SIST EN ISO 5136:2009

SLOVENSKI STANDARD
01-november-2009
1DGRPHãþD
SIST EN ISO 5136:2004
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Acoustics - Determination of sound power radiated into a duct by fans and other air-
moving devices - In-duct method (ISO 5136:2003)
Akustik - Bestimmung der von Ventilatoren und anderen Strömungsmaschinen in Kanäle
abgestrahlten Schallleistung - Kanalverfahren (ISO 5136:2003)
Acoustique - Détermination de la puissance acoustique rayonnée dans un conduit par
des ventilateurs et d'autres systèmes de ventilation - Méthode en conduit (ISO
5136:2003)
Ta slovenski standard je istoveten z: EN ISO 5136:2009
ICS:
17.140.20 Emisija hrupa naprav in Noise emitted by machines
opreme and equipment
23.120 =UDþQLNL9HWUQLNL.OLPDWVNH Ventilators. Fans. Air-
QDSUDYH conditioners
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 5136
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2009
ICS 17.140.20; 23.120 Supersedes EN ISO 5136:2003
English Version
Acoustics - Determination of sound power radiated into a duct by
fans and other air-moving devices - In-duct method (ISO
5136:2003)
Acoustique - Détermination de la puissance acoustique Akustik - Bestimmung der von Ventilatoren und anderen
rayonnée dans un conduit par des ventilateurs et d'autres Strömungsmaschinen in Kanäle abgestrahlten
systèmes de ventilation - Méthode en conduit (ISO Schallleistung - Kanalverfahren (ISO 5136:2003)
5136:2003)
This European Standard was approved by CEN on 20 July 2009.
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 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 Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2009 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 5136:2009: E
worldwide for CEN national Members.

Contents Page
Foreword .3
Annex ZA (informative) Relationship between this European Standard and the Essential
Requirements of EU Directive 98/37/EC .4
Annex ZB (informative) Relationship between this European Standard and the Essential
Requirements of EU Directive 2006/42/EC .5

Foreword
The text of ISO 5136:2003 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 5136:2009 by
Technical Committee CEN/TC 211 “Acoustics” the secretariat of which is held by DS.
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 February 2010, and conflicting national standards shall be withdrawn
at the latest by February 2010.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 5136:2003.
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association, and supports essential requirements of EC Directives.
For relationship with EC Directives, see informative Annexes ZA and ZB, which are integral parts of this
document.
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, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 5136:2003 has been approved by CEN as a EN ISO 5136:2009 without any modification.
Annex ZA
(informative)
Relationship between this European Standard and the Essential
Requirements of EU Directive 98/37/EC
This European Standard has been prepared under a mandate given to CEN by the European Commission
and the European Free Trade Association to provide a means of conforming to Essential Requirements of the
New Approach Directive 98/37/EC, amended by 98/79/EC on machinery.
Once this standard is cited in the Official Journal of the European Communities under that Directive and has
been implemented as a national standard in at least one Member State, compliance with the normative
clauses of this standard confers, within the limits of the scope of this standard, a presumption of conformity
with the relevant Essential Requirements of that Directive and associated EFTA regulations.
WARNING - Other requirements and other EU Directives may be applicable to the product(s) falling within the
scope of this standard.
Annex ZB
(informative)
Relationship between this European Standard and the Essential
Requirements of EU Directive 2006/42/EC
This European Standard has been prepared under a mandate given to CEN by the European Commission
and the European Free Trade Association to provide a means of conforming to Essential Requirements of the
New Approach Directive 2006/42/EC on machinery.
Once this standard is cited in the Official Journal of the European Communities under that Directive and has
been implemented as a national standard in at least one Member State, compliance with the normative
clauses of this standard confers, within the limits of the scope of this standard, a presumption of conformity
with the relevant Essential Requirements of that Directive and associated EFTA regulations.
WARNING — Other requirements and other EU Directives may be applicable to the product(s) falling within
the scope of this standard.
INTERNATIONAL ISO
STANDARD 5136
Second edition
2003-04-01
Acoustics — Determination of sound
power radiated into a duct by fans and
other air-moving devices — In-duct
method
Acoustique — Détermination de la puissance acoustique rayonnée
dans un conduit par des ventilateurs et d'autres systèmes de
ventilation — Méthode en conduit

