EN ISO 20270:2022

SLOVENSKI STANDARD
01-februar-2023
Akustika - Opredelitev virov zvoka in vibracij, ki jih prenaša konstrukcija -
Posredno merjenje blokiranih sil (ISO 20270:2019)
Acoustics - Characterization of sources of structure-borne sound and vibration - Indirect
measurement of blocked forces (ISO 20270:2019)
Akustik - Charakterisierung von Körperschall- und Schwingungsquellen - Indirekte
Messung von blockierten Kräften (ISO 20270:2019)
Acoustique - Caractérisation des sources de bruit solidien et de vibrations - Mesurage
indirect des forces bloquées (ISO 20270:2019)
Ta slovenski standard je istoveten z: EN ISO 20270:2022
ICS:
17.140.20 Emisija hrupa naprav in Noise emitted by machines
opreme and equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 20270
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2022
EUROPÄISCHE NORM
ICS 17.140.20
English Version
Acoustics - Characterization of sources of structure-borne
sound and vibration - Indirect measurement of blocked
forces (ISO 20270:2019)
Acoustique - Caractérisation des sources de bruit Akustik - Charakterisierung von Körperschall- und
solidien et de vibrations - Mesurage indirect des forces Schwingungsquellen - Indirekte Messung von
bloquées (ISO 20270:2019) blockierten Kräften (ISO 20270:2019)
This European Standard was approved by CEN on 23 October 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 20270:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Endorsement notice . 3

European foreword
The text of ISO 20270:2019 has been prepared by Technical Committee ISO/TC 43 "Acoustics” of the
International Organization for Standardization (ISO) and has been taken over as EN ISO 20270:2022 by
Technical Committee CEN/TC 211 “Acoustics” the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2023, and conflicting national standards shall be
withdrawn at the latest by April 2023.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 20270:2019 has been approved by CEN as EN ISO 20270:2022 without any modification.

INTERNATIONAL ISO
STANDARD 20270
First edition
2019-11
Acoustics — Characterization of
sources of structure-borne sound and
vibration — Indirect measurement of
blocked forces
Acoustique — Caractérisation des sources de bruit solidien et de
vibrations — Mesurage indirect des forces bloquées
Reference number
ISO 20270:2019(E)
©
ISO 2019
ISO 20270:2019(E)
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, 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.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 20270:2019(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Selection of degrees of freedom (DOFs) . 6
4.1 General . 6
4.2 Source receiver interface . 7
4.3 Contact DOFs. 7
4.4 Indicator DOFs . 8
4.4.1 General. 8
4.4.2 All indicator DOFs at contact area . 8
4.4.3 No indicator DOF at contact area . 8
4.4.4 Some indicator DOFs at contact area . 8
4.5 Validation DOFs . 8
5 Test arrangement . 8
5.1 General . 8
5.2 Representativeness of the receiver . 8
5.3 Design of test receiver . 9
5.4 Avoidance of secondary noise sources . 9
6 Measuring equipment .10
6.1 General .10
6.2 Multi-channel analyser .10
6.3 Vibration sensors .10
6.4 Means of excitation .10
7 Test procedure .10
7.1 General .10
7.2 Operational test .12
7.3 Frequency response function (FRF) test .12
7.3.1 General.12
7.3.2 Direct FRF measurement .12
7.3.3 Reciprocal FRF measurement.12
7.4 Preliminary test with artificial excitation .13
8 Analysis procedure .13
9 Uncertainties and validation .14
9.1 General .14
9.2 On-board validation .15
9.3 Preliminary validation using artificial excitation .15
10 Test report .15
Annex A (informative) Example of a test report: Electric rear axle drive in a passenger car;
transfer path analysis (TPA) and estimation of blocked forces in situ according to
ISO 20270:2019 .17
Annex B (informative) Tests for validity of measurement data .24
Annex C (informative) Case studies .26
Annex D (informative) Criteria for selection of indicator and validation DOFs .31
Annex E (informative) Prediction of sound and vibration .35
Bibliography .37
ISO 20270:2019(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 of 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 www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 1, Noise.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2019 – All rights reserved

