EN ISO 14163:1998

SLOVENSKI STANDARD
01-november-1999
Akustika - Smernice za varstvo pred hrupom z dušilniki (ISO 14163:1998)
Acoustics - Guidelines for noise control by silencers (ISO 14163:1998)
Akustik - Leitlinien für den Schallschutz durch Schalldämpfer (ISO 14163:1998)
Acoustique - Lignes directrices pour la réduction du bruit au moyen de silencieux (ISO
14163:1998)
Ta slovenski standard je istoveten z: EN ISO 14163:1998
ICS:
17.140.01 $NXVWLþQDPHUMHQMDLQ Acoustic measurements and
EODåHQMHKUXSDQDVSORãQR noise abatement in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL ISO
STANDARD 14163
First edition
1998-10-15
Acoustics — Guidelines for noise control
by silencers
Acoustique — Lignes directrices pour la réduction du bruit au moyen
de silencieux
A
Reference number
ISO 14163:1998(E)
ISO 14163:1998(E)
Contents
1 Scope .1
2 Normative references .1
3 Terms and definitions .2
4 Specification, selection and design considerations .4
4.1 Requirements to be specified .4
4.2 Selection and layout of silencers.4
4.3 Design of special silencers.5
5 Types of silencers, general principles and operational considerations .5
5.1 Overview.5
5.2 Acoustic and aerodynamic performance of silencers .7
5.3 Sound propagation paths .7
5.4 Acoustic installation effect .8
5.5 Abrasion resistance and protection of absorbent surfaces.9
5.6 Fire hazards and protection against explosion .9
5.7 Starting-up and closing-down of plants.9
5.8 Corrosion.9
5.9 Hygienic requirements and risk of contamination .9
5.10 Inspection and cleaning, decontamination.10
6 Performance characteristics of types of silencers.10
6.1 Dissipative silencers .10
6.2 Reactive silencers .22
6.3 Blow-off silencers .29
©  ISO 1998
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 the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
© ISO
ISO 14163:1998(E)
7 Measurement techniques . 30
7.1 Laboratory measurements . 30
7.2 Measurements in situ. 31
7.3 Measurements on vehicles. 31
8 Information on silencers. 31
8.1 Information to be provided by the user. 31
8.2 Information to be provided by the manufacturer . 32
Annex A (informative) Applications . 33
Annex B (informative) Effect of spectral distribution of sound on the declaration of attenuation
in one-third-octave or octave bands. 40
Annex C (informative) Operating temperatures of sound sources and temperature limits of
sound-absorbent materials. 42
Bibliography. 43
iii
© ISO
ISO 14163:1998(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 3.
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.
International Standard ISO 14163 was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee
SC 1, Noise.
Annexes A to C of this International Standard are for information only.
iv
© ISO
ISO 14163:1998(E)
Introduction
Whenever airborne sound cannot be controlled at the source, silencers provide a powerful means of sound
reduction in the propagation path. Silencers have numerous applications and different designs based on various
combinations of absorption and reflection of sound, as well as on reaction on the sound source. This International
Standard offers a systematic description of principles, performance data and applications of silencers.
v
INTERNATIONAL STANDARD  © ISO ISO 14163:1998(E)
Acoustics — Guidelines for noise control by silencers
1 Scope
This International Standard deals with the practical selection of silencers for noise control in gaseous media. It
specifies the acoustical and operational requirements which are to be agreed upon between the supplier or
manufacturer and the user of a silencer. The basic principles of operation are described in this International
Standard, but it is not a silencer design guide.
The silencers described are suitable, among others,
 for attenuating system noise and preventing crosstalk in heating, ventilation and air-conditioning (HVAC)
equipment;
 for preventing or reducing sound transmission through ventilation openings from rooms with high inside sound
levels;
 for attenuating blow-off noise generated by high-pressure lines;
 for attenuating intake and exhaust noise generated by internal combustion engines; and
 for attenuating intake and outlet noise from fans, compressors and turbines.