Reference number
ISO 5136:2003(E)
©
ISO 2003
ISO 5136:2003(E)
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ii © ISO 2003 — All rights reserved

ISO 5136:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
1.1 General. 1
1.2 Types of sound source. 1
2 Normative references . 2
3 Terms, definitions and symbols . 2
4 Uncertainty of the measurement method.7
5 Test facilities and instrumentation. 8
5.1 General requirements . 8
5.2 Duct specifications . 9
5.3 Instrumentation . 16
5.4 System calibration . 19
6 Test arrangement . 20
6.1 Sampling tube mounting. 20
6.2 Microphone position. 20
6.3 Operating condition control equipment .21
7 Test procedure . 21
7.1 Operating conditions . 21
7.2 Sound pressure level readings. 21
7.3 Measurements with and without flow straightener on the outlet side . 22
7.4 Inlet side measurements — Large fans: installation category D (according to
ISO 5801:1997). 22
8 Calculations. 23
8.1 Average sound pressure level. 23
8.2 Sound power level . 23
9 Information to be recorded . 24
10 Information to be reported . 24
Annex A (normative) Determination of the combined mean flow velocity and modal correction
C . 25
3,4
Annex B (normative) Determination of the signal-to-noise ratio of sound vs. turbulent pressure
fluctuation in the test duct . 31
Annex C (normative) Computational procedures for calculating the A-weighted sound power level
from one-third-octave-band sound power levels. 34
Annex D (informative) Example of calculation of C for a given duct diameter and mean flow
3,4
velocity. 35
Annex E (informative) Guidelines for the design and construction of an anechoic termination. 38
Annex F (informative) Evaluation of performance of anechoic terminations . 47
Annex G (informative) Sampling tube information. 50
Annex H (informative) Test method for small ducted fans. 54
Annex I (informative) Test method for large ducted fans. 58
Annex J (informative) Measurement of the swirl component . 65
Bibliography . 66

ISO 5136:2003(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 5136 was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 1, Noise.
This second edition cancels and replaces the first edition (ISO 5136:1990), which has been technically revised.
iv © ISO 2003 — All rights reserved

ISO 5136:2003(E)
Introduction
This International Standard describes a procedure for the measurement of sound pressure levels in the inlet
or outlet ducts of a fan and a method to use these sound pressure levels to calculate the sound power levels
radiated by the fan to the duct system.
Annex A lists values of coefficients for the determination of the combined mean flow velocity and modal
correction. Annex B specifies two procedures for the determination of the signal-to-noise ratio of sound versus
turbulence. A computational procedure for the calculation of the A-weighted sound power level from one-third-
octave band levels is given in Annex C. Annex D shows an example of the calculation of the combined mean
flow velocity and modal correction.
The sound power radiated into a duct by a fan or other air-moving device depends to some extent on the type
of duct, characterized by its acoustical impedance. For a measurement method, the test duct has, therefore,
to be clearly specified. In this International Standard, the test duct is of circular cross-section and terminated
anechoically. Details of typical anechoic terminations are given in Annex E. The sound power obtained under
these special conditions is a representative value for actual applications, as the anechoic termination forms an
impedance about midway between the higher and lower impedances found in practice. The sound power
radiated in actual applications can, in theory, be estimated from data on air-moving devices and duct
impedances. Since this information is at present incomplete, these effects are not usually considered in
acoustical calculations.
In order to suppress the turbulent pressure fluctuations at the microphone, the use of a long cylindrical
windscreen (“sampling tube”) is preferred. The microphone, with the sampling tube, is mounted at a radial
position such that the sound pressure is well related to the sound power by the plane wave formula to an
acceptable extent, even in the frequency range in which higher-order acoustic modes are possible.
The uncertainty of measurement (see Clause 4) is given in terms of the standard deviation to be expected if
the measurements were repeated in many different laboratories.
The procedures for measuring the operating conditions (performance measurements) are not specified in
detail in this International Standard. The operating conditions are specified in ISO 5801.
This International Standard is one of a series specifying different methods for determining the sound power
levels of fans and other air-moving devices.
In general, the sound powers radiated from a fan inlet or outlet into free space and into a duct are different
because of the reflection of sound energy at the fan inlet or outlet plane when there is no connected duct. The
in-duct method according to this International Standard is suitable for determining the sound power radiated
into a duct by a fan inlet or outlet. The sound power radiated into free space by a fan inlet or outlet should be
determined using the a reverberation room method (ISO 3741, ISO 3743), a free-field method (ISO 3744,
ISO 3745, ISO 3746) or a sound intensity method (ISO 9614).
INTERNATIONAL STANDARD ISO 5136:2003(E)

