IEC 60068-2-65:2013

IEC 60068-2-65 ®
Edition 2.0 2013-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Environmental testing –
Part 2-65: Tests – Test Fg: Vibration – Acoustically induced method

Essais d’environnement –
Partie 2-65: Essais – Essai Fg: Vibrations – Méthode induite acoustiquement

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IEC 60068-2-65 ®
Edition 2.0 2013-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Environmental testing –
Part 2-65: Tests – Test Fg: Vibration – Acoustically induced method

Essais d’environnement –
Partie 2-65: Essais – Essai Fg: Vibrations – Méthode induite acoustiquement

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX V
ICS 19.040; 29.020 ISBN 978-2-83220-641-6

– 2 – 60068-2-65 © IEC:2013
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviations. 7
3.1 Terms and definitions . 7
3.2 Symbols and abbreviations . 11
4 Acoustic environments and requirements for testing . 11
4.1 Acoustic environment for testing . 11
4.1.1 General . 11
4.1.2 Reverberant field . 13
4.1.3 Progressive wave field . 14
4.1.4 Cavity resonance . 14
4.1.5 Standing wave . 14
4.2 Sound sources . 14
4.3 Measuring apparatus . 14
4.3.1 General . 14
4.3.2 Acoustic measurements . 14
4.3.3 Vibration response measurements . 15
4.3.4 Analysis of results . 15
4.4 Requirements for testing . 15
4.4.1 Type of facility . 15
4.4.2 Mounting . 15
4.4.3 Specimen instrumentation . 16
4.4.4 Preparation of test control . 17
5 Recommended severities . 18
6 Preconditioning . 18
7 Initial measurements . 19
8 Testing . 19
8.1 Normal testing . 19
8.2 Accelerated testing . 19
9 Intermediate measurements . 19
10 Recovery . 19
11 Final measurements . 19
12 Information to be given in the relevant specification . 20
13 Information to be given in the test report . 20
Annex A (informative) Guidance for the test requirements. 22
Bibliography . 30

Figure 1 – Third-octave band spectrum for aeronautical applications . 12
Figure 2 – Octave band spectra for fans derived from [4] . 13
Figure 3 – Octave band spectrum for noisy industrial machinery derived from [4] . 13
Figure 4 – Typical locations of microphone checkpoints (1 – 6) on a fictitious surface
around a specimen . 17

60068-2-65 © IEC:2013 – 3 –
Figure A.1 – Typical microphone arrangement around a specimen in a reverberation
chamber. 22
Figure A.2 – Typical microphone checkpoint arrangement around a long cylindrical
specimen . 25

Table 1 – Tolerances for acoustic measurement . 14
Table 2 – Overall sound pressure level and duration of exposure . 18
Table A.1 – Octave band/room volume relationship . 23
Table A.2 – Reverberation room, ratios of dimensions . 23
Table A.3 – Examples of sound sources with waveforms and typical power outputs. 28
Table A.4 – Typical OASPL and exposure durations . 28

– 4 – 60068-2-65 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ENVIRONMENTAL TESTING –
Part 2-65: Tests –
Test Fg: Vibration –
Acoustically induced method
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60068-2-65 has been prepared by IEC technical committee 104:
Environmental conditions, classification and methods of test.
This second edition cancels and replaces the second edition, published in 1993, and
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
– minor technical and editorial changes were made throughout the document as originally
requested by the DE National Committee;
– following comments at the CD stage, particularly from the UK National Committee,
significant technical and editorial additions were made to the standard for acoustic testing
employing the progressive wave tube technique.