They are classified according to their types, performance characteristics and applications. Active and adaptive
passive noise-control systems are not covered in detail in this International Standard.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 3741, Acoustics — Determination of sound power levels of noise sources using sound pressures — Precision
methods for reverberation rooms.
ISO 3744, Acoustics — Determination of sound power levels of noise sources — Engineering methods for free-field
conditions over a reflecting plane.
ISO 7235, Acoustics — Measurement procedures for ducted silencers — Insertion loss, flow noise and total
pressure loss.
ISO 11691, Acoustics — Measurement of insertion loss of ducted silencers without flow — Laboratory survey
method.
ISO 11820, Acoustics — Testing of silencers in situ.
© ISO
ISO 14163:1998(E)
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
3.1
silencer
device reducing sound transmission through a duct, a pipe or an opening without preventing the transport of the
medium
3.2
dissipative silencer
absorptive silencer
silencer providing for broad-band sound attenuation with relatively little pressure loss by partially converting sound
energy to heat through friction in porous or fibrous duct linings
3.3
reactive silencer
general term for reflective and resonator silencers where the majority of the attenuation does not involve sound
energy dissipation
3.4
reflective silencer
silencer providing for single or multiple reflections of sound by changes in the cross-section of the duct, duct linings
with resonators, or branchings to duct sections with different lengths
3.5
resonator silencer
silencer providing for sound attenuation at weakly damped resonances of elements
NOTE The elements may or may not contain absorbent material.
3.6
blow-off silencer
silencer used in steam blow-off and pressure release lines throttling the gas flow by a considerable pressure loss in
porous material and providing sound attenuation by lowering the flow velocity at the exit and reacting on the source
of the sound (such as a valve)
3.7
active silencer
silencer providing for the reduction of sound through interference effects by means of sound generated by
controlled auxiliary sound sources
NOTE Mostly low-order modes of sound in ducts are affected.
3.8
adaptive passive silencer
silencer with passive sound-attenuating elements dynamically tuned to the sound field
3.9
insertion loss,
D
i
difference between the levels of the sound powers propagating through a duct or an opening with and without the
silencer
NOTE 1 The insertion loss is expressed in decibels, dB.
NOTE 2 Adapted from ISO 7235.
© ISO
ISO 14163:1998(E)
3.10
insertion sound pressure level difference
D
ip
difference between the sound pressure levels occurring at an immission point, without a significant level of
extraneous sound, without and with the silencer installed
NOTE 1 The insertion sound pressure level difference is expressed in decibels, dB.
NOTE 2 Adapted from ISO 11820.
3.11
transmission loss
D
t
difference between the levels of the sound powers incident on and transmitted through the silencer
NOTE 1 The transmission loss is expressed in decibels, dB.
NOTE 2 For standard test laboratories D equals D , whereas results for D and D obtained from in situ measurements may
t i t i
often differ due to limited measurement possibilities.
NOTE 3 Adapted from ISO 11820.
3.12
discontinuity attenuation
D
s
that portion of the insertion loss of a silencer or silencer section due to discontinuities
NOTE The discontinuity attenuation is expressed in decibels, dB.
3.13
propagation loss
D
a
decrease in sound pressure level per unit length which occurs in the midsection of a silencer with constant cross-
section and uniform longitudinal design, characterizing the longitudinal attenuation of the fundamental mode
NOTE The propagation loss is expressed in decibels per metre, dB/m.
3.14
outlet reflection loss
D
m
difference between the levels of the sound power incident on and transmitted through the open end of a duct
NOTE The outlet reflection loss is expressed in decibels, dB.
3.15
modes
spatial distributions (or transverse standing wave patterns) of the sound field in a duct that occur independently from
one another and suffer a different attenuation
NOTE The fundamental mode is least attenuated. In narrow and in lined ducts, higher-order modes suffer substantially
higher attenuation.
3.16
cut-on frequency
lower frequency limit for propagation of a higher-order mode in a hard-walled duct
NOTE 1 The cut-on frequency is expressed in hertz, Hz.
