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Technology for a Quieter America 5 Technology Although noise can be generated by turbulence in highspeed flows, most noise is generated by mechanical motion caused by forces acting on structures. The motion can be very complex—for example, in the case of a panel on a machine. Consequently, the coupling between the moving structure and air, required to generate noise, depends on details of the motion as well as frequency. Generally, low-frequency vibration is less efficient in generating sound than high-frequency vibration. Sound reaches the ear by propagation from the source to the receiver and can be complicated by reflections from nearby surfaces as well as atmospheric conditions outdoors. Some motion is unavoidable; for example, fan blades must move and vehicle tires must rotate. In many cases the function of the machine is unrelated to the noise generated. An example is the mechanical suspensions that attach airplane engines to an airplane but also allow the transmission of vibrations to the fuselage. This transmission into the fuselage and subsequent radiation of sound can be (and is) minimized by good design—which can also save money by reducing wear and fatigue. This chapter is concerned with new technologies in materials and systems to reduce noise, the modeling and analytical tools used to design products for reduced noise, and experimental methods of gathering and interpreting data to test and determine how much noise is generated by different product designs. It will be immediately obvious that there are enormous disparities among programs, facilities, and resources for addressing noises of different types. For example, although engineering tools may be available for reducing aircraft noise and highway noise, the former has been deemed a national priority, while the latter has received less attention. Resources allocated for noise reduction are not always commensurate with noise exposures and impacts. Many tools for designing and developing quieter products have become available in the past few decades, driven largely by increases in computational power and reductions in computational costs. Even so, access to new tools is as uneven as the allocation of resources; corporate budgets for capital equipment are generally tight and there is competition between departments for available funds. Furthermore, organizations that are doing only routine testing of products according to national and international standards find expensive new tools hard to justify. Thus, even though noise mechanisms in aircraft, automobiles, rapid transit and trains, consumer products, and industrial machinery are fundamentally similar, the availability and application of tools for addressing them are not. The question is whether ways can be found to give industry and academia access to these tools for the benefit of manufacturers, workers, and the public. AEROSPACE AND AEROACOUSTICS SOURCES OF AIRCRAFT NOISE Noise from aircraft includes both noise from airplanes and noise from helicopters. At commercial airports, airplanes are the major noise source and will be emphasized here because of the widespread annoyance issues that have affected the quality of life for many persons. Noise from helicopters is also an important issue and affects people living near heliports and in densely populated areas where helicopter flights are not uncommon. The Federal Aviation Administration was asked to prepare a report to Congress on nonmilitary helicopter noise (FAA, 2004). One important issue relates to noise metrics; the impulsive character of the noise requires that metrics in addition to the widely used day-night average sound level (DNL) be used to assess its effects on people. The noise heard when an airplane flies overhead comes from many sources, but the main contributors are engine noise and airframe noise. Engine noise comes from the fan/propeller, compressor, turbine, combustor, and jet exhaust. Airframe noise is produced mostly by airflows around lifting and control surfaces, such as flaps and slats, and around landing gears.
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Technology for a Quieter America The relative contribution of these sources depends on the engine and airframe designs and the operating conditions. For example, during takeoff, when the engines are at full thrust, jet noise is the largest contributor to the noise signature of an aircraft. At approach, when the engine is throttled back, noise comes more from the airframe. Other sources, such as the fan, are significant contributors during both takeoff and landing. Typical noise sources for a fixed-wing aircraft are shown in Figure 5-1. The noise received by an observer depends on the sources and propagation effects. The noise sources for a propeller-driven aircraft are shown in Figure 5-2. RESEARCH IN AEROACOUSTICS The aeroacoustics community has made significant progress over the years in understanding and reducing aircraft noise. Figure 5-3 shows comparative contributions from different noise sources for 1960s and 1990s engines. The figure, which originally appeared in Rolls-Royce (2005a), shows that the development of the turbofan engine and reduction in noise from individual engine components resulted in smaller, more evenly matched noise contributions from engine sources (SBAC, 2009). Over a period of 30 years, these improvements, coupled with advances in aircraft aerodynamics and weight technologies, have reduced aircraft noise by about 20 dB, which corresponds to a reduction in noise annoyance of about 75 percent (EU, 2007). The new Airbus A380, the largest commercial aircraft ever produced (average of 525 passengers), has takeoff and approach noise levels comparable to those of heavy road traffic, a lower noise level than in an underground train. The noise footprint of the A380 is about half that of older, large commercial aircraft (Rolls-Royce, 2005b). Despite these impressive results, airport community noise continues to be a significant environmental problem, and research and development (R&D) continue in the United States and Europe to meet increasingly stringent noise requirements set by regulatory bodies, such as the Federal Aviation Administration (FAA), the International Civil Aviation Organization (ICAO), and individual airports (Rolls-Royce, 2005b). Over the years, the FAA and ICAO have required comparable reductions. INFRASTRUCTURE AND PROGRAMS THAT SUPPORT RESEARCH AND APPLICATIONS A Note on Test Facilities Both the United States and Europe have first-class aero-acoustics test facilities. Anechoic flight simulation facilities, the most useful for testing both jet noise and airframe noise, are available on both sides of the Atlantic on a rental basis. In the United States, high-quality anechoic chambers for model-scale testing are available at the National Aeronautics and Space Administration (NASA) Langley and Glenn Research Centers, as well as at Boeing, General Electric, United Technologies, and some U.S. universities, such as Georgia Institute of Technology, which inherited Lockheed Georgia’s aeroacoustics facilities. Rolls Royce in England has used the NASA Glenn jet noise acoustic chambers, and Boeing researchers have used facilities in England. NASA FIGURE 5-1 Breakdown of typical noise sources for fixed-wing aircraft. Source: Posey (2008).
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Technology for a Quieter America FIGURE 5-2 Breakdown of typical noise sources for a rotorcraft configuration. Source: Burley (2008). Langley researchers have also used the Dutch anechoic wind tunnel to make helicopter noise measurements. Both the United States and Europe also have access to state-of-the-art flow measurement equipment (including particle imaging velocimetry) and modern phased microphone array systems. Most of these facilities have been described in great detail by Ahuja (1995). U.S. NOISE REDUCTION PROGRAMS The FAA and NASA have primary responsibility in the United States for R&D on aviation noise. The FAA focuses on the impacts of noise on communities, while NASA focuses on noise at its source—namely, aircraft engines and airframes. A recent congressionally requested report on aviation noise addresses (1) how well the FAA and NASA’s R&D plans are aligned and (2) the likelihood that noise reduction goals will be met (FAA, 2008). The FAA and NASA’s R&D plans, aligned through partnerships and planning and coordinating mechanisms, include a wide range of projects for addressing aviation noise. The FAA sponsors aviation noise R&D in noise measurement, noise effects, interrelationships between noise and pollutant emissions, and flight procedures and technologies to mitigate FIGURE 5-3 Noise sources for 1960s and 1990s jet engines. Source: Rolls-Royce (2005a).