Acoustics — Determination of sound power radiated into a duct
by fans and other air-moving devices — In-duct method
1 Scope
1.1 General
This International Standard specifies a method for testing ducted fans and other air-moving devices to
determine the sound power radiated into an anechoically terminated duct on the inlet and/or outlet side of the
equipment.
NOTE 1 For the sake of brevity, wherever the term “fan” occurs in the text, it means “fan or other air-moving device”.
The method is applicable to fans which emit steady, broad-band, narrow-band and discrete-frequency sound
and to air temperatures between − 50 °C and + 70 °C. The test duct diameter range is from 0,15 m to 2 m.
Test methods for small (d < 0,15 m) and large (d > 2 m) test ducts are described in the informative Annexes H
and I, respectively.
The maximum mean flow velocity at the microphone head for which the method is suitable depends on the
type of microphone shield used, and is as follows:
 foam ball 15 m/s;
 nose cone 20 m/s;
 sampling tube 40 m/s.
Above these values the suppression of turbulent pressure fluctuations by the microphone shield (see 3.9) may
be insufficient.
It is expected that sound power tests will be conducted in conjunction with airflow performance tests in
accordance with ISO 5801. The ducting arrangement will therefore normally incorporate a “star” type flow
straightener on the outlet side of the fan which will minimize swirl (see 7.3). Where it is permissible to delete
the straightener as, for example, with large fans to installation category C according to ISO 5801:1997, the
method is limited to a swirl angle of 15°. (An example of a method for determining the angle of swirl is given in
Annex J.)
NOTE 2 The installation categories defined in ISO 5801 imply that the fan is either ducted on the outlet side only
(category B), on the inlet side only (category C) or on both sides (category D).
1.2 Types of sound source
The method described in this International Standard is applicable to a sound source in which a fan is
connected to ducts on at least one side. It is also applicable to other fan/attenuator combinations or equipment
incorporating fans which can be considered as “black boxes”.
Examples of fans and other equipment covered by this International Standard are
 ducted centrifugal fans,
 ducted axial flow fans,
ISO 5136:2003(E)
 ducted mixed-flow fans,
 ducted air-handling units,
 ducted dust-collection units,
 ducted air-conditioning units, and
 ducted furnaces.
This International Standard is also applicable to other aerodynamic sources such as boxes, dampers and
throttle devices provided that a quiet air flow delivered by an auxiliary fan is available, and the signal-to-noise
ratio of sound pressures to turbulent pressure fluctuations in the test duct is at least 6 dB (see 7.2.1).
An alternative method to determine the sound power level of the flow-generated noise of such aerodynamic
sound sources, which does not require the measurement of sound pressure in a flow environment, is
described in ISO 7235. The method was originally devised for the determination of the flow noise level of
ducted silencers. The sound power is determined in a reverberation room connected to the test duct via a
transition element.
In the case of ducted fans with closely coupled attenuators, the signal-to-noise ratio of sound pressures to
turbulent pressures may be insufficient when using the in-duct method. Therefore the method described in
ISO 7235 is recommended for such fan/attenuator combinations.
This International Standard is not applicable to non-ducted fans or equipment.
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 266, Acoustics — Preferred frequencies
ISO 5801:1997, Industrial fans — Performance testing using standardized airways
IEC 60651:2001, Sound level meters
IEC 60942:1997, Electroacoustics — Sound calibrators
IEC 61260, Electroacoustics — Octave-band and fractional-octave-band filters
3 Terms, definitions and symbols
For the purpose of this document, the following terms and definitions apply. The symbols are given in Table 1.
3.1
fan inlet area
S
f1
surface plane bounded by the upstream extremity of the fan
NOTE 1 The inlet area is, by convention, taken as the gross area in the inlet plane inside the casing. No deduction is
made for motors, fairings or other obstructions.
2 © ISO 2003 — All rights reserved