NOTE 2 In a duct of circular cross-section, the cut-on frequency for the first higher-order mode is f = 0,57c/C where c is the
cC
speed of sound and C is the duct diameter. In a rectangular duct with larger dimension H, f = 0,5c/H.
cH
© ISO
ISO 14163:1998(E)
3.17
pressure loss
Δp
t
difference between the mean total pressures upstream and downstream of the silencer
NOTE 1 The pressure loss is expressed in pascals, Pa.
NOTE 2 Adapted from ISO 7235.
3.18
regenerated sound
flow noise
flow noise caused by the flow conditions in the silencer.
NOTE Sound power levels of regenerated sound and pressure losses measured in laboratory tests are related to a
laterally uniform flow distribution at the inlet of the silencer. If this uniform flow distribution is not attainable under in situ
conditions, for example because of the upstream duct design, higher levels of regenerated sound and higher pressure losses
will occur.
4 Specification, selection and design considerations
4.1 Requirements to be specified
4.1.1  In general, the sound pressure level (A-weighted, one-third-octave or full-octave) shall not exceed a specified
value at a specified position (e.g. at a work station, in the neighbourhood, or in a recreation room). The permissible
contribution from a sound source can then be determined in terms of the sound power level and the directivity index
of that source using sound propagation laws and requirements concerning the allocation of contributions to several
partial sound sources. The required insertion loss of the silencer is given by the difference between the permissible
and the actual sound power level of the source.
In simple cases where the sound immission is determined solely by the sound source to be attenuated, the
necessary insertion sound pressure level difference of the silencer can be calculated directly from the difference
between the permissible and the actual sound pressure level at the immission point. When the difference in
directivity indices with and without the silencer is negligible, the insertion sound pressure level difference equals the
insertion loss of the silencer.
4.1.2  The permissible pressure loss shall not be exceeded.
NOTE This requirement should be specified as clearly as possible. Instead of the imprecise specification "as small as
possible", a sensible limit value has to be found. Even if the pressure loss is considered as "not critical", a limit value should be
determined from the maximum permissible flow velocity that may not be exceeded for reasons of mechanical stability,
regenerated sound or energy consumption costs.
4.1.3  The permissible size of the silencer shall be kept as small as possible (for reasons of cost and weight).
NOTE There is a minimum size which (given the state of the art) cannot be reduced. This size depends on the required
reduction in sound level, the permissible pressure loss and on other restrictions concerning materials to be used (or avoided),
resistance to different kinds of stress, etc.
4.1.4  Additional requirements (concerning materials, durability, leakages, etc.) result from the application of the
silencer in hot, dusty, humid or aggressive gases, in pressure lines or for high sound pressure levels and vibration
levels, and from the combination of silencers with devices for the control of exhaust gas, sparks and particles.
4.2 Selection and layout of silencers
Specific information on silencers can be drawn from
 laboratory measurements made in accordance with ISO 7235;
 silencer manufacturers' test data;
© ISO
ISO 14163:1998(E)
 theoretical models to calculate propagation loss and insertion loss for silencers with circular or rectangular
cross-section;
 pressure loss and regenerated sound prediction methods.
The selection of a dissipative, a reactive or a blow-off silencer will be determined by its application or by reference
to the experience presented in this International Standard.
Results obtained from computer programs for the insertion loss of dissipative silencers depend on the assumptions
made concerning the magnitude and distribution of airflow resistance in the silencer and the acoustical effect of the
cover [18]. Certain geometrical features like off-setting of splitters or subdividing of absorbers are not easily accessible
for calculation. Calculations are most accurate for parameter variations concerning design as well as operating
conditions. Effects of flow on the performance of reactive silencers are taken into account by special highly
sophisticated computer software.
4.3 Design of special silencers
The design of a special silencer is usually an iterative process featuring the following stages:
a) rough specification of the dimensions of free ducts for the flow and of connected spaces for the distribution of
sound, for example using the manufacturers' declarations for similar silencers and taking into account the
essential requirements and restrictions;
b) construction of a model for predictive calculation or measurements;
c) use of the model and comparison of the results with requirements concerning sound level reduction and
pressure loss;
d) change of dimensions and sound-absorbent materials to fulfil requirements or to optimize the design;
e) constructional consideration of special requirements.