60068-2-65 © IEC:2013 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
104/591/FDIS 104/597/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 60068 series, published under the general title Environmental
testing, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 60068-2-65 © IEC:2013
INTRODUCTION
Acoustic noise may produce significant vibration in components and equipment. In the acoustic
noise field, sound pressure fluctuations impinge directly on the specimen and the response
may be different to that produced by mechanical excitation.
Items particularly sensitive to acoustic noise include relatively lightweight items whose
dimensions are comparable to an acoustic wavelength in the frequency range of interest and
whose mass per unit area is low, such as dish antennas and solar panels, electronic devices,
printed circuit boards, optical elements, etc.
Acoustic testing is applicable to components, equipment, functional units and other products,
hereinafter referred to as “specimens”, which are liable to be exposed to and/or are required to
function in conditions of high sound pressure levels. It should be noted that, under service
conditions, the specimen may be subjected to simultaneous mechanical and acoustical
excitation.
High sound pressure levels may be generated by jet engines and other aircraft propulsion
systems, rocket motors, high-powered gas circulators, turbulent gas flow around aircraft or
launchers, etc. This part of IEC 60068 deals with acoustic testing in compressible gases and
can also be used to simulate the excitation response caused by turbulence resulting from high-
velocity separated gas flows.
The intent of the test procedure contained in this standard is to produce a high intensity
acoustic noise field by either reverberant methods (known as reverberant chamber testing) or
by progressive wave methods (known as progressive wave tube testing).
Testing for the effects of vibration caused by acoustic noise demands a certain degree of
engineering judgement and this should be recognized both by the manufacturer/supplier and
the purchaser of the specimen. Based on the guidance provided in this standard, the writer of
the relevant specification is expected to select the most appropriate method of test and values
of severity, taking account of the nature of the specimen and its intended use.
Since the acoustic levels occurring during testing are high enough to be damaging to human
hearing, appropriate protective measures need to be taken to reduce the noise exposure of
operators performing the test to a level regarded as permissible from the standpoint of hearing
conservation.
60068-2-65 © IEC:2013 – 7 –
ENVIRONMENTAL TESTING –
Part 2-65: Tests –
Test Fg: Vibration –
Acoustically induced method
1 Scope
This part of IEC 60068 provides standard procedures and guidance for conducting acoustic
tests in order to determine the ability of a specimen to withstand vibration caused by a
specified sound-pressure level environment to which it is, or is liable to be, subjected.
For sound pressure level environments of less than 120 dB acoustic tests are not normally
required.
This standard determines the mechanical weakness and/or degradation in the performance of
specimens and to use this information, in conjunction with the relevant specification, to decide
on their acceptability for use. The methods of test may also be used as a means of establishing
the mechanical robustness or fatigue resistance of specimens.
Two procedures are described for conducting tests and for measurement of the sound
pressure levels within the acoustic noise field and considers the need for measurement of the
vibration responses at specified points on the specimen. It also gives guidance for the
selection of the acoustic noise environment, spectrum, sound pressure level and duration of
exposure.
The progressive wave tube method is relevant to material where aerodynamic turbulence will
excite part, or all, of the total external surface. Such applications include aircraft panel
assemblies where the excitation exists on one side only. The reverberant chamber method is
relevant where it is preferable to induce vibration onto the entire external surface of equipment
by distributed excitation rather than fixed points by means of electro-dynamic shakers.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
IEC 61672-1, Electroacoustics – Sound level meters – Part 1: Specifications
ISO/IEC 17025:2005, General requirements for the competence of testing and calibration
laboratories
3 Terms, definitions, symbols and abbreviations
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

– 8 – 60068-2-65 © IEC:2013
3.1.1
acoustic horn
tube with increasing cross-section of generally exponential envelope, used to couple an
acoustic source to the test volume, for example the inside of a reverberation room, thus
achieving the maximum transfer of sound energy
Note 1 to entry: Each acoustic horn has individual transfer characteristics which affect the sound spectrum.
3.1.2
analysis integration time
time duration over which a signal is averaged
Note 1 to entry: See Clause A.8.
3.1.3
bandwidth
difference between the nominal upper and lower cut-off frequencies
Note 1 to entry: It may be expressed
a) in hertz,
b) as a percentage of the pass-band centre frequency, or
c) as the interval between the upper and lower nominal cut-off frequencies in octaves.
3.1.4
overall sound pressure level
OASPL
value computed from the third-octave or octave band sound pressure levels L
i
m
L /10
i
L = 10 log 10
∑
G 10
i=1
where
L is the overall sound pressure level in dB;
G
L is the sound pressure level in the ith third-octave or octave band;
i
m is the number of third-octave or octave bands.
3.1.5
centre frequency
geometric mean of the nominal cut-off frequencies of a pass-band
Note 1 to entry: The nominal upper and lower cut-off frequencies of a filter pass-band are defined as those
frequencies above and below the frequency of maximum response of a filter at which the response to a sinusoidal
signal is 3 dB below the maximum response.
½
Note 2 to entry: The geometric mean is equal to (f × f ) , where f and f are the cut-off frequencies.
1 2 1 2
3.1.6
constant-bandwidth filter
filter which has a bandwidth of constant value when expressed in hertz; it is independent of the
centre frequency of the filter
3.1.7
cut-off frequency (of acoustic horn)
frequency below which an acoustic horn becomes increasingly ineffective
Note 1 to entry: This cut-off frequency is a main characteristic of an acoustic horn.

