  Noise management is another evolving concern in plant design and siting. With recent advances and accumulated experience in acoustic science and engineering, noise abatement capabilities have been significantly improved. An analysis of Northland Power's 110 MW Iroquois Falls cogeneration plant in Ontario provides an excellent case study. 
The plant sits only 750 feet east of the nearest residential district. An adjacent embankment forms a natural noise barrier between the plant and the nearby residences.
The Iroquois Falls plant provides up to 300,000 lb/hr steam to the Abitibi-Consolidated paper mill and generates electricity for sale to Ontario Hydro. The plant consists of two General Electric LM6000 gas turbines, two heat recovery steam generators (HRSGs) and a single steam turbine. Variations in steam demand are met using the auto-extraction steam turbine and HRSG duct burners. Full steam condensing capabilities allow the plant to meet monthly electrical output targets.
Noise levels were critical in the design of Iroquois Falls because the power plant is located only 2,500 feet east of the paper mill and only 750 feet from the nearest house. Northland Power considered noise impacts from the very beginning. "We'd had some problems at another plant near a residential area where we had to go back after construction to implement noise control measures," said Dino Gliosca, Northland Power project engineer. "That cost us a lot of money and time, so we decided that the best approach for future projects would be to include noise management concerns from the outset." Through its engineering contractor, Northland Power worked with ATCO Noise Management to write acoustical specifications into the project bid documents. During plant design, Northland Power and ATCO had to consider noise generated from seven major sources: two gas turbines, two combustion turbine air inlet filters, two HRSGs and one steam turbine-generator with associated steam vent noise. Calculations based on data supplied by the equipment manufacturers revealed that the total sound power level (PWL) of the equipment inside the plant would be 119.7 dBA (Table 1). If this acoustic energy were allowed to freely propagate, the noise level at the nearest residence would have been 63.3 dBA. The guidelines in effect at the time of plant design required that the power plant not exceed the lowest measured ambient sound level at the nearest residence, which was 40 dBA. Northland Power relied on both design elements and natural topography to meet the 40 dBA guideline. Although the total acoustic energy was 119.7 dBA, the critical figure for the acoustic design was the fact that the equipment generated a sound pressure level (SPL) of 95.5 dBA at the inside walls. [The sound power level (PWL) is a function of the sound pressure level (SPL) and the physical environment. For example, a radio and orchestra might produce the same SPL at a certain distance, but the orchestra emits substantially higher amounts of acoustic energy (PWL) with a correspondingly greater impact on the environment.] To achieve the exterior noise requirements, transmission losses must be considered, which represent the difference between the noise level measured on the source side of a noise barrier and the level measured on the receiver side. The American Society for Testing Materials has developed the Standard Transmission Classification System (STC), which enables comparisons of various types of acoustical structures according to their transmission loss properties. The higher the STC, the better a wall or roof insulates against noise. The STC standard applies to frequencies from 125 to 4,000 Hz. Noise emitted from industrial machinery, however, often has a significant component at frequencies below this range, requiring multiple-layer construction for effective sound attenuation. The STC system does not sufficiently consider the importance of low frequency attenuation, with the result that barriers and buildings that appear to have adequate STC ratings often do not achieve the desired outcome. To obtain good acoustic performance (STC greater than 35), walls must have a relatively high transmission loss and high absorption on their internal surfaces. As a general rule, the heavier and thicker the wall, the greater the attenuation of the sound or higher the TL. This is because it is difficult for sound waves in air to move or excite a dense, heavy wall. Sound transmission through walls, floors or ceilings varies with sound frequency, and the weight and stiffness of the construction. This gives rise to the effect known as the "mass law" in acoustics, which states that for each doubling of the surface weight of the wall, there will be about 5 or 6 dBA less transmitted sound. The mass law also states that for each doubling of the frequency (Hz) there will be about 5 or 6 dBA less transmitted sound. 
A typical acoustic wall structure (Figure 1) over a steel frame starts with a perforated wall liner, usually made of metal. The correct size and spacing of holes in the liner are important because they act as resonating sound absorbers. When sound impinges on the holes, some of the sound is absorbed into the cavities and the rest is reradiated. Because the sound energy is bounced back toward the source in semi-circular waves, sound is actually diffused and noise levels are reduced. Resonance is particularly effective at absorbing low frequency noise.
