The treatment of industrial noise by means of ventilated enclosures and acoustic screens is a necessity in different contexts e.g. manufacturing, construction, extraction of raw materials (mines and quarries), supply of water and energy, in particular electricity - not only Establishments Classified for Environmental Protection - (ECEP) -:
- to guard the environment of an industrial site against the nuisance caused to the neighborhood by excessive noise levels[1]
- to protect workers against risks due to noise[2]
ITS participated in the treatment of industrial noise by means of ventilated enclosures and acoustic screens built on an electricity production site in Southeast Asia. Several hardware have namely required high-performing treatment of noise:
- gas turbines (2 for the project under consideration), installed outdoors, for which the construction of ventilated enclosures was appropriate
- generators (2 for the project under consideration), installed outdoors, for which the construction of acoustic screens was appropriate
In terms of acoustics, the treatment of industrial noise by means of ventilated enclosures and acoustic screens requires the construction of structures with basically two complementary functions:
- sound reduction i.e. the ability to oppose the transmission of sounds from inside the space thus delimited to the outside ; a sound reduction index of around 40 dB at medium and high frequencies is often required for projects of the nature of the one described here
- sound absorption i.e. the ability not to reflect sound waves - on the side of the noise source - otherwise an increase in the sound level would occur: on the side of the noise source and therefore - all other things being equal - on the other side, which would be counterproductive in terms of soundproofing[3] ; a sound absorption coefficient close to 100% at medium and high frequencies is often achievable for projects of the nature of the one described here
For these 4 sources of noise mentioned above, the soundproofing devices were based on steel constructions, with a frame and filling elements limiting the propagation of noise, mainly:
- metallic, modular, removable acoustic insulation panels (a detailed presentation is available here)
- metal acoustic door-sets, with or without glazing (a detailed presentation is available here)
In the case of the 2 gas turbines, the evacuation of the heat power dissipated by the mechanical components and the other enclosed hardware has made it necessary to install high-performance ventilation systems comprising fans and ventilation silencers for the suction and discharge (a detailed presentation is available here).
The treatment of industrial noise by means of ventilated enclosures and acoustic screens must take into account various requirements:
- those related to noise limitation, possibly with objectives in the near acoustic field - e.g. a sound pressure level of less than 80 dB(A) at 1 meter, which is a considerable challenge for such high noise emissions -, and in the far acoustic field i.e. at specified acoustic locations within the perimeter from the industrial site or beyond:
- in the case of alternators, i.e. generators (usually: already equipped with a ventilated enclosure), for which noise treatment is based on the implementation of acoustic screens (anti-noise walls), a limitation of the propagation of noise by the interposition of an obstacle whose by-pass - by the sound waves - must be limited, not only due to the acoustic attenuation of the panels, but also due to the dimensions of the screen and the relative positioning of source of noise and locations to be protected from excessive sound levels
- in the case of combustion turbines, for which noise treatment is based on the construction of buildings (enclosed and covered envelopes), noise reduction “at the source” is at stake, i.e. the sound power level is reduced, which induces - all other things being equal - a reduction in sound pressure levels throughout its environment
- those related to the limitation of overheating, with a maximum temperature rise objective:
- in the case of alternators i.e. generators (usually: already equipped with a ventilated enclosure), for which the treatment of noise is based on the implementation of acoustic screens (noise barrier walls), ventilation by natural convection is often preferred (requiring then - generally in the lower part of the constructions - soundproof air inlets of appropriate section)
- in the case of combustion turbines, for which noise treatment is based on the construction of buildings (enclosed and covered structures), an additional constraint sometimes concerns the maximum electrical power consumed by the fans (which must also often meet the requirements of the regulations for EXplosive ATmospheres (ATEX), and also the specific soundproofing devices required by the sound emissions of such equipment
- those related to the operation and maintenance of soundproof equipment, for which an ergonomic design taking into account the needs in terms of access, dismantling, durability is required, as well as careful manufacturing
- those related to the site, in relation - when it comes to outdoor constructions – to the issues of resistance to earthquakes and bad weather (in this case: very strong winds and heavy rain; in other cases: extremely low temperatures, snow)
In addition, depending on the project, the scope of supply can be extended to connected equipment (e.g. electrical, detection, firefighting, air filtration).
For all this, the sizing and construction must adapt to local rules and codes - which vary from one project to another - and respect the quality standards of the most demanding customers.
Everything counts: (in addition to studies) the quality of raw materials, welds and assemblies, protection against corrosion, air and water tightness to obtain high-performance structures in terms of soundproofing and which resist the ravages of time.
The treatment of industrial noise by means of ventilated enclosures and acoustic screens constitutes a vast field of intervention for ITS (which has for this, like its commercial partners, a solid experience), whether - as in the this case - power generation facilities or projects in other sectors.
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[1] in France, a regulatory non-compliance can have different origins:
- exceeding of the admissible limit for emergence: this is the difference between the A-weighted equivalent continuous pressure levels of the ambient noise - considered industrial site considered in operation - and of the residual noise - in the absence of noise generated by the considered industrial site, but measured over its operating period) in zones with regulated emergence (ZRE); depending on the context: 5 or 6 dB(A) during the day and 3 or 4 dB(A) during the night
- presence of a tonal component: when, considering an unweighted third-octave spectrum due to the operation of the indtsrial site, the level in a frequency band exceeds by 5 dB or more the average level of the two adjacent frequency bands from 400 Hz to 6300 Hz or 10 dB that of the two adjacent frequency bands from 63 to 315 Hz
- exceeding the permissible level at the property boundary, set by prefectural decree - typically 70 dB(A) during the day, 60 dB(A) during the night –
[2] in France, the respect of the regulation is determined in accordance with European directive 2003/10/EC, for the daily noise exposure level L EX, 8hours.: 80 dB(A) and 85 dB(A) are the thresholds, respectively lower and upper, triggering the action; 87 dB(A) is the authorized limit value
[3] compared to propagation in a free acoustic field (i.e. in non-vacuum space: the sound propagation there is spherical), the presence of walls that perfectly reflect the sounds cause more or less significant increases in the sound pressure level:
- 3 dB for 1 reflective plane (e.g. 1 ordinary industrial floor)
- 6 dB for 1 reflecting plane (e.g. 1 ordinary industrial floor) and 1 non-absorbing wall, at the intersection of which a noise source would be positioned
- 9 dB for 1 reflecting plane (e.g. 1 ordinary industrial floor) and 2 non-absorbing walls, at the intersection of which a noise source would be positioned