SIST EN ISO 7235:2009

Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss (ISO 7235:2003)

This International Standard specifies methods for determining the insertion loss, in frequency bands, of ducted silencers with and without airflow, the sound power level, in frequency bands, of the flow noise (or regenerated sound) generated by ducted silencers, the total pressure loss of silencers with airflow, and the transmission loss, in frequency bands, of air-terminal units. The measurement procedures are intended for laboratory measurements at ambient temperature. Measurements on silencers in situ are specified in ISO 11820. It is to be noted that the results determined in a laboratory according to this International Standard will not necessarily be the same as those obtained in situ (installation), as different sound and flow fields will yield different results. For example, the pressure loss will be lower under laboratory conditions than in situ, but will be comparable between different laboratories. This International Standard is applicable to all types of silencer including silencers for ventilating and airconditioning systems, air intake and exhaust of flue gases, and similar applications. Other passive air-handling devices, such as bends, air-terminal units or T-connectors, can also be tested using this International Standard.

Akustik - Labormessungen an Schalldämpfern in Kanälen - Einfügungsdämpfung, Strömungsgeräusch und Gesamtdruckverlust (ISO 7235:2003)

Acoustique - Modes opératoires de mesure en laboratoire pour silencieux en conduit et unités terminales - Perte d'insertion, bruit d'écoulement et perte de pression totale (ISO 7235:2003)

L'ISO 7235:2003 prescrit les méthodes pour déterminer
la perte d'insertion de silencieux en conduit en présence et en l'absence d'écoulement d'air, par bande de fréquences, le niveau de puissance acoustique du bruit d'écoulement (ou bruit régénéré) généré par des silencieux en conduit, par bande de fréquences, la perte de pression totale du silencieux avec écoulement d'air, et la perte de transmission des unités terminales, par bande de fréquences.
Les modes opératoires de mesure sont destinés aux mesurages en laboratoire, à température ambiante. Les mesurages sur des silencieux in situ sont spécifiés dans l'ISO 11820.
Il convient de noter que les résultats obtenus en laboratoire conformément à l'ISO 7235:2003 ne seront pas nécessairement identiques à ceux obtenus in situ (installation), étant donné que différents champs acoustiques et champs de propagation donneront des résultats différents. La perte de pression sera, par exemple, inférieure dans les conditions de laboratoire à celle obtenue in situ mais sera comparable entre différents laboratoires.
L'ISO 7235:2003 s'applique à tout type de silencieux, y compris les silencieux pour ventilateurs et systèmes de conditionnement d'air, épuration des fumées et applications similaires. Il est également possible de soumettre à l'essai selon la présente Norme internationale d'autres dispositifs passifs de traitement de l'air, tels que des coudes, des unités terminales ou des raccords en T.
Elle ne s'applique pas aux silencieux réactifs utilisés pour les véhicules à moteur.

Akustika - Laboratorijski merilni postopki za dušilnike v kanalih in elementih za dovod in odvod zraka - Dodano dušenje, hrup zaradi pretoka in padec celotnega tlaka (ISO 7235:2003)

General Information

• Status: Published
• Public Enquiry End Date: 09-Jun-2009
• Publication Date: 12-Oct-2009
• ICS: 17.140.01 - Acoustic measurements and noise abatement in general
• ICS: 91.120.20 - Acoustics in building. Sound insulation
• ICS: 91.140.30 - Ventilation and air-conditioning systems
• Technical Committee: AKU - Acoustics
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 28-Aug-2009
• Due Date: 02-Nov-2009
• Completion Date: 13-Oct-2009
• Directive: 2006/42/EC - Directive 2006/42/EC of the European Parliament and of the Council of 17 May 2006 on machinery, and amending Directive 95/16/EC (recast)
• Directive: 98/37/EC - Machinery
• Mandate: M/BC/CEN/88/13 - Standardization request to CEN/CENELEC relating to machinery
• Ref Project: EN ISO 7235:2009 - Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss (ISO 7235:2003)

Relations

• Replaces: SIST EN ISO 7235:2004 - Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss (ISO 7235:2003)
• Effective Date: 01-Nov-2009