ISO 5136:2003(E)
NOTE 2 Where motors, fairings or other obstructions extend beyond an inlet or outlet flange at which the performance
for ducted installation is to be determined, the casing should be extended by a duct of the same size and shape as the
inlet or outlet and of sufficient length to cover the obstruction. The test airway dimensions should be measured from the
plane through the outermost extension of the obstruction as if this were the plane of the inlet or outlet flange.
NOTE 3 The fan inlet area is expressed in square metres (m ).
NOTE 4 Adapted from ISO 5801:1997.
3.2
fan outlet area
S
f2
surface plane bounded by the downstream extremity of the fan
NOTE 1 The outlet area is, by convention, taken as the gross area in the outlet plane inside the casing. No deduction is
made for motors, fairings or other obstructions.
NOTE 2 Some free-outlet fans without casings have no well-defined outlet area. For the purpose of determining the
fan's dynamic pressure, a nominal area may then be defined and stated, e.g. the area within the ring of a propeller wall fan
or the circumferential outlet area of an open-running centrifugal impeller. The corresponding fan dynamic pressure and fan
pressure will also be nominal and should be so described.
NOTE 3 The fan outlet area is expressed in square metres (m ).
NOTE 4 Adapted from ISO 5801:1997.
3.3
ducts
any of the airways defined in 3.3.1, 3.3.2 and 3.3.3
3.3.1
test duct
duct in which the fan sound power is measured
NOTE The test duct has an anechoic termination.
3.3.2
terminating duct
duct opposite to the test duct, if both sides of the fan are ducted
NOTE The terminating duct has an anechoic termination.
3.3.3
intermediate duct
duct fitted on the intake side and on the discharge side of the fan to ensure desired flow conditions
NOTE The intermediate duct connects to the test duct or the terminating duct, if necessary by a transition section
(see Figure 7).
3.4
measurement plane
radial plane in the test duct in which the microphone diaphragm is located
3.5
sound pressure level
L
p
p
L = 10 lg dB (1)
p
p
ISO 5136:2003(E)
where p is the root mean square value of the sound pressure and the reference sound pressure p is equal
to 20 µPa
NOTE 1 The width of a restricted frequency band should be indicated, for example, octave-band sound pressure level,
one-third-octave-band sound pressure level.
NOTE 2 L , L and L are the sound pressure levels at each of the three measurement positions in the test duct.
p1 p2 p3
L is the spatially averaged sound pressure level obtained from averaging over the measurement positions in the test
pm
duct. It may also be obtained from a continuous circumferential traverse (see 7.2.4).
L is the spatially averaged sound pressure level at the measurement plane, corrected for the combined free-field
p
response C (see Table 1 and 8.1).
NOTE 3 The sound pressure level is expressed in decibels (dB).
3.6
sound power level
L
W
P
L = 10 lg dB (2)
W
P
where P is the sound power and the reference sound power P is equal to 1 pW
NOTE 1 The width of a restricted frequency band should be indicated, for example, octave-band sound power level,
one-third-octave-band sound power level.
NOTE 2 The sound power level is expressed in decibels (dB).
3.7
fan sound power
sound power radiated into the test duct by the fan
3.8
frequency band range of interest
one-third-octave bands with centre frequencies between 50 Hz and 10 000 Hz
NOTE For information only, the frequency range of interest may be extended up to 20 000 Hz. For fans which radiate
predominantly high- or low-frequency sound, the frequency range of interest may be limited in order to reduce the costs of
the test facilities and procedures. The limits of the restricted frequency range shall be given in the test report.
3.9
microphone shield
device designed to protect a microphone placed in a moving airstream from self-generated wind noise and
turbulent pressure fluctuations
NOTE 1 See Clause 4, Note 5.
NOTE 2 The three types are listed in order of preference in 3.9.1, 3.9.2 and 3.9.3.
3.9.1
sampling tube
turbulence screen
metal tube with a longitudinal slit, covered by a porous material within which the microphone is positioned,
designed to reduce the response of the microphone to self-induced wind noise and to turbulent pressure
fluctuations of the air pressure within the duct
See Figure 1.
4 © ISO 2003 — All rights reserved

