Space systems — Acoustic testing

ISO 19924:2017 provides guidance for test providers and interested parties to implement acoustic tests of aerospace systems, subsystems, modules and units for applicable spacecraft programs. ISO 19924:2017 specifies a framework to meet test and process requirements and acts as a supplement to ISO 15864. The acoustic test system, the technical requirements and the procedures for acoustic tests in reverberant chambers are described. Furthermore, the criteria for the manual test interruption and evaluation are also described. The technical requirements in ISO 19924:2017 can be tailored to fulfil the objectives of tests.

Systèmes spatiaux — Essais acoustiques

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Status
Published
Publication Date
27-Sep-2017
Current Stage
9060 - Close of review
Start Date
03-Mar-2028
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INTERNATIONAL ISO
STANDARD 19924
First edition
2017-09
Space systems — Acoustic testing
Systèmes spatiaux — Essais acoustiques
Reference number
ISO 19924:2017(E)
©
ISO 2017

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ISO 19924:2017(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
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copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved

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ISO 19924:2017(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 3
5 Test purpose . 4
6 General . 4
7 Test system . 4
7.1 Test facility . 4
7.2 Equipment requirement . 5
7.2.1 Chamber system . 5
7.2.2 Sound source system . 6
7.2.3 Control system . 6
7.2.4 Measurement system . 6
8 Test technical requirements . 6
8.1 Laboratory environment . 6
8.2 Test condition and tolerance . 7
8.2.1 Test condition . 7
8.2.2 Fill effect . 7
8.2.3 Test level tolerances . 8
8.3 Specimen configuration requirements . 8
8.4 Specimen installation requirements . 8
8.5 Control requirements . 8
8.6 Measurement requirements .10
8.6.1 Structure response measurement .10
8.6.2 Sound measurement .10
8.7 Safety .10
9 Test procedure .10
9.1 Test flow .10
9.2 Test procedure .11
9.2.1 Before the test .11
9.2.2 Test implementation . .12
9.2.3 After the test .12
10 Test interruption and handling .12
10.1 Test interruption .12
10.2 Interruption handling .13
11 Test data and result evaluation .13
11.1 Test data .13
11.2 Result evaluation .13
12 Test documents .13
Annex A (informative) Methods for calculating the payload fill effect .14
Bibliography .17
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ISO 19924:2017(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 14, Space systems and operations.
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INTERNATIONAL STANDARD ISO 19924:2017(E)
Space systems — Acoustic testing
1 Scope
This document provides guidance for test providers and interested parties to implement acoustic
tests of aerospace systems, subsystems, modules and units for applicable spacecraft programs. This
document specifies a framework to meet test and process requirements and acts as a supplement to
ISO 15864.
The acoustic test system, the technical requirements and the procedures for acoustic tests in
reverberant chambers are described. Furthermore, the criteria for the manual test interruption and
evaluation are also described.
The technical requirements in this document can be tailored to fulfil the objectives of tests.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 15864:2004, Space systems — General test methods for space craft, subsystems and units
ISO 14620, Space systems — Safety requirements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
3.1
acoustic reverberation chamber
acoustic chamber built in hard and highly reflective surface walls such that the sound field therein
becomes diffused
3.2
diffuse sound field
sound field that has uniform energy density in a given region so that all directions of energy flux at all
parts of the region are equally probable
3.3
sound pressure
p
root mean square value of instantaneous sound pressure over a given time interval, unless specified
otherwise
Note 1 to entry: Normally given in Pa.
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ISO 19924:2017(E)

3.4
sound pressure level
SPL
L
p
expressed by
Lp=20lg /p
()
p 0
where
p is root mean square value of instantaneous sound pressure (3.3) over a given time interval (Pa);
p is reference pressure at threshold (Pa), p = 20 μPa.
0 o
3.5
overall sound pressure level
OASPL
value computed from one-third-octave (3.13) or octave band sound pressure levels, L
i
m
L /10
i
L =10lg 10
g

i=1
where
L is the overall sound pressure level in dB;
g
L is the sound pressure level (3.5) in one-third-octave or octave band;
i
m is the number of one-third-octave or octave bands.
3.6
bandwidth
difference between the nominal upper and lower cut-off frequencies
3.7
centre frequency
geometric mean of the nominal cut-off frequencies of a pass-band
Note 1 to entry: The definition of octave (3.12) and third-octave bands preferred centre frequency values refers
to ISO 266.
3.8
cut-off frequency of acoustic horn
frequency below which an acoustic horn becomes increasingly ineffective
3.9
measurement point
specific points spatially distributed in the sound field at which sound pressure levels (3.4) are measured
during test
3.10
control point
measurement points (3.9), spatially distributed inside the reverberant chamber, whose signals are used
for the sound pressure level test control
3.11
multipoint control
control achieved by using the average of the signals at the control points (3.10)
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ISO 19924:2017(E)