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Technology for a Quieter America the impact of noise on communities. Much of this R&D is funded through partnerships with universities, other federal agencies (including NASA), and industry. NASA’s R&D can eventually lead to new technologies for substantially quieter aircraft. However, industry will have to integrate the research results into production-ready aircraft designs. The FAA and NASA have worked with interagency planning and coordinating groups to establish objectives for the nation’s aeronautical R&D and for specific research on the environmental impacts of next-generation aviation technologies. The strategic plans for the National Airspace System indicate how each agency’s R&D will contribute to meeting noise reduction goals, which are designed to reduce public exposure to aviation noise primarily by reducing noise at its source (GAO, 2008). In 1994, NASA initiated a seven-year program, the Advanced Subsonic Transport (AST) Noise Reduction Program, to develop technology to reduce jet transport noise by 10 dB relative to 1992 levels. Most of the goals of AST were met by 2001. However, because of an anticipated annual increase of 3 to 8 percent in passenger and cargo operations well into the twenty-first century and the slow introduction of new noise reduction technology into the fleet, the global impact of world aircraft noise is expected to remain essentially constant until 2020 (or perhaps 2030) and thereafter begin to increase. Therefore, NASA has begun planning with FAA, industry, universities, and environmental interest groups in the United States for a new noise reduction initiative. One of the most important noise reduction technology programs in the United States is the so-called Quiet Technology Demonstrator (QTD1) Program, a partnership among Boeing, Rolls Royce, and American Airlines initiated in 2000 (Bartlett et al., 2004). A second phase, QTD2, a partnership among NASA, General Electric, Goodrich, and ANA, was begun in 2005. These programs have validated new, advanced noise reduction technologies, including nacelle inlet acoustical treatments and chevrons on engine exhaust ducts. FIGURE 5-4 QTD2 noise reduction technologies. Source: Herkes (2006). Copyright Boeing. All rights reserved. After rigorous testing, including measurements taken on the ground, in the passenger cabin, and on the airframe (Herkes, 2006), many noise reduction technologies, including nozzle chevrons, spliceless inlet linings, extended lining locations, and redesigned wing anti-icing systems (see Figure 5-4), as well as smooth fairings to reduce landing gear noise (see Figure 5-5), have been incorporated into existing airplanes and designs for future Boeing airplanes. Thus, Boeing’s newer airplanes are significantly quieter for both passengers and airport communities (Herkes et al., 2006). A third phase, QTD3, is in the planning stages at Boeing. Over the years the FAA has defined requirements for the reduction of aircraft noise emissions in terms of stages (1–4). The metric for describing the noise emissions is the effective perceived noise level in decibels (EPNdB), and well-defined microphone positions are used for the measurement. Note that this is quite different from the immission metric (DNL) used to describe the effects of aircraft noise on communities. The goal of NASA’s current Subsonic Fixed Wing Project is to reduce aircraft noise by 42 EPNdB cumulative below Stage 3 for conventional, small, tube-with-wing twin-jet aircraft, what NASA calls “N + 1 generation” aircraft, by 2012 to 2015 (Collier and Huff, 2007). An even more ambitious goal, set for the 2018 to 2020 period, is to reduce aircraft noise by 52 EPNdB cumulative below Stage 3 for N + 2 generation aircraft, which NASA envisions as an unconventional hybrid wing-body aircraft (see Figure 5-6). In addition to reducing noise, NASA expects dramatic improvements in the emission and performance of these aircraft. EUROPEAN NOISE REDUCTION PROGRAMS Driven by increasingly stringent noise requirements and strong competition from the United States, Europe has set FIGURE 5-5 Toboggan landing gear fairings for reducing landing gear noise tested in QTD2. Source: Herkes (2006). Copyright Boeing. All rights reserved.
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Technology for a Quieter America FIGURE 5-6 Goals of the N+1 and N+2 generation aircraft. Source: Collier and Huff (2007). very ambitious goals for reducing aircraft noise by 2020 (Collier and Huff, 2007). For example, as shown in Figure 5-7, the Advisory Council for Aeronautical Research in Europe (ACARE) has set a goal of a 50 percent reduction in noise annoyance (relative to their 2000 counterparts) for aircraft entering into service in 2020. This is equivalent to a 10-EPNdB reduction in the day-evening-night averaged sound level from fixed-wing airplanes. Along with the noise reduction, there must be a 50 percent reduction in specific fuel consumption (again relative to engines introduced into service in 2000). FIGURE 5-7 Noise reduction objectives and technology plans set by ACARE. Source: EU (2007).
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Technology for a Quieter America Over the years a number of ambitious programs for reducing aircraft noise and, more recently, reducing aircraft emissions have been launched in Europe. Significant investments have been made under the so-called Framework Programs in which European Union (EU) industry and researchers in many countries work together to perform well-funded, coordinated R&D. A total of 20 aircraft noise R&D initiatives were launched in Europe between 1998 and 2006 with considerable participation by industry, small and medium enterprises (SMEs), research establishments, and government agencies (see Figure 5-8). Four of the most noteworthy programs are (1) the Silence(R) Program, (2) the Silent Aircraft Initiative (SAX-40), (3) the EnVIronmenTALly (VITAL) Friendly Aero Engine, and (4) the EU Clean Sky Initiative. Each program is described in detail in a Society of British Aerospace Companies (SBAC) Aviation and Environment Briefing Paper (SBAC, 2009).1 The summaries that follow are based on the SBAC briefing paper, a discussion at a Council of Academies of Engineering and Technical Sciences workshop in June 2008 (CAETS, 2008), and a presentation at the Workshop on Technologies for a Quieter America (Ahuja, 2008).2 The SILENCE(R) Program SILENCE(R), the largest European aircraft noise research project ever undertaken, was a six-year program that began in 2001. Coordinated by Snecma, a French company, the €112 million program was a collaboration of 51 partners, including all major European airframe and engine manufacturers, major research institutes, and universities. The program addressed both engine noise (including jet noise, fan noise, compressor noise) (see Figure 5-9) and landing gear noise and airframe noise (see Figure 5-10). Technologies for reducing jet noise included the ultra high bypass ratio fan; low-noise core and fan nozzles designed to improve the mixing of exhaust and bypass flows; internal and external exhaust plugs; and technologies to attenuate fan noise, including a zero-splice passive liner, active noise control technologies, and a negatively scarfed intake design to reflect fan noise away from the ground (see Figure 5-11). In flight tests the negatively scarfed fan was shown to reduce perceived noise by about 2.5 dB for an observer at a 60 degree angle to the engine (Rolls-Royce, 2005a). Acoustical liners have traditionally been constructed from two or three pieces to facilitate manufacturing and maintenance. However, a continuous, zero-splice design greatly improved the absorption of fan noise, and the new technology is now being used in Rolls-Royce’s Trent 900 engine on the Airbus A380. The change has resulted in a 4- to 7-dB reduction in fan-tone noise on takeoff and a 2-dB reduction in fan noise on approach (Coppinger, 2007). (A similar device was demonstrated by QTD2 in the United States.) An active noise control system was also successfully demonstrated (SBAC, 2009). The system consisted of microphones mounted in the fan duct and actuators mounted on the stator vanes. The microphones measured fan noise and sent signals to the actuators, which generated “antinoise” (sound waves that were out of phase with the sound waves generated by the fan), canceling out the fan noise. To reduce landing gear noise, some new low-noise designs for the nose and main landing gears were investigated. Ultimately, the noise was reduced by shielding the gears from each other and aligning them in the direction of the flow. Two aligned nose landing gears were demonstrated to be as much as 3 dB quieter than two independent gears (Coppinger, 2007). Some of the noise technologies validated in SILENCE(R) are now in production engines. Others are either undergoing further work in R&D programs by individual manufacturers or have been carried over to other projects (e.g., VITAL, described below). Silent Aircraft Initiative (SAX-40) The Silent Aircraft Initiative (2006) (SAX-40) was a £2.3 million three-year research project run by Cambridge University and the Massachusetts Institute of Technology—with input from industry and government. SAX-40 culminated in a revolutionary concept design for a very quiet aircraft (see Figures 5-12 and 5-13). The concept design includes an airframe and engines designed specifically for a steep, low-speed climb and a low-noise approach that reduces both the amount of noise generated and the ground area of noise exposure. Some of the noise reduction technologies are listed below: a novel three-fan design that allows UHBR and hence lower jet noise low fan speeds that emit less noise extensive use of acoustic liners to absorb fan noise engines embedded in the fuselage, with intakes above the wings, to shield much of the engine noise from the ground variable area nozzles that allow engines to operate with low-speed, low-noise exhaust jets at takeoff and on ascent and then can be optimized for minimum fuel burn and carbon dioxide emissions at cruise elimination of flaps and slats low-noise fairing on the undercarriage 1 SBAC is the Society of British Aerospace Companies. After a merger of three companies, it is now ADS, which is Aerospace|Defence|Security. 2 Ahuja, K.K. 2008. Summary of the Aircraft Noise Day of the CAETS Workshop on Transportation Noise Sources in Europe, June 2–4, 2008, Southampton, United Kingdom. Presentation at the Workshop on New Technologies for a Quieter America, National Academy of Engineering, Washington, DC, June 11–12, 2008. Unpublished. A summary of the CAETS workshop is available online at http://www.noisenewsinternational.net/docs/caets-2008.pdf.