5 Types of silencers, general principles and operational considerations
5.1 Overview
Silencers are used to
 prevent pulsations and oscillations of the gas at the source,
 reduce conversion of the pulsations and oscillations into sound energy, and
 provide conversion of sound energy into heat.
Table 1 — Typical advantages and shortcomings of different types of silencers
Type of silencer Advantages Shortcomings
Dissipative silencer Broad-band attenuation, little pressure Sensitive to contamination and
loss mechanical destruction
Reactive silencers:
Resonator type Tuned attenuation, insensitive to Narrow-band attenuation, sensitive to flow
contamination
Reflective type Robust element, application for large Greater pressure loss, acoustic pass
pressure pulsations, high sound levels, bands (frequency bands with little or no
contaminated flow, strong mechanical attenuation), flow sensitivity of acoustical
vibrations performance
© ISO
ISO 14163:1998(E)
The resulting insertion loss for a silencer mounted in a duct will in general depend on all three of these mechanisms.
According to the dominant attenuation mechanisms involved, silencers may be classified as (see table 1):
 dissipative silencers,
 reactive silencers, including resonator and reflective silencers,
 blow-off silencers, and
 active silencers.
5.1.1 Dissipative silencers
These provide broad-band sound attenuation by conversion of sound energy into heat with relatively little pressure
loss. Precautions shall be taken to prevent coating or clogging of the surface of the absorbent material when
dissipative silencers are used in ducts carrying gases contaminated with dust or encrusting material. Porous
absorbers made of fine fibrous material or thin-walled structures may be mechanically destroyed by high amplitudes
of alternating pressure.
5.1.2 Resonator silencers (reactive)
These reduce the conversion of gas pulsations and oscillations into sound energy and absorb sound. Single
resonators are mounted as side branches in duct walls. Groups of resonators are used as duct linings or splitter
elements (baffles) in ducts, thus causing a limited pressure drop. Resonances are mostly tuned to low and
intermediate frequencies, where attenuation is needed. The performance is limited to a narrow frequency band, is
sensitive to grazing flow and may (under certain unfavourable conditions) be negative so that a tone is generated.
5.1.3 Reflective silencers (reactive)
These reduce the conversion of gas pulsations and oscillations into sound energy. They are usually chosen for their
robustness in applications where purely dissipative silencers are less suitable, and where greater pressure loss is
permissible. This is the case, for example, with gas flows carrying dust, or with higher flow velocities and pressure
pulsations, and for applications with strong mechanical vibrations. The maximum attenuation and the frequency
where it occurs will be affected by the flow. It is possible that in some frequency bands only little or even negative
attenuation is encountered.
5.1.4 Blow-off silencers
These are mounted on steam and pressurized air release lines and are effective by reaction on the source of
sound, such as a valve, and by lowering the exit flow velocity through an expanded surface area while conversion of
sound into heat is usually of little significance. Large pressure losses require the silencer to have a good mechanical
stability. Its performance can be affected by material carried by the gas. There is also a danger of icing.
5.1.5 Active silencers
These mainly consist of speaker sets driven by amplifiers with input from suitable microphones. Control is effected
by a high-performance computer, the controller. These are specialist devices not dealt with in this International
Standard. Active silencers are most effective at low frequencies where passive dissipative silencers offer little
attenuation [32].
NOTE Active systems are presently offered exclusively as individual solutions tailored for particular applications and are
thus not discussed in this International Standard.
