Daniel P. Duffy 2016-06-06 17:43:19
Anyone whose ears have been on the receiving end of a loud, piercing noise, knows that sound pollution is a serious problem. All of us, at one time or another, have had our hearing so assaulted. Sound attenuation is a major concern. In the past few decades there have been major strides in the technology of noise abatement, but not every advance has to be a great leap forward. This is especially true in the case of replacement components that improve on existing systems. They are employed in response to a toughening of noise pollution regulations or changes in the facility’s local environment that make the upgrades necessary. It’s these incremental improvements and advances in acoustic customization and retrofitting that add up to big improvements in mitigating noise emissions. What are primary sources of this noise? For the distributed power industry, gensets, turbines, and engines are often as noisy as they are useful. Some of these systems can generate noise levels that are uncomfortable or even potentially damaging to hearing. The solution to this problem is the use of silencers and sound attenuation enclosures. And since ease of use can be as important as functional utility, these noise abatement systems are a valuable and necessary part of any distributed energy system. Noise is created by a generator because there is no such thing as 100% efficiency. To begin with, fuel is ignited in an internal combustion engine, converting chemical energy into mechanical energy. If this conversion were 100% efficient, all of the potential chemical energy in the fuel would be converted into the mechanical energy of driving the pistons. But it does not; due to the three laws of thermodynamics, some of that energy will always be lost to heat—even in the most efficient of engines. This heat translates into an expanding bubble of air, also known as “noise.” And that is just the initial phase of fuel consumption. Heat is created (and energy wasted) due to friction during the reciprocal motion of the pistons. In addition to heat, this friction creates vibration, and with it noise. In short, due to the laws of physics, there can be no such thing as a silent engine or turbine. So how much noise is too much? The operator’s choice of a generator will, in no small part, depend on its noise level. This is not just for personal comfort—maximum noise levels are often mandated by local codes and ordinances—violations of which can result in law suits, fines, and expensive legal fees). Even in remote areas, noise is a concern. National parks and wilderness areas have strict limits on noise generated during camping, ATV operation, recreational vehicle use, and other outdoor events. Measuring Noise Levels For noise to be measured, rated, and regulated, it needs to be quantified. This is done with the decibel rating. This is a logarithmic scale, not a linear rating. So for each change in 10 decibels (dB), noise is actually 10 times greater. Ten decibels is, therefore, 10 times more than 1 decibel, 20 is 100 times more than 1, and 30 is 1,000 times more than 1, etc. And, in keeping with its logarithmic nature, each addition of 3 decibels results in a doubling of the sound’s acoustic energy. So, the addition of two separate sources of noise operating at the same decibel level will result in a joint decibel reading 3 decibels higher than each individual noise source. For example, a machine that generates 50 decibels is 10 times louder than one operating at 40 decibels. Decibels measure sound, the manifestation of pressure waves vibrating and traveling through the material medium of air. (Though technically sound can also travel through water or solid objects.) Table 1 compares various decibel levels with resultant human pain levels and related sources of sound. Since the decibel scale is logarithmic, as a simple rule of thumb, the addition of 3 dBA means a measured doubling of the acoustical energy. (Therefore, 80 dBA + 80 dBA = 83 dBA.) However, an addition of 10 dBA represents a perceived doubling of the sound pressure. The decibel range of an operating generator increases with its power output. This is obvious since increases in power output require larger and more powerful engines. At the low end of the decibel scale are portable, small (with 1,000-W inverters) generators whose noise levels range from the quietest models operating at about 45 decibels (somewhere between a quiet room and a rainstorm), to 75 decibels (between city traffic and an operating vacuum cleaner), for a 10,000-W generator operating at 15 hp. To go from 45 to 65 decibels, a generator’s power needs to increase from about the minimum 1,000 to 6,000 W. The increase from 65 to 75 decibels results from an additional 4,000 W of power. Sound is one thing, but what people hear is noise. Noise is sound as perceived by the human ear. More specifically, noise is an unwanted and undesirable sound as perceived by the listener. It is the potentially unpleasant nature of sound that requires that adoption of laws and regulations to minimize noise as much as possible and to limit human exposure to noise pollution. As the human pain and discomfort column on Table 1 indicates, sound and noise levels can generate a great deal of energy. The energy of sound and its resulting pressure are measured on an “A-weighted” scale, which takes into account the elevation where the sound occurs (from sea level to the top of a mountain), and the density of the air. The energy of a sound wave is shown graphically by the height (amplitude) of the wave itself. For example, larger commercial generators can operate at 115 decibels (between a chain saw and pneumatic riveter) and produce 10,000,000 micro-Pascals of pressure, while the sound waves from the smaller portable generators described above operating at about 50 decibels generate only 8,000 micro-Pascals. The pitch of a sound is not the same thing as its energy. Energy on the decibel scale measures the overall loudness of sound. Pitch describes how high or low it sounds. Pitch is