EN ISO 3744:1995

SLOVENSKI STANDARD
01-april-1997
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Acoustics - Determination of sound power levels of noise sources using sound pressure -
Engineering method in an essentially free field over a reflecting plane (ISO 3744:1994)
Akustik - Bestimmung der Schalleistungspegel von Geräuschquellen aus
Schalldruckmessungen - Hüllflächenverfahren der Genauigkeitsklasse 2 für ein im
wesentlichen freies Schallfeld über einer reflektierenden Ebene (ISO 3744:1994)
Acoustique - Détermination des niveaux de puissance acoustique émis par les sources
de bruit a partir de la pression acoustique - Méthode d'expertise dans des conditions
approchant celles du champ libre sur plan réfléchissant (ISO 3744:1994)
Ta slovenski standard je istoveten z: EN ISO 3744:1995
ICS:
17.140.01 $NXVWLþQDPHUMHQMDLQ Acoustic measurements and
EODåHQMHKUXSDQDVSORãQR noise abatement in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL
STANDARD
Second edition
1994-05-O 1
Acoustics - Determination of sound
power levels of noise sources using sound
pressure - Engineering method in an
essentially free field over a reflecting plane
Acoustique - Dgtermination des niveaux de puissance acoustique 6mis
par les sources de bruit P partir de la pression acoustique - M&hode
d’expertise dans des conditions approchant celles du champ libre sur plan
r6fkchissant
Reference number
IS0 3744:1994(E)
IS0 3744:1994(E)
Contents
Page
Scope .
Normative references .
....................................................................................... 3
Definitions
...............................................................
Acoustic environment
......................................................................... 5
Instrumentation
........................ 5
Installation and operation of source under test
................................... 7
Measurement of sound pressure levels
Calculation of surface sound pressure level and sound power
level .
Information to be recorded .
IO Information to be reported . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Annexes
. . . . . . . . 14
A Qualification procedures for the acoustic environment
B Microphone array on the hemispherical measurement surface
C Microphone array on the parallelepiped measurement surface
. . . . . . . . . . . . . . . . . . . .
D Guidelines for the detection of impulsive noise
. . . . . . 30
E Guidelines for the determination of the directivity index
F Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0 IS0 1994
Ail rights reserved. 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 per-
mission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii
IS0 3744:1994(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work
of preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 3744 was prepared by Technical Committee
ISO/TC 43, Acoustics, Sub-Committee SC 1, Noise.
This second edition cancels and replaces the first edition
(IS0 3744: 1981), which has been technically revised.
Annexes A, B and C form an integral part of this International Standard.
Annexes D, E and F are for information only.

IS0 3744:1994(E)
Introduction
0.1 This International Standard is one of the IS0 3740 series, which
specifies various methods for determining the sound power levels of
machines, equipment and their sub-assemblies. When selecting one of
the methods of the IS0 3740 series, it is necessary to select the most
appropriate for the conditions and purposes of the noise test. General
guidelines to assist in the selection are provided in IS0 3740. The
IS0 3740 series gives only general principles regarding the operating and
mounting conditions of the machine or equipment under test. Reference
should be made to the noise test code for a specific type of machine or
equipment, if available, for specifications on mounting and operating con-
ditions.
0.2 This International Standard specifies a method for measuring the
sound pressure levels on a measurement surface enveloping the source,
and for calculating the sound power level produced by the source. The
enveloping surface method can be used for any of three grades of accu-
racy (see table O.l), and is used in this International Standard for grade 2
accuracy.
The use of this International Standard requires certain qualification criteria
to be fulfilled , as described in table 0.1. If the relevant qualification criteria
cannot be met, other basic standards with different environmental re-
quirements are suggested (tableO.l; see also IS0 3740 and IS0 9614).
Noise test codes for specific families of machines or equipment should
be based without any contradiction on the requirements of one or more
of the IS0 3740 series or IS0 9614.
Free-field conditions are usually not encountered in typical machine rooms
where sources are normally installed. If measurements are made in such
installations, corrections may be required to account for background noise
or undesired reflections.
The methods specified in this International Standard permit the determi-
nation of sound power level both as an A-weighted value and in frequency
bands.
The A-weighted value calculated fro m frequency band data may differ
from that determ ined f ram measu red A-weighted sound press ure levels.
