
orifices, a portion of the discharge flow, and t
...