ISO 20270:2019(E)
Introduction
This document has been developed in response to demand from mechanical industries for an agreed
method of specifying the "source strength" of sources of structure-borne sound and vibration. Quantities
[2]
which independently characterize a source are the free velocity and blocked force: ISO 9611 specifies
a measurement procedure for the former in which the machine, a vibration source, is mounted on soft
mounts to approximate free suspension. The blocked forces are the forces the operating machine would
exert when constrained by a perfectly rigid foundation. They can potentially be measured directly by
inserting force transducers in between the operating machine and a rigid foundation. However, this
document describes an indirect method for measurement of blocked forces using an inverse method.
Whereas the measurement of free velocity requires the source to be resiliently mounted and direct
measurement of blocked forces requires the machine mounts to be blocked, the indirect measurement,
as defined in this document, can theoretically be carried out with the source attached to any receiver
structure. Essentially the same measurement techniques are used in the diagnosis of structure-borne
sound using "transfer path analysis" (TPA), also called "source path contribution" analysis (SPC).
A method of characterizing sources of structure-borne sound and vibration by the indirect measurement
of blocked forces at the points of connection to supporting, or receiver, structures is described in this
document. The measurement method is applied in situ, which means that the source is connected to a
receiver structure while the measurements are performed. In theory, the use of any receiver structure
is valid provided the vibration source mechanisms of the specimen remain representative of those in a
real installation. Therefore, the receiver structure can be part of a real installation, such as a machine
foundation or a building, but can also be a specially designed test stand if it provides representative
dynamic loading for the source.
The method specifies a two-stage measurement procedure comprising, first, a passive test in which
frequency response functions (FRF) of the assembled source-receiver structure are measured, and
secondly, measurement of vibration in an operational test. The blocked forces are obtained by solving
the inverse problem. It is well known that inverse solutions of this type can result in very large errors,
particularly if there is inconsistency in the input data. Such errors vary significantly depending on the
case and the skill of the operator. Therefore, a means of estimating the uncertainties in the blocked
force, through a process called on-board validation, forms an essential part of this measurement
procedure.
The blocked forces are obtained in narrow frequency bands that can subsequently be converted to
approximate octave or third octave frequency bands.
[3]
The in situ blocked force method is intended to complement the reception plate method of EN 15657 .
The reception plate method offers a simplified approach in which forces and velocities are effectively
averaged over the feet of an operating machine by mounting on a standard plate. The approximations
allow measurements to be simplified but information about distribution and phase of the forces and
velocities is lost. This document aims to provide an alternative for structure borne sound sources not
compatible with the reception plate approach or where more detail is needed about the distribution of
the forces.
The blocked forces obtained from this document can be used for the following purposes:
a) obtaining data for preparing technical specifications for vibrationally active components (sources);
b) obtaining input data for prediction of vibration in, or sound radiated sound from, structures
connected to the source;
c) obtaining diagnostic information about the contribution of particular blocked forces to a target
vibration or sound pressure (in situ transfer path analysis).
Prediction of sound and vibration in a new assembly [as in b) above] does not form a normative part of
this document, although guidelines for prediction are provided in Annex E. For prediction purposes,
extra data are needed in addition to the measured blocked forces. Specifically, the frequency response
functions (FRFs) of the new assembly (which consists of the source connected to the new receiver
ISO 20270:2019(E)
structure) need to be known. These FRFs can in principle be measured (if the assembly is available
for measurement), calculated (for example using numerical methods) or calculated by combining the
FRFs of the separate source and the receiver structures (dynamic substructuring) whether measured
or calculated.
vi © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 20270:2019(E)
Acoustics — Characterization of sources of structure-
borne sound and vibration — Indirect measurement of
blocked forces
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing
this document using a colour printer.
1 Scope
This document specifies a method where a vibrating component (a source of structure-borne sound or
vibration) is attached to a passive structure (or receiver) and is the cause of vibration in, or structure-
borne sound radiation from, the assembly. Examples are pumps installed in ships, servo motors in
vehicles or machines and plant in buildings. Almost any vibrating component can be considered as a
source in this context.
Due to the need to measure vibration at all contact degrees of freedom (DOFs) (connections between
the source and receiver), this document can only be applied to assemblies for which such measurement
is possible.
This document is applicable only to assemblies whose frequency response functions (FRFs) are linear
and time invariant.
The source can be installed into a real assembly or attached to a specially designed test stand (as
described in 5.2).
The standard method has been validated for stationary signals such that the results can be presented
in the frequency domain. However, the method is not restricted to stationary signals: with appropriate
data processing, it is also applicable to time-varying signals such as transients and shocks (provided
linearity and time invariance of the FRFs are preserved).
This document provides a method for measurement and presentation of blocked forces, together
with guidelines for minimizing uncertainty. It provides a method evaluating the quality of the results
through an on-board validation procedure but does not comment on the acceptability or otherwise of
the results.
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 7626-1, Mechanical vibration and shock — Experimental determination of mechanical mobility —
Part 1: Basic terms and definitions, and transducer specifications
ISO7626-2, Mechanical vibration and shock — Experimental determination of mechanical mobility —
Part 2: Measurements using single-point translation excitation with an attached vibration exciter
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO 20270:2019(E)
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
blocked force
dynamic force applied by an operational source (3.4) to a perfectly rigid receiver (3.5) structure
3.2
frequency response function
FRF
frequency-dependent ratio of the motion-response Fourier transform to the Fourier transform of the
excitation force of a linear system
Note 1 to entry: Excitation can be harmonic, random or transient functions of time. The test results obtained with
one type of excitation can thus be used for predicting the response of the system to any other type of excitation.
Note 2 to entry: Motion may be expressed in terms of velocity, acceleration or displacement; the corresponding
frequency-response function designations are mobility, accelerance and dynamic compliance or impedance,
effective (i.e. apparent) mass and dynamic stiffness, respectively.
[SOURCE: ISO 2041:2018, 3.1.53]
3.3
in situ blocked force vector
f ()f
c
complex blocked force (3.1) at the contact degrees of freedom (DOFs) (3.8), arranged into an n × 1 vector
at each frequency according to:
 ff 
()
c,1
 