0.3 In this International Standard, the computation of sound power level
from sound pressure level measurements is based on the premise that the
sound power output of the source is directly proportional to the mean-
square sound pressure averaged over time and space.

IS0 3744: 1994(E)
A. d.
I a1318 u.1 - uvervlew or InternatIonal StatWards tor determlnatlon
of sound power levels of noise sources using enveloping surface
methods over a reflecting plane and giving different grades of
accuracy
IS0 3746
IS0 3745 IS03744
Precision Engineering Survey
Parameter
method method method
Grade 1 Grade 2 Grade 3
Test environment
Hemi-anechoic Outdoors or Outdoors or
room indoors indoors
Criterion for suitability of test en- K2 < 0,5 dB K2 < 2 dB K2 < 7 dB
vironmentl)
Volume of sound source Preferably less No restriction; No restriction;
than 0,5 % of limited only by limited only by
test room vol- available test available test
ume environment environment
Character of noise
Any (broad-band, narrow-band, discrete-frequency,
steady, non-steady, impulsive)
Limitation for background noise’) ti 2 10 dB (if hL 2 6 dB (if hL>,3dB
possible, ex- possible, ex-
ceeding 15 dB) ceeding 15 dB)
K, < 0,4 dB K, < 1,3 dB K, <3dB
> 42)
Number of measurement points 2 10 a 92)
Instrumentation:
- Sound level meter at least a) type 1 as a) type 1 as a) type 2 as
complying with
specified specified specified
in IEC 651 in IEC 651 in IEC 651
- Integrating sound level meter b) type 1 as b) type 1 as b) type 2 as
at least complying with specified specified specified
in IEC 804 in IEC 804 in IEC 804
- Frequency band filter set at c) class 1 as c) class 1 as
least complying with specified specified
in IEC 225 in IEC 225
Precision of method for deter- aR, mining &A expressed as standard K2 < 5 dB)
deviation of reproducibility q G 4 dB (if
5dB < 7 dB)
If discrete
tones are pre-
dominant, the
value of oR is
1 dB greater.
1) The values of K, and K2 given shall be met in each frequency band within the fre-
quency range of interest for determining the sound power spectrum. For determining
A-weighted sound power levels, the same criteria apply to K,* and KzA.
2) Under given circumstances (see 7.2 to 7.4), it is permissible to use a reduced num-
ber of microphone positions.
This page intentionally left blank

INTERNATIONAL STANDARD IS0 3744: 1994(E)
Acoustics - Determination of sound power levels of
- Engineering
noise sources using sound pressure
method in an essentially free field over a reflecting
plane
This International Standard is applicable to noise
1 Scope
sources of any type and size (e.g. device, machine,
component, sub-assembly).
I .I General
NOTE 3 Measurements according to this International
Standard may be impracticable for very tall or very long
This International Standard specifies a method for sources such as chimneys, ducts, conveyors and multi-
source industrial plants.
measuring the sound pressure levels on a measure-
ment surface enveloping a noise source, under es-
sentially free-field conditions near one or more
1.3 Test environment
reflecting planes, in order to calculate the sound
power level produced by the noise source. It gives
The test environment that is applicable for measure-
requirements for the test environment and instru-
ments made in accordance with this International
mentation, as well as techniques for obtaining the
Standard is an essentially free field near one or more
surface sound pressure level from which the sound
reflecting planes (indoors or outdoors).
power level of the source is calculated, leading to re-
sults which have a grade 2 accuracy.
It is important that specific noise test codes for vari-
1.4 Measurement uncertainty
ous types of equipment be established and used in
accordance with this International Standard. For each
Determinations made in accordance with this Inter-
type of equipment, such noise test codes will give
national Standard result, with few exceptions, in
detailed requirements on mounting, loading and op-
standard deviations of reproducibility of the A-
erating conditions for the equipment under test as
weighted sound power level equal to or less than
well as a selection of the measurement surface and
I,5 dB (see table 1).
the microphone array as specified in this International
A single value of the sound power level of a noise
Standard.
source determined according to the procedures given
NOTE 1 The noise test code for a particular type of in this International Standard is likely to differ from the
equipment should give detailed information on the particular
true value by an amount within the range of the
surface that is selected, as the use of differently shaped
measurement uncertainty. The uncertainty in deter-
measurement surfaces may yield differing estimates of the
minations of the sound power level arises from sev-
sound power level of a source.
eral factors which affect the results, some associated
with environmental conditions in the measurement
laboratory and others with experimental techniques.