Multiple acoustic layers are used if the wall must achieve very high acoustic performance. Septum layers, which are dense structures with high transmission loss characteristics, are typically placed between the acoustic layers to achieve such performance. The outermost layer of the wall structure is a protective, leak-proof facing such as metal cladding or brick. The perforated liner, acoustic material and septum layers are effective at attenuating air-borne noise. However, structure-borne noise, which refers to mechanical vibrations carried from machinery to a building's structure, must be considered in the wall design as well. For example, an engine bolted onto a metal skid that is bolted to the floor transmits huge amounts of acoustical energy to the structure. Vibrations from rattling machinery travel easily through solid structures like wood, steel, concrete or masonry. Elastic vibration isolation elements are used to prevent the vibration from reaching the structure, thereby reducing structure-borne noise transmission. 
Topography and location played a large role in Northland Power's plant design. Because the plant was located in a low-lying area along a river bank at an elevation below the town of Iroquois Falls, the adjacent hill formed a natural noise barrier. Particular attention, however, had to be devoted to the north and west sides of the plant, which faced the paper mill and closest residences. The south and east sides of the plant, on the other hand, faced only river and forest, so noise radiating in that direction was not critical to the plant's acoustic target.
ATCO designed a complete noise control system, consisting of an acoustic building envelope, acoustically treated ventilation system and acoustic doors (Figure 2). The project team designed the building and ventilation system to achieve a noise target of 35 dBA at the nearest residence. The same target was established for the combined air inlet filters and enclosure ventilation exhaust openings. The sum of the two identical noise targets of 35 dBA was 38 dBA, allowing for the remaining noise contribution to come from the HRSG exhausts. To achieve the 38 dBA target at the nearest receptor, the plant incorporated various design elements. For the acoustic building envelope, ATCO used three different acoustic wall and roof assemblies. The north wall, which directly faces the residences, used a higher-level acoustic wall to provide an STC of 57. The roof and west walls, which also needed relatively high attenuation, provided an STC of 44. The east and south walls, facing away from the residences, provided an STC of only 35. The acoustic wall and roof assemblies consist of sound-absorbing and reflective materials, including a perforated liner, insulation layers, mass layers, decoupling materials and exterior cladding. These assemblies are not panels, but seamless whole walls or roofs that were built in situ from outside the building structure. The acoustic ventilation system was designed to meet the requirement of 20 air changes per hour and a static pressure of 0.50 inches water gauge. To minimize the amount of noise escaping through ventilation openings, the openings had to be placed where they would have minimum impact on residences. Instead of locating the ventilation exhaust fans on the roof, where they normally would be situated and where the noise would carry directly to the residences, they were placed on the south wall facing away from the homes. The air intakes were located on the west wall facing the residences, but only 4 feet above the ground. Because the plant level is 82 feet below the level of the houses, the hill acts as a natural barrier, reducing noise while ensuring optimum cross-ventilation. All ventilation openings-36 in total-were fitted with custom-designed silencers. ATCO also targeted noise escaping into the environment through engine intake and exhaust ducts, cracks under doors, at panel joints, pipe penetrations and other openings. For example, the plant design minimized noise leakage from equipment doors by using manually operated acoustic double-leaf doors with special compressible seals. 
Interior equipment was specified to meet an 85 dBA noise level requirement at 3 feet. The owner ordered enclosures for the GE LM6000s and opted for an upgrade to the enclosures' ventilation exhaust silencers. The upgrade was required because the turbine packager called for placement of the exhaust silencers on the plant roof, where their noise would affect nearby homes. By upgrading the silencers, money was saved since the owner didn't need to install an acoustic barrier on the roof to control the noise from the standard package's exhaust silencers. Northland controlled noise from the HRSGs by specifying thermal insulation from the supplier and by making effective use of site layout. The high level of attenuation of the west and north walls enabled the owner to move the HRSGs from the east side of the facility, behind the shelter of the old river bank, to the northeast corner, where they were directly in-line with residences. Furthermore, given that the gas turbines are axially aligned with the HRSGs, the gas turbines and associated intake structures are effectively shielded by the HRSG building and can be placed farthest from the residential neighborhood. 
Noise control project costs will vary with a number of factors, including plant layout, plant siting and choice of equipment. The billed amount, however, is not necessarily indicative of actual acoustical costs; for example, plant walls and roof would have been erected anyway. At Iroquois Falls, it is estimated that the acoustic portion of the design represented less than 1 percent of the total plant cost of approximately $100 million. Furthermore, adding the acoustic elements did not add any time to the design or construction schedule, according to Northland's Gliosca.
"In terms of noise performance, Iroquois Falls has had smooth sailing since going operational," said Gliosca. "As a passive system, there are few operational or maintenance concerns, and you'd be hard-pressed to know the plant was running, even when standing within 10 feet of the facility." Table 1: Unattenuated Noise Levels
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