ISO 3747:2010

INTERNATIONAL ISO
STANDARD 3747
Third edition
2010-12-01
Acoustics — Determination of sound
power levels and sound energy levels
of noise sources using sound pressure —
Engineering/survey methods for use in
situ in a reverberant environment
Acoustique — Détermination des niveaux de puissance acoustique et
des niveaux d'énergie acoustique émis par les sources de bruit à partir
de la pression acoustique — Méthode d'expertise et de contrôle pour
une utilisation in situ en environnement réverbérant

Reference number
©
ISO 2010
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2010
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2010 – All rights reserved

Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.2
3 Terms and definitions .2
4 Test environment.6
5 Instrumentation and measurement equipment .6
6 Location, installation and operation of noise source under test .7
7 Measurement procedure.8
8 Calculation of sound power levels and sound energy levels.11
9 Measurement uncertainty.15
10 Information to be recorded.18
11 Test report.20
Annex A (normative) Evaluation of the excess of sound pressure level at a given distance .21
Annex B (informative) Recommendations for the location of the reference sound source and the
microphones, if only one position is used for the reference sound source.22
Annex C (normative) Sound power level and sound energy level under reference meteorological
conditions .27
Annex D (normative) Calculation of A-weighted sound power levels and A-weighted sound
energy levels from octave-band levels .29
Annex E (informative) Guidance on the development of information on measurement uncertainty.31
Bibliography.42

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 3747 was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 1, Noise.
This third edition cancels and replaces the second edition (ISO 3747:2000), which has been technically
revised.
iv © ISO 2010 – All rights reserved

Introduction
[2]
This International Standard is one of the series ISO 3741 to ISO 3747, which specify various methods for
determining the sound power levels and sound energy levels of noise sources including machinery, equipment
and their sub-assemblies. The selection of one of the methods from the series for use in a particular
application depends on the purpose of the test to determine the sound power level or sound energy level and
[1] [1]
on the facilities available. General guidelines to assist in the selection are provided in ISO 3740 . ISO 3740
to ISO 3747 give only general principles regarding the operating and mounting conditions of the machinery or
equipment for the purposes of the test. It is important that test codes be established for individual kinds of
noise source, in order to give detailed requirements for mounting, loading, and operating conditions under
which the sound power levels or sound energy levels are to be obtained.
The method given in this International Standard is based on a comparison of the sound pressure levels in
octave frequency bands of a noise source under test with those of a calibrated reference sound source;
A-weighted sound power levels or sound energy levels may be calculated from the octave-band levels. The
method is applied where the noise source is found in situ and as such is suitable for larger pieces of stationary
equipment which, due to their manner of operation or installation, cannot readily be moved.
The method specified in this International Standard permits the determination of the sound power level and
the sound energy level in octave bands from which the A-weighted value is calculated.
This International Standard describes a method giving results of either ISO 12001:1996, accuracy grade 2
(engineering grade) or ISO 12001:1996, accuracy grade 3 (survey grade), depending on the extent to which
the requirements concerning the test environment are met. For applications where greater accuracy is
[2] [5] [17]-[19]
required, reference can be made to ISO 3741 , ISO 3744 or an appropriate part of ISO 9614 . If the
relevant criteria for the measurement environment specified in this International Standard are not met, it might
[17]-[19]
be possible to refer to another standard from this series, or to an appropriate part of ISO 9614 .

INTERNATIONAL STANDARD ISO 3747:2010(E)

Acoustics — Determination of sound power levels and sound
energy levels of noise sources using sound pressure —
Engineering/survey methods for use in situ in a reverberant
environment
1 Scope
1.1 General
This International Standard specifies a method for determining the sound power level or sound energy level of
a noise source by comparing measured sound pressure levels emitted by a noise source (machinery or
equipment) mounted in situ in a reverberant environment, with those from a calibrated reference sound source.
The sound power level (or, in the case of noise bursts or transient noise emission, the sound energy level)
produced by the noise source, in frequency bands of width one octave, is calculated using those
measurements. The sound power level or sound energy level with frequency A-weighting applied is calculated
using the octave-band levels.