© ISO
ISO 14163:1998(E)
5.2 Acoustic and aerodynamic performance of silencers
The attenuation required from a silencer is described in terms of the insertion loss, D, if no particular immission
i
point is defined, or in terms of the insertion sound pressure level difference, D , at a particular position. It is
i
p
specified in one-third-octave bands or full-octave bands. According to the laboratory standard ISO 7235, the
attenuation shall be measured in one-third-octave bands. Full-octave-band values may be calculated using
equation (1):
D
13/,k
 3 
−
 
10dB
D =− 10 (1)
10lg dB
1/1 ∑
 
k=1
 
where D to D are the attenuation values in the three one-third octaves of a full-octave band, in decibels, and
1/3,1 1/3,3
D is the resulting full-octave-band value. Declaring attenuation values in full octaves will suffice for broad-band
1/1
noise and for silencers with broad-band effect. For tonal noise and for resonator silencers with narrow band effect,
the attenuation data should be given in one-third-octave bands.
NOTE 1 Octave-band attenuation data may strongly depend upon the spectrum of the sound (see annex B).
A necessary parameter for the selection of a silencer is the permissible pressure loss in the flow. It shall not exceed
the total pressure loss Δp which depends on the mean flow velocity and density of the gas and on the flow condition
t
as described by equation (2):
ρ
ΔΔpv=+ζζ (2)
()
t1
where
ζ is the total pressure loss coefficient as defined in ISO 7235 for uniform flow conditions at both ends of the
silencer;
Δζ is the additional pressure loss coefficient due to flow conditions in situ deviating from the laboratory
conditions (values are to be estimated empirically);
r is the density of the gas, in kilograms per cubic metre, kg/m ;
v is the mean flow velocity in the inlet cross-section, in metres per second, m/s.
NOTE 2 It is common for definitions of the total pressure loss coefficient to differ from the one given in ISO 7235. It is
therefore necessary to check the definitions before using any values. For example, a different definition is the one considering
the flow velocity in the narrowest cross-section of the silencer instead of v . This will result in much lower values for ζ.
Other parameters to be considered which affect the acoustic and aerodynamic performance are
 the regenerated sound,
 the maximum dimensions available for the silencer, and
 the necessary durability of the silencer under exposure to flow, pressure pulsations and mechanical vibration.
5.3 Sound propagation paths
It is possible for sound propagating from a source to an immission point to follow several paths beside the direct one
through the silencer (Figure 1, path 1). The additional paths are:
a) radiation from the housing of the source (path 2);
b) radiation from duct walls before the silencer (path 3);
© ISO
ISO 14163:1998(E)
c) radiation from the shell of the silencer (path 4); and
d) propagation of structure-borne sound along and past the silencer (path 5).
Sound propagation along these flanking paths shall be prevented by providing housings and duct walls with
sufficient sound insulation and by inserting vibration isolation devices for interrupting the propagation path for
structure-borne sound.
Figure 1 — Sound propagation paths (schematic)
5.4 Acoustic installation effect
For certain applications and silencer types, the sound attenuation provided by a silencer depends on the
characteristics of the source connected to the inlet side and the characteristics of the termination connected to the
outlet side. Such an installation effect occurs especially on reactive silencers or on all types of silencers for low
frequencies.
It is also important that either the source or the termination is reactive, i.e. non-absorbing. When these conditions
are fulfilled, unfavourable resonance effects can be expected in the system that will lead to strong coupling between
different parts of the system. Formally, this type of installation effect can be described via equation (3):
LL=−D−D+E (3)
(rad) (source)
WW tm
where
(rad) is the level of sound power radiated from duct end, in decibels, dB,
L
W
L (source)is the level of sound power radiated from source into duct with anechoic termination, in decibels,
W
dB;
D is the transmission loss (see 3.11), in decibels, dB;
t
is the reflection loss at the duct outlet (see 3.14 and 6.2.2.2), in decibels, dB;
D
m
E is the acoustic installation effect, in decibels, dB; in dissipative systems; the magnitude of E
generally does not exceed 10 dB.
The reaction of reflected sound on the source described by E can result in an increase or a decrease of sound
emission.
© ISO
ISO 14163:1998(E)
NOTE For strongly reactive systems, E can be a large positive quantity in narrow frequency bands, which implies that the
silencer system actually amplifies the sound power radiated from the source.
5.5 Abrasion resistance and protection of absorbent surfaces
Abrasion of the materials used in dissipative silencers may lead to particles of the infill being carried in the gas flow.