ISO 5136:2003(E)
NOTE 1 The sampling tube is the preferred microphone shield for measurements according to this International
Standard.
NOTE 2 To minimize self-induced wind noise, the outer surface of the tube should be smooth and free of any
discontinuities (see Figure 1). The slit and covering of the sampling tube should be designed to reduce the response of the
microphone to turbulent pressure fluctuations in the air stream emanating from the fan being tested.

Key
1 nose cone
2 slit-tube
3 microphone
4 slit
5 porous material
Figure 1 — Schematic of a sampling tube for a 13 mm (1/2 inch) microphone
3.9.2
nose cone
microphone shield designed to substitute the normal protection grid of the microphone and used in high-
velocity air flows with low turbulence and little swirl having a streamlined shape with the least possible
resistance to airflow and a fine wire mesh around its periphery allowing sound pressure transmission to the
microphone diaphragm, whilst a truncated cone behind the mesh reduces the air volume in form of the
diaphragm
See Figure 2.
Figure 2 — Schematic of a nose cone
3.9.3
foam ball
ball of open-pored foam with a cylindrical hole of appropriate diameter for insertion of the microphone and
preamplifier, designed not to affect the directivity of the microphone
See Figure 3.
Figure 3 — Schematic of a foam ball
ISO 5136:2003(E)
3.10
frequency range of plane-wave sound propagation in a duct with circular cross section
frequencies, in hertz, below the cut-on frequency of the first cross mode, f , as given by
1,0
cU
f=−0,586 1 (3)
1,0

Dc

where
c is the speed of sound, approximately 340 m/s;
D is the duct diameter, in metres;
U is the mean flow velocity, in metres per second.
Table 1 — Symbols
C correction in decibels supplied by the manufacturer to be added to the calibrated microphone
response to obtain the free field response.
C frequency response correction in decibels of the sampling tube microphone shield at normal
incidence to be added to the calibrated microphone response. (See 5.3.3 and 5.3.4.)
C combined mean flow velocity and modal correction in decibels for the frequency response

3,4
required by the use of the sampling tube microphone shield. (See tables in Annexes A, H and I.)
C = C + C + C combined frequency response correction, expressed in decibels.

1 2 3,4
c
speed of sound in the test duct, in metres per second.
U mean flow velocity in the test duct, in metres per second.
fluid density, in kilograms per cubic metre, in the duct.
ρ
d diameter, in metres, of the fan inlet (d ), fan outlet (d ), test duct (d and d in Figure 5),
1 2 3 6
intermediate ducts (d ), terminating ducts (d in Figure 6 and d in Figure 7).
4 6 3
l
length of the ducts and transitions (see Figures 5 to 7).
r radial distance, in metres, from the test duct centreline to the microphone centreline.
r dimensionless pressure reflection coefficient defined as the ratio of the sound pressure
a
amplitude of the sound wave reflected from the anechoic termination to the sound pressure
amplitude of the incident wave.
b, h cross dimensions, in metres, of the rectangular fan inlet or fan outlet.
S
cross-sectional areas of ducts or duct sections, in square metres.
NOTE 1 In the first edition of ISO 5136 (1990), two correction terms C and C were used to account for the effect of the flow and the
3 4
modal distribution in the sound field on the response of the sampling tube. In the present edition, these two effects are incorporated in
the new combined correction term C .
3,4
NOTE 2 U < 0 for inlet side measurements; U > 0 for outlet side measurements.