1.2 Types of noise and noise sowrces
If a particular noise source were to be transported to
each of a number of different laboratories, and if, at
The method specified in this International Standard is
each laboratory, the sound power level of that source
suitable for measurements of all types of noise.
were to be determined in accordance with this lnter-
national Standard, the results would show a scatter.
NOTE 2 A classification of different types of noise
The standard deviation of the measured levels could
(steady, non-steady, quasi-steady, impulsive, etc.) is given
in IS0 2204. be calculated (see examples in IS0 7574-4:1985, an-

IS0 3744: 1994(E)
location of microphone positions, sound source location, in-
nex B) and would vary with frequency. With few ex-
tegration times, and determination of environmental cor-
ceptions, these standard deviations would not exceed
rections, if any. The standard deviations are also affected
those listed in table 1. The values given in table 1 are
by errors associated with measurements taken in the near
standard deviations of reproducibility, Q, as defined
field of the source; such errors depend upon the nature of
in IS0 7574-l. The values of table 1 take into account
the sound source, but generally increase for smaller meas-
the cumulative effects of measurement uncertainty in
urement distances and lower frequencies (below 250 Hz).
applying the procedures of this International Standard,
but exclude variations in the sound power output
5 If several laboratories use similar facilities and instru-
caused by changes in operating conditions (e.g. rota-
mentation, the results of sound power determinations on a
tional speed, line voltage) or mounting conditions.
given source in those laboratories may be in better agree-
ment than would be implied by the standard deviations of
table 1.
The measurement uncertainty depends on the stan-
dard deviation of reproducibility tabulated in table 1
6 For a particular family of sound sources, of similar size
and on the degree of confidence that is desired. As
with similar sound power spectra and similar operating
examples, for a normal distribution of sound power
conditions, the standard deviations of reproducibility may
levels, there is 90 % confidence that the true value
be smaller than the values given in table 1. Hence, a noise
of the sound power level of a source lies within the
test code for a particular type of machinery or equipment
range + 1,645 OR of the measured value and a 95 %
making reference to this International Standard may state
confidence that it lies within the range + I,96 CR of
standard deviations smaller than those listed in table 1, if
the measured value. For further examples, reference substantiation is available from the results of suitable inter-
laboratory tests.
should be made to the IS0 7574 series and IS0 9296.
7 The standard deviations of reproducibility, as tabulated
in table 1, include the uncertainty associated with repeated
Table 1 - Estimated values of the standard
measurements on the same noise source under the same
deviations of reproducibility of sound power
conditions (for standard deviation of repeatability, see
levels determined in accordance with this IS0 7574-l). This uncertainty is usually much smaller than
the uncertainty associated with interlaboratory variability.
International Standard
However, if it is difficult to maintain stable operating or
Octave-band One-third-octave Standard
mounting conditions for a particular source, the standard
centre band centre deviation of
deviation of repeatability may not be small compared with
frequencies reproducibility
frequencies
the values given in table 1. In such cases, the fact that it
OR
was difficult to obtain repeatable sound power level data on
the source should be recorded and stated in the test report.
Hz Hz dB
8 The procedures of this International Standard and the
63 50 to 80 5 ‘1
standard deviations given in table 1 are applicable to meas-
125 100 to 160 3
urements on an individual machine. Characterization of the
250 200 to 315 2
sound power levels of batches of machines of the same
500 to 4 000 400 to 5 000 I,5
family or type involves the use of random sampling tech-
8 000 6300to10000 23
niques in which confidence intervals are specified, and the
results are expressed in terms of statistical upper limits. In
A-weighted I,5 2)
applying these techniques, the total standard deviation must
be known or estimated, including the standard deviation of
1) Normally for outdoor measurements; many rooms
production, as defined in IS0 7574-1, which is a measure
are not qualified for this frequency band.
of the variation in sound power output between individual
machines within the batch. Statistical methods for the
2) Applicable to a source which emits noise with a
characterization of batches of machines are described in
relatively “flat” spectrum in the frequency range
IS0 7574-4.