 
ff()
 
c,2
f ()f =
 
c

 
 
ff()
 
cn,
 
where ff() is the complex Fourier spectrum component of the blocked force at frequency f and at
ci,
contact degree of freedom (DOF) i
Note 1 to entry: Forces can be considered as generalized forces, that is, including rotational components like
moments.
3.4
source
active substructure which contains the mechanisms of structure-borne sound or vibration generation
and comprises all parts of the assembly (3.6) on the active side of the source-receiver interface (3.7)
Note 1 to entry: Typically, the source is a separable component although this is not a requirement for the method.
Note 2 to entry: See Figure 1.
3.5
receiver
passive substructure comprising all parts of the assembly (3.6) on the passive side of the source-receiver
interface (3.7)
Note 1 to entry: The receiver may comprise the remaining parts of an assembled machine other than the source,
a test bench or a foundation structure such as a building.
2 © ISO 2019 – All rights reserved

ISO 20270:2019(E)
Note 2 to entry: By definition, there are no source mechanisms within the receiver so it is a purely passive
structure.
Note 3 to entry: See Figure 1.
3.6
assembly
installation comprising the source (3.4) and receiver (3.5) connected together
Note 1 to entry: See Figure 1.
Key
1 source (active structure)
2 receiver (passive structure)
3 assembly
s internal source excitation (not accessible)
in situ blocked force vector at the set of contact DOFs, c
f
c
v validation velocity (or acceleration) vector at the set of validation DOFs, v
v
v indicator velocity (or acceleration) vector at the set of validation DOFs, r
r
Y typical structural FRF between validation DOFs, v, and contact DOFs, c
vc
Y typical structural FRF between indicator DOFs, r, and contact DOFs, c
rc
H typical vibro-acoustic FRF between prediction DOFs, a, and contact DOFs, c (see NOTE 3)
ac
p structure-borne sound predicted at DOFs, a, in the fluid around the receiver (see NOTE 3)
a
NOTE 1 Indicator DOFs can be located anywhere on the receiver, including the source-receiver interface.
NOTE 2 The obtained blocked force vector can be used to predict vibration in, and radiated sound from, the
receiver structure (see Annex E).
NOTE 3 A vibration source (1) connected to a passive receiver (2) causes vibration (v ) in, or structure-borne
r
sound (p ) radiated from, the assembly (3) at interfaces (r, v) and (a), respectively. The internal excitation, s, is
a
unknown, requiring the source to be characterized at the source-receiver interface by blocked forces f , inferred
c
from v and the assembly FRF matrix Y . Additional structural, Y , and vibro-acoustic FRFs, H , can be used for
r rc vc ac
validation and prediction purposes.
Figure 1 — Test assembly
3.7
source-receiver interface
hypothetical surface which separates the source (3.4) structure from the receiver (3.5) structure
ISO 20270:2019(E)
3.8
contact degrees of freedom
contact DOFs
DOFs located on the source receiver interface through which structure-borne sound or vibration is
transmitted from the source (3.4) to the receiver (3.5) structure
Note 1 to entry: n is the number of DOFs and c is the subscript used for contact DOFs.
Note 2 to entry: See 4.3 for a full definition.
3.9
indicator degrees of freedom
indicator DOFs
DOFs on the receiver (3.5) at which vibration responses are measured
Note 1 to entry: m is the number of DOFs and r is the subscript used for indicator DOFs.
Note 2 to entry: See 4.4.
3.10
validation degrees of freedom
