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)

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

Diese Internationale Norm legt Verfahren zur Bestimmung der folgenden Größen fest:
- Einfügungsdämpfungsmaße von durchströmten und nicht durchströmten Kanalschalldämpfern in Frequenz-bändern;
- Schallleistungspegel des von Kanalschalldämpfern erzeugten Strömungsgeräusches in Frequenzbändern;
- Gesamtdruckverlust von durchströmten Schalldämpfern und
- Durchgangsdämpfungsmaße von Luftdurchlässen in Frequenzbändern.
Die Messverfahren sind zur Anwendung in einem Prüfstand bei Umgebungstemperatur vorgesehen. Vor-Ort-Messungen (Messungen im Einsatzfall) an Schalldämpfern werden durch ISO 11820 festgelegt.
Es ist anzumerken, dass die in einem Prüfstand erzielten Ergebnisse von Messungen nach dieser Internationalen Norm aufgrund von abweichenden Schall- und Strömungsfeldern nicht notwendigerweise mit denen von Vor-Ort-Messungen (Messungen im Einsatzfall) übereinstimmen. Zum Beispiel ist bei Laboratoriumsbedingungen der Druckverlust geringer als vor Ort, aber er ist vergleichbar groß zwischen verschiedenen Laboratorien.
Diese Internationale Norm gilt für alle Bauarten von Schalldämpfern einschließlich der Schalldämpfer für Lüftungs- und Luftkonditionierungsanlagen, Zuluftsysteme und Abgasanlagen und ähnliche Anwendungsfälle. Weitere passive luftführende Systeme, z. B. Krümmer, Luftdurchlässe und T-Stücke, können ebenfalls nach dieser Internationalen Norm gemessen werden.
Diese Internationale Norm gilt nicht für Reflexionsschalldämpfer (reaktiv wirkende Schalldämpfer), die in Kraftfahrzeugen zum Einsatz kommen.
ANMERKUNG 1   Anhang A beschreibt den Aufbau der Schallquelle zur Schallfeldanregung. Anhang B enthält Anforderungen an Übertragungselemente. In Anhang C sind Einzelheiten zu Kanalwänden und Grenzdämpfungsmaß angegeben. Anhang D enthält die Umrechnung von Dämpfungswerten in Terzbändern in solche in Oktavbändern. In Anhang E sind Messungen an großen Kulissenschalldämpfern beschrieben. In Anhang F ist die Prüfung der Längsdämpfung festgelegt. Anhang G gibt Hinweis

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)

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: Withdrawn
• Publication Date: 31-Dec-2003
• Withdrawal Date: 27-Aug-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: 9900 - Withdrawal (Adopted Project)
• Start Date: 28-Aug-2009
• Due Date: 20-Sep-2009
• Completion Date: 28-Aug-2009
• Directive: 98/37/EC - Machinery
• Mandate: M/BC/CEN/88/13 - Standardization request to CEN/CENELEC relating to machinery
• Ref Project: EN 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)

Relations

• Replaced By: 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)
• Effective Date: 01-Nov-2009
• Replaces: SIST EN ISO 7235:1997 - Acoustics - Measurement procedures for ducted silencers - Insertion loss, flow noise and total pressure loss (ISO 7235:1991)
• Effective Date: 01-Jan-2004

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN ISO 7235:2004
01-januar-2004
1DGRPHãþD
SIST EN ISO 7235:1997
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 7235:2003
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 7235:2004 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 7235:2004

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SIST EN ISO 7235:2004
EUROPEAN STANDARD
EN ISO 7235
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2003
ICS 91.120.20 Supersedes EN ISO 7235:1995
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 June 2003.
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 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 Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2003 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7235:2003 E
worldwide for CEN national Members.

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SIST EN ISO 7235:2004
EN ISO 7235:2003 (E)
CORRECTED  2003-09-24
Foreword
This document (EN ISO 7235:2003) has been prepared by Technical Committee ISO/TC 43
"Acoustics" in collaboration with 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 2004, and conflicting national
standards shall be withdrawn at the latest by February 2004.
This document supersedes EN ISO 7235:1995.
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 EU
Directive(s).
For relationship with EU Directive(s), see informative annex ZB, which is an integral part 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, Czech
Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and
the United Kingdom.
Endorsement notice
The text of ISO 7235:2003 has been approved by CEN as EN ISO 7235:2003 without any
modifications.
NOTE Normative references to International Standards are listed in Annex ZA (normative).
2

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SIST EN ISO 7235:2004
EN ISO 7235:2003 (E)
Annex ZA
(normative)
Normative references to international publications
with their relevant European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of
any of these publications apply to this European Standard only when incorporated in it by
amendment or revision. For undated references the latest edition of the publication referred to
applies (including amendments).
NOTE Where an International Publication has been modified by common modifications, indicated
by (mod.), the relevant EN/HD applies.
Publication Year Title EN Year
ISO 3741 1999 Acoustics - Determination of sound EN ISO 3741 1999
power levels of noise sources using
sound pressure - Precision methods
for reverberation rooms
ISO 3746 1995 Acoustics - Determination of sound EN ISO 3746 1995
power levels of noise sources using
sound pressure - Survey method using
an enveloping measurement surface
over a reflecting plane
ISO 9614-3 2002 Acoustics - Determination of sound EN ISO 9614-3 2002
power levels of noise sources using
sound intensity - Part 3: Precision
method for measurement by scanning
3

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SIST EN ISO 7235:2004
EN ISO 7235:2003 (E)
Annex ZB
(informative)
Relationship of this document with EC Directives
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 Directive(s) :
Machinery Directive 98/37/EC, amended by Directive 98/79/EC.
Compliance with this document provides one means of conforming with the specific essential
requirements of the Directive concerned and associated EFTA regulations.
WARNING: Other requirements and other EC Directives may be applicable to the product(s)
falling within the scope of this document.
4

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SIST EN ISO 7235:2004


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

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SIST EN ISO 7235:2004
ISO 7235:2003(E)
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©  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,
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ii © ISO 2003 — All rights reserved

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SIST EN ISO 7235:2004
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

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SIST EN ISO 7235:2004
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

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SIST EN ISO 7235:2004
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

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SIST EN ISO 7235:2004

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SIST EN ISO 7235:2004
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
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SIST EN ISO 7235:2004
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

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SIST EN ISO 7235:2004
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
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SIST EN ISO 7235:2004
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

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SIST EN ISO 7235:2004
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 parallel baffles installed in a
substitution duct, or an air-terminal unit, ready for installation in the test facility, including its housing and its
inlet and outlet openings to be connected to ducts
NOTE 1 Examples of silencers are given in Figure 1 and Annex E. Other elements to which the method of this
International Standard is applicable are listed in Clause 1.
NOTE 2 For “parallel baffles”, the term “splitters” is also common.

a)  Parallel-baffle silencer without transitions

b)  Off-set silencer
© ISO 2003 — All rights reserved 5

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SIST EN ISO 7235:2004
ISO 7235:2003(E)

c)  Circular silencer with concentric pod

d)  Flexible silencer

e)  Silencer with spark arrestor

f)  Elbow silencer
NOTE A centreline is only drawn for test objects with a rotationally symmetrical airway cross section.
Figure 1 — Examples of silencers
6 © ISO 2003 — All rights reserved

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SIST EN ISO 7235:2004
ISO 7235:2003(E)
4 Symbols
Symbols are listed in Table 1. The meanings of indices used in this International Standa
...