60068-2-65 © IEC:2013 – 9 –
3.1.8
diffuse sound field
sound field which, in a given region, has statistically uniform energy density, for which the
directions of propagation at any point are randomly distributed
Note 1 to entry: In a diffuse sound field, the sound pressure level measured with a directional microphone would
give the same results whatever its orientation.
[SOURCE: IEC 60050-801:1994 [1] , definition 801-23-31, modified – Addition of the Note 1 to
entry]
3.1.9
electro-pneumatic transducer
hydraulic-pneumatic transducer
most generally employed laboratory source of acoustic noise to simulate sound pressure levels
encountered in a high operational ambient acoustic noise environment
Note 1 to entry: This transducer consists of a pneumatic transducer supplied with pressurized gas modulated by
an electromagnetic or hydraulic valve.
Note 2 to entry: This type of transducer provides a continuous spectrum of energy over a wide frequency band
with random amplitude distribution and is capable of providing a shaped sound spectrum to meet the specifications
in acoustic testing (see Clause A.5).
3.1.10
grazing incidence
angle between the direction of the acoustic wave and either the surface of the specimen and/or
the sensing surface of the acoustic transducer, 0 ° being parallel and 90 ° normal to the
surface
3.1.11
frequency interval
ratio of two frequencies
[SOURCE: IEC 60050-801:1994, definition 801-30-07]
3.1.11.1
octave
interval between two frequencies which have a ratio of 2
3.1.11.2
one-third octave
1/3
1/3
interval between two frequencies which have a ratio equal to 2
Note 1 to entry: Octave and third-octave frequency bands are defined by their geometric centre frequencies in
ISO 266 [2].
3.1.11.3
one-twelfth octave
1/12
1/12
interval between two frequencies which have a ratio equal to 2
3.1.12
measuring points
specific points in the sound field at which sound pressure is measured for the conduct of the
test
______________
Figures in square brackets refer to the bibliography.

– 10 – 60068-2-65 © IEC:2013
Note 1 to entry: Measurements may be made at points within the specimen in order to assess its behaviour but
these are not considered as measuring points in the sense of this standard.
3.1.12.1
checkpoints
points located on a fictitious surface surrounding the specimen and at a fixed distance from it
3.1.12.2
reference points
points chosen from the checkpoints, whose signals are used to control the test so that the
requirements of this standard are satisfied
3.1.13
multipoint control
control achieved by using the average of the signals at the reference points
Note 1 to entry: When using multipoint control, each microphone signal relates to the sound pressure level at one
position. The average sound pressure level L can be computed as given in IEC 60050-801:1994, definition 801-
AV
31-36, when
n
L /10
i
L = 10 log 10
∑
AV 10
n
where
n is the number of reference points;
L is the sound pressure level in the ith third-octave or octave band.
i
3.1.14
narrowband frequency filter
band-pass filter for which the pass-band is generally smaller than third-octave
3.1.15
broadband frequency
wide band filter
band-pass filter for which the pass-band is relatively wide or broad, in general larger than an
octave
3.1.16
progressive wave tube
tube along which sound waves propagate from the acoustic source, which is coupled to a
suitable test section by an acoustic horn
Note 1 to entry: The tube is terminated by an acoustically absorptive termination placed at the end of the test
section to minimize reflection of the progressive acoustic waves in the frequency range of interest (see Clause A.2).
3.1.17
proportional-bandwidth filter
filter which has a bandwidth that is proportional to the frequency
Note 1 to entry: Octave bandwidth, third-octave bandwidth, etc. are typical bandwidths for proportional-bandwidth
filters.
3.1.18
reverberation chamber (or room)
chamber or room which has hard, highly reflective surfaces such that the sound field therein
becomes diffuse
Note 1 to entry: The geometry of the chamber or room may influence the test. Information on reverberant
chambers is given in Clause A.1.