measured by frequency, or the number of times a sound wave occurs (as measured in cycles per second, or Hertz). As a result of the physical properties of air at sea level, sound travels through air at a fixed speed. This is equivalent to 1,126 feet per second, otherwise referred to as Mach 1. Given this fixed speed, a sound wave’s frequency is the inverse of its wavelength (the distance between the peaks or troughs of adjacent sequential waves). So a sound wave with a frequency of 100 hertz would have a wavelength of 112.6 feet (or, 1,126 ft. per second per 100 Hz = 112.6 ft.). The shorter the wavelength, the higher the frequency, and the lower the frequency, the longer the wavelengths. So, the decibel/energy and hertz/pitch values objectively define the physical characteristics of a sound wave. But, what about the subjective response of the human ear? The range of human hearing has been divided into octave bands, which are further subdivided into one-third octave bands. An octave is defined as the frequency interval between one musical pitch and another with half or double its frequency. In other words, a frequency has an octave width when the its upper band frequency is twice that of its lower band frequency. High pitch noise on the other hand (sound with very high frequency and very short wavelength of 0.5 ft.) is defined as being at least 2,000 Hz. At this frequency, a listener experiences pain and discomfort and can suffer permanent hearing loss from prolonged exposure. Low pitch noise (sound with very low frequency and very long wavelengths of up to 36 ft.) is defined as occurring between 30 and 250 Hz. The last physical factor affecting the impact of noise pollution is distance. Because of divergence, sound naturally attenuates over longer distances from the source of the noise. This is a physical expression of the inverse square law. The inverse square law states that the power intensity (of sound, light, or whatever) varies inversely according to the square of the distance from the source. Mathematically, this is expressed as A = 1/r^2. Assuming no reflective surfaces for sound waves to bounce off on and change direction, and a sound source that propagates in all directions, the sound’s energy (as measured in decibels) will decrease with the square of the relative distance from the source. So, a listener to a sound source twice as far away will experience a four times reduction in decibels, compared to the energy at the source itself. As measured on the decibel scale, this is a reduction of 6 decibels (remember a doubling or halving occurs every 3 dB). Or more simply put, a sound source 10 times the distance will result in a decibel level of 100th the energy at the source (10^2 = 100). Mechanical Means of Acoustical Attenuation What are the goals of sound reduction? As defined by most local codes and ordinances, the goal is to avoid noise disturbance in residential areas. In areas zoned for industrial activity, the goal is a more modest avoidance of potential hearing loss. Typically, local governments set noise limits according to location, hours of the day, zoning, permitting, and such. These limits are set at property boundaries so that no noise crosses these lines in exceedance of 60 decibels in residential areas, 55 decibels in noise-sensitive zones, and 80 decibels in commercial and industrial zones—or otherwise disturbs the peace and quiet of a neighbor 50 feet away. A noise-sensitive zone typically means any area designated by the planning commission for the purpose of ensuring exceptional quiet (parks, schools, and such). But sufficient distance is not always available for natural noise attenuation. This is especially true in the tight confines of urban and even suburban locations. And so, the manufacturers of generators augment their equipment with mechanical means of acoustical attenuation. Each method has an attenuation coefficient, which provides a rating for its sound reduction capability. Specifically, the attenuation coefficient measures the energy loss of the sound wave as it propagates through air or water. The attenuation coefficient is also measured in decibels. One method of achieving attenuation is the installation of an enclosure around the source of the sound with interior surfaces capable of absorbing and annulling noise. These surfaces absorb sound energy instead of reflecting it back, concerting the energy into vibration and waste heat. As there is no perfect sound reflecting surface, there is no perfect absorbing surface—some of the sound gets reflected back even with the best absorbers. How much energy gets absorbed and how much gets reflected is a function of several factors. The first factor is the incident angle that the wave impacts on the surface. The second factor is the roughness of the surface, with more porous and pitted surfaces trapping and absorbing more sound. The geometry and shape of the pitted surface can be designed to maximize absorption. The third factor is the softness of the surface. Dense and hard materials reflect sound, while low-density, soft materials absorb it. Manufacturers of gensets, engines, and turbines will tend to equip their equipment with sound reduction enclosures. But, often these enclosures are provided by third-party manufacturers who specialize in the design, manufacture, and installation of soundproof enclosures. Their capabilities may range from only weather protection to thorough sound elimination. Major Suppliers of Sound Attenuation Equipment Girtz Industries manufactures Z-GUARD, stationary equipment enclosures for specialized applications with unique post and panel designs for stationary sound attenuated enclosures. These structures include several advanced features (such as completely removable, fully welded roofing to prevent water intrusion), which make for strong enclosures resistant to extreme weather conditions and seismic activity. This inherent design strength is reinforced by the use of carbon steel, aluminum, and stainless materials. Exterior protection is provided by coatings up