3.12
octave
1/1 Oct
interval between two centre frequencies (3.7) which have a ratio equal to 2
3.13
one-third-octave
1/3 Oct
1/3
interval between centre frequencies (3.7) which have a ratio equal to 2
3.14
test level tolerances
allowance of superior limit and inferior limit of a test level
3.15
closed-loop control
feedback control
system where the output acts upon the process in such a way as to reduce the difference between the
measured value and the desired set-point value to zero
[SOURCE: ISO 16484-2:2004, 3.41]
3.16
open-loop control
control action not using any automatic means of deviations from the target value
3.17
statistical DOF
number of independent variables in an estimate of some quantity
3.18
root mean square
RMS
obtained by squaring the amplitude at each instant, obtaining the average
of the squared values over the interval of interest and then taking the square root of this average
3.19
power spectral density
PSD
measure of the distribution of the energy (squared amplitude) of the signal as a function of frequency
3.20
broadband reverberant field
includes signals over a relative large frequency range of 22,5 Hz ~ 10 000 Hz (1/3 oct)
4 Abbreviated terms
All abbreviated terms in Table 1 are applied to this document.
Table 1 — Abbreviated terms
Oct Octave
DOF Degree of freedom
PSD Power spectral density
RMS Root mean square
SPL Sound pressure level
OASPL Overall sound pressure level
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ISO 19924:2017(E)

5 Test purpose
The purpose of acoustic test is to demonstrate the ability of the test specimen to endure acoustic
levels imposed by the launch vehicle and to validate unit random vibration test levels. Depending on
the product development stage, it can include qualification of the design with margin, detection of
workmanship defects, flaw of material and manufacturing failures.
There are two types of acoustic tests. The first test addresses the specimen functional compatibility
with acoustic environment.
The second test addresses mechanical resistance of the structures. In this case, the use of accelerometers
shall be required to measure responses to low levels before and after high load testing.
6 General
a) If there is no condition for a reverberant field acoustic test (RFAT), a direct field acoustic test
(DFAT) may be applied.
b) For compact specimen, random vibration may replace the acoustic test if analysis and/or heritage
data show that the payload responses are clearly dominated by random vibration compared to the
acoustic field. It is important to make a decision knowing that vibration tests do not reach high
frequency contents; whether the structure is sensitive to acoustic loads and the region that the
equipment is embedded shall also be considered. The decision shall be made by customers.
c) Generally, all structures and components requiring acoustic testing should be subjected to a
broadband reverberant field. The corresponding acoustic random noise source shall have an
approximate normal amplitude distribution.
7 Test system
7.1 Test facility
In general, an acoustic reverberation test facility is composed of a chamber system, gas supply system,
sound source system, control system and measurement system. An example of acoustic reverberant
test facility is shown in Figure 1.
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ISO 19924:2017(E)

Figure 1 — General composition of the acoustic reverberation test facility
7.2 Equipment requirement
7.2.1 Chamber system
Chamber system general requirements are as follows.
a) Generally, the shape of the chamber is a polyhedron. The surface of the reverberation chamber shall
be smooth and rigid enough.
b) The maximum OASPL of the chamber system shall be higher than the requirement of the tests to be
performed.
c) The chamber shall have enough channels that are connected to the control and measurement
system outside the chamber to meet the requirement of the customers.
d) The volume of the reverberation chamber should be 10 times bigger than the volume of the test
specimen if it is possible. In all cases, the volume of the reverberation chamber shall be large enough
to achieve the adequate test environment taking into account the volume of the test specimen. The
geometrical size of the reverberation chamber also defines the homogeneity of the SPL in the low
frequency third-octave bands. Chamber size shall be considered when low-frequency noise loading
is an essential test objective.
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ISO 19924:2017(E)

7.2.2 Sound source system
Generally, the sound source system is composed of modulators, power amplifiers and horns.
a) The achievable sound spectrum shall meet the test requirements.
b) Taking into account the reverberation time of the chamber and the additional damping of acoustic
modes introduced by test articles and test installations, the sound power of the sound source
system (i.e. number and type of the sound modulators and horns) shall meet the test requirements.
c) The operational frequency range of the modulators and power amplifiers shall meet the test
requirements.
d) In order to well represent the broadband frequency test requirements, different cut-off frequency
horns can be used.
7.2.3 Control system
Generally, the control system is composed of the controller, the control microphones and signal
conditioners or signal pre-amplifiers.
a) The control system shall have the function of multi-inputs average control.
b) The control system shall be able to store controlled time history of SPL.
c) The control system should allow calibration.
d) The measurement range of the microphones shall meet the test requirements.
e) 1/1 octave or 1/3 octave closed-loop control or open-loop control method may be used.
f) It should be possible to control power spectral density on an equal scale according to the test
requirements.
g) The control system should have the function t
...

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