100 Hz to 10 000 Hz.
2 Normative references
NOTES
4 The standard deviations listed in table 1 are associated
The following standards contain provisions which,
with the test conditions and procedures defined in this
through reference in this text, constitute provisions
International Standard and not with the noise source itself.
of this International Standard. At the time of publi-
They arise in part from variations between measurement
cation, the editions indicated were valid. All standards
laboratories, changes in atmospheric conditions if outdoors,
are subject to revision, and parties to agreements
the geometry of the test room or outdoor environment, the
based on this International Standard are encouraged
acoustical properties of the reflecting plane, absorption at
to investigate the possibility of applying the most re-
the test room boundaries if indoors, background noise, and
cent editions of the standards indicated below.
the type and calibration of instrumentation. They are also
Members of IEC and IS0 maintain registers of cur-
due to variations in experimental techniques, including the
size and shape of the measurement surface, number and rently valid International Standards.

IS0 3744:1994(E)
IS0 354:1985, Acoustics - Measurement of sound 3.2 sound pressure level, Lp: Ten times the log-
absorption in a reverberation room. arithm to the base 10 of the ratio of the square of the
sound pressure to the square of the reference sound
IS0 2204:I 979, Acoustics - Guide to International pressure. Sound pressure levels are expressed in
decibels.
Standards on the measurement of airborne acoustical
noise and evaluation of its effects on human beings.
The frequency weighting or the width of the fre-
quency band used, and the time weighting (S, F or I,
IS0 3745:1977, Acoustics - Determination of sound
see IEC 651) shall be indicated. The reference sound
power levels of noise sources - Precision methods
pressure is 20 PPa (2 x low5 Pa).
for anechoic and semi-anechoic rooms.
NOTE 10 For example, the A-weighted sound pressure
IS0 374711987, Acoustics - Determination of sound
level with time weighting S is GAS.
power levels of noise sources - Survey method us-
ing a reference sound source.
3.2.1 time-averaged sound pressure level, Lpeq +
Sound pressure level of a continuous steady sound
IS0 4871 :1984, Acoustics - Noise labelling of ma-
that, within a measurement time interval T, has the
chinery and equipment.
same mean-square sound pressure as a sound under
consideration which varies with time:
IS0 6926:1990, Acoustics - Determination of sound
power levels of noise sources - Requirements for
the performance and calibration of reference sound
Lpeq,T= 10 Ig [+[,,,, dt] dB
sources.
IS0 7574-I :1985, Acoustics - Statistical methods for
= IO Ig [+[y di] dB . . .
(1)
determining and verifying stated noise emission val-
ues of machinery and equipment - Part 1: General
considerations and definitions.
Time-averaged sound pressure levels are expressed
in decibels and shall be measured with an instrument
IS0 7574-4:1985, Acoustics - Statistical methods for
which complies with the requirements of IEC 804.
determining and verifying stated noise emission val-
ues of machinery and equipment - Part 4: Methods
NOTES
for stated values for batches of machines.
11 Time-averaged sound pressure levels are usually A-
I EC 225: 1966, Octave, half-octave and third-octave
weighted and denoted by LpAeq,* which is usually abbrevi-
band filters intended for the analysis of sounds and
ated to LpA.
vibrations.
12 In general, the subscripts “eq” and “T” are omitted
since time-averaged sound pressure levels are necessarily
I EC 651: 1979, Sound level meters.
determined over a certain measurement time interval.
I EC 804: 1985, Integrating-averaging sound level me-
ters. 3.2.2 single-event sound pressure level, LP ,S:
Time-integrated sound pressure level of an isolated
I EC 942: 1988, Sound calibrators. single sound event of specified duration T (or speci-
fied measurement time 7’) normalized to To = I s. It is
expressed in decibels and is given by the following
formula:
r m r) 1
Lp.,s= IO lg[+-[Fdt]dB
3 Definitions
. . .
(2)
For the purposes of this International Standard, the
following definitions apply.
3.2.3 measurement time interval: A portion or a
3.1 sound pressure, p: A fluctuating pressure
multiple of an operational period or operational cycle
superimposed on the static pressure by the presence
for which the time-averaged sound pressure level is
of sound. It is expressed in pascals.
determined.
NOTE 9 The magnitude of the sound pressure can be
3.3 measurement surface: A hypothetical surface
expressed in several ways, such as instantaneous sound
of area S, enveloping the source, on which the meas-
pressure, maximum sound pressure, or as the square root
urement points are located. The measurement sur-
of the mean-square sound pressure over designated time
and space (i.e. over the measurement surface). face terminates on one or more reflecting planes.