1.2 Types of noise and noise sources
The method specified in this International Standard is suitable for all types of noise (steady, non-steady,
fluctuating, isolated bursts of sound energy, etc.) defined in ISO 12001. The method is primarily applicable to
sources which emit broad-band noise. It can, however, also be used for sources which emit narrow-band
noise or discrete tones, although there is a possibility that the measurement reproducibility is then degraded.
The noise source under test can be a device, machine, component or sub-assembly, especially one which is
non-movable.
1.3 Test environment
The test environment that is applicable for measurements made in accordance with this International Standard
is a room where the sound pressure level at the microphone positions depends mainly on reflections from the
room surfaces (see 4.1). In measurements of ISO 12001:1996, accuracy grade 2 (engineering grade),
background noise in the test environment is low compared to that of the noise source or reference sound
source (see 4.2).
1.4 Measurement uncertainty
Information is given on the uncertainty of the sound power levels and sound energy levels determined in
accordance with this International Standard, for measurements made in octave bands and for A-weighted
frequency calculations performed on them. The uncertainty conforms with that of either ISO 12001:1996,
accuracy grade 2 (engineering grade) or ISO 12001:1996, accuracy grade 3 (survey grade), depending on the
extent to which the requirements concerning the test environment are met.
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 5725 (all parts), Accuracy (trueness and precision) of measurement methods and results
ISO 6926, Acoustics — Requirements for the performance and calibration of reference sound sources used
for the determination of sound power levels
ISO 12001:1996, Acoustics — Noise emitted by machinery and equipment — Rules for the drafting and
presentation of a noise test code
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
IEC 60942:2003, Electroacoustics — Sound calibrators
IEC 61260:1995, Electroacoustics — Octave-band and fractional-octave-band filters
IEC 61672-1:2002, Electroacoustics — Sound level meters — Part 1: Specifications
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
sound pressure
p
difference between instantaneous pressure and static pressure
[22]
NOTE 1 Adapted from ISO 80000-8:2007 , 8-9.2.
NOTE 2 Sound pressure is expressed in pascals.
3.2
sound pressure level
L
p
ten times the logarithm to the base 10 of the ratio of the square of the sound pressure, p, to the square of a
reference value, p , expressed in decibels:
p
L = 10 lg dB (1)
p
p
where the reference value, p , is 20 µPa
[21]
[ISO/TR 25417:2007 , 2.2]
NOTE 1 If specific frequency and time weightings as specified in IEC 61672-1 and/or specific frequency bands are
applied, this is indicated by appropriate subscripts; e.g. L denotes the A-weighted sound pressure level.
pA
[22]
NOTE 2 This definition is technically in accordance with ISO 80000-8:2007 , 8-22.
2 © ISO 2010 – All rights reserved

3.3
time-averaged sound pressure level
L
p,T
ten times the logarithm to the base 10 of the ratio of the time average of the square of the sound pressure, p,
during a stated time interval of duration, T (starting at t and ending at t ), to the square of a reference value,
1 2
p , expressed in decibels
t
⎡⎤
⎢⎥
pt()dt
∫
⎢⎥
T
t
⎢ 1 ⎥
L = 10 lg dB (2)
pT,
⎢ ⎥
p
⎢⎥
⎢⎥
⎢⎥
⎣⎦
where the reference value, p , is 20 μPa
NOTE 1 In general, the subscript “T ” is omitted since time-averaged sound pressure levels are necessarily determined
over a certain measurement time interval.
NOTE 2 Time-averaged sound pressure levels are often A-weighted, in which case they are denoted by L , which is
pA,T
usually abbreviated to L .
pA
[21]
NOTE 3 Adapted from ISO/TR 25417:2007 , 2.3.