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Technology for a Quieter America FIGURE 5-8 Aircraft noise research initiatives undertaken in Europe under the Framework Programs. Source: LEMA (2008). FIGURE 5-9 Engine/nacelle noise reduction technologies. UHBR = ultra high bypass ratio. Source: SBAC (2009).
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Technology for a Quieter America FIGURE 5-10 Aircraft noise reduction technologies. Source: SBAC (2009). FIGURE 5-11 Negatively scarfed intake reflects fan noise away from the ground. Source: The Jet Engine, 2005. Reprinted with permission from Rolls Royce, 2005. FIGURE 5-12 SAX-40 silent aircraft. Source: SBAC (2009). Copyright Silent Aircraft Initiative.
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Technology for a Quieter America FIGURE 5-13 SAX-40 engine design. Source: SBAC (2009). Copyright Silent Aircraft Initiative. SAX-40 is predicted to achieve a reduction in noise of 25 dB based on current standards and also a reduction in fuel consumption of about 25 percent for a typical flight. Although these results are impressive, the SAX-40 is a concept design only. Further work must be done to confirm the feasibility and develop and validate the novel technologies. EnVIronmenTALly (VITAL) Friendly Aero Engine The VITAL program is a four-year, €90 million, EU-wide R&D program that began in January 2005 and has 53 partners. The partners, major stakeholders in the European aviation industry, include all major engine manufacturers, Airbus, and equipment makers, as well as innovative small businesses, universities, and research centers. The goal of this Snecma-led program is to integrate the results and benefits in noise reductions of the SILENCE(R) program with the emission reductions achieved in the Affordable Near Term Low Emissions and Component vaLidator for ENvironmentally friendly Aero Engine programs. By the end of VITAL, there should be a noise reduction of 8 dB per aircraft operation and an 18 percent reduction in carbon dioxide emissions, compared to engines in service prior to 2000. To reduce engine noise, very high bypass ratio engines with novel low-noise, low-speed fan designs are being studied. One of these designs, the contrarotating turbo fan, is shown in Figure 5-14. VITAL also plans to demonstrate a low-pressure compressor and turbine technologies designed for low noise and weight and compatible with the novel fan designs. An overview of the VITAL project was given by Bone (2009) at a European Engine Technology Workshop in Warsaw, Poland. EU Clean Sky Initiative The goal of the Clean Sky Initiative is to create a radically innovative air transport system with a reduced environmental impact based on less noise and gaseous emissions and better fuel economy. The specific objective is to reduce carbon dioxide emissions by about 40 percent, nitrogen oxide emissions by 60 percent, and noise by 50 percent in time for a major fleet renewal in 2015. The approach is to conduct an overall assessment of individual technologies at FIGURE 5-14 Schematic drawing of contrarotating turbo fan design to be studied in VITAL. Source: EU (2007).
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Technology for a Quieter America the fleet level to ensure the earliest possible deployment of research results. The budget for Clean Sky is up to €800 million from the 7th Research Framework Program, which will be matched by funds from industry. The total budget could be as high as €1.6 billion. The research partners include all major aeronautical players in Europe, almost 100 organizations that are active in aeronautical R&D and many SMEs, research centers, and universities. The technical and geographical scope of a typical team is shown in Table 5-1. The program is organized around six technical areas, called integrated technology demonstrators (ITDs), that will (1) perform preliminary studies, (2) select research areas, and (3) lead large-scale demonstrations either on the ground or in flight. The ITDs are “smart” fixed-wing aircraft, “green” regional aircraft, “green” rotorcraft, sustainable and “green” engine systems for “green” operations, and eco-design. OVERALL OBJECTIVES OF ALL AERONAUTICS RESEARCH PROGRAMS The goal of all of the programs described above is to have a “silent” aircraft in the future, that is, for the average sound pressure levels from all aircraft noise sources not to exceed sound pressure levels from other sources beyond airport boundaries during departure and arrival operations. In the next 20 years, newly designed aircraft are likely to be introduced at a rapid rate. These aircraft will likely be based on current aircraft but designed to achieve significant reductions in noise and carbon emissions. In the longer term (beyond 2025), further reductions in noise and carbon emissions are likely to require the development of entirely new aircraft and engine configurations. TABLE 5-1 Team Members Available to Work on European Noise Reduction Programs X-3 Team Partners Country Ain Shams University Egypt Alenia Italy ANOTEC Spain A2 Acoustics Sweden Budapest University of Technology and Economics (U.T.E.) Hungary COMOTI Romania Czech Technical University (T.U.) Czech Republic EADS CRC Denmark EPLF (Ecole Polytechnique Fédérale de Lausanne) Switzerland Federal University of Santa Catarina Brazil FFT (Free Field Technologies) Belgium Gediminas T.U. (Technical University) Republic of Lithuania INASCO Greece Institute of Aviation Poland Instituto Superior Tecnico Portugal ISVR United Kingdom National Aviation University Ukraine NLR Holland ONERA France Trinity College Ireland The enabling technologies for both phases of development are becoming apparent. It appears that one version of the futurist aircraft, based on lessons learned from SILENCE(R) and SAX-40, will mimic a hybrid wing/body (HWB) configuration. As NASA continues to work toward the introduction of a new generation of highly fuel efficient large aircraft as early as 2020, it is already planning wind tunnel tests of low-noise HWB aircraft (Figure 5-15 shows a typical HWB). Convinced that the HWB is the only way it can meet its goals, NASA is providing funding for Boeing to study improvements to the configuration to further reduce noise and improve fuel burn. NASA’s subsonic N+2 research is now focused on a cargo version of the HWB, and if all goes well, an HWB freighter could be available by 2020, with a passenger version to follow within 10 years. According to a report in Aviation Weekly (2009), Boeing, with funding from NASA and the U.S. Air Force, will test two low-noise HWB configurations—N2A and N2B—in a wind tunnel in 2011. N2A has padded engines mounted above the aft fuselage. N2B has embedded engines and S-duct inlets for lower drag. Both designs incorporate hybrid laminar flow control to further reduce drag. For NASA to achieve its goals of aircraft noise of 42 EPNdB cumulative below Stage 3 for the N+1 generation aircraft, considerable research will be needed in the following areas: target next-generation single aisle ultra-high-bypass engines noise reduction technologies for fans, landing gears, and propulsion airframe aeroacoustics lightweight acoustic treatment in multifunctional structures FIGURE 5-15 Hybrid wing/body aircraft with vertical tails on either side of the engines to shield jet noise. Source: NASA (2002).
