Which domains of applications for silencers in the context of a soundproofing project?

Silencers are devices reducing sound transmission in a duct, a pipe or an opening, without preventing the transport of a fluid. They therefore constitute basic components within the panoply of means to fight against noise both in industry, in the context of the preservation of the environment or in the building.

Those equipments allowing to control noise emissions in a gaseous medium, are appropriate (inter alia):

  1. for attenuation of the noise emissions by systems and for prevention of parasitic coupling produced by heating, ventilation and air conditioning equipments (they are thus useful in the context of noise reduction of equipments in residential buildings, hotels, hospitals, whether being equipment installed in an equipment room or bieng equipment installed outdoor such as heat pumps, air conditioners, air coolers ...)
  2. for the prevention or reduction of noise transmission through ventilation openings, from rooms with high internal noise levels (they are thus employed to prevent or limit the sound nuisance in relation to the ventilation of technical premises, of parkings, of power generation units whether cogeneration plants, emergency gensets or power plants and also in industry with respect to various processes, such as dust control facilities, chips evcuation facilties)
  3. attenuation of air intake noise and of exhaust noise emitted by internal combustion engine (the air intake noise is usually treated with dissipative silencers while the exhaust noise uses reactive mufflers)
  4. attenuation of air intake noise and of exhaust noise emitted by compressors and turbines equipped with fan
  5. attenuation of industrial process control valves
    1. control valves noise

What are the input data useful for the design of a silencer ?

The input data useful for the design of a silencer are mainly the type of fluid (and its degree of physicochemical aggressiveness with respect to the envisaged silencer), its thermodynamic state (flow rate, pressure, temperature, density), the level of acoustic performance required (insertion loss, remaining sound power), the level of aerodynamic performance required (total pressure loss often called pressure drop), and the space available (dimensions of the upstream network, space available for the silencer, downstream interfacing if applicable). In very numerous cases, the reaction to fire of considered materials  is also taken into account.

In the case of realization in the food industry, in pharmaceuticals, or in hospitals, specific constraints related to hygiene are involved and may necessitate the use of particular absorbing fillings (with waterproof washable linings) or the use of stainless steels.

Other parameters may be involved such as the authorized mass, the longevity required or the available budget.

Which ventilation silencer in buildings and for industrial applications at room temperature?

Regarding ventilation silencers in buildings and for industrial applications at room temperature, they usually consist of an outer casing (mostly made of non-stainless steel, most of the time: galvanized) and of an absorbing filling (quite often: mineral wool) covered by a surfacing (mostly: a fiber glass) and also by a perforated protection (mostly: made of galvanized steel). For the outer casing, if required: a powder coating usually provides the best possible protection against corrosion: classification up to C5 according to ISO 12944.

As far as simple dissipation silencers are concerned, they are soundproofing equipment belonging to the category of dissipative silencers for which the sound absorption material is placed exclusively on the inside of the outer envelope. In most cases, such silencers are round and are intended to be inserted in networks with ducts being themselves of circular cross section (silencers with rectangular cross section, possibly square can be envisaged in some cases and are intended to be inserted into networks of ducts being themselves of rectangular or enventually square cross section). The acoustic performance of such silencers (insertion loss) is highly dependent on the considered frequency and is linked to the fluid velocity, to the acoustic thickness of the lining, its nature (in particular: the flow resistance of the filling and of its covers if any), to the free inner dimension for the passage of fluid, and also to the length of the silencer. Such silencers are characterized by a total pressure loss (also called pressure drop) being low because exclusively dependent on the roughness of the wall and on the length of the silencer (usually negligible if the flow velocity of the fluid is low enough).

Regarding splitter silencers (also called baffle silencers), they are soundproofing equipment belonging to the category of dissipative silencers for which the sound absorption material is held in frames (usually metallic: in non-stainless steel) forming splitters between which the fluid flows. Most of the time, such silencers have a rectangular cross section and are intended to be inserted into networks of ducts being themselves of rectangular cross-section (silencers with circular cross-section, with circular central splitter and, where appropriate with one or more ring-shaped intermediate splitters can be envisaged in some cases and are intended to be inserted into networks of ducts being themselves of circular cross-section). The acoustic performance of such silencers (insertion loss) is very dependent on the considered frequency and is related to the fluid velocity, to the thickness of the splitters (baffles), to their nature (including: fow resistance of the filling and of its cover if any), to their spacing, as well as to the length of the silencer. Such silencers generate a total pressure loss (also known as pressure drop) mainly related to the thickness of the splitters, to their spacing, and also to their geometry (profiling of upstream and downstream extremities if any) as well as to the length of the silencer.

