ITS participated in the limitation of industrial noise pollution by means of enclosures and noise barrier walls, for an electricity production site located in the Middle East, as in the context of many similar projects carried out elsewhere in the world.
Industrial noise pollution must be combated in many sectors, e.g. rubber, plastics, metallurgy, textiles & clothing, food processing, electronics, electrical equipment and machines, wood, paper, furniture, chemicals, pharmaceuticals, transport equipment, extraction, energy.
Depending on the context, its impacts well-being to a greater or lesser extent:
- personnel on the site in question, being likely to expose the workers to a daily sound exposure level greater than 85 dB(A), then in contravention of the regulations[1]
- residents (sometimes: even at long distance), which can cause the limits authorized by law in terms of noise to be exceeded[2]
When it is a question of limiting the noise at the source i.e. when one wishes to reduce the sound power level, which is an intrinsic characteristic (not dependent on the environment of the noise source), a sound enclosure (according to some language habits: cowling, canopy, hood, carterization - for a machine -) - when the dimensions of the equipment to be installed inside are large: resembling a building - constitutes a particularly efficient mean, if different conditions are fulfilled:
- implementation of an envelope based on airtight constructive systems (and therefore avoiding the propagation of sound):
- at the interfaces between constituent elements e.g. panels, doors, removable elements
- at wall junctions e.g. walls, roof
- use of sub-assemblies that sufficiently oppose the propagation of noise
- for common surfaces (soundproofing panels, soundproof doorsets, acoustic glazed frames): with a sufficiently high sound reduction index[3] (expressed in dB); a weighted sound reduction index Rw=35 dB as for the most standard elements marketed by ITS allows many applications in industry
- for silencers to be fitted in the openings intended for air renewal or for the evacuation of the heat power dissipated by the machines and enclosed components: with a sufficiently high insertion loss[3] (expressed in dB ; an insertion loss greater than 10 dB from 250 Hz as for the mots standard silencers marketed by ITS allows many applications in industry
- implementation, on the side where the noise source is located, of surfaces in sufficient quantity and quality to limit the amplification of sound levels inside the sound enclosure (otherwise the overall efficiency of the device will be reduced (all other things being equal):
- inner face of the sub-assemblies constituting the envelope or elements fixed to the walls or suspended from the roof: with a sufficiently high sound absorption coefficient[3],[4] ; a sound absorption coefficient of 1.00 in the 1/1 octave bands centered on 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz as for the most standard panels marketed by ITS allows many applications in industry
In the case of a sound enclosure that is homogeneous with regard to the intrinsic performance of its sub-assemblies (e.g. if the proportion of the surfaces of the elements with a lower airborne noise insulation capacity is sufficiently low), the sound levels (depending on the frequency ) measurable at a given distance are (at locations at which the effects of ground reflections are comparable) quite similar, and can be as low as 80 dB(A) at 1 meter (even lower with special constructions), even with a source sound power level of enclosed noise exceeds 130 dB(A) (as in the case of the gas turbine for the project basing this article), which illustrates a remarkable overall efficiency.
When it comes to limiting the propagation of noise in specified locations within an industrial site (e.g. workstations, circulations), at the property line or beyond in the environment (e.g. zones with a regulated emergence - ZRE -) - without reducing the sound power level - a noise barrier wall (according to some language habits: screen) is effective, if different conditions are met:
- construction meeting the requirements mentioned above for a sound enclosure, with the exception of what relates to the roof
- creation of a work with:
- an appropriate location (place, geometry and wall course) with regard to the positioning of noise sources (emission points) and of locations to be protected (reception points) to be considered
- sufficient dimensions (length and height)
- consideration (and, in case of need: a treatment aiming at make theim sufficiently absorbing) of hard surfaces that reflect sound (inside a building as well as outside)
Even in the case of a noise barrier being homogeneous with regard to the intrinsic performance of its sub-assemblies (as mentioned above for a sound enclosure), the sound levels (depending on the frequency) measurable in different locations are very variable:
- depending on the distance from each noise source[5]
- depending on the distance from the locations specified for the measurements[5]
- depending on the characteristics of sound propagation on the path of the sound waves (taking into account the possible presence of obstacles, atmospheric absorption, and - at long distances - the curvature of the sound rays)
- depending on the characteristics (surface, ability to reflect sound according to frequency) of the hard vertical surfaces at the noise emission point and at the reception point (e.g. equipment, building facades) without forgetting those located along sound waves path
In the case of noise barriers protecting traffic aisles from noise emissions from the exhaust duct for the project basing this article, the sound pressure level can be reduced to a value below the regulatory limit[1].
The acoustical effectiveness of the sound enclosures and noise barriers intended to limit industrial noise pollution win the context of this project was based on metal constructions, with products and construction systems marketed for a long time by ITS (soundproofing panels, acoustic doorsets, ventilation silencers with dissipative baffles).
The determination, on a case-by-case basis, depending on the input data and the objectives of each project, of the elements constituting sound enclosures and noise barriers, and of the overall performance that can be expected from their assembly in the locations specified in acoustic near field (i.e. near the noisy installation) or far field (at higher distances: within the limits of the considered industrial site or beyond) constitutes a key step in the study of the limitation of industrial noise pollution, for which ITS has both a qualified human resource, a considerable experience database, state-of-the-art computational tools as well extensive and diversified practice[6].
However, soundproofing is not the only expected functionality of sound enclosures and noise barriers such as those discussed in the project reported in this article.
The resistance to climatic conditions (snow, wind, rain) and earthquakes of these large constructions, their stability in the event of partial dismantling during maintenance or repair operations on the enclosed hardware, and their ventilation equipment for the management of thermal inputs (in the case of sound enclosures: with noisy fans that must also be soundproofed, in the case of noise barriers walls using the phenomenon of convection, with fresh air intakes equipped with silencers) constitute as much challenges for enclosures and noise barriers walls.
The realization of such a project in the context of the construction of a power plant in a place far from the pre-fabrication site of the enclosures and noise barriers implies, by adapting to local building codes and regulations (more or less specific), to overcome difficulties of all kinds (technical, logistical and organizational).
ITS and its business partners have a long experience in this field, as (moreover) for the implementation of enclosures and noise barriers in other industrial sectors (e.g. printing, corrugated cardboard, metal processing).
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[1] where European directive 2003/10/EC is applicable, 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
[2] where french law is applicable :
- at the limit of property, a maximum sound level is set by a prefectural decree e.g. 60 dB(A) in limit of property
- in Regulated Emergence Areas (REAs) difference (using A-weighting) between the noise level of ambient noise (facillity in operation) and the residual noise level (in the absence of noise produced by the facility), is legally limited: depending on the case 5 or 6 dB(A) during the day and 3 or 4 dB(A) during the night
[3] in the entire frequency band of interest, variable according to the context (there is a fairly broad consensus to consider that human auditory perception concerns the range 20 Hz -20 kHz, the decibel A being the single overall sound level unit taking into account such a physiology as a whole, the frequencies - rounded - 31 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1kHz, 2kHz, 4kHz, 8 kHz, 16 kHz being central for bands of 1 /1 octave to which the human ear has a different sensitivity basing A-weighting)
[4] (between 0 and 1 i.e. 100%) unless determined in a reverberation room in which case values greater than 1 can be measured
[5] with respect to the noise barrier wall i.e. in relation to the acoustic screen, in relation to each of its edges: vertical ends, as regards the length and horizontal end as regards the height (the difference between direct path and path taking into account the edges bases the efficiency which also depends on the dimensional characteristic of the noise source: point, linear, surface and its directivity)