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Technology for a Quieter America To meet the goal of aircraft noise of 52 EPNdB cumulative below Stage 3 for the N+2 generation aircraft, considerable research will be needed in the following areas: noise reduction from wing shielding of engines drooped leading edge continuous-mold line flaps landing gear fairings long-duct, low-drag acoustic liners distortion-tolerant fans with active noise control Objectives for the Air Transport System Meeting NASA’s Goals If NASA can meet its targets for the next three generations of aircraft, successively quieter aircraft would enter into service by 2015, 2020–2025, and 2030–2035, respectively (GAO, 2002). The likelihood of meeting these targets depends on a number of factors. First, federal funding will have to be available not only for NASA’s research but also for later-stage R&D, which NASA expects will be conducted by others. Second, even if funding is available, the development of noise reduction technologies may be limited by concerns about global warming, because advances in noise reduction technologies could make it more difficult to achieve reductions in aircraft emissions of greenhouse gases. Third, manufacturers must be willing to integrate newly developed technologies into aircraft and engine designs. Finally, airlines must purchase new aircraft or retrofit existing aircraft with the new technologies in sufficient numbers to achieve targeted reductions in exposure to aviation noise. If the FAA and NASA’s noise reduction goals are not met, this could impede efforts to reduce congestion by expanding the capacity of the National Airspace System (FAA, 2007). U.S. and European Visions of the Future In 2002 the Federal Transportation Advisory Group published Aeronautics Research and Technology for 2050: Assessing Visions and Goals, which compares civil aeronautics in Europe and the United States. Although the United States recognizes that its national well-being depends on a national transportation system with a strong aviation element, there is no explicit goal to ensure the primacy of the U.S. aeronautics industry. On the contrary, competitiveness is central to the European vision, so much so that it appears in the title of the document that defines this vision: European Aeronautics: A Vision for 2020—Meeting Society’s Needs and Winning Global Leadership (DG Energie et Transport and DG Recherche, 2001). NASA’s Blueprint (2002) and the European Aeronautics vision both specify that the ultimate goal in terms of operational impact is that aircraft noise be reduced to the point at which it is no longer a nuisance beyond airport boundaries and that airports be free of operational restrictions related to noise. The European Aeronautics vision highlights two areas not emphasized in any U.S. visions: (1) the quality and affordability of air transportation and (2) the global primacy of the aeronautics industry (FTAG, 2002a; NRC, 2002). According to the GAO report, by including quality and affordability issues, the European vision acknowledges the importance of structuring R&D programs to focus on providing air transportation services that users want to buy and can afford. NASA’s original goals issued in 1997 included reducing the cost of air travel by 50 percent in 20 years. However, this goal fell out of favor with Congress, which argued that meeting customer demands is an industry responsibility and not an appropriate goal for NASA’s research. Congress then reduced NASA’s aeronautics budget to eliminate research related to this goal (GAO, 2002). The European Aeronautics document foresees the future in the following way: In 2020, European Aeronautics is the world’s number one. Its companies are winning more than 50% shares of world markets for aircraft, engines, and equipment. The public sector plays an invaluable role in this success story. Crucially, they are coordinating a highly effective European framework for research cooperation, while funding programs that put the industry on more equal terms with its main rivals. Future Operational Procedures Limiting—on a yearly basis—the cumulative noise footprint in areas surrounding airports will effectively limit the capacity of the national aerospace system. Present departure and arrival procedures, which were developed when a limited range of navigational aids was available, are far from optimal from an environmental point of view. Therefore, in combination with new “silent” aircraft, the introduction of new approach, navigation, and flight management systems will make environmentally friendly procedures feasible. FINDINGS AND RECOMMENDATIONS A generation ago, “Higher, Farther, Faster” was the imperative for the future of air transport. Today, it is “More Affordable, Safer, Cleaner, and Quieter.” This change reflects the new emphasis on combining cost effectiveness with safety and environmental objectives. Significant investment is being made on both sides of the Atlantic to meet the demands of the market as well as the needs of the community. In the United States much of this effort has been led by NASA; in Europe significant investments have been made under the Framework Programs, in which EU industry and researchers in many countries work together in well-funded, coordinated R&D programs. The major challenge in the development of noise reduction technology for the future is that the design requirements
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Technology for a Quieter America blies than common-wall assemblies, and it would be helpful if the NRC of Canada provided data on the wide range of floor/ceiling assemblies in the built environment. The housing industry would also benefit from the development of theory and testing to characterize improvements to the design of acoustical materials such as the dynamic stiffness of resilient underlayers and how this information can be used to evaluate the IIC rating of floor/ceiling assemblies. Recent advances in the incorporation of damping into panelized building materials such as drywall should also be rated to provide a better understanding of how damping works in building sound isolation systems; this would also encourage product development. New material concepts should also be explored, such as distributed absorbers composed of heavy lumped masses embedded in a lossy sheet binder, which has been shown to improve the sound isolation of low-frequency airborne noise, and nanogels that offer high sound absorption and partial translucency. CLASSROOMS American National Standard S12.60, Acoustical Performance, Criteria, Design Requirements, and Guidelines for Schools is the first widely used standard for acoustical conditions in classrooms (available online at http://asastore.aip.org). This standard establishes limits for sound isolation between spaces; background sound produced by mechanical, electrical, and plumbing equipment and systems; and reverberation time. For the most part, sound isolation and reverberation control methods and materials are well known. However, this is not true for in-room unitary HVAC (heating, ventilation, and air-conditioning) units or classroom ventilation units. Currently, sound produced by these units exceeds the American National Standards Institute (ANSI) recommended maximum sound pressure level of 35 dB(A), thus requiring the use of central air distribution using air handlers, air heating and cooling methods, and air distribution terminal units. The cost of these systems, according to manufacturers, school building owners, and designers, is considerably higher than the cost of typical classroom ventilation units. Arguments by classroom equipment manufacturers to exclude or significantly raise permitted sound levels in order to permit the use of noisier conventional units have not persuaded the standards and education communities; the standard has not been modified. Nevertheless, the cost of school buildings and the need for flexibility are important issues. Hence, quiet design concepts for classroom ventilation units should be investigated. So far, manufacturers have had only limited success in developing units that are comparable in cost to more conventional central system equipment. Certain manufacturers of electroacoustical products (microphones, loudspeakers, etc.) have argued that their systems can be used in place of more expensive architectural solutions to background sound, sound isolation, and