NOTE Little is known about the particle number concentration in the gas stream for longer operation of silencers.
If the surface of a sound-absorbent material is mechanically damaged, low flow velocities will suffice to carry away
large numbers of particles through erosion. This process may even result in the depletion of a whole absorbent
element (such as a splitter).
To protect the sound-absorbing infill of silencers against moisture, water or pollutants carried in the gas (in particular
in hospitals and in the food processing industry), foils are used for airtight sealing. Such foils not only reduce the
attenuation performance at high frequencies (typically above 1 kHz) but may also rupture during plant operation. A
difference in total (i.e. static and dynamic) pressures inside and outside the sealed element causes stress in the foil.
High temperatures and impacting sharp (and hot) particles increase the risk of damage. Thus, the protection of
sound-absorbing infill by means of foil needs careful consideration of foil thickness, temperatures, flow velocities
and contamination of the gas.
5.6 Fire hazards and protection against explosion
There is a particular danger of fire being started or transmitted by ventilation silencers for technical equipment if oil
aerosols are carried. This applies particularly to chemical laboratories, large kitchens and engine-testing
installations. Organic substances like flour or milk powder may form explosive mixtures with air, and this shall be
taken into account where dust-carrying gases flow through the silencer.
In all these applications, and in accordance with many building codes, "non-combustible" materials shall be used for
the silencer. Collections of grease, oil or dust in the absorbent material shall be prevented by using appropriate
shapes and arrangements of silencers. Resonator silencers without absorbent material and with precautions
against dust deposit are also suitable to meet fire- and explosion-protection requirements.
5.7 Starting-up and closing-down of plants
Silencers in technical plants may cause problems when the plant is started up or closed down. Sufficient space shall
be provided for the expansion of components of the silencer to allow for considerable changes in pressure and/or
temperature. Particularly in the case of pressure variations and for foil covers, pressure relief shall be possible in
the absorbent lining.
In the starting-up and closing-down phases of plants, there are frequently temperatures below the dew point,
especially inside the absorbent linings and on the inside of the silencer housing. Collection of moisture should be
prevented (for instance by "dry-running" the plant); particular corrosion problems may arise. Condensed liquid
should be allowed to drain.
5.8 Corrosion
Sheet metal shells, covers and partitions of silencers as well as mounting flanges shall be protected from the effects
of weather, acids in exhaust gases, and differences in voltage potentials of different materials. Corrosion can be
prevented by selection of particular material (e.g. aluminium) or by application of protecting covers (e.g. rubber).
5.9 Hygienic requirements and risk of contamination
Special requirements shall be met, for example,
 in cleanrooms,
 in food-processing plants,
 in hospitals,
© ISO
ISO 14163:1998(E)
 in power plants.
Hygienic problems can arise when dust is deposited on the adhesive surfaces of sound-absorbent linings,
particularly in combination with humidity. Viable particles (bacteria) can also pose a problem, especially if the air
temperature is elevated. Nuclear contamination may occur in nuclear power plants.
Smooth surfaces shall be used for silencer linings in such critical plants. Large cavities and protruding edges shall
be avoided because they encourage the collection of dust and damp and enhance the pressure loss.
5.10 Inspection and cleaning, decontamination
Provision for the inspection, cleaning or replacement of silencers or splitters should be made where needed.
The special requirements in most applications of HVAC equipment make cleaning or decontamination necessary at
intervals. It is therefore necessary that elements (splitters) can be dismounted for cleaning (decontamination) or
replacing. In this case, the silencer housing shall be cleaned as well. Splitters can be cleaned using pressurized air,
steam jets, brushes and solvents, or decontamination fluid, depending on the design.
A dust deposit forming on splitters after a certain operating time in dusty flow will lead to a considerable decrease in
insertion loss. Here too, provisions should be made to allow for cleaning at intervals.
6 Performance characteristics of types of silencers
6.1 Dissipative silencers
6.1.1 Simple dissipative silencers
A simple dissipative silencer is a straight duct with a sound-absorbent lining, of circular or rectangular cross-section
and without any fittings (see Figure 2).