60068-2-65 © IEC:2013 – 11 –
3.1.19
sound absorption coefficient
fraction of incident sound power not reflected from the surface of a material at a given
frequency and under specified conditions
Note 1 to entry: Sound absorption is the property possessed by materials and objects for converting sound energy
to heat.
[SOURCE: IEC 60050-801:1994, definition 801-31-02, modified – word order of definition
reversed, Note 1 to entry replaces previous NOTE and bears no relation]
3.1.20
sound pressure
p
root mean-square of instantaneous sound pressures over a given time interval, unless
specified otherwise
Note 1 to entry: Sound pressure characterizes the variation of pressure about the static pressure, produced by
acoustic waves, which are variations of pressure caused by disturbances in a gaseous medium.
[SOURCE: IEC 60050-801:1994, definition 801-21-20, modified – addition of Note 1 to entry]
3.1.21
sound pressure level L
p
p
L = 20 log dB
p 10
p
o
3.2 Symbols and abbreviations
NOTE Where appropriate, a cross-reference to the definition is given.
OASPL overall sound pressure level in dB (derived from 801-22-07), see 3.1.4;
L overall sound pressure level in dB (see 3.1.4);
G
L sound pressure level in third-octave or octave band in dB (see 3.1.4);
i
L sound pressure level in dB (see 3.1.21);
p
L average sound pressure level in dB (see 3.1.13);
AV
p r.m.s. sound pressure in N/m or Pa (see 3.1.20);
–5
p international reference sound pressure, standardized as 2 x 10 Pa or 20 µPa in
o
air (IEC 61672-1), 1 µPa in other media;
DOF statistical degrees of freedom, given by:
N = 2B × T
d e a
where
B is the frequency resolution;
e
T is the effective averaging time.
a
4 Acoustic environments and requirements for testing
4.1 Acoustic environment for testing
4.1.1 General
An acoustic test is conducted in order to determine the ability of a specimen to operate or
survive in a specified high-intensity acoustic noise field. In practice, the fluctuating pressure
environment exerted on a specimen under consideration may be a complex combination of
progressive waves and reverberant acoustic fields. Standing waves, formed within structures
and cavities exposed to noise may resonate and produce very high local sound pressure levels.
It is, therefore, necessary to select the most appropriate type of acoustic test for the specimen.

– 12 – 60068-2-65 © IEC:2013
The selection may be based upon real measured data from field tests or flight trials or be
obtained from general levels specified for particular equipment applications, for example as in
Figures 1, 2 and 3. The applied test spectrum may contain energy above and below the
frequencies given in the figures.
NOTE For further information on sound pressure levels associated with aircraft environments, see ISO 2671 [3].

–9
3 dB/Octave –3 dB/Octave
Upper limit
–16
Lower limit
10 dB/Octave –10 dB/Octave
50 250 1 250 8 000
Third octave band centre frequency  (Hz)
IEC  382/13
Figure 1 – Third-octave band spectrum for aeronautical applications
Third octave band sound pressure levels  (dB)
relative to test overall sound pressure level

60068-2-65 © IEC:2013 – 13 –
Upper limit
Lower limit
–10
Axial flow
fans
–20
Upper limit
Lower limit
–30 Centrifugal
fans
–40
–50
63 125 250 500 1 000 2 000 4 000 8 000
Octave band centre frequency  (Hz)
IEC  383/13
Figure 2 – Octave band spectra for fans derived from [4]

Upper limit
–10
Lower limit
–20
–30
–40
63 125 250 500 1 000 2 000 4 000 8 000
Octave band centre frequency  (Hz)
IEC  384/13
Figure 3 – Octave band spectrum for noisy industrial machinery
derived from [4]
4.1.2 Reverberant field
A reverberant field is generally used for specimens intended to be located in enclosed spaces,
when the pressure fluctuations seen by the specimens are evenly distributed. However, it may
Octave band sound pressure levels  (dB)
Octave band sound pressure levels  (dB)
relative to test overall sound pressure level
relative to test overall sound pressure level