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 7235:2009
01-november-2009
1DGRPHãþD
SIST EN ISO 7235:2004
Akustika - Laboratorijski merilni postopki za dušilnike v kanalih in elementih za
dovod in odvod zraka - Dodano dušenje, hrup zaradi pretoka in padec celotnega
tlaka (ISO 7235:2003)
Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal
units - Insertion loss, flow noise and total pressure loss (ISO 7235:2003)
Akustik - Labormessungen an Schalldämpfern in Kanälen - Einfügungsdämpfung,
Strömungsgeräusch und Gesamtdruckverlust (ISO 7235:2003)
Acoustique - Modes opératoires de mesure en laboratoire pour silencieux en conduit et
unités terminales - Perte d'insertion, bruit d'écoulement et perte de pression totale (ISO
7235:2003)
Ta slovenski standard je istoveten z: EN ISO 7235:2009
ICS:
17.140.01 $NXVWLþQDPHUMHQMDLQ Acoustic measurements and
EODåHQMHKUXSDQDVSORãQR noise abatement in general
91.120.20 $NXVWLNDYVWDYEDK=YRþQD Acoustics in building. Sound
L]RODFLMD insulation
91.140.30 3UH]UDþHYDOQLLQNOLPDWVNL Ventilation and air-
VLVWHPL conditioning
SIST EN ISO 7235:2009 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------

SIST EN ISO 7235:2009

---------------------- Page: 2 ----------------------

SIST EN ISO 7235:2009
EUROPEAN STANDARD
EN ISO 7235
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2009
ICS 91.120.20 Supersedes EN ISO 7235:2003
English Version
Acoustics - Laboratory measurement procedures for ducted
silencers and air-terminal units - Insertion loss, flow noise and
total pressure loss (ISO 7235:2003)
Acoustique - Modes opératoires de mesure en laboratoire Akustik - Labormessungen an Schalldämpfern in Kanälen -
pour silencieux en conduit et unités terminales - Perte Einfügungsdämpfung, Strömungsgeräusch und
d'insertion, bruit d'écoulement et perte de pression totale Gesamtdruckverlust (ISO 7235:2003)
(ISO 7235:2003)
This European Standard was approved by CEN on 20 July 2009.
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 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 Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2009 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7235:2009: E
worldwide for CEN national Members.

---------------------- Page: 3 ----------------------

SIST EN ISO 7235:2009
EN ISO 7235:2009 (E)
Contents Page
Foreword .3
Annex ZA (informative) Relationship between this European Standard and the Essential
Requirements of EU Directive 98/37/EC .4
Annex ZB (informative) Relationship between this European Standard and the Essential
Requirements of EU Directive 2006/42/EC .5

2

---------------------- Page: 4 ----------------------

SIST EN ISO 7235:2009
EN ISO 7235:2009 (E)
Foreword
The text of ISO 7235:2003 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 7235:2009 by
Technical Committee CEN/TC 211 “Acoustics” the secretariat of which is held by DS.
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 February 2010, and conflicting national standards shall be withdrawn
at the latest by February 2010.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 7235:2003.
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association, and supports essential requirements of EC Directives.
For relationship with EC Directives, see informative Annexes ZA and ZB, which are integral parts of this
document.
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, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 7235:2003 has been approved by CEN as a EN ISO 7235:2009 without any modification.
3

---------------------- Page: 5 ----------------------

SIST EN ISO 7235:2009
EN ISO 7235:2009 (E)

Annex ZA
(informative)

Relationship between this European Standard and the Essential
Requirements of EU Directive 98/37/EC
This European Standard has been prepared under a mandate given to CEN by the European Commission
and the European Free Trade Association to provide a means of conforming to Essential Requirements of the
New Approach Directive 98/37/EC, amended by 98/79/EC on machinery.
Once this standard is cited in the Official Journal of the European Communities under that Directive and has
been implemented as a national standard in at least one Member State, compliance with the normative
clauses of this standard confers, within the limits of the scope of this standard, a presumption of conformity
with the relevant Essential Requirements of that Directive and associated EFTA regulations.
WARNING - Other requirements and other EU Directives may be applicable to the product(s) falling within the
scope of this standard.
4

---------------------- Page: 6 ----------------------

SIST EN ISO 7235:2009
EN ISO 7235:2009 (E)
Annex ZB
(informative)

Relationship between this European Standard and the Essential
Requirements of EU Directive 2006/42/EC
This European Standard has been prepared under a mandate given to CEN by the European Commission
and the European Free Trade Association to provide a means of conforming to Essential Requirements of the
New Approach Directive 2006/42/EC on machinery.
Once this standard is cited in the Official Journal of the European Communities under that Directive and has
been implemented as a national standard in at least one Member State, compliance with the normative
clauses of this standard confers, within the limits of the scope of this standard, a presumption of conformity
with the relevant Essential Requirements of that Directive and associated EFTA regulations.
WARNING — Other requirements and other EU Directives may be applicable to the product(s) falling within
the scope of this standard.