Key:
1 Shell
2 Sound-permeable cover
3 Flow duct
4 Sound-absorbent material
Figure 2 — Dissipative silencer (schematic)
© ISO
ISO 14163:1998(E)
A sound-absorbent element consists of one or more layers of absorbent material and a sound-permeable cover.
Fine mineral, metal or plastic fibres and open-pore structures made of foam, sintered metal or concrete are used as
absorbent material. In coarse-grained structures, the viscosity of the air will have a smaller effect than turbulence. In
this case, pressure differences will increase with the square of the flow velocity. Such non-linear effects can be
found in silencers with flow through or tangential to the absorber. For covering fibre materials and foams subject to
high stress, perforated sheet metal, diamond mesh or rib mesh combined with close-meshed wire screen. glass or
steel fibre cloth should be used. For moderate stress conditions, thin foil, fibre glass or plastic fleece should be
used.
The transmission loss D (or insertion loss D ;. see 3.11) of the simple dissipative silencer can be described by
t i
DD=+Dl (4)
ts a
where
D is the discontinuity attenuation, in decibels, dB;
s
D is the propagation loss along the silencer, in decibels per metre. dB/m;
a
l is the length of the silencer, in metres, m.
The discontinuity attenuation can be calculated from laboratory measurements on two different lengths l and l of a
1 2
type of silencer. If the insertion losses D and D are measured for l and l without the influence of flanking
i1 i2 1 2
transmission within or around the silencer, the discontinuity attenuation D can be determined from equation (5):
s
Dl−D l
i1 2 i2 1
D= (5)
s
ll−
The propagation loss is determined from such measurements as:
DD−
i2 i1
D= (6)
a
ll−
For a qualitative estimate of the propagation loss D , Piening's proportionality can be used:
a
U
D∝α (7)
a
S
where
U is the length, in metres, m, of the duct perimeter lined with sound-absorbent material;
S is the cross-sectional area of the duct, in square metres, m ;
a is the sound absorption coefficient of the lining.
The greater the ratio of the surface area Ul of the absorber to the cross-section S of the duct, and the higher the
absorption coefficient a of the duct lining, the more effective the dissipative silencer will be. Small sound-reflecting
surfaces will reduce the effect only slightly.
The free area S of the cross-section is dependent on the maximum permissible flow velocity. This flow velocity shall
not be exceeded because of its relationship to the service life of the silencer, the pressure loss and the regenerated
sound. If the area is adapted to connecting ducts, the cross-section may also be round or rectangular. Equation (7)
indicates that narrow, rectangular openings with the larger sides being sound-absorbent are to be preferred.
Openings like this will also suppress beam formation which occurs when the distance between the walls exceeds
half the wavelength of the sound.
© ISO
ISO 14163:1998(E)
A high sound absorption coefficient is only possible when the thickness of the lining is at least one-eighth of the
sound wavelength. This criterion can be fulfilled in simple dissipative silencers even for low frequencies if a
sufficiently large cross section is available at the location where the silencer is to be mounted. The proportionality in
equation (7) to the sound absorption coefficient of the lining breaks down when the duct width becomes significantly
smaller than half the wavelength of the sound to be attenuated. Furthermore, the formula does not apply at high
frequencies when the sound propagates like a beam without hitting the lining at all.
A sound-absorbent material is characterized by its airflow resistivity r [29] (ranging for silencer applications from
4 4
5 kN�s/m to 50 kN�s/m ). The airflow resistivity is related to the fibre diameter and material bulk density according
to equation (8):
32/
r 
h
c
r ∝ (8)
 
r
 
a
u
where
r is the bulk density, in kilograms per cubic metre, kg/m , of the compressed absorber material;
c
r is the bulk density, in kilograms per cubic metre, kg/m , of the uncompressed absorber material,
u
h is the viscosity of the gas, in newton seconds per square metre, N⋅s/m ;
a is the average diameter of the fibres, in metres, m.