3.4
single event time-integrated sound pressure level
L
E
ten times the logarithm to the base 10 of the ratio of the integral of the square of the sound pressure, p, of an
isolated single sound event (burst of sound or transient sound) over a stated time interval T (starting at t and
ending at t ) to a reference value, E , expressed in decibels
2 0
t
⎡⎤2
⎢⎥
pt()dt
∫
⎢⎥
t
⎢⎥1
L = 10 lg dB (3)
E
⎢⎥
E
⎢⎥
⎢⎥
⎢⎥
⎣⎦
2 −10 2
where the reference value, E , is (20 µPa) s = 4 × 10 Pa s
NOTE 1 This quantity can be obtained by L + 10 lg(T /T ) dB, where T = 1 s.
p,T 0
NOTE 2 When used to measure sound immission, this quantity is usually called “sound exposure level” (see
[21]
ISO/TR 25417:2007 ).
3.5
measurement time interval
T
portion or a multiple of an operational period or operational cycle of the noise source under test for which the
time-averaged sound pressure level is determined
NOTE Measurement time interval is expressed in seconds.
3.6
comparison method
method by which the sound power level or sound energy level of a noise source under test is determined from
a comparison of the sound pressure levels produced by the source under test with those of a reference sound
source of known sound power output, when both sources are operated in the same environment
3.7
reverberant sound field
that portion of the sound field in the test room over which the influence of sound received directly from the
source is negligible
3.8
reference sound source
sound source meeting specified requirements
NOTE For the purposes of this International Standard, the requirements are those specified in ISO 6926:1999,
Clause 5.
3.9
calibration position
position, well-defined relative to reflecting surfaces, in which the reference sound source has been calibrated
3.10
excess of sound pressure level at a given distance
ΔL
f
difference, at a given distance, between the sound pressure level of a sound source in a given room and the
sound pressure level that would be expected in a free sound field, expressed in decibels
[20]
NOTE This term and its definition differ from that given in ISO 14257:2001 , 3.6, which relates to an average
difference over a given distance range.
3.11
frequency range of interest
for general purposes, the frequency range of octave bands with nominal mid-band frequencies from 125 Hz to
8 000 Hz
NOTE For special purposes, the frequency range can be extended or reduced, provided that the test environment,
reference sound source, and instrument specifications are satisfactory for use over the modified frequency range. Any
change to the frequency range of interest is clearly indicated in the test report. Measurements are not valid if the
A-weighted levels are predominantly determined by high or low frequencies outside the frequency range of interest.
3.12
reference box
hypothetical right parallelepiped terminating on the floor of the test environment on which the noise source
under test is located, that just encloses the source including all the significant sound-radiating components
and any test table on which the source is mounted
3.13
measurement distance
d
m
distance from the nearest point of the reference box to a microphone position
NOTE Measurement distance is expressed in metres.
3.14
background noise
noise from all sources other than the noise source under test
NOTE Background noise includes contributions from airborne sound, noise from structure-borne vibration, and
electrical noise in the instrumentation.
4 © ISO 2010 – All rights reserved

3.15
background noise correction
K
correction applied to the measured octave-band sound pressure levels at each microphone position to
account for the influence of background noise
NOTE 1 Background noise correction is expressed in decibels.
NOTE 2 The background noise correction is frequency dependent; the correction in the case of a frequency band is
denoted K , where f denotes the relevant mid-band frequency.
1f
3.16
sound power
P
through a surface, product of the sound pressure, p, and the component of the particle velocity, u , at a point
n
on the surface in the direction normal to the surface, integrated over that surface
[22]
[ISO 80000-8:2007 , 8-16]
NOTE 1 Sound power is expressed in watts.
NOTE 2 The quantity relates to the rate per time at which airborne sound energy is radiated by a source.
3.17
sound power level
L
W
ten times the logarithm to the base 10 of the ratio of the sound power of a source, P, to a reference value, P ,
expressed in decibels
P
L = 10 lg dB (4)
W
P
where the reference value, P , is 1 pW
NOTE 1 If a specific frequency weighting as specified in IEC 61672-1 and/or specific frequency bands are applied, this
should be indicated by appropriate subscripts; e.g. L denotes the A-weighted sound power level.
WA
[22]
NOTE 2 This definition is technically in accordance with ISO 80000-8:2007 , 8-23.