5

---------------------- Page: 7 ----------------------

SIST EN ISO 7235:2009

---------------------- Page: 8 ----------------------

SIST EN ISO 7235:2009


INTERNATIONAL ISO
STANDARD 7235
Second edition
2003-08-01


Acoustics — Laboratory measurement
procedures for ducted silencers
and air-terminal units — Insertion loss,
flow noise and total pressure loss
Acoustique — Modes opératoires de mesure en laboratoire pour
silencieux en conduit et unités terminales — Perte d'insertion, bruit
d'écoulement et perte de pression totale




Reference number
ISO 7235:2003(E)
©
ISO 2003

---------------------- Page: 9 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


©  ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland

ii © ISO 2003 — All rights reserved

---------------------- Page: 10 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 2
4 Symbols . 7
5 Test facilities and requirements for instrumentation. 9
5.1 Purpose and types of test facilities. 9
5.2 Equipment for acoustic testing of silencers . 9
5.3 Equipment for acoustic testing of air-terminal units . 15
5.4 Equipment for flow testing. 16
5.5 Equipment for dynamic testing . 21
6 Test procedures . 22
6.1 General. 22
6.2 Insertion loss. 22
6.3 Transmission loss. 24
6.4 Sound power level of the flow noise (or regenerated sound) . 24
6.5 Volume flow rate and pressure loss coefficient . 25
7 Information to be recorded . 29
7.1 Description of the test object . 29
7.2 Instrumentation . 29
7.3 Sound-source equipment. 30
7.4 Test, substitution and transmission ducts.30
7.5 Transitions. 30
7.6 Anechoic termination . 30
7.7 Reverberation room . 30
7.8 Acoustical test results. 30
7.9 Measurement uncertainty. 31
8 Information to be reported . 31
Annex A (normative) Design of the sound field excitation equipment and qualification tests . 32
Annex B (normative) Transmission element. 34
Annex C (normative) Duct walls and limiting insertion loss . 37
Annex D (normative) Conversion of one-third-octave-band attenuation values to octave-band
attenuation values. 40
Annex E (normative) Measurements on large parallel-baffle silencers . 41
Annex F (normative) Test of longitudinal attenuation . 43
Annex G (informative) Anechoic terminations. 44
Annex H (informative) Examples of measurement arrangements. 46
Bibliography . 48

© ISO 2003 — All rights reserved iii

---------------------- Page: 11 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 7235 was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 1, Noise.
This second edition cancels and replaces the first edition (ISO 7235:1991), which has been technically revised.
iv © ISO 2003 — All rights reserved

---------------------- Page: 12 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
Introduction
This International Standard specifies the substitution method for determining the insertion loss of ducted
silencers and a method for determining the transmission loss of air-terminal units.
In the substitution method, the sound pressure level of the transmitted wave is first determined for the test
object and then when the test object has been replaced by the substitution duct. The sound pressure level of
the transmitted wave can be measured
 in a reverberation room,
 in a test duct after the silencer, or
 in an essentially free field.
The methods are listed in order of preference.
The acoustic performance of silencers depends on the modal composition of the sound field at the inlet and
on reflections at the outlet side, on flanking transmission and on level differences between signals and flow
noise (or regenerated sound).
This International Standard describes configurations at the inlet side providing for a predominant fundamental
mode that suffers the least attenuation. For the outlet side, it describes anechoic terminations and
measurement procedures which are not sensitive to reflections or which allow for specified corrections.
Furthermore, this International Standard gives guidance on the suppression of flanking transmission and noise
signals.
The transmission loss of an air-terminal unit is determined from the results of measurements in a
reverberation room and theoretical reflection coefficients of a substitution duct.
The insertion loss of a silencer is generally affected by the airflow. The insertion loss is therefore preferably
measured with superimposed airflow if the silencer is to be used in ducts with high flow velocity.
For absorptive silencers where the maximum internal flow velocity falls short of 20 m/s, the flow will hardly
have an effect on the insertion loss. In practice, non-uniform flow distributions will occur. Therefore, the limit
velocity of 20 m/s may correspond to a design velocity of 10 m/s to 15 m/s.
An airflow through a silencer regenerates noise. This flow noise (or regenerated sound) establishes the lowest
sound pressure level that can be achieved after the silencer. It is, therefore, necessary to know the sound
power level of the flow noise (or regenerated sound) behind the silencer. This is preferably determined in a
reverberation room connected to the object via a transmission element.
In accordance with this International Standard, the total pressure loss of a silencer to be used with flow is to
be determined. It is, therefore, useful to equip the test facility with the instruments and devices necessary for
the determination of the total pressure loss.