6 © ISO 2003 — All rights reserved

ISO 5136:2003(E)
4 Uncertainty of the measurement method
Determination of sound power made in accordance with this International Standard will tend to result in an
uncertainty of sound power level given in terms of the values of the standard deviation of reproducibility given
in Table 2. The standard deviations given in this table reflect the cumulative effects of all causes of
measurement uncertainty such as source location, duct end reflections, duct transitions, instrument calibration,
sound pressure to sound power computing and sampling errors. The standard deviations given in the table
are those which would be expected if the measurement of a single fan were repeated in many different
laboratories. They do not include variations in the sound power radiated by the fan itself caused, for example,
by changes in the mounting arrangements. Care should be taken to obtain a specified time average in
accordance with the requirements laid down in 7.2.2.
Table 2 — Values of the standard deviation of reproducibility for the sampling tube
One-third-octave band Standard deviation of
centre frequency reproducibility, σ
R
Hz dB
50 3,5
63 3
80 to100 2,5
125 to 4 000 2
5 000 2,5
6 300 3
8 000 3,5
10 000 4
NOTE The standard deviations given in Table 2 are
derived from information in references [3], [5] and [19].

The procedures of this International Standard and the standard deviations given in Table 2 are applicable to
measurements on an individual piece of equipment. Characterization of the sound power levels of batches of
equipment of the same family or type involves the use of random sampling techniques in which confidence
intervals are specified, and the results are expressed in terms of statistical upper limits. In applying these
techniques, the total standard deviation must be known or estimated, including the standard deviation of
production as defined in ISO 7574-1, which is a measure of the variation in sound power output between
individual pieces of equipment within the batch. Statistical methods for the characterization of batches of
equipment are described in ISO 7574-3 and ISO 7574-4.
The measurement uncertainty may be lowered by careful construction of the test set-up, by eliminating
transition ducts, and by use of more absorptive terminating ducts.
For a particular family of sound sources, of similar size and with similar sound power spectra, the standard
deviation of reproducibility could be smaller than the values given in Table 2. Hence, a test code for a
particular type of equipment may state standard deviations smaller than those listed in Table 2 if
substantiation is available from the results of suitable interlaboratory tests.
At high frequencies, particularly above 4 000 Hz, the standard deviation data quoted in Table 2 can
underestimate the actual standard deviations when the noise spectrum being measured decreases rapidly
with frequency. Under these conditions, the high-frequency sound pressure levels sensed by the microphone
can be of small magnitude compared with those at low frequencies, and electrical noise, particularly from the
frequency analyser, can interfere with the sound signal at these high frequencies. In order to achieve
reproducible determinations of sound power (with standard deviations in Table 2) it may be necessary to
repeat the high-frequency sound measurement by passing the microphone signal through a high pass filter
before it is analysed by the frequency analyser.
ISO 5136:2003(E)
NOTE 1 When octave-band sound power levels are calculated, the uncertainty of each octave-band level will not be
greater than that of the largest uncertainty of the three constituent one-third-octave bands.
NOTE 2 For a normal distribution, 68 % of all data lie within an interval ± σ , and 95 % lie within ± 2σ .
R R
NOTE 3 The uncertainty will increase in the presence of swirling flows.
NOTE 4 If discrete frequency components are present or if measurements are not averaged over a sufficiently long
period (see 6.2.2), the uncertainty will be greater than that indicated.
NOTE 5 A microphone exposed to high air velocity will give a falsely high reading. This is rectified by fitting a shield
such as a sampling tube, a nose cone or a foam ball. These are limited in their use (see 1.1) according to the mean flow
velocity. Whilst the foam ball is omni-directional and reduces the wind-generated noise in all directions, a nose cone has to
be aligned with the flow to reduce the wind-generated noise. Only the sampling tube, however, reduces the false noise
generated by turbulent fluctuations of pressure to a sufficient degree. It is, therefore, the preferred solution for all cases.
The uncertainties given in Table 2 refer to the sampling tube only and can be expected to increase for other shields.
NOTE 6 The standard deviations listed in Table 2 are associated with the test conditions and procedures defined in this
International Standard and not with the noise source itself. They arise partly from variations between measurement
laboratories in the geometry of the test facility, background noise, turbulent pressure fluctuations, and the type and
calibration of instrumentation. They are also due to variations in experimental measurement techniques, including spatial
averaging and integration times.
NOTE 7 If several laboratories use similar facilities and instrumentation, the results of sound power determinations on
a given source in those laboratories may be in better agreement than would be inferred by the standard deviations of
Table 2.
Measurements above 10 000 Hz may be reported, but are not considered part of this International Standard.
The extrapolated values of the standard deviation given in Table 3 are suggested.
Table 3 — Extrapolated values
One-third-octave band Standard deviation of
centre frequency reproducibility, σ
R
Hz dB
12 500 4,5
16 000 5
20 000 5,5
5 Test facilities and instrumentation
5.1 General requirements
The test arrangement shall consist of the fan to be tested, an intermediate duct, the test duct with anechoic
termination, and the instrumentation (see Figures 5 to 7). If a fan usually used with duct work on both sides is
to be tested, a termination duct with anechoic termination plus an intermediate duct shall be connected
opposite to the side on which the sound power is determined.
All connections between the fan and the ducts shall be firm, unless a vibration-isolating coupling is an inherent
part of the fan. The test ducts shall include provisions for mounting the microphone and sampling tube at the
locations specified in 6.2.
Suitable provisions shall also be made for controlling the desired operating conditions of the fan.
It is recognised that acoustic and fan performance measurements are to be performed at the same time, and
that the test arrangements of this International Standard and those of ISO 5801 should be in conformity. This
requires that the common part as defined in ISO 5801 be introduced at the fan inlet and/or outlet.
8 © ISO 2003 — All rights reserved

