In the field of industrial acoustics, reducing noise reverberation within a machine room is a common challenge, especially when the space has not been constructed using industrial soundproofing panels with an absorbing face, as the reflection of sound waves off hard surfaces leads to an amplification of sound levels that is highly detrimental:
- regarding the sound levels inside the premises to which personnel assigned to the monitoring and repair of machinery are exposed, who must be protected against the risk of hearing impairment or even total hearing loss. European Directive 2003/10/EC concerning the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise) is a reference document, fixing:
- limit exposure values for workers to noise: exposure level L EX,8h = 87 dB (A) and peak sound pressure pcrête = 200 Pa or 140 dB (C) ref. 20 μPa
- lower exposure values for workers to noise triggering action: exposure level L EX,8h = 80 dB (A) and peak sound pressure pcrête = 112 Pa or 135 dB (C) ref. 20 μPa
- higher exposure values for workers to noise triggering action: exposure level L EX,8h = 85 dB (A) and peak sound pressure pcrête = 140 Pa or 137 dB (C) ref. 20 μPa
- regarding noise levels outside the premises, which - all other things being equal, will be higher if the noise levels inside the premises are high - whereas it is necessary to protect the surrounding area from any nuisance. French regulations, if applied, distinguish between the daytime period (7:00 a.m. to 10:00 p.m. except Sundays and public holidays) and the nighttime period (10:00 p.m. to 7:00 a.m. as well as Sundays and public holidays):
- in some cases, a prefectural decree (when it is not ministerial) sets the noise levels not to be exceeded at the property boundary: this is particularly the case for Facilities Classified for the Protection of the Environment (FCPE) e.g. 70 dB(A) during the day and 60 dB(A) at night
- emergence i.e. the difference between the equivalent continuous A-weighted pressure levels of ambient noise (considered industrial facility in operation) and residual noise (in the absence of the considered industrial facility, but measured over the period of operation of the considered industrial facility) is limited in the Regulated Emergence Zones (REZ), which generally include, near the considered industrial facility: homes, building zones and areas occupied by third parties e.g. Public Access Buildings (PAB), other industrial establishments. If the ambient noise level in the REZ (including the considered industrial facility noise) is between 35 dB(A) and 45 dB(A) then the admissible emergence is 6 dB(A) during the day and 4 dB(A) at night. If the ambient noise level in the REZ (including considered industrial facility) is greater than 45 dB(A) then the admissible emergence is 5 dB(A) during the day and 3 dB(A) during the night.
- in the case of a Facility Classified for the Protection of the Environment (FCPE), the marked tone, the presence of which is linked to the difference between the sound pressure level in a given 1/3 octave frequency band and the adjacent frequency bands, is considered undesirable if it exceeds 10 dB in the frequency range 50 Hz-315 Hz or 5 dB in the frequency range 400 Hz-8000 Hz
In the absence of sound-absorbing materials, this amplification of sound levels within a space can reach (provided that all surfaces are made of concrete and the ceiling or roof height is significant) 5 to 10 dB (this value varies depending on to frequency, like the sound-absorption capacity of acoustic materials, as well as on the distance from the noise sources to be accounted) because, at any given location, the sound pressure level:
- is not equal to the level that would be measured in a "free field" i.e. outdoors, with no obstacles to sound propagation (referred to as a direct sound field)
- is higher than this lower limit due to the contribution of the "reverberant sound field," which arises from the presence of physical boundaries such as walls and a roof (in addition to the floor) that reflect sound waves, causing the associated acoustic energy to be added to that of the direct sound field
Thus, reducing noise reverberation in a machine room involves limiting the impact of sound wave reflections off the walls, with the goal of minimizing the overall sound level. In France, from a regulatory standpoint, this is required under the Order of August 30, 1990 -issued pursuant to Article R. 235-11 of the Labor Code - which concerns the acoustic treatment of workplaces, applying:
- to premises where machinery or equipment is to be installed that could expose workers to a daily noise exposure level exceeding 85 dB(A)
- whenever it is determined, through predictive acoustic modeling, that reverberation would cause an increase in a worker’s daily noise exposure level of 3 dB(A) or more
ITS will contribute to reducing noise reverberation in a machine room at an industrial site in Western Europe, which houses 6 high-capacity compressors, to the opration of which is associated a sound pressure level of 100 dB(A) - each - at a distance of 1 meter in a free acoustic field (a level that is further increased by the presence of multiple units - though it is physically impossible for any single point to be 1 meter away from all of them simultaneously, given their very large dimensions -).
Given that this is a structure made of cast-in-place concrete (which has a sound absorption coefficient of only about 1%) and is, moreover, of large dimensions (a floor area exceeding 1,000 m² and a height greater than 11 m), the conditions are such that - in the absence of sound-absorbing materials - there will be a significant amplification of sound levels, estimated to be in the range of 5 to 8 dB(A).
While applying the criterion based on exposure levels (and their increase due to reverberation) is difficult in such cases - given that it depends on the exposure duration for personnel who do not work in fixed shifts - there are other compelling reasons to reduce noise reverberation within the machinery room and keep internal noise levels sufficiently low:
- to enable workers exposed to noise to communicate with one another (as much as possible) and to hear warning signals
- to keep external noise levels sufficiently low. As mentioned above, any increase in the sound level inside such a machinery room - all other factors being equal - results in an increase in the sound level outside. This creates challenges, particularly regarding the sizing of silencers for the room’s ventilation openings (required to dissipate the significant heat generated by compressors operating simultaneously) and the need to prevent noise disturbance to the surrounding environment
For this reason, the walls and ceiling will be fitted with sound-absorbing panels (offering an acoustic absorption coefficient close to 100% at high frequencies) of the type detailed here.
Installed using a metal framework, these panels are suitable for reducing noise reverberation in machine halls and technical areasin many contexts, involving (when it does not come to compressors), other noisy equipment such as internal combustion engines (e.g. generator sets), electric motors, or pumps.
On-site measurements (reverberation time, sound decay), study (including predictive acoutics computtaions), marketing of solutions: one can sollicit ITS for any project aiming at reducing noise reverberation within an industrial facility's machine room.
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On-site measurements, study, marketing of solutions: one can sollicit ITS for any project aiming at reducing noise reverberation within an industrial facility's machine room. |
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