validation DOFs
DOFs on the receiver (3.5) structure (not at the contact area) at which "spare" vibration responses are
measured so as to provide a comparison for the on-board validation
Note 1 to entry: p is the number of DOFs and v is the subscript used for validation DOFs.
Note 2 to entry: See 4.5.
Note 3 to entry: The validation is described in Clause 9.
3.11
indicator velocity vector
v ( f )
r
complex velocity (or acceleration) at the indicator DOFs (3.9), arranged into an m × 1 vector at each
frequency according to:
vf()
 
r,1
 
vf()
 
 r,2 
v f =
()
 
r

 
 
vf
()
 
 rm, 
where v ( f ) is the complex Fourier spectrum component of the velocity (or acceleration) at frequency f
r,j
and at indicator DOFs j
Note 1 to entry: Consistent quantities shall be used throughout: either velocity and mobility, or acceleration and
accelerance.
3.12
measured validation velocity vector
v ( f )
v
4 © ISO 2019 – All rights reserved

ISO 20270:2019(E)
complex velocity (or acceleration) at the validation DOFs (3.10), arranged into a p × 1 vector at each
frequency according to:
vf()
 
v,1
 
vf
 ()
 v,2 
v ()f =
 
v

 
 
vf
()
 vp, 
 
where v ( f ) is the complex Fourier spectrum component of the velocity (or acceleration) at frequency f
v,k
and at indicator degree of freedom k
3.13
predicted validation velocity vector
v ' f
()
v
complex velocity (or acceleration) vector which has the same form as the measured validation velocity
vector (3.12) but contains predicted rather than measured data
Note 1 to entry: It is calculated according to Clause 8.
3.14
operational test
test in which vibration responses are measured at the indicator (3.9) and validation DOFs (3.10) while
the source (3.4) is in operation under a given set of operational conditions (3.16)
3.15
operational test using artificial excitation
test in which vibration responses are measured at the indicator (3.9) and validation DOFs (3.10) in the
same way as for an operational test (3.16) except that the source (3.4) is switched off and excitation is
provided by an instrumented hammer or shaker
3.16
operational conditions
defined set of circumstances under which the source (3.4) operates for the operational test (3.14),
including speed, load and any other settings or conditions particular to the source which can affect
source operation
3.17
artificial excitation
set of circumstances similar to operational conditions (3.16) except that the source (3.4) is switched off and
the source structure is excited artificially by a controlled force from an instrumented hammer or shaker
3.18
background noise conditions
conditions similar to operational conditions (3.16) except that the source (3.4) is switched off while any
other auxiliary equipment required to operate or load the source, e.g. hydraulic pumps, generators or
actuators, and/or other secondary sources of noise, e.g. wind noise, are active
3.19
on-board validation
procedure used for determining the quality of the blocked force (3.1) data
Note 1 to entry: The on-board validation is described in Clause 9.
3.20
frequency response function test
FRF test
test in which the response to a unit point force (mechanical mobility or accelerance) matrix is measured
with the source (3.4) switched off, i.e. under passive conditions
ISO 20270:2019(E)
3.21
inversion frequency response function matrix
inversion FRF matrix
Y
rc
m × n matrix of FRFs (3.2) in which the columns correspond to the contact DOFs (3.8) and the rows to
the indicator DOFs (3.9) according to:
Yf Yf
() ()
 