© ISO 2003 — All rights reserved v

---------------------- Page: 13 ----------------------

SIST EN ISO 7235:2009

---------------------- Page: 14 ----------------------

SIST EN ISO 7235:2009
INTERNATIONAL STANDARD ISO 7235:2003(E)

Acoustics — Laboratory measurement procedures for ducted
silencers and air-terminal units — Insertion loss, flow noise and
total pressure loss
1 Scope
This International Standard specifies methods for determining
 the insertion loss, in frequency bands, of ducted silencers with and without airflow,
 the sound power level, in frequency bands, of the flow noise (or regenerated sound) generated by ducted
silencers,
 the total pressure loss of silencers with airflow, and
 the transmission loss, in frequency bands, of air-terminal units.
The measurement procedures are intended for laboratory measurements at ambient temperature.
Measurements on silencers in situ are specified in ISO 11820.
It is to be noted that the results determined in a laboratory according to this International Standard will not
necessarily be the same as those obtained in situ (installation), as different sound and flow fields will yield
different results. For example, the pressure loss will be lower under laboratory conditions than in situ, but will
be comparable between different laboratories.
This International Standard is applicable to all types of silencer including silencers for ventilating and air-
conditioning systems, air intake and exhaust of flue gases, and similar applications. Other passive air-handling
devices, such as bends, air-terminal units or T-connectors, can also be tested using this International
Standard.
This International Standard is not applicable to reactive silencers used for motor vehicles.
NOTE 1 Annex A specifies the sound field excitation equipment. Annex B gives requirements for the transition element.
Annex C gives details of duct walls and limiting insertion loss. Annex D specifies how to convert one-third-octave band
attentuation values to octave band values. Annex E gives requirements for measurements on large parallel-baffle
silencers. Annex F specifies a test of longitudinal attenuation. Annex G gives guidelines on anechoic terminations and
Annex H shows examples of measurement arrangements.
NOTE 2 Acoustic testing of air-terminal devices and fan-coil units is to be carried out as described for air-terminal units.
NOTE 3 Sound power measurements on air-terminal units are specified in ISO 5135. Measurements of the pressure
loss of air-terminal units are described in EN 12238, EN 12239 and EN 12589.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 3741:1999, Acoustics — Determination of sound power levels of noise sources using sound pressure —
Precision methods for reverberation rooms
© ISO 2003 — All rights reserved 1

---------------------- Page: 15 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
ISO 3746, Acoustics — Determination of sound power levels of noise sources using sound pressure —
Survey method using an enveloping measurement surface over a reflecting plane
ISO 5167-1, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 1: General principles and requirements
ISO 5221, Air distribution and air diffusion — Rules to methods of measuring air flow rate in an air handling
duct
ISO 9614-3, Acoustics — Determination of sound power levels of noise sources using sound intensity —
Part 3: Precision method for measurement by scanning
IEC 60651:2001, Sound level meters
IEC 60804:2000, Integrating-averaging sound level meters
IEC 60942:1997, Electroacoustics — Sound calibrators
IEC 61260, Electroacoustics — Octave-band and fractional-octave-band filters
3 Terms and definitions
For the purposes of this document, the following definitions apply.
3.1
insertion loss
D
i
〈of the test object〉 reduction in the level of the sound power in the duct behind the test object due to the
insertion of the test object into the duct in place of a substitution duct, given by the equation
D=−LL (1)
i WWII I
where
L is the level of the sound power in the frequency band considered, propagating along the test duct or

WI
radiating into the connected reverberation room when the test object is installed;
L is the level of the sound power in the frequency band considered, propagating along the test duct or