– 14 – 60068-2-65 © IEC:2013
also be used for testing the enclosures themselves, for example nose cone fairings of large
launch vehicles, etc., where no other more suitable simulation is possible. Reverberant fields
may arise in enclosures, from excitation of the boundary structures by turbulent gas flow or
flow separation over a surface, radiated propulsion noise, and within for example, gas-cooled
reactor pressure vessels (see Clause A.1).
4.1.3 Progressive wave field
A progressive wave field is used where the acoustic sound pressure sweeps over the surface
of the specimen. Examples of the occurrence of this environment include externally carried
items on aircraft, rocket engine heat shields, aircraft panels or tail surfaces (see Clause A.2).
4.1.4 Cavity resonance
A cavity resonance can occur as a result of turbulent flow over the cavities or when they are
exposed to acoustic excitation. Examples include aircraft landing gear wheel cavities when
wheels are lowered for landing or combustion chambers (see Clause A.3).
4.1.5 Standing wave
A standing wave may produce very high tonal sound pressure levels (see Clause A.4).
4.2 Sound sources
Guidance on the selection of an appropriate sound source for testing is given in Clause A.5.
4.3 Measuring apparatus
4.3.1 General
Measuring apparatus is required to monitor the sound pressure field around the specimen and,
if necessary, to measure the acoustically induced vibrations in the specimen. These
measurements require being analysed with respect to their frequency content (see 4.3.3).
4.3.2 Acoustic measurements
The monitoring instrumentation system shall be capable of measuring sound pressure levels in
the frequency range between 22,4 Hz and 11 200 Hz in either octave or third-octave bands,
with centre frequencies between 31,5 Hz/25 Hz (octave/third-octave) and 8 kHz/10 kHz.
This instrumentation system shall have a nominally flat frequency response within ± 5 % over
the frequency range of interest within the tolerances given in Table 1.
Table 1 – Tolerances for acoustic measurement
Frequency range Tolerance of frequency response
Hz dB (relative to the required test severity)
22,4 – 125 ± 1
126 – 2 500
± 2
2 501 – 11 200
± 3
The microphones used shall be capable of random incidence measurements for reverberant
chamber testing and grazing incidence measurements for progressive wave testing. In either
case, they should be capable of measuring peak values of at least three times the maximum
rated r.m.s. value.
60068-2-65 © IEC:2013 – 15 –
The instrumentation shall be capable of measuring sound pressure levels at least 10 dB higher
than the specified test level. This capability refers both to the overall level and to individual
frequency band levels.
4.3.3 Vibration response measurements
The monitoring of the vibration of the specimen, where specified by the relevant specification,
may be performed on the basis of acceleration and/or strain measurements. Also interface
forces, displacement or velocity response may also be monitored, if appropriate.
The monitoring equipment shall be capable of measuring overall vibration response at least in
the frequency range between 16 Hz and 2 000 Hz. This instrumentation shall have a nominally
flat frequency response over the frequency range of interest and be suitable for the application
and the type of measurement.
4.3.4 Analysis of results
The measured data obtained from 4.3.2 and, if appropriate, 4.3.3, shall be analysed for
frequency composition:
a) acoustic measurements shall be analysed with a resolution of at least one octave or,
preferably, third-octave, bands;
b) vibration response measurements usually require finer resolution analysis.
The frequency resolution bandwidths shall be prescribed by the relevant specification for the
particular application.
4.4 Requirements for testing
4.4.1 Type of facility
The service or operational space-time behaviour of the sound field to be simulated influences
the choice of testing. The facilities encompassed by this procedure are the reverberation room
or chamber and the progressive wave tube. Other types of specialist facilities are described in
Annex A and the principles of this standard may be used as the basis for test procedures for
those alternative facilities.
The type of facility to be used shall be specified in the relevant specification.
If a combined test is required in which the specimen is exposed simultaneously to a high
intensity acoustic environment and some other environmental parameter, the acoustical portion
of the testing shall be in accordance with this standard. Combined testing may include acoustic
and extreme or varying temperatures as well as mechanical vibrations to augment the acoustic
excitation at low frequencies.
4.4.2 Mounting
4.4.2.1 Reverberation chambers
The specimen shall be located in the centre of the reverberation room in such a way as to
avoid, as far as possible, parallelism between walls (including floor and ceiling) and the main
surfaces of the specimen. The specimens (and its mechanical support, if appropriate) shall be
supported or suspended elastically inside the reverberation room. The relevant specification
shall prescribe, as necessary, the preferred points of mounting or attachment.
The resonance frequency of the specimen on its suspension shall be less than 25 Hz or a
quarter of the lowest frequency of interest, whichever gives the lower value.
The distance between the checkpoints and the surface of the specimen shall be greater than
half the wavelength of the lowest frequency of interest or half the distance of the specimen