… Yf()
rc rc
11 12 rc
1 n
 
 
Yf() 

rc
 
Y ()f =
rc
 
 
 
 
Yf() Yf( ))(Yf )
rc rc rc
 
mm12 mn
where Yf() is the complex mobility (or accelerance) at frequency f for excitation at contact DOF c
i
rc
ji
and response at indicator DOF r
j
Note 1 to entry: Consistent quantities shall be used throughout: either velocity and mobility, or acceleration and
accelerance.
Note 2 to entry: The mobility (accelerance) shall be dimensionally consistent with the contact DOFs and particular
care is required if rotational components (moments) are included in the definition of the blocked force vector.
3.22
validation frequency response function matrix
validation FRF matrix
Y
vc
p × n matrix of FRFs (3.2) in which the columns correspond to the contact DOFs (3.8) and the rows to the
validation DOFs (3.10):
Yf Yf
() ()
 … Yf 
()
vc vc
vc
11 12
1 n
 
Yf  
()

vc
 
Y f =
()
vc
 
 
 
 
Yf Yf … Yf
() ( )) ()
vc vc vc
pp12 pn
 
where Yf() is the complex mobility (or accelerance) at frequency f for excitation at contact DOF c
vc i
ki
and indicator at validation DOF v
k
3.23
direct excitation
excitation applied to the contact DOFs (3.8) for the FRF test (3.20), as opposed to reciprocal
excitation (7.3.3)
4 Selection of degrees of freedom (DOFs)
4.1 General
Selection of the appropriate DOFs is an essential part of the procedure which can have an important
bearing on the reliability of the results. It is difficult to provide comprehensive guidelines since every
case is unique, however, some general guidelines are given below.
The main sources of error are likely to be related to inconsistency or incompleteness of the data set:
a) incompleteness due to transmission via DOFs not included in the definition of the contact DOFs;
6 © ISO 2019 – All rights reserved