to marine-grade environment level C5; per ISO 12944. Their modular design with standardized dimensions allows for future add-ons with the placement of these units side by side, limiting the need for a large footprint. Sound attenuation levels can be customized for client needs. Harco Manufacturing exhaust silencers are built-in spark arrestors to accomplish both sound attenuation, and provide spark-arresting performance. Having patented the first low-profile or “Hockey Puck” style engine exhaust silencers, Harco provides a full line of residential-grade through super critical-grade engine exhaust silencers, spark arrestors, spark arresting silencers, and supporting products. A compact design makes it suitable for situations in which space constraints are a major concern. Harco was also the first manufacturer to develop a low-profile, catalyst exhaust silencer, the SFH-F series, with filters mounted inside the silencer itself. Maxim Silencers is a supplier of industrial-grade silencers for noise control in the oil and gas industry and the power generation market. Maxim manufactures the QAC line of catalytic silencers. It combines silencer technology with catalytic housing, providing both noise and emission controls. Their Maxim Quick Access is designed for natural gas internal combustion engine emissions control. Its unified construction saves space and ensures overall quality of construction. It is designed to accommodate either three-way, non-selective catalytic reduction (NSCR), which reduces NOx, CO, NMHC/VOCs, and formaldehyde on rich-burn engines, or a two-way oxidizing element, which reduces CO, NMHC/VOCs, and formaldehyde on lean burn/clean-burn engines. Silencer configurations come in either industrial or “hospital plus” grades. MIRATECH, a global company, is a leading provider of cost-effective, reliable, and mission-critical emission and acoustical solutions for natural gas and diesel reciprocating engines used in natural gas production, oil and gas drilling, power generation, rail, marine, and fluid pumping. Headquartered in Tulsa, MIRATECH manufactures from three locations in North America. Its approach is customer-centric, utilizing advanced engineering, industry knowledge, innovative design solutions, and superior product quality control. It produces a wide and varied product line of industrial silencers including emission-control silencers, exhaust silencers, compressor/blower silencers, vent silencers, and vacuum pump silencers. In addition to silencers, MIRATECH has a comprehensive customer offering that includes catalysts, housings, monitoring systems, and related services that address and reduce engine exhaust pollutants such as NOx, CO, VOC, diesel particulate, HAPs, and noise. MIRATECH specializes in the design, engineering, manufacturing, and delivery of total acoustic or emission turnkey solutions that fit unique customer needs. MIRATECH’s capabilities extend to project size, as well as product quality. Their in-house engineering and project management team are capable of providing highly customized solutions, even for large-scale projects. Their teams are practiced at the art of delivering total acoustic and emission turnkey solutions. In response to an ever-changing technological and more stringent regulatory environment, MIRATECH is always pushing for better solutions and products. In doing so, its engineers are adopting new techniques, shapes, and tools to improve noise mitigation products. Mehmood Ahmed, Director of Acoustical Engineering for MIRATECH explains one new approach: Three-dimensional [3D] acoustical modeling software that was used in the automotive industry is making its way into the industrial silencer market. Since the analysis previously needed comparatively large amounts of computing power, it was generally not practical to utilize it in relatively larger industrial silencer applications that required many points of analysis. This is changing, and currently it is relatively feasible to adopt 3D acoustical modeling for large silencers. As an early adopter of such technology, MIRATECH has been utilizing it to help the engineers think outside the box in applying new shapes and elements to improve acoustical performance. Even though this analysis is still resource-intensive, it is helping industrial silencer companies develop better products. Not only has it improved performance of their standard product range, but it also has led to introducing higher grade silencers that reduce noise to even lower levels required for many of today’s critical installations. Again, although the 3D acoustical modeling technique is resource-intensive, our engineering group has developed in-house noise modeling codes that allow them to quickly tune the silencer for low- and mid-frequency bands. Having the right tools at hand helps us efficiently select from a wide variety of acoustic elements for possible design consideration. MIRATECH is also a leader in the application of cutting-edge technology such as Computational Fluid Dynamics (CFD) analysis to establish pressure drop across the silencer. This differs from the traditional approach of relying on empirical formulas, which can be generic. This method allows design engineers to push the boundaries to improve flow distribution and reduce turbulence, thus reducing flow generated noise that adversely affects the overall performance of a silencer. Better flow distribution allows for smaller bodies or improves acoustical performance. As a result, several different shapes of silencers have been introduced over the last decade to improve space utilization, improve acoustics, and reduce cost. Two of the leading shapes are disk and oval silencers. New designs are also being introduced based on advanced data gathering techniques. Again, Ahmed notes: The advancement in handheld sound meters and application software allow users to capture and analyze a lot more information than just overall sound levels. In the recent years, it has become feasible to easily measure noise in narrow bands, which assists in identifying