[21]
[ISO/TR 25417:2007 , 2.9]
3.18
sound energy
J
integral of the sound power, P, over a stated time interval of duration T (starting at t and ending at t )
1 2
t
J = Pt()dt (5)
∫
t
NOTE 1 Sound energy is expressed in joules.
NOTE 2 The quantity is particularly relevant for non-stationary, intermittent sound events.
[21]
[ISO/TR 25417:2007 , 2.10]
3.19
sound energy level
L
J
ten times the logarithm to the base 10 of the ratio of the sound energy, J, to a reference value, J , expressed
in decibels
J
L = 10 lg dB (6)
J
J
where the reference value, J , is 1 pJ
NOTE If a specific frequency weighting as specified in IEC 61672-1 and/or specific frequency bands are applied, this
should be indicated by appropriate subscripts; e.g. L denotes the A-weighted sound energy level.
JA
[21]
[ISO/TR 25417:2007 , 2.11]
4 Test environment
4.1 Criterion for acoustic adequacy of test environment
The test environment is where the noise source under test is found in situ, i.e. either where the source is built
or where it normally operates. The method of test specified in this International Standard is for application in a
reverberant sound field. The test environment shall therefore be sufficiently reverberant to cause the directivity
of the source under test to have an insignificant influence on the sound pressure levels measured according to
7.5 and 7.6. The indicator, excess of sound pressure level at a given distance, ΔL , shall be determined in
f
accordance with Annex A, and shall have a magnitude of at least 7 dB in regions where the requirement for a
reverberant sound field is fulfilled. This indicator serves as the parameter by which to assess the
measurement uncertainty, see Clause 9.
4.2 Criterion for background noise
At each microphone position, the octave-band sound pressure levels due to background noise shall be at
least 6 dB and preferably more than 15 dB below the octave-band sound pressure levels from the noise
source under test and from the reference sound source.
NOTE If it is necessary to make measurements where the difference between the sound pressure levels of the
[7] [17] [18]
background noise and the sources is less than 6 dB, ISO 3746 , ISO 9614-1 or ISO 9614-2 can be used.
5 Instrumentation and measurement equipment
5.1 General
The instrumentation system, including the microphones and cables, shall meet the requirements of
IEC 61672-1:2002, class 1, and the filters shall meet the requirements of IEC 61260:1995, class 1. The
reference sound source shall meet the requirements given in ISO 6926.
5.2 Calibration
Before and after each series of measurements, a sound calibrator meeting the requirements of
IEC 60942:2003, class 1 shall be applied to each microphone to verify the calibration of the entire
measurement system at one or more frequencies within the frequency range of interest. Without any
adjustment, the difference between the readings before and after each series of measurements shall be less
than or equal to 0,5 dB. If this value is exceeded, the results of the series of measurements shall be discarded.
6 © ISO 2010 – All rights reserved

The calibration of the sound calibrator, the compliance of the instrumentation system with the requirements of
IEC 61672-1, and the compliance of the reference sound source with the requirements of ISO 6926, shall be
verified at intervals in a laboratory making calibrations traceable to appropriate standards.
Unless national regulations dictate otherwise, it is recommended that the sound calibrator should be calibrated
at intervals not exceeding 1 year, the reference sound source should be calibrated at intervals not exceeding
2 years, and the compliance of the instrumentation system with the requirements of IEC 61672-1 should be
verified at intervals not exceeding 2 years.
6 Location, installation and operation of noise source under test
6.1 Source location and installation
Since the test procedure is designed for use in situ, the installation and location of the noise source under test
have to be those where the source is found. However, the sound power or sound energy emitted by a source
can be affected by the manner of installation and its location, for instance, relative to nearby walls or other
reflecting surfaces.
Many small sound sources, although themselves poor radiators of low-frequency sound, can, as a result of the
method of mounting, radiate more low-frequency sound when their vibrational energy is transmitted to
surfaces large enough to be efficient radiators. In such cases, resilient mountings should if practicable be
interposed between the noise source under test and the supporting structure, so that the transmission of
vibration to the support and the reaction on the source are both minimized. In this case, the mounting base
should be rigid (i.e. have a sufficiently high mechanical impedance) to prevent it from vibrating excessively
and radiating sound. Such resilient mounts should not be used if the noise source under test is not resiliently
mounted in typical usage.