IS0 3744:1994(E)
3.4 surface sound pressure level, L&: The energy- 3.14 background noise: The no ise from all sources
average of the time-averaged sound pressure levels
other than the source under test.
at all the microphone positions on the measurement
surface, with the background noise correction K, NOTE 15 Background noise may include contributions
from airborne sound, structure-borne vibration, and elec-
(3.15) and the environmental correction K2 (3.16) ap-
trical noise in instrumentation.
plied. It is expressed in decibels.
3.15 background noise correction, K,: A correction
3.5 sound power, W: The rate per unit time at
term to account for the influence of background noise
which airborne sound energy is radiated by a source.
on the surface sound pressure level; K, is frequency
It is expressed in watts.
dependent and is expressed in decibels. The cor-
rection in the case of A-weighting is denoted K,*.
3.6 sound power level,.&: Ten times the logarithm
to the base 10 of the ratio of the sound power radi-
3.16 environmental correction, K2: A correction
ated by the sound source under test to the reference
term to account for the influence of reflected or ab-
sound power. It is expressed in decibels.
sorbed sound on the surface sound pressure level;
The frequency weighting or the width of the fre- K2 is frequency dependent and is expressed in deci-
quency band used shall be indicated. The reference bels. The correction in the case of A-weighting is de-
sound power is 1 pW (lo-l2 W). noted K2*.
NOTE 13 For example, the A-weighted sound power
3.17 impulsive noise index (impulsiveness): A
level is &A.
quantity by means of which the noise emitted by a
source can be characterized as “impulsive”. (See an-
3.7 free field: A sound field in a homogeneous,
nex D.) It is expressed in decibels.
isotropic medium free of boundaries. In practice, it is
a field in which reflections at the boundaries are neg-
3.18 directivity index, DI: A measure of the extent
ligible over the frequency range of interest.
to which a source radiates sound predominantly in
one direction. (See annex E.) It is expressed in deci-
3.8 free field over a reflecting plane: A sound field
bels.
in a homogeneous, isotropic medium in the half-space
above an infinite, rigid plane surface on which the
source is located.
4 Acoustic environment
3.9 frequency range of interest: For general pur-
poses, the frequency range of interest includes the
4.1 General
octave bands with centre frequencies from 125 Hz to
8 000 Hz.
The test environments that are applicable for meas-
urements according to this International Standard are:
NOTE 14 For special purposes, it is permissible to extend
or reduce the frequency range of interest at either end,
a) a laboratory room which provides a free field over
provided the test environment and instrument accuracy are
a ref letting plane;
satisfactory for use over the extended or reduced frequency
range. For sources which radiate predominantly high (or
b) a flat outdoor area that meets the requireme nts
low) frequency sound, it is permissible to extend or reduce
of 4.2 and annex A;
the frequency range of interest, in order to optimize the test
facility and procedures.
c) a room in which the contributions of the
reverberant field to the sound pressures on the
3.10 reference box: A hypothetical surface which is
measurement surface are small compared with
the smallest rectangular parallelepiped that just en-
those of the direct field of the source.
closes the source and terminates on the reflecting
plane or planes.
NOTE 16 Conditions described under c) above are usually
met in very large rooms as well as in smaller rooms with
3.11 characteristic source dimension, 4: Half the
sufficient sound-absorptive materials on their walls and
length of the diagonal of the box consisting of the
ceilings.
reference box and its images in adjoining reflecting
planes.
4.2 Criterion for adequacy of the test
3.12 measurement distance, d: The distance from environment
the reference box to a box-shaped measurement sur-
face.
As far as is practicable, the test environment shall be
free from reflecting objects other than a reflecting
3.13 measurement radius, r: The radius of a plane so that the source radiates into a free field over
hemispherical measurement surface. a reflecting plane.
IS0 3744:1994(E)
Annex A describes procedures for determining the
5.3 Microphone windbreak
magnitude of the environmental correction K2, to ac-
count for deviations of the test environment from the
If measurements are to be made outdoors, a
ideal condition. For this International Standard, the
windbreak is recommended. Ensure that the
environmental correction K2* (see tableO. and 8.4)
windbreak does not affect the accuracy of the instru-
shall be numerically less than or equal to 2 dB. For
mentation.
spectral quantities determined according to this Inter-
national Standard, K2 for each frequency band of in-
terest shall not exceed 2 dB.