WII
radiating into the connected reverberation room when the substitution duct replaces the test object.
NOTE 1 The insertion loss is expressed in decibels (dB).
NOTE 2 For measurements according to this International Standard, the insertion loss of a silencer equals its
transmission loss.
3.2
transmission loss
D
t
〈of an air-terminal unit〉 difference between the levels of the sound powers incident on and transmitted through
the test object
NOTE 1 The transmission loss is expressed in decibels (dB).
NOTE 2 Adapted from ISO 11820:1996.
2 © ISO 2003 — All rights reserved

---------------------- Page: 16 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
3.3
face velocity
v
f
velocity in front of the test object
q
V
v = (2)
f
S
1
where
3
q is the volume flow rate, in cubic metres per second (m /s);
V
2
S is the inlet (or face) cross-sectional area of the test object, in square metres (m )
1
NOTE The face velocity is expressed in metres per second (m/s).
3.4
total pressure loss
∆p
t
〈of the test object〉 difference between the total pressures upstream and downstream of the test object
NOTE The total pressure loss is expressed in pascals (Pa).
3.5
total pressure loss coefficient
ζ
total pressure loss divided by the face velocity pressure upstream of the test object, given by the formula
∆p
t
ζ = (3)
1
2
ρ v
1 f
2
where
∆p is the total pressure loss, in pascals (Pa);
t
3
ρ is the air density upstream of the silencer, in kilograms per cubic metre (kg/m );
1
v is the face velocity, in metres per second (m/s) (see 3.3)
f
3.6
front
position relative to the direction of the sound propagation of the sound signal to be measured, corresponding
to the “source side”
3.7
behind
position relative to the direction of the sound propagation of the sound signal to be measured, corresponding
to the “receiving side”
3.8
test duct
straight, rigid duct of constant cross section in front of and behind the test object
© ISO 2003 — All rights reserved 3

---------------------- Page: 17 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
3.9
transition
duct element which connects two duct sections with different duct cross sections to each other
NOTE Transitions which are part of a silencer as supplied by the manufacturer/supplier are considered part of the
test object.
3.10
anechoic termination
device intended to reduce sound reflections at the receiving-side end of the test duct
3.11
transmission element
connection from the test duct behind the test object to a reverberation room, transmitting a certain fraction of
the sound energy from the duct into the room
3.12
substitution duct
rigid, non-absorbing duct element and having the same length and the same connecting cross sections as the
test object
3.13
reverberation room
test room meeting the requirements of ISO 3741
[ISO 3741:1999]
3.14
regenerated sound
flow noise
noise caused by the flow conditions in the test object
NOTE Adapted from ISO 14163:1998.
3.15
background noise level
sound pressure level at the indicating instrument when measurements are made with the substitution duct in
place and the loudspeaker is switched off
NOTE 1 The background noise level is expressed in decibels (dB).
NOTE 2 Adapted from ISO 11200:1995.
NOTE 3 The main elements in background noise are
 flow noise from the fan,
 flow noise generated at the microphone,
 flow noise from the duct system,
 structure-borne sound from the fan propagating along the duct walls to the measurement position,
 airborne sound radiated from the fan or from the loudspeaker equipment into the test room and transmitted through
the duct walls to the microphone, and
 electrical noise in the measurement equipment.
NOTE 4 Flanking transmission of sound from the loudspeaker or of flow noise generated by the test object is not part
of the background noise, but determines the limiting insertion loss.
4 © ISO 2003 — All rights reserved

---------------------- Page: 18 ----------------------

SIST EN ISO 7235:2009
ISO 7235:2003(E)
3.16
reflection coefficient
r
ratio of the reflected sound pressure amplitude to the sound pressure amplitude of the sound wave incident on
the reflecting object
NOTE Adapted from ISO 5136:1990.
3.17
frequency range of interest
one-third-octave bands with centre frequencies from 50 Hz to 10 000 Hz
NOTE For certain applications, it may be sufficient to measure in the frequency range between 100 Hz and 5 000 Hz.
3.18
limiting insertion loss
maximum insertion loss which can be determined in a given test installation without flow
NOTE 1 The limiting insertion loss is expressed in decibels (dB).
NOTE 2 The limiting insertion loss is generally determined by the flanking transmission along the duct walls.
3.19
test object
complete silencer, as supplied by the manufacturer/supplier, one or several parall
...