The influence of temperature and pressure on the specific airflow resistance R = rd of a material layer of thickness
S
d is approximately described bv equation(9):
12,
R p R
 T
S 0 S
(9)
=
 
rrc T p c

 0
Tp, T,p
where
T is the absolute temperature, in kelvins, K;
T is the reference temperature, in kelvins, K;
p is the gas pressure, in pascals, Pa;
p is the reference pressure, in pascals, Pa;
rc the characteristic impedance of the gas, in N⋅s/m , for plane waves.
is
Typical temperatures to be expected for various sound sources and temperature limits for various sound-absorbent
materials are listed in annex B.
Examples for the propagation loss in ducts of circular cross-section with linings of different thickness are shown in
Figure 3. They are based on rigorous calculations without flow and typical data for the airflow resistivity of mineral
wool. The thickness of the lining has a strong effect on the attenuation performance at low frequencies.
In some circumstances it is necessary to protect the environment from the silencer infill or the infill from the gas
flow. This can be done by thin impervious or perforated covers. For broad-band attenuation, the effective mass per
unit area of the cover should be kept as small as possible. The effective mass is either the weight of an impervious
cover or the mass of the air oscillating near the perforated cover divided by the fraction of open area.
NOTE Often a surface weight of the impervious cover of less than 0,033 kg/m or a porosity of the perforated cover of
more than 30 % is sufficient.
Ensure that impervious covers do not stick to the infill or, in the case of multiple layers of different covers to the
perforated cover which will reduce the mobility.
© ISO
ISO 14163:1998(E)
Free duct diameter: D = 0,2 m
Airflow resistivity of isotropic absorber: r = 12 kN�s/m
Specific airflow resistance of lining surface modelling the effect
of a dust deposit or a close-fitting porous cover: R = 0,2 kN�s/m
s
Figure 3 — Calculated propagation loss D vs. frequency f for a simple dissipative silencer with circular
a
cross-section and lining thickness t
For enhanced low-frequency attenuation, thicker impervious covers or perforated covers with lower porosity are
sometimes used.
Frequent starting-up and closing-down of furnaces may lead to humidity collecting in flue gas silencers (see A.2.4).
Plastic foil cannot completely prevent steam diffusion and will allow water to collect in the absorber, particularly
when the foil is damaged.
Absorbers shall be mechanically and thermally stable and their shape or structure shall not change due to
mechanical vibrations throughout their agreed service life.
6.1.2 Splitter silencers
6.1.2.1 General considerations
The factors that determine the acoustic performance of splitter silencers are, essentially, the same as those for
simple dissipative silencers described in 6.1.1.
A splitter silencer generally consists of a transition element which serves to expand the duct cross-section, a
midsection containing sound-absorbent splitters (or baffles) and gaps or airways to channel the flow, and a second
transition element to concentrate sound and flow to the original duct cross-section. This is illustrated in Figure 4. In
special cases, the transition elements at both sides are omitted or are not to be considered part of the silencer if so
agreed by the parties involved.
© ISO
ISO 14163:1998(E)
Key:
1 Entry cross-section
2 Transition elements
3 Sound-permeable cover
4 Sound-absorbent material (splitter)
Figure 4 — Splitter silencer
Providing a number of parallel splitters and a sufficient free area S can help to achieve high sound attenuation
according to equation (7) at small pressure loss.
Depending on the frequency range, the insertion loss of a splitter silencer results from the contributions of a
discontinuity attenuation at the inlet and the propagation loss along the splitters (see Figure 5). At low frequencies,
when the diameter of the connected duct is less than half a wavelength and propagation of higher-order modes is
inhibited, the discontinuity attenuation is negligible. At high frequencies, when the transition element allows for
nearly random sound incidence on the splitters, the discontinuity attenuation usually lies between 6 dB and 10 dB
and may exceed the propagation loss.
An additional discontinuity attenuation effective for splitters, where the internal structure changes along the
propagation path, is usually small.
Figure 5 — Decay of sound pressure level L along a path length x taken by th
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