In some cases, the metallic parts of dissipative silencers for industrial applications at room temperature can be made of stainless steel (SS 304, SS 316, SS 321) or even replaced by plastic parts in extreme cases where the transported fluid is extremely corrosive (silencers for smoke ducts at low temperature for example).

As required, the absorbing filling may consist of acoustic foam or of polyester wool and and may for some applications be protected by a waterproof surfacing (polyurethane film and for extreme cases: painting).

The prediction of acoustic and aerodynamic performance of dissipative silencers such as ventilation silencers in buildings and for industrial applications at room temperature can be performed with the simulation software SILDIS.

Which silencer for industrial applications at high temperature ?

Regarding silencers for industrial applications at high temperature, they usually consist of a double shell outer casing (mostly with the outer shell made of non-stainless steel and with the inner shell made of stainless steel) and of an absorbing filling (quite often: mineral wool) covered by a surfacing (mostly: a glass fabric) and also by a perforated protection (mostly: made of stainless steel). For the outer casing, if required: a powder coating usually provides the best possible protection against corrosion: classification up to C5 according to ISO 12944.

As far as simple dissipation silencers are concerned, they are soundproofing equipment belonging to the category of dissipative silencers for which the sound absorption material is placed exclusively on the inside of the outer envelope. In most cases, such silencers are round and are intended to be inserted in networks with ducts being themselves of circular cross section (silencers with rectangular cross section, possibly square can be envisaged in some cases and are intended to be inserted into networks of ducts being themselves of rectangular or enventually square cross section). The acoustic performance of such silencers (insertion loss) is highly dependent on the considered frequency and is linked to the fluid velocity, to the acoustic thickness of the lining, its nature (in particular: the flow resistance of the filling and of its covers if any), to the free inner dimension for the passage of fluid, and also to the length of the silencer. Such silencers are characterized by a total pressure loss (also called pressure drop) being low because exclusively dependent on the roughness of the wall and on the length of the silencer (usually negligible if the flow velocity of the fluid is low enough).

Regarding splitter silencers (also called baffle silencers), they are soundproofing equipment belonging to the category of dissipative silencers for which the sound absorption material is held in frames (usually metallic: in non-stainless steel) forming splitters between which the fluid flows. Most of the time, such silencers have a rectangular cross section and are intended to be inserted into networks of ducts being themselves of rectangular cross-section (silencers with circular cross-section, with circular central splitter and, where appropriate with one or more ring-shaped intermediate splitters can be envisaged in some cases and are intended to be inserted into networks of ducts being themselves of circular cross-section). The acoustic performance of such silencers (insertion loss) is very dependent on the considered frequency and is related to the fluid velocity, to the thickness of the splitters (baffles), to their nature (including: fow resistance of the filling and of its cover if any), to their spacing, as well as to the length of the silencer. Such silencers generate a total pressure loss (also known as pressure drop) mainly related to the thickness of the splitters, to their spacing, and also to their geometry (profiling of upstream and downstream extremities if any) as well as to the length of the silencer.

In some cases, all the metallic parts of dissipative silencers for industrial applications at high temperature can be made of stainless steel (SS 304, SS 316, SS 321).

The prediction of acoustic and aerodynamic performance of dissipative silencers for industrial applications at high temperature can be performed with the simulation software SILDIS.

Which performance in terms of acoustics for silencers ?

La performance acoustique de silencieux (à température ambiante: ventilation, climatisation, process industriels variés ou en haute température: process industriels variés, cheminées, échappements...) est très dépendante de la fréquence et est liée principalement d'une part à la performance du revêtement absorbant compte tenu de l'écartement des voies d'air et de la longueur du silencieux (caractérisée par la perte par propagation) ainsi le cas échéant (pour des silencieux autres qu'à dissipation simple) qu'à la géométrie des séparateurs (caractérisée par la perte par réflexion) et d'autre part aux phénomènes de by-pass: transmission d'énergie acoustique au travers de l'enveloppe du silencieux ainsi le cas échéant (pour des silencieux autres qu'à dissipation simple) qu'au travers des séparateurs eux-mêmes, et enfin au bruit regénéré en relation avec la vitesse de passage du fluide.