6 Installation and operation of source
under test
NOTE 17 If it is necessary to make measurements in
spaces in which KZA exceeds 2 dB, see tableO. and 8.4 or
IS0 3746 or IS0 9614.
6.1 General
The manner in which the source under test is installed
4.3 Criterion for background noise
and operated may have a significant influence on the
sound power emitted by the source. This clause
Averaged over the microphone positions, the level of
specifies conditions that minimize variations in the
background noise shall be at least 6 dB and preferably
sound power output due to the installation and oper-
more than 15 dB below the sound pressure level to
ating conditions of the source under test. The in-
be measured (see table 0.1 and 8.3).
structions of a noise test code, if any exists, shall be
followed in so far as installation and operation of the
NOTE 18 If the difference between the sound pressure
source under test is concerned.
levels of the background noise and the source noise is less
than 6 dB, see tableO. and 8.3 or IS0 3746. The effects
Particularly for large sources, it is important that a
of wind which may increase the background noise should
be minimized. noise test code specify which components, sub-
assemblies, auxiliary equipment, power sources, etc.
are to be included in the reference box.
5 Instrumentation
6.2 Source location
The source to be tested shall be installed with respect
5.1 General
to the reflecting plane or planes in one or more lo-
cations as if it were being installed for normal usage.
The instrumentation system, including the micro-
If several possibilities exist, or if typical installation
phones and cables, shall meet the requirements for
conditions are unknown, special arrangements shall
a type 1 instrument specified in IEC 651 or, in the
be made and described in the test report. In locating
case of integrating-averaging sound level meters, the
the source within the test environment, it is important
requirements for a type 1 instrument specified in
to allow sufficient space so that the measurement
IEC 804. The filters used shall meet the requirements
surface can envelop the source under test in accord-
of IEC 225.
ance with the requirements of 7.1.
The source under test shall be located at a sufficient
5.2 Calibration
distance from any reflecting wall or ceiling or any re-
flecting object so that the requirements given in an-
During each series of measurements, a sound nex A are satisfied on the measurement surface.
calibrator with an accuracy of + 0,3 dB (class 1 as
The typical installation conditions for some sources
specified in IEC 942) shall be applied to the micro-
involve two or more reflecting surfaces (see figures
phone to verify the calibration of the entire measuring
C.7 and C.8; for example, an appliance installed
system at one or more frequencies over the fre-
against a wall) or free space (e.g. a hoist) or an open-
quency range of interest.
ing in an otherwise reflecting plane (so that radiation
The compliance of the calibrator shall be verified with may occur on both sides of the vertical plane). De-
the requirements of IEC 942 once a year and the tailed information on installation conditions and the
compliance of the instrumentation system with the configuration of the microphone array should be
requirements of IEC 651 (and IEC 804 in the case of based on the general requirements of this Inter-
integrating systems) at least every 2 years in a lab- national Standard and specific noise test codes for
oratory making calibrations traceable to appropriate such sources.
standards.
The source shall only be installed near two or more
The date of the last verification of the compliance with reflecting surfaces when this is truly representative
the relevant IEC standards shall be recorded.
of normal use.
IS0 3744:1994(E)
6.3 Source mounting 6.4 Auxiliary equipment
In many cases, the sound power emitted will depend Care shall be taken to ensure that any electrical
upon the support or mounting conditions of the conduits, piping, or air ducts connected to the source
source under test. Whenever a typical condition of under test do not radiate significant amounts of sound
mounting exists for the equipment under test, that energy into the test environment.
condition shall be used or simulated, if feasible.
If practicable, all auxiliary equipment necessary for the
operation of the source under test and which is not a
If a typical condition of mounting does not exist or
part of the source (see 6.1) shall be located outside
cannot be utilized for the test, care shall be taken to
the test environment.
avoid changes in the sound output of the source
caused by the mounting system employed for the
If impracticable, the auxiliary equipment shall be in-
test. Steps shall be taken to reduce any sound radi-
cluded in the reference box and its operating condi-
ation from the structure on which the equipment may
tions described in the test report.
be mounted.