reverberation. Most of these are one-way systems; the teacher speaks into a microphone and students wear hearing assistance devices. These systems generally do not work for student-to-student communication or student-to-teacher communication. The use of electroacoustical solutions to architectural acoustics problems is hotly disputed in the architectural acoustics profession. However, there may be a place for electroacoustical devices in classrooms, particularly for hearing-disabled students or those who have different learning styles. GREEN ACOUSTICAL DESIGN The importance of Leadership in Energy and Environmental Design (LEED) certification5 for newly constructed buildings and for the reuse/rehabilitation of existing buildings is rapidly becoming a focal point of building design. Whereas only two years ago little attention was paid to green design, including acoustics in the green design of buildings, it is now being addressed in some cases, notably in classrooms and hospitals. Up to now, acoustics has played a minor role in the LEED rating of a building, although significant contributions to LEED ratings have been possible through high-recycled-content products, such as acoustical ceilings, duct silencer fill, and the use of acoustical products produced near project sites. It is expected that the availability of green acoustical products will increase over time. Green factors affect all building systems, which in turn affect the acoustics of a building. In a post-occupancy survey of building acoustics (see Muehleisen, 2009), it was found that bad acoustics was at the top of the list of undesirable features (acoustics, thermal comfort, air quality, lighting, etc.) for all buildings and was considered even more undesirable for green buildings. Some features of green buildings that are considered important for reasons other than acoustics include more use of natural lighting, natural ventilating systems, use of hard interior surfaces, maximum use of windows (especially when they must open), and the lack of conventional (porous) acoustical materials. All of these features tend to degrade the acoustical quality of workspaces. Some green features include lower partial-height partitions, which may be used to extend natural light farther into an open-plan building space. However, this can reduce speech privacy between workstations. Another feature is the use of green materials that, in many cases, absorb less sound than conventional materials. However, this can result in an excessively reverberant environment and reflections from the ceiling can compromise speech privacy in open-plan offices. Natural ventilating systems are considered to be desirable in green buildings, but they can transmit noise throughout a building. The ability to open windows is considered desirable but can result in transportation noise entering a building and being transmitted through the ventilation system. Lack of conventional acoustical materials in buildings can affect 5 LEED is an initiative of the U.S. Green Building Council.
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Technology for a Quieter America speech privacy, as mentioned above, and can also affect speech intelligibility in conference rooms. Electronic sound masking, widely used in open- and closed-plan offices since the 1960s, is now necessary as a means of maximizing speech privacy. But, although electronic sound masking can go a long way toward ensuring acceptable speech privacy, it is usually not a sufficient solution. Green solutions to office workstation partition height and sound absorption will have to be developed. The requirement for more natural ventilation, including opening of windows, just adds to the challenge. Razavi (2009) has reported on some acoustical improvements in green building ventilation systems, but the noise control engineering and architectural acoustics community face a major challenge in integrating good acoustical conditions into green buildings. The Green Guide for Health Care and the Green Guide for Schools establish design objectives for acoustical building characteristics, including reverberation, sound isolation, and ambient sound in building spaces (http://www.gghc.org; http://www.buildgreenschools.org). LEED points6 are added if these objectives are met using green materials and methods. AIR DISTRIBUTION SYSTEMS The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has been the pioneer and sole standard bearer in the development of standards and estimation methods for sound produced by air distribution systems. Much to the organization’s credit, it has funded most of the research that is now the exclusive basis for estimating and evaluating sound produced by building ventilation systems. Designers of building mechanical systems rely on the ASHRAE Guide (ASHRAE, 2007), a handbook updated every five years that covers all aspects of the design of building mechanical systems (thermal and ventilation). The ASHRAE Guide includes a chapter on design goals for sound produced by building mechanical systems. The design goals are divided into noise criteria, room criteria ratings, and A-weighted sound pressure levels. The algorithms for estimating sound in building spaces are based on work by Reynolds and Bledsoe (1989). Little progress has been made since 1989, when the algorithms were published, despite efforts by TC 2.6 (the ASHRAE committee on sound and vibration) to improve the situation. In fact, it has been generally agreed that the previously used general method of estimating the sound power level of ventilation fans should be dropped from the Guide because of its unreliability. It would be beneficial if industry and academia formed a partnership to study the acoustical literature, produce some additional theory and testing, and include new information in the ASHRAE Guide. This would reinforce the tools used by the mechanical engineering profession to address sound produced by new green mechanical systems, such as numerous small fans operating in parallel in lieu of a single large fan, new concepts in passive induction units that replace fan-powered terminal units, and the development of new, quiet classroom ventilators (discussed above). ENTERTAINMENT VENUES The rapid increase in multifamily urban dwellings is likely to increase demand for public entertainment venues, both inside and outside buildings, particularly small venues that can nurture a sense of community. Small venues can provide opportunities for the performing arts in intimate, attractive performance spaces. Entertainment in the broadest sense includes music, cinema, and theater but also dining and parks. The proliferation of small entertainment venues would open the door to commercial opportunities in lighting, sound system equipment, and computer-controlled software, all of which have been addressed in the marketplace. However, the proximity of entertainment venues to living spaces, and community annoyance from sound that sometimes results, can be a significant challenge. Rather than prohibiting such proximity, communities and developers should be guided by planning guidelines and codes that protect residences with only minimal compromises in performance or entertainment. Conflicts that arise between performers and the public were discussed and resolved in a decision by the U.S. Supreme Court in 1989 (Ward, 1989). FINDINGS AND RECOMMENDATIONS More people are probably affected by noise inside buildings—such as sound transmission in multifamily buildings, noise (and reverberation) in classrooms, noise in residences from road, rail, and air traffic, and noise in hospitals—than in any other environment. Clearly, trade associations and professional societies will play important roles in the design and construction of quieter interior spaces. Recommendation 5-5: The acoustics and noise control communities should actively promote the inclusion of noise criteria in requirements for Leadership in Energy and Environmental Design (LEED) certification of buildings, not only to improve the noise environment but also to ensure that the acoustical environment is not degraded. Design standards (e.g., building codes) must be improved to ensure that good acoustical practices are followed in the construction of buildings. Recommendation 5-6: The National Institutes of Health and/or the Facilities Guidelines Institute should fund the de- 6 LEED certification involves awarding points for various aspects of green designs.