ISO 20270:2019(E)
b) inconsistency due to differences in location or direction of the frequency response function (FRF)
excitation compared with the actual operational forces.
Therefore, selection of the appropriate DOFs is important.
It is advisable to agree on the contact DOFs, indicator DOFs and validation DOFs with the client prior to
testing. Also, a preliminary test is recommended (as described in 7.4) in order to test and, if necessary,
refine the selection of contact DOFs.
Particular care is required in determining the DOFs to be included at the interface since small details
can have a strong influence on the results. Examples of particular interface types are provided in
Annex C.
At each DOF, it is essential to adopt a sign convention for the direction of the force and velocity (or
acceleration) and this convention shall be adopted consistently between the operational and FRF tests.
Large errors can result from errors in sign.
NOTE See ISO 7626-1 for advice on polarity of transducers.
4.2 Source receiver interface
The source receiver interface is a hypothetical surface between the source and receiver structures.
The part of the interface where there is solid contact between the source and receiver is known as the
contact area. The contact area need not be continuous and typically consists of one or more points,
lines or areas of contact, such as flanges. The contact area typically coincides with the connections
between separable components, such as a pump and its support structure. However, the choice of the
interface is arbitrary provided that all the source mechanisms which generate structure-borne sound
and vibration are on the source side of the interface.
4.3 Contact DOFs
The contact area typically consists of one or more points, lines or areas of contact at which the source
and receiver structures are physically connected. The n contact DOFs are selected so as to account for
the excitation and coupling between the receiver and the source through these connections. In order
to select the correct DOFs, it is important to understand how the receiver structure is coupled to, and
excited by, the source. Important DOFs can include moments, and in-plane forces as well as normal
forces. Omission of important DOFs can lead to significant errors in the calculation of blocked forces. On
the other hand, inclusion of unnecessary DOFs increases the possibility for inversion errors, particularly
if the corresponding FRF data quality is poor, which is more likely for DOFs that are difficult to excite,
such as moments. Experimentation prior to data acquisition can be required to determine relevant
DOFs, for example using the artificial excitation procedure (see 7.4) combined with onboard validation
[27]
(see 9.2). Additionally, the "Interface Completeness Coefficient" may be employed to help define the
contact DOFs.
For point contact, excitation can occur, in general, in up to six DOFs at each point (three forces and three
moments on orthogonal axes). Continuous line interfaces may, for example, be represented by a set of
discreet points distributed along the line. Small contact areas (small in comparison with a structural
wavelength) may be represented as single equivalent points with up to six DOFs, or as a grid of points. In
all cases, sufficient accelerometers need to be employed so as to capture the dynamics of the structures
in all significant DOFs.
Each contact DOF may correspond directly to an accelerometer. Alternatively, the contact DOFs may
be obtained by combining the signals from several accelerometers, for example by subtracting signals
[30,18]
to give rotational DOFs using the "Finite Difference Method" . Other methods of defining contact
DOFs from combinations of accelerometer signals include, but are not limited to, the "Virtual Point
[26] [31]
Transformation" and "Interface Mobilities" .
ISO 20270:2019(E)
4.4 Indicator DOFs
4.4.1 General
The indicator DOFs may coincide fully or partially with the contact DOFs (4.3). Indicator DOFs may
be located anywhere on the receiver including, ideally, at the contact interface. The system shall be
"determined" or "over-determined", which means that there shall be at least as many indicator DOFs as
contact DOFs (i.e., m ≥ n). The indicator DOFs may be fully coincident with the contact DOFs, partially
coincident or not coincident, in other words all, none or some of the indicator DOFs may be at the
contact area.
4.4.2 All indicator DOFs at contact area
In this case, the indicator DOFs are the same as the contact DOFs. The inversion FRF matrix is then
[28]
square and symmetrical. For reasons not fully understood , this arrangement often appears to
provide better results than when the indicator DOFs are away from the interface. However, this option
demands direct excitation at the contact DOFs during the FRF measurement and is not always possible.
4.4.3 No indicator DOF at contact area
In this case, the indicator DOFs are all located away from the contact area. It is thus strongly advised
to over-determine the system by adding more indicator DOFs than contact DOFs, typically by a factor
between 2 and 3. The measured responses should have as much linear independence as possible and
therefore it is advisable to select indicator DOFs which capture different aspects of structural response,
for example by using well-spaced locations and different directions. If reciprocal excitation is to be used
for FRF measurement, then the ease with which these DOFs can be excited in the FRF test is also a factor
to consider since practical difficulties in excitation are a common reason for poor quality FRF data.
4.4.4 Some indicator DOFs at contact area
A third option is to locate some of the indicator DOFs at the contact interface and some elsewhere on the
receiver. In this case, it is also advisable to over-determine the system.
4.5 Validation DOFs
The validation DOFs shall be selected so as to provide responses which are, as far as possible, linearly
independent from those at the indicator DOFs (4.4) which are used in the solution (Clause 8). They shall
not be located at the contact area. They shall be at different locations and/or in different directions to
any of the indicator DOFs so as to provide as much linear independence as possible.
5 Test arrangement
5.1 General
The test may be conducted in situ, i.e. with the source installed in a real installation, or on a specially
designed test stand. The factors to consider in the choice of test arrangement are:
a) representativeness of the receiver in terms of its effect on source mechanisms;
b) design of the test receiver structure for ease of access, avoidance of resonances and non-linearities;
c) the need to avoid secondary noise sources.
5.2 Representativeness of the receiver
Provided the source mechanisms remain constant, the blocked force is theoretically an independent
property of the source and therefore is, in principle, not affected by the installation. However, dynamic
8 © ISO 2019 – All rights reserved

ISO 20270:2019(E)
loading of the source by the receiver structure can influence source mechanisms, for example due to
quasi-static deformation of a gearbox under load which can affect gear misalignment. There is little
information available on any such dynamic loading effects; however, it is advisable to ensure that the
test arrangement is representative of any intended installation in order to minimize such effects.
In the case of an in situ test, the test environment is representative by definition. However, some sources
are designed for a range of receivers, in which case it can be desirable to test the same source on a range
of receiver structures representing the intended installations.
If the test receiver is different from that of the intended installation, either because a special test rig
is used or because the intended receivers’ properties are variable, then it is necessary to consider the
representativeness of the receiver. The assumption in this document is that source mechanisms are
not unduly affected if
...