NOTES
6.5 Operation of source during test
19 Many small sound sources, although themselves poor
radiators of low-frequency sound, may, as a result of the
During the measurements, the operating conditions
method of mounting, radiate more low-frequency sound
specified in the relevant test code, if one exists for
when their vibrational energy is transmitted to surfaces
the particular type of machinery or equipment under
large enough to be efficient radiators. In such cases, if
test, shall be used. If there is no test code, the source
practicable, resilient mounting should be interposed be-
shall be operated, if possible, in a manner which is
tween the device to be measured and the supporting sur-
typical of normal use. In such cases, one or more of
faces so that the transmission of vibration to the support
the following operating conditions shall be selected:
and the reaction on the source are both minimized. In this
case, the mounting base should have a sufficiently high
- device under specified load and operating condi-
mechanical impedance to prevent it from vibrating and ra-
diating sound excessively. Such resilient mounts should not tions;
be used if the device under test is not resiliently mounted
in typical field installations.
- device under full load (if different from above);
20 Coupling conditions, e.g. between prime movers and
- device under no load (idling);
driven machines, may exert considerable influence on the
sound radiation of the source under test.
- device under operating conditions corresponding
to maximum sound generation representative of
normal use;
6.3.1 Hand-held machinery and equipment
- device with simulated load operating under care-
Such machinery and equipment shall be suspended
fully defined conditions;
or guided by hand, so that no structure-borne sound
is transmitted via any attachment that does not be-
- device under operating conditions with character-
long to the machine under test. If the source under
istic work cycle.
test requires a support for its operation, the support
The sound power level of the source may be deter-
structure shall be small, considered to be a part of the
mined for any desired set of operating conditions (i.e.
source under test, and described in the machine test
loading, device speed, temperature, etc.). These test
code.
conditions shall be selected beforehand and shall be
held constant during the test. The source shall be in
the desired operating condition before any noise
6.3.2 Base-mounted and wall-mounted
measurements are made.
machinery and equipment
If the noise emission depends on secondary operating
Such machinery and equipment shall be placed on a
parameters, such as the type of material being proc-
reflecting (acoustically hard) plane (floor, wall). Base-
essed or the type of tool being used, as far as is
mounted machines intended exclusively for mounting
practicable, those parameters shall be selected that
in front of a wall shall be installed on an acoustically
give rise to the smallest variations and that are typical
hard floor surface in front of an acoustically hard wall.
of the operation. The noise test code for a specific
Table-top equipment shall be placed on the floor at
family of machines shall specify the tool and the ma-
least I,5 m from any wall of the room, unless a table
terial for the test.
or stand is required for operation according to the test
For special purposes it is appropriate to define one or
code for the equipment under test. Such equipment
more operating conditions in such a way that the
shall be placed in the centre of the top of the test ta-
ble . noise emission of machines of the same family is

IS0 3744:1994(E)
highly reproducible and that the operating conditions
and/or to be measured in large open areas under sat-
which are most common and typical for the family of
isfactory acoustical conditions, a large measurement
machines are covered. These operating conditions
distance is usually selected and in this case the
shall be defined in specific test codes. hemispherical measurement surface is preferred. For
directivity measurements, a hemispherical or partial
If simulated operating conditions are used, they shall
hemispherical measurement surface is required.
be chosen to give sound power levels representative
of normal usage of the source under test. For measurements on a series of similar sources (for
example, machines of the same type or machines
If appropriate, the results for several separate operat-
from the same family), the use of the same shape of
ing conditions, each lasting for defined periods of
measurement surface is required.
time, shall be combined by energy-averaging to yield
the result for a composite overall operating procedure.
NOTE 21 The specific noise test code pertinent to the
particular source under investigation should be consulted for
The operating conditions of the source during the
detailed information.
acoustical measurements shall be fully described in
the test report.
The construction of the reference box, the size and
shape of the measurement surface, as well as the
measurement distance, d, or the radius of the hemi-
sphere, I-, shall be described in the test report.