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Technology for a Quieter America velopment of improved materials for hospital environments, where traditionally used materials may harbor and promote the growth of bacteria and other harmful biological agents. MODELING, SIMULATION, AND DATA MANAGEMENT Perhaps the greatest change in technologies for noise reduction has occurred because of increased computational power, which has changed the way products are designed, tested, and analyzed. We now have tools for defining and manipulating structures and mechanisms, for modeling and simulation, for laboratory measurements on prototypes, and for processing and interpreting voluminous amounts of data. Mechanism analysis programs compute the motions and forces of gears, cams and followers, cranks, and sliders that are the sources of audible energy in many products. It is said that a sewing machine contains more interesting mechanisms per dollar than any other product. The forces that these mechanisms place on their supports lead to vibrations in the product structure that are analyzed using finite element analysis. These vibrations in turn cause radiated sound, analyzed by boundary element analysis. The computer also has just as important a role in the experimental testing that is part of the product engineering process. Accelerometer arrays allow the measurement and display of the natural modes of structural vibration, and postprocessing using modal analysis programs is used to test the validity of both the measured modes and those computed using finite element analysis. Microphone arrays allow the quantification and display of the radiation of sound from the product using software for acoustic intensity and acoustical holography. As discussed below, the existence of these technologies does not mean that product companies are able to take advantage of them. Cost—in terms of the acquisition of the software/hardware and the commitment to the training and retention of specialized personnel—can be a problem, particularly to smaller companies. Making these new methods more affordable and available to companies is a challenge to be met. MODELING AND SIMULATION Traditional modeling for sound has been based on “canonical problems” representing different aspects of a sound source. Simple examples include radiation from bending waves on a plate to estimate sound from machine or equipment housing and a simple monopole source of sound to represent the radiation of sound from the unsilenced inlet of a compressor. These models can be useful aids to understanding, but they cannot deal with all aspects of design. Some modeling procedures are oriented toward describing and analyzing mechanisms. These models, which can compute motions and forces attributable to cams, followers, and other components, enable computation of forces at supporting points, combined with structural finite element analysis, to predict vibrations of the structure. Other software packages can use information about structure and vibration to compute radiated sound. Although at one time these capabilities were available only in distinct packages, software companies today offer them as an integrated package. In some products, airflow and heat transfer, accompanied by noise from fans and airflow, represent a different kind of interaction between mechanisms, product geometry, and sound production. Progress toward an integrated procedure is not as advanced as in the example cited above, but there is little doubt that integration will be achieved in the near future. DATA MANAGEMENT AND ANALYSIS Microphone arrays are now commonly used to characterize radiation from a structure. The analysis of these data can take the form of acoustic intensity or acoustical near-field holography. Data rates are typically 50 kilobits per second for 24-bit words; thus, a 10-second recording is 1.2 megabits of data and a 100-microphone array will generate 120 megabits of data per experiment. This kind of data collection can be done with modern (even ordinary) computers, but keeping track of all of these data for later processing can be a challenge. Generating the intensity and/or hologram graphs for N channels of data may require as many as N(N − 1)/2 cross spectra for these data records of a simple 10-second experiment that will be repeated many times. Similar issues arise in collecting and processing vibration data to correlate with acoustical data. Accelerometers are the most widely used sensors, but new scanning, three-axis laser vibrometers are increasingly being used. The latter have signal processing, in the form of cross spectra between channels, “built in” to the system. A laser vibrometer channel is much more expensive than an accelerometer channel, but in some situations being able to analyze data without physical contact between the sensor and the structure or the airflow can be valuable. CONSUMER PRODUCTS MAKING PRODUCTS QUIETER AND SELLING QUIET U.S.-made white goods (major household appliances such as refrigerators, dishwashers, and cookers), health care devices, personal care products, and other products are mostly sold on the domestic market; the export sector is relatively small. In addition, foreign competitors are moving into U.S. markets and challenging U.S. companies abroad. The sound and sound quality of products is important for market acceptance, and technology for improving sound and/or producing quieter products is important for maintaining U.S. competitiveness. (See Chapter 6.)
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Technology for a Quieter America Although the current economic situation may slow reductions in product noise, there is little doubt that consumers have concluded that quieter products are better built and have “real quality” and not just better “sound quality.” On the other hand, the market has not favored developments that result in increased prices. Thus many consumer products become commodities with different manufacturers meeting the same price points and offering very similar products. In some product sectors, however, consumers are willing to pay more for products with extra features or materials. For example, new countertop cookers and refrigerators with brushed steel exteriors and countertops made of granite have become status symbols and statements of achievement. Kitchens are becoming gathering places where these products are displayed. These upscale products (made both in the United States and abroad) are generally quieter and have profit margins sufficient to support extra engineering and manufacturing costs. But these products, although growing, remain a smaller part of the market. There is still a need to make the technology for better noise control more available in the manufacturing environment where cost constraints are very important. PRODUCT SOUND QUALITY Metrics for product sound are important for controlling noise exposure, measuring customer satisfaction, and guiding design. The acceptability of the sound of a product is influenced by user expectations, context, and signal content or information. Unfortunately, noise control professionals have labeled product sound as “product noise,” implying that any sound from a product is undesirable. Perhaps as a reaction to the notion of product sound as product noise, the most attention has been paid to metrics, such as A-weighted sound pressure level, that measure noise exposure, annoyance, and hearing impairment and reflect negative reactions to sound. However, hearing scientists (psychologists and engineers) and product designers are aware that A-weighted sound level is an imperfect measure for predicting product sound acceptability. Recent work has focused on defining physical metrics that can select out certain sound signal features that are separately audible and are likely to be associated with positive or negative reactions to sound. In some cases the link between metrics and design is very strong. Product engineers in the automotive industry can sit at a workstation, manipulate signals by filtering and other means, and decide that certain signal features (tones, modulation, and transients) should be changed to achieve a more desirable sound for the driver and passengers in a car. The “sound quality” programs used allow them to modify signals and process the resulting signals to determine changes in 20 to 30 physical metrics. The changes in values are an indication of how the sound should be evaluated as design changes are made. In this case the first evaluation is made by an engineer or a product designer. Jury (listening panel) studies are a useful mechanism for designing for better sound quality. Listeners are presented with a group of sounds from real or virtual products and asked to rate them in terms of acceptability. The number of sounds, their order, the number of listeners, and the scaling of responses are all part of the experimental design. In a sense the jury is a measuring instrument, the output of which is a measure of sound quality. But to anticipate the effect of future design changes on sound quality, either the jury study must be repeated or a correlation must be found between physical metrics and the jury’s response. Historically, acousticians have associated perceptual aspects of sound with individual physical metrics. Thus, the perception of loudness correlates well with the physical metric of “loudness.” A similar correlation between the perception of annoyance and the metric “noisiness” was developed for jet aircraft and later applied to other noise sources. But as the perceptions become more complex, involving expected, informative, and hedonistic dimensions, the correlation between any single physical metric and perception breaks down, and one is required to look for patterns of acceptability or sound quality of a product, and that correlation will be different for each product. This has been expressed as “a good lawnmower does not sound like a good washing machine.” Physical metrics in use include tonality (the presence of tones in the signal), spectral balance (high-frequency versus low-frequency content), fluctuation strength (presence of modulation), and roughness (nonharmonic dissonant components) as well as loudness and noisiness. One sound quality program evaluates nearly 20 such physical metrics to form a profile of values to correlate with jury judgments of product sounds. Products for which such metrics profiles have been used to correlate with jury study judgments of sound quality include washing machines, dishwashers, vacuum cleaners, cookers, and room air cleaners. The metrics profile that best correlates with good sound quality (or most acceptable) will be different for different products, but there are certain features of the sound that are undesirable for any product. Loudness, noisiness, tonality, and fluctuation strength are all undesirable if too strong. Modulation is an interesting example because it is very desirable in music as vibrato or tremolo but undesirable in a product sound. The reason seems to be that modulation captures our attention—desirable in music, undesirable in a product. There is little cost to generating a profile of 20 or more metrics since this only requires running the same sound samples that are to be presented to a jury through the signal processing algorithm for each metric. Using a larger set of physical metrics can give some reassurance that nothing has been missed, but making sense of the profiles can be difficult. If the metrics profile for each sound is labeled with the jury evaluation for that sound, it is possible to combine the metrics into a smaller set of variables using the method of principal component analysis.