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from the wall, whichever is the lesser. If this is not possible and it becomes necessary to
position a microphone closer than half the wavelength, then the measured noise levels may be
subject to large variations due to reflections from the specimen and this shall be considered
when assessing the results of tests.
If a structural member is required, either between the specimen and the elastic suspension, or
for attaching the elastic suspension itself, care shall be taken to avoid distortion of the noise
field or the introduction of extraneous vibration.
Any connections to the specimen such as cables, pipes, etc. shall be so arranged that they
impose similar restraint and mass to that when the specimen is installed in its operational
position. In order to achieve this, it may be necessary to fasten the cables, pipes, etc. to the
mounting fixture.
4.4.2.2 Progressive wave tubes
Test specimens shall be mounted within the working section, either on a soft suspension or by
the service attachment, such that the excitation is applied over the whole of the external
surface. Alternatively, the item may be mounted as part of the wall of the working section when
only one side shall be excited. Where the test specimen is provided with specific means of
mounting, the support system shall be attached to these points. Where no specific means of
attachment are provided, the support system shall be connected to the specimen in such a way
that it does not interfere with the free movement of independent parts or provide additional
restraint or damping to panels or other structural parts. The rigid body modes of the system
shall be lower than 25 Hz or one-quarter of the lowest test frequency, whichever is the lesser.
Care shall be exercised to ensure that no spurious acoustic or vibratory inputs are introduced
by the test support system or ancillary structure. Any connections to the specimen, such as
cables or pipes shall be arranged so they impose dynamic restraint and mass similar to that
when installed in-service.
Test specimens such as panels shall be mounted in the wall of the duct such that the required
test surface is exposed to the acoustic excitation. This surface shall be flush with the inner
surface of the duct so as to prevent the introduction of cavity resonance or local turbulence
effects.
The distance between the checkpoints shall be half the distance of the specimen from the wall
or shall be greater than half the wavelength of the lowest frequency of interest, whichever is
the lesser. If this is not possible and it becomes necessary to position a microphone closer
than half the wavelength, then the measured noise levels may be subject to large variations
due to reflections from the specimen and this shall be considered when assessing the results
of the test.
When testing panel assemblies, the control microphones should be preferably mounted flush in
the duct wall opposite to the test specimen. Other positions within the working section may be
selected, provided that the microphone is positioned so that it responds only to grazing
incidence waves and that the necessary corrections are applied to the measured levels.
4.4.3 Specimen instrumentation
Where appropriate, the relevant specification shall state the number, type and location of
transducers (accelerometers, microphones, strain gauges, etc.) applied to the specimen.
The proof of calibration for each transducer shall be available.
Microphones for use in reverberation chambers shall be calibrated for random incident noise
and for grazing incident noise when used in progressive wave tubes.

60068-2-65 © IEC:2013 – 17 –
4.4.4 Preparation of test control
4.4.4.1 Number and location of checkpoints
For specimens located entirely within either a reverberant chamber or progressive wave tube,
there shall be at least three control microphones to measure the sound pressure levels around
the specimen. To determine the number of checkpoints, the size of the specimen shall be
considered with respect to the size of the sound field known to be homogeneous. The number
and position of the microphones, which shall be located on the major orthogonal axes of the
specimen and of the fictitious surface, shall be prescribed by the relevant specification (see
Figure 4).
For specimens located in the wall of a progressive wave tube, control may be achieved with
either a single microphone or, for example with large specimens, with microphones distributed
over the surface area occupied by the specimen.

Z
Test specimen
M1
M4
M6
M3
M2
M5
Fictitious surface
–Z
M  Microphone
IEC  385/13
Figure 4 – Typical locations of microphone checkpoints (1 – 6)
on a fictitious surface around a specimen
4.4.4.2 Control of spectrum
The responses from each control microphone shall be subjected to octave or third-octave
analysis as prescribed in the relevant specification. The average level in each band shall be

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obtained as in 3.1.13. The overall average value shall then be calculated from the band levels.
The band levels and overall level of the averaged spectrum shall be within the specified level
limits given in Figures 1, 2 or 3, or the spectrum prescribed by the relevant specification. The
averaged values shall remain within the specified limits for the duration of the testing.
The analysis integration time, as prescribed by the relevant specification, shall be sufficiently
long to ensure statistical confidence in the results (see Clause A.8).
Where test durations are of sufficient length, real time analysis of the responses of the control
microphones shall be carried out at intervals during the test in order to ensure that the sound
pressure levels are within the specified limits.
NOTE 1 The maximum allowable variation in band level and overall sound pressure level measured by each
microphone may be prescribed by the relevant specification.
NOTE 2 If the relevant specification prescribes one-third octave analysis, then it will also need to provide the one-
third octave spectr
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