Coupling conditions, e.g. between prime movers and driven machines, can exert considerable influence on
the sound radiation of the noise source under test. It may be appropriate to use a flexible coupling, but similar
considerations apply to these as to resilient mounts.
6.2 Auxiliary equipment
If practicable, all auxiliary equipment necessary for the operation of the noise source under test, but which is
not an integral part of the source itself, including any electrical conduits, piping, air ducts, etc., connected to
the source under test, shall be located outside the test environment. If this is impractical, care shall be taken
to minimize any sound radiated into the test environment from such equipment. The noise source under test
shall be taken to include all significant sources of sound emission, including auxiliary equipment which cannot
either be removed or adequately quietened, and the reference box (see 7.2) shall be extended appropriately.
6.3 Operation of source during test
The sound power or sound energy emitted by a source can be affected by the load applied, the running speed,
and the conditions under which it is operating. The source shall be tested, wherever possible, under
conditions that are reproducible and representative of the noisiest operation in typical usage. The
specifications given in a noise test code, if any exists, shall be followed, but in the absence of a noise test
code one or more of the following modes of operation shall be selected for the test(s):
a) source under specified load and conditions;
b) source under full load [if different from a)];
c) source under no load (idling);
d) source at maximum operating speed under defined conditions;
e) source operating under conditions corresponding to maximum sound generation representative of normal
use;
f) source with simulated loading, under defined conditions;
g) source undergoing a characteristic work cycle under defined conditions.
The source shall be stabilized in the desired operating condition, with any power source or transmission
system running at a stable temperature, prior to the start of measurements for sound power level or sound
energy level determination. The load, speed, and operating conditions shall either be held constant during the
test, or varied through a defined cycle in a controlled manner.
If the sound power or sound energy emission depends on secondary operating parameters, e.g. the type of
material being processed, the design of cutting tool, or the humidity, those parameters shall be selected, as
far as is practicable, that give the smallest variations and that are typical of normal use. If simulated loading
conditions are used, they shall be chosen such that the sound power levels or sound energy levels of the
source under test are representative of normal use.
7 Measurement procedure
7.1 General
For determination of either the sound power level of a noise source emitting stationary noise or the sound
energy level of a source which emits bursts of noise, two sets of measurements of sound pressure levels shall
be made in the test environment, first with the noise source under test operating and then with the reference
sound source operating; in some circumstances (see 7.3.1) the measurements with the reference sound
source have to be repeated for different locations of the source. The specifications given in a noise test code,
if any exists, shall be followed, but in the absence of a noise test code the procedures described hereafter
shall be followed for the test(s).
7.2 Characterization of noise source under test
A preliminary aural examination of the noise emitted by the source under test shall be made to determine
whether sound emitted from one component predominates. If so, the geometric centre of that component shall
be assumed to be the acoustic centre of the source for the purpose of the test (see 7.3.2), and a reference
box shall be delineated which just encloses that component and terminates on the floor on which the source
under test is mounted. If no component appears to emit sound more than any other, any component which
clearly emits no sound shall be excluded from consideration, the acoustic centre of the source shall be taken
to be the geometric centre of the remainder, and the reference box shall be delineated accordingly.
Preliminary measurements shall be used to determine whether the sound emitted by the source is too
directional for the method of this International Standard to be applied. The source directivity shall be evaluated
by measuring sound pressure levels of the source (at intervals of 2 m or less along a given side for a large
source) at a distance of 1 m from the reference box and a height normally of 1,5 m. If the source emits sound
predominantly in an upward direction, the height at which the sound pressure levels are measured shall be
sufficient to ensure that the microphone positions are in a direct line of sight to the acoustic centre of the
source. If the A-weighted range of sound pressure levels so measured varies by no more than ±2 dB, the
source shall be considered to be omnidirectional; if the variations exceed this amount, the source shall be
considered to be directional. If the range of these directivity measurements exceeds ±7 dB, the engineering
grade limits on source directivity are exceeded and the reported grade of accuracy shall not exceed that of
ISO 12001:1996, accuracy grade 3 (survey grade) (see Clause 9).
