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. If the operating principles (which can be combined) are mainly only two in number, the fields of application are numerous and varied.
The two main operating principles of silencers (dissipation or reflection - for reactive silencers -) refer to phenomena specifically linked to the modalities of propagation of sound waves in the ducts making aeraulic networks or pressurized fluid piping systems. The choice of one, the other or a combination of the two (in the same soundproofing device, or in the form of two distinct sub-assemblies - installed in series -) makes it possible to deal with the various problems justifying the use of such noise reduction equipment.
Principle of dissipative silencers
Dissipative silencers are those whose efficiency is based on the presence of an internal sound-absorbing lining, e.g. mineral wool (rock, basalt, glass) or polyester (more rarely: foam), maintained by a metal frame (the most often: made of steel), and which can - depending on the context - cover the (internal) periphery of a duct and/or constitute splitters (baffles) i.e. blocks between which the fluid circulates (sometimes: concentric - for a cylindrical silencer - or parallel - for a muffler with an often rectangular section, more rarely: circular - ; depending on the service conditions, a surfacing (glass clothl, fabric, metallic knit) and a perforated protection (in general: it is a steel sheet ) avoiding defibration due to erosion (related to the speed of circulation of the fluid, and sometimes to its composition). Generally effective over a wide frequency band - but with less performance at both low and high frequencies -, they are suitable for soundprrofing in many contexts:
- when the fluid is air under quasi-atmospheric conditions: the most common applications are those encountered in the building sector (aeraulic networks for air renewal and air conditioning of premises) and others, often more demanding, have to do with the industrial sector (air condensers - the number of fans to be treated can be counted by tens -, cooling towers - the diameter of the fans can exceed 10 meters, air intake of turbomachines - the inlet section of air can exceed 100 m2 -) without forgetting installations of all kinds for which fans are likely to create noise disturbance e.g. paint booths, tunnels
- for high-temperature fluids (combustion gas, steam): noise reduction devices for gas turbines and industrial chimneys for various processes, downstream stage of an exhaust silencer for a heat engine or gas decompression
Principle of reactive silencers
Reactive silencers are those whose efficiency is based on geometric variations of the internal parts e.g. modifications of section - including by means of chambers connected by tubes, presence of perforated elements through which the passage of the fluid is provided, changes in the direction of fluid flow. Generally used for low frequency noise attenuation, they are suitable for noise reduction in many contexts:
- when the fluid is air at quasi-atmospheric conditions: for compressors
- for high temperature fluids (combustion gas): upstream stage of exhaust silencer for diesel or gas engine
Domains of applications for silencers
Silencers being equipments allowing to control noise emissions in a gaseous medium, are appropriate (inter alia):
- for attenuation of the noise emissions by systems and for prevention of parasitic coupling produced by heating, ventilation and air conditioning - HVAC - 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 ...)
- 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 facilities)
- limitation of air intake noise and of exhaust noise emitted by internal combustion engines (the air intake noise is usually treated with dissipative silencers while the exhaust noise uses reactive mufflers). Generating sets for the propulsion of boats as well as for the supply of electricity in the event of a power outage (e.g. in hospitals, data centers) or in a context of continuous energy production e.g. cogeneration, methanization are concerned.
- reduction of air intake noise and of exhaust noise emitted by compressors and turbines equipped with fans
- sound level decrease of industrial process control valves
Intervention of ITS with regard to application relating to silencers
With regard to the domains of application of silencers in the context of a soundproofing project, ITS can (depending on the case):
- provide advice on the choice of a noise reduction device appropriate to the context, as part of an acoustic design office assignment
- size a dissipative or reactive silencer by defining its acoustic performance (i.e. dynamic insertion loss, self-noise) and aerodynamic performance (i.e. total pressure loss) with specific software
- market the corresponding soundproofing hardware and also associated equipment: air filters for tubomachines and engines, exhaust gas pollution control systems (catalytic reduction)
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