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Technology for a Quieter America Manufacturers would like to have a single metric such as A-weighted sound level that would enable them to claim their products have better sound than their competitors and can also be used in product development. Organizations such as Consumers Union that routinely evaluate products for sound would also like such a metric. Unfortunately, the correlation between any single metric and sound quality and the outcome of jury studies has not been generally accepted by the acoustics community, so claims that one product has “better sound” than another cannot be supported by physical metrics, even though improvement in the sound quality of a particular product in a particular organization is possible. For more information on product sound quality, see Lyon (2000, 2004) and Lyon and Bowen (2007). R&D IN SUPPORT OF QUIETER PRODUCTS Sound is very important for some products (e.g., automobiles), and companies spend heavily in terms of facilities and personnel to make these products quiet and pleasing. But in the past 40 years or so, the price of an automobile has risen by a factor of more than 10, while the price of a dishwasher has risen by a factor of 3 to 4. One result is that while the automobile companies have developed large staffs and good facilities for sound, most appliance companies have not (with one notable exception). In typical appliance and health care products companies, engineers are “jacks of all trades,” working one day on problems of airflow or heat transfer and the next on product sound. Also, these engineers may have significant motivation to move around in a company where the path upward is through management and not technical expertise. Another factor that affects nonautomotive producers is the pace of model changes. Appliances, health care, and personal care products go through much more frequent changes, so consumers will replace older products or choose to buy a newer product because of a desired feature. The effect of this is to compress development schedules and to limit the transfer of a new development (e.g., a quieter way to support a small motor) into the new model. It would appear that simpler products such as a sleep apnea device should have noise issues that are simpler. But this product has a couple of brushless DC motors, a fan, an air pump, and valves, each of which produces audible sound in a device that is in someone’s bedroom at night. In addition, cost and utility constraints mean the enclosure is lightweight and stiff, a perfect construction for the efficient radiation of sound. The manufacturer probably buys the motors from a manufacturer in China and finds it impossible to convince his supplier to do the engineering to make the motor quieter. There are other trends that are not helpful in terms of product sound. Design for manufacturing has a cachet that is attractive to industry because of lower assembly costs and easier model changes. One such method is “layering,” in which an assembly is achieved by placing components into the supporting structure in a sequence that minimizes the need for reconfiguring the assembly. When this method was applied to a popular electric mixer, its noisiness was significantly increased because of the increased tolerances in the drive train gearing inherent in this method of assembly. The basic message is that issues of product sound are very complex and do not become simpler and easier to handle because a product is simpler and less costly. Indeed, the situation may be quite the opposite. But there are good tools for meeting the need. The question is: are they being used and, if not, why not? TOOLS FOR QUIET PRODUCT DESIGN AND TESTING Most companies now use computer-aided design (CAD) software to visualize their product designs and to anticipate problems of parts interference and fit before a prototype is built. These CAD programs can be interfaced with certain computer-aided engineering programs like finite element analysis for structural analysis (stiffness, resonant modes, mass distribution) or dynamic analysis for mechanism forces. But these programs (discussed above) while useful, are limited in their assistance in designing for quiet function. For example, a fan can be analyzed using a computer fluid dynamics (CFD) program, which most likely does not reflect the actual flow environment of a typical product. Also, these programs are very expensive to run, and considerable expertise is needed to run them. Most consumer products companies will not make the investment in personnel or funds to have their products analyzed in this way. Some CFD providers will work with manufacturers on a consulting basis to provide such analyses, but the process remains expensive and the idealized calculations may not provide the information needed for design decisions. Manufacturers are more likely to invest in experimental facilities than software for analysis for several reasons. First, the cost of experimental equipment has been coming down and its capabilities are increasing. Multichannel systems of microphones and vibration sensors (accelerometers) involving dozens of sensors are now commonplace, and the software to analyze the patterns of sound and vibration, such as acoustical near-field holography and modal analysis, is widely available. Also, experimental work is generally more relied on in product development than is analysis. The ability to keep engineers in place long enough to become proficient in the use of both hardware and software remains an issue but seems to be much less of an issue than for the analytic software. WHAT’S NEXT? Although the current economic situation may slow sound improvements, there seems little doubt that consumers have become convinced that quiet products are better built and have “real quality” and not just better “sound quality.” So
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Technology for a Quieter America the issue of better sound as a marketing feature will not go away, and the need to support the industry in its attempts to meet this marketing and technical challenge will not go away. Thus the technology for better sound must be more available in the manufacturing environment where cost constraints are very important. ACTIVE NOISE CONTROL The most efficient and cost-effective way of reducing noise is to design equipment to produce less noise. If this strategy has been fully implemented and additional noise reduction is needed, add-on measures must be applied. Active noise control is one of these measures. Most noise sources produce noise in a wide frequency range. Passive noise control measures (such as silencers, acoustic enclosures, wrappings, barriers, etc.) usually provide sufficient noise reduction at middle and high frequencies (approximately 200 Hz and above), and they are robust, reliable, and cost effective. However, they are ineffective at low frequencies (below about 200 Hz). At these low frequencies, active control becomes an alternative; it may be the only solution for frequencies below 100 Hz. Noise sources such as gas turbines and large reciprocating compressors produce high levels of low-frequency noise. Almost without exception, the noise control of such sources requires a combination of both passive and active measures. The passive measures attenuate the mid and high frequencies, and the active measure attenuates the low frequencies. There are four major active noise control strategies: Reducing the sound radiation efficiency of the sound source by placing a secondary source (loudspeaker in an enclosure) in its immediate vicinity and driving it with an electric signal that produces the same magnitude but opposite phase fluctuating volume flow as the primary noise source. In this case the air volume pushed out of the primary source during the positive cycle fills the void generated by the receding volume of the secondary source and, conversely, the receding volume flow of the primary source is supplied by the outflow from the secondary source. This strategy, which reduces the radiation efficiency of the original source and effectively reduces the noise level at all locations, is sometimes referred to as “global” noise reduction. Creating a limited “zone of silence” in the vicinity of the receiver (the person to be protected) by sensing the local sound pressures, driving the loudspeaker with an electric signal (located as near to the receiver as practicable) that produces a sound pressure of the same magnitude and opposite in phase as the primary signal. This is the only situation where “noise cancellation” is appropriate. This active noise control strategy, in almost all cases, is inferior to the first strategy because its effectiveness is limited to a single area. Because this strategy does not affect the sound power output of the primary source and creates a secondary source, the overall noise level is increased in locations where cancellation does not occur. A good practical application of this strategy are noise-canceling headphones, such as those manufactured by the Bose Corporation that achieve a significant reduction in