ITS contributed to the prevention of sound annoyance for neighbors by installing a noise barrier at a power plant in Western Europe. The objective was to limit the propagation of noise emitted by a very high-power industrial generator (coupled with a combined-cycle combustion turbine with a power output of approximately 700 MW) at locations situated not only on the edge of industrial property, but also at greater distances, which is a requirement for obtaining and maintaining an operating permit. Regarding French regulations, distinction is made 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):
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- 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
As for the noise barrier wall, it will be a metal structure, featuring a framework and:
- for fixed or easily removable parts, by elements: industrial soundproofing panels with an absorbing face (modular)
- for acces: (rotating) acoustic doorsets with, for translucent parts (with glazing guaranteeing residual safety in the event of accidental breakage): acoustic window sets
Naturally, given that this is a high-rise structure, the utmost attention has been paid to structural calculations to ensure it can withstand wind loads and seismic risks.
Other soundproofing solutions (ranging in scale from enclosures to full sound-insulated buildings: also marketed by ITS) create an airtight envelope and therefore require, to evacuate the heat dissipated by the noisy equipment housed within, a complex ventilation system, including air inlets and outlets, each fitted with a silencer to maintain acoustic performance, as well as fans (of which the positioning depends on the enclosure’s ventilation strategy whether operating under positive or negative pressure) which must themselves also be soundproofed, as they are inherently noisy (and, what is more, requiring electrical connections and monitoring).
This is not the case when a noise barrier is installed, such a structure allowing for natural ventilation i.e. the venting of hot air from the upper section, driven by the draught effect, as hot air has a lower density, provided that openings are included at the base to allow for the intake of fresh air, which must be equipped (they alone) with ventilation silencers whose insertion loss (an indicator of acoustic performance) may be significantly lower than that of enclosures or soundproofed buildings, given that the overall performance is markedly superior (soundproofed louvers which are more compact and less expensive than baffle-type silencers are often suitable in such a context).
For a noise barrier wall - as is the case with enclosures and soundproofed buildings - sound reduction aka attenuation (a barrier's ability to resist the transmission of sound through its constituent partition) and acoustic absorption (a barrier's ability to prevent the reflection of sound waves on the side facing the noise sources, thereby avoiding the amplification of noise) are important, but they are not the only criteria that must be considered.
Regarding acoustic performance, it should be borne in mind that it depends not only on frequency (as is the case with any soundproofing system) but also on geometric parameters related both to the noise sources under consideration and to the locations where a reduction in noise level is desired:
the respective distances from the sound screen that the noise barrier wall is
the respective heights
the respective distances from the vertical edges of the sound screen
Such effects can be accounted for using various levels of modeling sophistication, the most simplified formulation for evaluating the effectiveness of a noise wall, ΔL (in terms of sound wave diffraction), assuming a source small enough to be considered a point source, being probably as follows:
ΔL = 10 log10(3 + 20N) dB, where N is the Fresnel number; N = 2ΔL/λ
ΔL = the path length difference of the sound rays relative to the direct path (by analogy with optics)
λ = the wavelength under consideration; λ = c/f
c = the speed of sound in air (≈ 340 m/s for clean, dry air at a temperature of 15°C and a pressure of 1 atmosphere = 1.01325 bar = 101325 Pa)
f = frequency (Hz)
In practice, when it comes to preventing sound annoyance for neighbors, such formulations and models (even much more sophisticated ones) should be considered with great caution, even when the noise source to be mitigated is broken down into smaller components small enough to be treated as point sources (a challenging task, as it requires data that is often unavailable for what concerns the precise location of individual noise sources and their respective sound emissions in terms of sound power level and directivity).
The effectiveness of a noise barrier wall depends not only on the topography of the installation site (and its surroundings) but also - beyond the phenomenon of sound waves diffraction mentioned above - on potential sound waves reflections on hard surfaces (which can occur at various points along the acoustic path, not just at the source) as well as meteorological conditions.
Consequently, this device offers limited overall performance, particularly regarding industrial noise (effectiveness is often minimal at low frequencies), which restricts its use to specific scenarios, as in the context of the project discussed in this article, where it was employed to supplement the factory-standard soundproofing of an alternator by adding an acoustic enclosure: an noise barrier wall was used appropriately, here.
When it comes to preventing sound annoyance (noise disturbances) for the surrounding community, clients expect more from ITS and its commercial partners than just robust structures delivered on time and at an affordable price (though these qualities are indeed provided). They know they can rely on an expertise that enables the optimization of noise-reduction measures (along with the limtation of associated costs and other constraints) firmly rooted in decades of practical experience with studies and projects in the field of industrial soundproofing.
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When it comes to preventing sound annoyance, ITS clients know they can rely on expertise that enables the optimization of noise-reduction solutions firmly grounded in decades of practical experience with studies and projects in the field of industrial soundproofing |
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