7.3 Locations of the reference sound source
7.3.1 General
Normally, one location for the reference sound source is sufficient. For noise sources under test which are
large or which have two or more clearly distinguishable sound sources far apart from one another, two or
more locations may need to be used (see 7.3.3).
8 © ISO 2010 – All rights reserved

7.3.2 One location
If one location is sufficient, the reference sound source shall be positioned as close as possible to the acoustic
centre of the noise source under test. For a directional source, the reference sound source should be located
to simulate the emission pattern of the noise source under test or, if this is impractical, a position for the
reference sound source on top of the source under test shall be chosen. If this is not possible, a location shall
be selected alongside the source under test, at a height and position which best simulate the emission pattern
of the source under test. In the latter case, positions closer than 0,5 m to the surface of the reference box
should be avoided. For an omnidirectional source under test, care shall be taken to ensure that the reference
sound source can emit sound equally in all directions.
NOTE 1 The more reverberant the test environment is, in other words, the larger ΔL is, the less critical is the selection
f
of the location for the reference sound source. However, if a position is selected for the reference sound source for which
it is not calibrated, the reproducibility of the sound power level or sound energy level determined is degraded, see
Clause 9.
NOTE 2 Each reflecting surface within λ/2 (half a wavelength) of the reference sound source can increase its sound
power, resulting in a possible underestimate of the noise source sound power level of up to 3 dB. Conversely, placing the
reference sound source less than 0,5 m from the edge or edges of the reflecting floor plane can reduce its sound power
output below 400 Hz, resulting in a possible overestimate of the noise source sound power level of up to 3 dB.
NOTE 3 Further guidance on selection of the location for the reference sound source is given in Annex B.
7.3.3 More than one location
The number of locations required for the reference sound source depends on the ratio a/d , where a is the
m
largest dimension of the reference box and d is the measurement distance, see 7.4.1. The number of
m
locations to be used is specified in a) to c).
a) If a/d >1 and if the source under test is omnidirectional, several locations shall be used for the reference
m
sound source along the sides of the source under test, separated by a distance equal to d .
m
b) If a/d >1 and if the source under test has clearly definable sound emission areas, one location of the
m
reference sound source shall be used for each emission area.
c) If a/d u1 and if the source under test is omnidirectional, but it is impossible to use a location on top of
m
the source under test, four locations for the reference sound source shall be used, one adjacent to each
vertical side of the reference box.
7.4 Microphone positions
7.4.1 General
The aim is to position the microphones around the sides of the noise source under test so that each position is
situated similarly in relation to the sound-emitting areas of the source, i.e. for a particular microphone position,
either there is a line of sight to each sound-emitting area or each area is screened. Positions to be avoided
are those to which only a few parts of the source are emitting sound.
In total, three or four microphone positions are to be used (see 7.4.3), distributed as evenly as possible
around the noise source under test. The same positions and microphone orientations are to be used for
measurements made on the noise source under test, the reference sound source, and the background noise.
The measurement distance, d , from the respective microphone positions to the nearest point of the reference
m
box is to be selected so that, if possible, the microphone positions are in a part of the test environment where
the sound field is reverberant, i.e. a region in which ΔL W 7 dB (see Table 2).
f
No microphone position shall be closer than 0,5 m to any boundary surface of the test environment. If the
environment is sufficiently spacious, and if the noise source under test is located away from all boundary
surfaces, the microphones shall be positioned round all four vertical sides of the reference box. The
microphone positions shall be spaced at least 2 m from each other. If the ceiling is high and sound absorbing,
and frequencies greater than 2 000 Hz are important, if possible, at least two of the microphone positions shall
be above the source.
7.4.2 Zoning
The test environment shall be divided into zones in which differences can be denoted in the sound-radiating
patterns in the horizontal plane around the noise source under test (ST) and the reference sound source
(RSS) in its various locations. The zones are used in choosing locations for the reference sound source and
the microphone positions. The different zones are identified and given notations in Table 1.