Cette performance peut être exprimée en terme de différence (avec et sans le silencieux) de niveaux globaux de pression acoustique pondérés A ou de niveaux de pression acoustique par bandes d'octave à des emplacements spécifiés (tels que valeur maximale à 1 m de du plan de sortie du silencieux, valeur moyenne sur une surface enveloppante) - également appelée différence de niveau de pression acoustique d'insertion - ou bien en terme de différence de niveaux globaux de puissance acoustique pondérés A ou de niveaux de puissance acoustique par bandes d'octave du plan de sortie du silencieux (ou de la bouche, ou de la source sonore insonorisée) - également appelée perte d'insertion du silencieux - .

Les silencieux dissipatifs (ou encore: silencieux à dissipation) sont des dispositifs atténuant les sons à large bande.

A titre d'ordres de grandeur (et vis à vis d'un spectre de bruit de type "bruit rose"), une différence de niveau jusqu'à 10 dBA peut en général être obtenue sans exigence particulière, tandis qu'une différence de niveau de 10 à 20 dBA requière un silencieux type sans by-pass important, tandis qu'une différence de niveau de 20 à 30 dBA nécessite un silencieux type avec des dispositifs de cloisonnements transverses du revêtement absorbant et un montage élastique, et tandis qu'une différence de niveau de 30 à 40 dB met en jeu des silencieux à haute performance soigneusement conçus et montés (une différence de niveau encore supérieure doit faire intervenir une construction spéciale ou 2 silencieux installés en série avec un espacement suffisant).

La prévision des performances acoustiques de silencieux dissipatifs peut être effectuée avec le logiciel de simulation SILDIS.

Les mesurage sur silencieux peuvent être effectués (selon le cas) selon la norme NF EN ISO 7235 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 ou selon la norme INF EN SO 11820 Acoustique - mesurage sur silencieux in situ.


Acoustic performance of silencers (at room temperature: ventilation, air conditioning, various industrial processes or at high temperature: various industrial processes, stacks, exhausts ...) is very frequency dependent and is primarily related on the one hand to the performance of the absorbing filling given the spacing of the airways and the length of the silencer (characterized by the propagation loss) and also where appropriate (for silencers other than with simple dissipation) to the geometry of the splitters (characterized by the reflection loss) and also to bypass phenomena: acoustic energy transmission through the casing of the silencer as well as, if appropriate (for silencers other than with simple dissipation) through splitters themselves, and finally to noise regenerated in relation to the speed of passage of the fluid.

This performance can be expressed in terms of difference (with and without silencer) of overall A weighted sound pressure levels or of
sound pressure levels per octave bands at specified locations (such as maximum value at 1 m from the outlet plane of the silencer, average value of an enveloping surface) - also known as insertion difference of sound pressure level - or in terms of difference of overall A-weighted sound power levels or of sound power levels per octave bands of the outlet of the silencer (or of the mouth, or of the silenced noise source) - also known as insertion loss of the silencer -.

Dissipative silencers (or: dissipation silencers) are devices allowing an attenuation of sounds on a wide frequency range.

For orders of magnitude (and with respect to a noise spectrum like "pink noise"), a level difference of up to 10 dBA can usually be obtained without special requirements, while a level difference from 10 to 20 dBA requires a standard silencer without significant by-pass, while a level difference of 20 to 30 dBA requires a standard silencer with transverse partitionning devices of the absorbing filling and a resilient mounting, and while that a level difference of 30 to 40 dB involves high performance silencers carefully designed and installed (a higher level difference shall involve a special achievement or 2 silencers installed in series with sufficient spacing).

The prediction of acoustic performance of dissipative silencers can be performed with the simulation software SILDIS.

Measurements on silencers can be made (as applicable) according to NF EN SO 7235 Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss or according to standard NF EN ISO 11820 Acoustics - Measurements on silencers in situ

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