dominant frequency noise rather than looking at wide frequency band measurements. For example, this has allowed engineers to utilize specific design elements such as a tuned Helmholtz resonator to cancel noise associated with engine fundamental frequency. A good example of MIRATECH’s approach to the development of solutions for difficult acoustic challenges can be seen in a recent project for a prominent US-based Vacuum Truck Manufacturer. The specifications for this project were tight, with demanding standards for acoustic performance, space, weight, and cost. Previously, the typical way to improve performance for industrial silencers was to increase their size. However, tight space constraints and limited exterior dimensions prohibited this approach. Instead, 3D acoustical modeling was used to design and refine the internal elements of the silencers to achieve optimum results. Then, CFD analysis was used to streamline the flow characteristics inside the silencers to minimize turbulence or flow restrictions, which contribute to the silencer’s backpressure. This approach resulted in reduced noise levels as confirmed through acoustic testing and exceeded the customer’s needs. Robinson Custom Enclosures specializes in unique noise reduction applications. The company applies flexible in-house manufacturing expertise to provide customized solutions, whether they are high-structural-integrity, or semi-portable applications. The high-strength steel containers range 20–40 feet in length. They can provide weatherproof enclosures and containers packaged around a customer’s generator and accessories, in addition to sound attenuation. Universal AET manufactures both the SU Series and U5 Series of absorptive silencers. The SU series comes in three models (SU5, SU4, and SU3). Made with mild steel construction with a primer-coated exterior, all models are designed with an annular flow path with either full or partly blocked line of sight. The U5 Series Absorptive Silencer is designed as a highly efficient straight-through absorptive silencer. This makes it especially well-suited for inlet service on small rotary positive or centrifugal blowers, or the discharge of vacuum pumps. It also features mild steel construction with enamel paint exteriors. Universal’s absorptive silencers with discharge less than 15 psig are used for sound attenuation of inlet and discharge of high-speed, low-pressure centrifugal compressors and blowers, industrial fan inlets and discharges, high-pressure centrifugal compressors inlets, gas turbine inlets, dry vacuum pump discharge units, certain low-pressure vents, high-frequency noise sources, and the inlets of turbocharged reciprocating engines. Soundown has been in the business of manufacturing noise control materials for over 25 years. It has developed a portfolio of products for the effective treatment of packaged power units and sound attenuated enclosures. The company’s products include acoustic insulation and vibration-damping materials, as well as a range of machinery-isolation solutions. Their experience and diverse product line allows them to meet specification and/or regulatory compliance levels. J&A Enterprises, Sundown’s sister company, also provides noise and vibration engineering support for new projects or existing applications. Soundown offers a variety of acoustical foam products from the standard polyether to high-temp polyimide foams. These absorption materials are effective at reducing the amount of reverberant noise in an enclosure, which translates into lower overall sound pressure levels. The absorption treatments are particularly effective when installed in conjunction with baffles to stop noise exiting the sound shield through ventilation and other unsealed penetrations. Barrier composite insulation (composites consisting of foam decoupler and absorption layers sandwiched around a TuffMass—mass loaded vinyl—barrier) is also available from Soundown, and these floating acoustic membranes are available in variable thicknesses and weights in response to requirements for noise control, space, weight, and cost. In doing so, they can specify a composite that will most effectively treat airborne noise radiating from machinery such as engines, generators, pumps, etc. Standard thicknesses range from ½ to 3 inches, and can be finished with a range of facing options. PSA is also available, as are precut kits. Soundown’s TuffMass and damping materials have a very low-gauge thickness (0.062 to 0.625 in.), which makes them ideal for enclosures with low clearance between the machinery and shell. In addition to materials that muffle noise, isolation mounts are key components in reducing mechanical noise from most pieces of equipment. This vibration will transmit noise to booth the sound shield and the equipment’s structural foundation. Soundown’s engineers study the dynamic characteristics of machinery by using six degrees of freedom analysis to ensure smooth operation of the system. Soundown also applies advanced materials science to the problem of noise abatement. Their Sylomer, Sylodyn, and Sylomer HD are microcellular urethanes designed with various stiffness and damping properties that allow them to be easily customized for specific applications. Unlike traditional rubber products, Sylomer materials exhibit no surge frequencies, show less creep and stiffening over time, and require no profiling since their volume compressible Sylomer materials are commonly supplied as pads or strips that can be placed directly beneath machinery to provide vibration isolation. Machines with high dynamic forces and/or requiring lower natural frequencies of the isolation system are traditionally mounted on an additional base or foundation for increased mass and stability. Fully decoupling the foundation with Sylomer materials significantly improves the vibration isolation and reduces forces that can be transmitted to or from adjacent areas. BE Daniel P. Duffy, P.E., writes on the topics of energy and the environment.
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