sound pressure level in the ear canal. Increasing the low-frequency sound attenuation of tuned dissipative silencers by placing actuators (loudspeakers) in the cavity behind the thin porous lining as described by Vér (2000). The sound pressure is sensed behind the porous lining by a microphone and entered into a control system that feeds the loudspeaker with a signal so that for a wide frequency band it produces (nearly) zero sound pressure immediately behind the porous liner. This condition maximizes the sound pressure gradient across the liner and consequently its ability to absorb sound. In a passive silencer this condition occurs only at single frequencies where the depth of the airspace is one-quarter the acoustic wavelength and at odd multiples of that frequency (frequency, f, and wavelength, λ, are related by f = c / λ, where c is the speed of sound). When the noise is produced by the sound radiation of a structure exited to vibration by localized dynamic forces (such as the attachment points of the wing of an airplane to a ring frame), the most efficient way to obtain global noise reduction is to mount a shaker at the attachment point and feed it by a control system to produce nearly zero vibration (i.e., render nearly zero power input to the structure). Here, again, the noise that is attributable to the vibration force is reduced at all locations. One early example of active control was the electronic sound absorber (Olson and May, 1953), which was a microphone, phase inverter, and loudspeaker that could be used to create a “zone of silence” around the head of a factory worker. At that time all of the circuits were analog, and phase shift through the system was critical. It was not until digital signal processing became feasible that applications began to be developed. Active control of sound is effective only when the wavelength of the sound is long compared with the dimensions of the volume in which cancellation is desired. For example, the most successful application of the technology is in active headsets where cancellation of sound in the (small-volume) ear canal is desired. Another example is cancellation in the cabin of a turboprop commuter airplane, which requires a large number of microphones and loudspeakers and is only effective at low frequencies. This limitation of cancellation to low frequencies also
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Technology for a Quieter America has implications for sound perception, sound quality, and hazard to hearing. A listener may perceive the sound as lacking in low frequencies. Hence, it may sound “hissy.” The A-frequency weighting network already attenuates low-frequency sound, and therefore additional attenuation through active control may not produce a significant decrease in the A-weighted sound level. According to current standards, a small decrease in A-weighted sound level produces only a small decrease in the hazard to hearing. APPLICATIONS OF ACTIVE CONTROL Despite the complexity of active control and the above limitations, the technology has been applied in a number of cases. Some examples are given below. Active headsets provide noise reduction and both comfort and protection from hazardous noise for the user. The Federal Railroad Administration has demonstrated both active control in locomotive cabs and proof of principle for active control of exhaust stack noise from idling locomotives. Hansen (2005) developed an active control system to control sound propagation in the exhaust stack of a spray dryer unit in a dairy factory. Scheuren (2005) discussed a number of engineering applications of active control, including wind tunnel buffering, control of combustion burners, noise control in gas turbines, and modification of sound in the cabin of automobiles. Cancellation of the blade passage tone in a small axial flow fan was achieved by Sommerfeldt and Gee (2003) by using four small cancellation loudspeakers placed around the fan. There are a number of applications of active control in the aerospace industry; these have been described by Maier (2009). Gorman et al. (2004) produced noise reduction on the flight deck of an airplane, and Cabell et al. (2004) have shown how active control can be used to control chevrons and produce noise reduction of a jet engine exhaust. Finally, Fuller et al. (2009) reduced noise from a portable generator set by using active control. Impediments to Commercial Development Despite the long history of the development of active control technology and digital processing systems, there are few devices (except for active headsets) on the market today. Some of the barriers to commercial development are expense and reliability as well as the materials used and characteristics of transducers, amplifiers, and materials. Active control systems are expensive to implement because of the required microphones (or accelerometers), loudspeakers (or force transducers), and electronic control systems. If a universal control system were to be developed, it would have to be versatile because the control algorithm will depend on the type of noise being canceled (e.g., a single-frequency tone, a tone in noise, or broadband noise). Reliability is also an issue in complex systems. For high-intensity noise sources, high-powered amplifiers and special loudspeakers may be required. There is also the problem that the materials used for transducers (microphones, accelerometers, loudspeakers, force transducers) must, in many cases, withstand hostile environments. Examples are hot exhaust gases and turbulent flow. There is a rich literature on active control of sound and vibration. This includes books (Hansen and Snyder, 1997; Nelson and Elliott, 1993), technical articles (Nelson and Elliott, 1993; Tichy, 1996), and conference proceedings papers (ACTIVE, 2009; Fuller, 2002). Recommendation 5-7: Research agencies should fund university research on active noise control to address situations where the use of traditional noise-control materials is problematic or where they are not suitable for attenuating noise in the appropriate frequency range. Investigations into hybrid active-passive and adaptive-passive noise control systems and the development of low-cost microphones and loudspeakers that can be used in hostile environments should also be funded. SUMMARY Active controls of sound and vibration have been under development for many years, but few products on the market have incorporated them, and many barriers must still be overcome. In this chapter, technologies for controlling noise from a large variety of sources have been described. Clearly, aircraft noise control technology is much more advanced than technologies for addressing other noise sources, and the funds expended to reduce the noise of airplanes themselves as well as mitigation measures around airports are far greater than for other noise sources. Road traffic noise has been controlled mostly by constructing noise barriers, but work is being done on promising technologies for reducing noise generated by tire/road interaction. Technologies are available for reducing noise from rail-guided vehicles, and these will become more important as the nation develops light rail systems and high-speed trains. Technologies for the built environment will also become more important as building construction is driven by LEED certification and “green” principles. REFERENCES 23 CFR 772. Procedures for Abatement of Highway Traffic Noise and Construction Noise. Available online at http://www.fhwa.dot.gov/hep/23cfr772.htm. ACTIVE. 2009. Collected papers from the ACTIVE series of international symposia on active control of sound and vibration: 1995, 1997, 1999, 2002, 2004, 2006, and 2009. Available online at http://www.atlasbooks.com/marktplc/00726.htm. Ahuja, K.K. 1995. Aeroacoustic Performance of Open-Jet Wind Tunnels with Particular Reference to Jet/Collector Interactions. Final Report AEDC-SBIR-94-02. Reston, VA: American Institute of Aeronautics and Astronautics. Anjali, J., and R. Ulrich. 2007. Sound Control for Improved Outcomes in
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Feedback Control of a Morphing Chevron for Takeoff and Cruise Noise Reduction. Proceedings of ACTIVE 04, The 2004 International Symposium on Active Control on Sound and Vibration, Williamsburg, VA, September 20–22. Available online on the ACTIVE 2009 CD at http://www.bookmasters.com/marktplc/00726.htm. CAETS (International Council of Academies of Engineering and Technological Sciences). 2008. The Design of Low-Noise Vehicles for Road, Rail, and Air Transportation. Sponsored by the Royal Swedish Academy of Engineering Sciences and the Royal Academy of Engineering (U.K.), Institute of Sound and Vibration Research, Southampton, U.K., June 2–4. Available online at http://www.noisenewsinternational.net/docs/caets-2008.pdf. Cavanaugh, W.J., and J.A. Wilkes. 1998. Architectural Acoustics: Principles and Practice. New York: John Wiley & Sons. Cavanaugh, W.J., G.C. Tocci, and J.A. Wilkes, eds. 2010. Architectural Acoustics: Principles and Practice, 2nd ed. Hoboken, NJ: John Wiley & Sons. 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