Table 1 — Zones of the test environment
Direct line of sight to sound-emitting Distances from the
Effect on the sound Notation of the
areas microphone position
power estimation zone
a
of ST of RSS d
m
Yes No — Strong overestimate ++
No Yes — Strong underestimate −
Yes Yes d < d Overestimate +
m(ST) m(RSS)
Yes Yes d > d Underestimate −
m(ST) m(RSS)
d ≈ d Negligible
m(ST) m(RSS)
Yes Yes +/−
(within 10 %) over- or underestimate
Either strong
No No — ++/−
over- or underestimate
a
(ST) indicates source under test; (RSS) indicates reference sound source.

7.4.3 Selection of microphone positions
If the noise source under test is omnidirectional and the reference sound source is located on top of it, all the
zones fall into one or other of the notations + or +/− and one microphone position shall be selected on each
free side of the source under test, in compliance with the requirements of 7.4.1.
In all other cases, microphone positions shall be sought in a +/− zone. If this is not possible, one microphone
position shall be selected in a + zone, one in a +/− zone and one or two positions in − zones.
Unless the test environment is highly reverberant (no acoustic treatment of walls or ceiling, no large absorbing
obstacles) no microphone positions should be located in a ++/− zone.
NOTE It can prove impossible to have all microphone positions in direct line of sight of the reference sound source
(see B.2 and B.4).
7.5 Measurement of sound pressure levels for a noise source which emits steady
or non-steady noise
Time-averaged sound pressure levels from the noise source under test, for each octave band within the
frequency range of interest, L′ , shall be obtained, at each microphone position, i (i = 1, 2 … n), and from
pi(ST)
the reference sound source, L′ . A suitable averaging time for the reference sound source is 30 s. If the
pi(RSS)
sound output from the noise source under test is as stable as that of the reference sound source, then a
similar averaging time is satisfactory, but if it is less stable or undergoes periodic cycles, a longer averaging
time is required.
In addition, either immediately before or immediately after the sound pressure levels from the noise source
under test are measured, the time-averaged sound pressure levels of the background noise for each octave
band, L , shall be obtained at each microphone position, over the same measurement time interval as that
pi(B)
used for the noise source under test.
10 © ISO 2010 – All rights reserved

7.6 Measurement of sound pressure levels for a noise source which emits impulsive noise
Single event sound pressure levels from the noise source under test for each octave band within the
frequency range of interest, L′ , shall be obtained, at each microphone position, i (i = 1, 2 … n), either for
Ei(ST)
one single sound event at a time (in which case the process shall be repeated N times, where N W 5), or from
several successive, N, sound events (where again N W 5). The measurement time interval shall be long
enough to contain all that part of the noise of the event(s), including the decay, which make a significant
contribution to the single event sound pressure level. The time-averaged sound pressure levels from the
reference sound source, L′ , shall also be measured, with an averaging time of 30 s.
pi(RSS)
In addition, either immediately before or immediately after the sound pressure levels from the noise source
under test are measured, the time-averaged octave-band sound pressure levels of the background noise,
L , shall be obtained once at each microphone position, over a representative time interval.
pi(B)
8 Calculation of sound power levels and sound energy levels
8.1 Corrections for background noise
The background noise correction, K , at the ith microphone position and in each octave band, shall be
1i
calculated using Equation (7):
−0,1ΔL
pi
K =−10 lg(1− 10 ) dB (7)
1i
where
Δ=LL′ −L
pipi (ST) pi(B)
in which
L′ is the measured (uncorrected) octave-band time-averaged sound pressure level at the ith
pi(ST)
microphone position, with the noise source under test (ST) in operation, in decibels,
L is the octave-band time-averaged sound pressure level of the background noise (B)
pi(B)
measured at the ith microphone position, in decibels.
If, at all microphone positions, ΔL W 6 dB, the measurement is valid according to this International Standard.
pi
If, at any microphone position, ΔL > 15 dB, K is assumed equal to zero at that position. For values of ΔL
pi 1i pi
between 6 dB and 15 dB, corrections shall be calculated according to Equation (7).