ISO 5136:2003(E)
NOTE 1 The presence of the “star-type” flow straightener on the fan outlet side is necessary for the measurement of
the aerodynamic fan performance according to ISO 5801. However, the swirling flow entering the flow straightener may
generate excess noise at the microphone position which may or may not be of higher level than the sound pressure level
produced by the fan under test. On the other hand, without a flow straightener, the swirling flow around the measurement
microphone may generate excess flow noise which may or may not be of higher level than the sound pressure level
produced by the fan under test. For this reason, comparative sound measurements with and without the “star-type” flow
straightener in position are specified (see 7.3).
NOTE 2 Examples of designs of anechoic terminations and throttling devices are given in Annex E.
5.2 Duct specifications
5.2.1 Construction of ducts and transitions
The ducts shall be straight, coaxial with the inlet or outlet of the fan, and of uniformly circular cross section.
The ducts and transitions shall be manufactured either from steel having a minimum thickness of 1 mm or
from a material of equivalent mass per unit area and rigidity which ensures an acoustically hard and smooth
interior surface.
The ducts and transitions should preferably be treated with a vibration-damping material on the outside.
NOTE This International Standard specifies test ducts with circular cross sections. Future International Standards
may involve ducts with other cross sections.
5.2.2 Duct lengths
Duct lengths shall be as specified in Figure 5.
5.2.3 Duct cross-sectional area
The duct cross-sectional areas shall be as specified in Table 4, where the inlet area S or outlet area S is the
f1 f2
area on the side to which the respective duct is connected.
Table 4 — Cross-sectional areas of ducts
Duct Cross-sectional area
min. max.
Intermediate 1 S 1 S
f1 f1
Inlet side Test 1 S 2,1 S
f1 f1
Terminating 1 S 2,1 S
f1 f1
0,95 S 1,07 S
Intermediate
f2 f2
Outlet side Test 0,7 S 2,1 S
f2 f2
Terminating 0,7 S 2,1 S
f2 f2
5.2.4 Transition ducts
The test duct or terminating duct shall be coupled directly to the intermediate duct or, where there is a change
of cross-sectional area, indirectly by means of a transition duct. The diameter ratio of the transition shall lie
within the limits specified in Table 4.
ISO 5136:2003(E)
For acoustic reasons (see references [9] and [26]), the length of the transition shall be such that the minimum
length of transition, l , conforms to
min
lS
min l
=−1 (4)
lS
0s
where
l = 1 m;
S is the larger area;
l
S is the smaller area.
s
For aerodynamic reasons, the outlet transition shall have an
...