7 Measurement of sound pressure levels
7.1 Selection of the measurement surface
7.2 Hemispherical measurement surface
To facilitate the location of the microphone positions
The hemisphere shall be centred in the middle of the
on the measurement surface, a hypothetical refer-
box consisting of the reference box and its images in
ence box shall be defined. When defining the dimen-
adjoining reflecting planes (point Q in figure 1). The
sions of this reference box, elements protruding from
radius, I-, of the hemispherical measurement surface
the source which are not significant radiators of sound
shall be equal to or greater than twice the character-
energy may be disregarded. These protruding ele-
istic source dimension, 4, and not less than 1 m.
ments should be identified in specific noise test codes
for different types of equipment. The microphone
NOTE 22 The radius of the hemisphere should be one
positions lie on the measurement surface, a hy-
of the following values (in metres): I, 2, 4, 6, 8, IO, 12, 14
pothetical surface of area S which envelops the
or 16. Some of these radii may be so large that the envi-
source as well as the reference box and terminates
ronmental requirements given in annex A cannot be satis-
on the reflecting plane(s).
fied; such large values of the radii may not be used.
The location of the source under test, the measure-
7.2.1 Area and key microphone positions on the
ment surface and the microphone positions are de-
hemispherical measurement surface
fined by a coordinate system with the horizontal axes
x and y in the ground plane parallel to the length and
If there is only one reflecting plane, the microphone
width of the reference box. The characteristic source
positions lie on the hypothetical hemispherical surface
dimension, a& is shown in figure 1.
of area S = 2nr2, enveloping the source and terminat-
One of the following two shapes shall be used for the ing on the reflecting plane. If the source under test is
measurement surface: in front of a wall, S = xr2. If it is in the corner,
s = 0,57Tr2. The key microphone positions of the
a) a hemispherical surface or partial hemispherical hemispherical surface are shown in figures B.l and
B.2 in annex B. Figure B.l specifies the locations of
surface of radius r;
10 key microphone positions, each associated with
equal areas, on the surface of the hemisphere of ra-
b) a rectangular parallelepiped whose sides are par-
dius I-. The hemispherical array of figures B.l and B.2
allel to those of the reference box; in this case, the
has been selected to minimize the errors which can
measurement distance, d, is the distance between
be caused by interference between the sound wave
the measurement surface and the reference box.
reaching the microphone directly and the wave re-
For sources usually mounted and/or to be measured
flected by the reflecting plane.
in rooms or spaces under unfavourable acoustical
conditions (for example, many reflecting objects and If a source is installed adjacent to more than one re-
high levels of background noise), the selection of a flecting plane, reference shall be made to
small measurement distance is appropriate and usu- figure B.3 a) and figure B.3 b) in annex B to define a
ally dictates the selection of a parallelepiped meas- suitable measurement surface and the microphone
urement surface. For sources usually mounted positions.

IS0 3744: 1994(E)
a) Reference box on one reflecting plane
=
(2,/z)* + (z*/z)* + 13’
II
Reference box on
b) two reflecting planes
=&jijqTg
c) Reference box on three reflecting planes
Figure 1 -
Examples illustrating reference boxes and characteristic source dimensions, 4, with respect
to the origin of the coordinate system, Q

IS0 3744: 1994(E)
In special cases (i.e. for families of machines, such as surface, a hypothetical surface of area S enveloping
construction equipment or earthmoving machinery, the source whose sides are parallel to the sides of the
which are to be measured in a moving state or in a reference box and spaced out a distance d (measure-
driving mode) a different number and arrangement of ment distance) from the box.
microphone positions can be used. However, this is
only possible if preliminary investigation has shown The microphone positions on the parallelepiped
that the resulting sound power level value is equal to measurement surface are shown in figures C.l to C.8
or larger by less than 1 dB than that determined with in annex C. The area S of the measurement surface
the array specified in this International Standard. according to figures C.2 to C.6 is given by the formula
S=4(ab+bc+ca) . . .
(3)
7.2.2 Additional microphone positions on the
where
hemispherical measurement surface
a = 0,5Z, +d
Sound pressure level measurements are required at
b = 0,5l, +d
additional microphone positions on the hemispherical
measurement surface if
c=l+d
a) the range of sound pressure level values meas-
E,, l2 and Z3 are the length, width and height of the
ured at the key microphone positions (i.e. the dif-
reference box.
ference in decibels between the highest and
lowest sound pressure levels) exceeds the num-
If a source is installed adjacent to more than one re-
ber of key measurement points, or
flecting plane, reference shall be made to figures C.7
and C.8 in annex C to define a suitable measurement
b) the source radiates noise with a high directivity,
surface. The calculation of the area S of the meas-
or
urement surface
...