If, at any microphone position, ΔL < 6 dB, the accuracy of the result(s) is reduced. The maximum correction
pi
to be applied to these measurements is 1,3 dB. The result may, however, be reported and may be useful for
determining an upper boundary to the sound power level of the noise source under test. If such data are
reported, it shall be clearly stated in the text of the report, as well as in graphs and tables of results, that the
background noise requirements of this International Standard have not been fulfilled.
8.2 Mean time-averaged sound pressure levels for a noise source which emits steady
or non-steady noise
The mean corrected time-averaged sound pressure level for the noise source under test in each octave band,
L , for the chosen mode of operation, shall be calculated using Equation (8):
p(ST)
n
⎡⎤
1 0,1L
pi(ST)
L = 10 lg⎢ 10 ⎥ dB (8)
p(ST) ∑
n
⎢⎥
i=1
⎣⎦
where
LL=−′ K
pip(ST) i(ST) 1i
in which
L′ is the measured (uncorrected) octave-band time-averaged sound pressure level at the ith
pi(ST)
microphone position, with the noise source under test in operation, in decibels,
K is the octave-band background noise correction at the ith microphone position, in decibels.
1i
The mean corrected time-averaged sound pressure level for the reference sound source in each octave band,
L , shall be calculated using Equation (9):
p(RSS)
n
⎡⎤
1 0,1L
pi(RSS)
⎢ ⎥ (9)
L = 10 lg 10 dB
p(RSS) ∑
n
⎢⎥
i=1
⎣⎦
where
′
LL=−K
pi(RSS) pi(RSS) 1i(RSS)
in which
L′ is the octave-band time-averaged sound pressure level of the reference sound source,
pi(RSS)
measured at the ith microphone position, corrected for speed, temperature and static
pressure according to the manufacturer's specifications, but without correction for
background noise, in decibels;
K is the octave-band background noise correction at the ith microphone position for the
1i(RSS)
measurement of the reference sound source, in decibels.
For several locations of the reference sound source, the mean corrected time-averaged sound pressure level
of the reference sound source at each location j (j = 1 … m) in each octave band, L , shall be calculated
pj(RSS)
using Equation (10):
n
⎡⎤
1 0,1L
pji(RSS)
L = 10 lg⎢ 10 ⎥ dB (10)
pj(RSS) ∑
n
⎢⎥
⎣⎦i=1
where
′
LL=−K
pji(RSS) pji(RSS) 1i(RSS)
in which L′ is the uncorrected octave-band time-averaged sound pressure level of the reference sound
pji(RSS)
source located at the jth position, measured at the ith microphone position, in decibels.
12 © ISO 2010 – All rights reserved

8.3 Sound power levels
8.3.1 One location of reference sound source
The sound power level of the noise source under test in each octave band, L , for the meteorological
W
conditions at the time and place of the test, shall be calculated using Equation (11):
LL=−L +L (11)
WW(RSS) p(RSS) p(ST)
where L is the sound power level of the calibrated reference sound source in the respective octave
W(RSS)
band, in decibels.
The sound power level, L , corresponding to the reference static pressure of 1,013 25 × 10 Pa and
W
ref,atm
reference atmospheric temperature 23,0 °C shall be calculated, if required, according to Annex C.
8.3.2 Several locations of reference sound source
The sound power level of the noise source under test in each octave band, from m locations of the reference
sound source, L , for the meteorological conditions at the time and place of the test, shall be calculated from
W
the following Equation (12):
⎡⎤mm⎡ ⎤
110,1LL0,1
Wj(RSS) pj(RSS)
⎢⎥⎢ ⎥
L=−10lg 10 10lg 10+L dB (12)
W p(ST)
∑∑
⎢⎥mm⎢ ⎥
jj==11
⎣⎦⎣ ⎦
where L is the sound power level of the calibrated reference sound source in the respective octave
Wj(RSS)
band, in decibels, in a calibration position similar to the one used for the test.
The sound power level, L , corresponding to the reference static pressure of 1,013 25 × 10 Pa and
W
ref,atm
reference atmospheric temperatur
...